WEBVTT 00:00:00.000 --> 00:00:02.940 align:middle line:90% [SQUEAKING] 00:00:02.940 --> 00:00:04.410 align:middle line:90% [RUSTLING] 00:00:04.410 --> 00:00:07.840 align:middle line:90% [CLICKING] 00:00:07.840 --> 00:00:11.280 align:middle line:90% 00:00:11.280 --> 00:00:15.420 align:middle line:84% DAVID KAISER: So today, we're going 00:00:15.420 --> 00:00:20.080 align:middle line:84% to pick up where we were on the most recent classes. 00:00:20.080 --> 00:00:21.960 align:middle line:84% So in the last few class sessions, 00:00:21.960 --> 00:00:27.090 align:middle line:84% we were looking at some changes in high energy particle theory 00:00:27.090 --> 00:00:29.880 align:middle line:84% with the development of things like quantum chromodynamics 00:00:29.880 --> 00:00:33.030 align:middle line:84% and so on and ways to try to make sense of very high energy 00:00:33.030 --> 00:00:36.570 align:middle line:84% interactions among elementary particles or at least 00:00:36.570 --> 00:00:39.120 align:middle line:90% particles smaller than an atom. 00:00:39.120 --> 00:00:41.290 align:middle line:84% And then in our most recent class session, 00:00:41.290 --> 00:00:47.770 align:middle line:84% we looked at some of the shifts within the fields of study, 00:00:47.770 --> 00:00:51.840 align:middle line:84% including the emergence of this relatively new subfield called 00:00:51.840 --> 00:00:55.680 align:middle line:84% particle cosmology, which really came together starting 00:00:55.680 --> 00:00:58.620 align:middle line:84% in the mid 1970s partly because of some really 00:00:58.620 --> 00:01:01.770 align:middle line:84% exciting new ideas, but also, as we've 00:01:01.770 --> 00:01:06.360 align:middle line:84% looked at in some detail, because of some broader 00:01:06.360 --> 00:01:10.290 align:middle line:84% institutional even geopolitical shifts in the physics 00:01:10.290 --> 00:01:14.470 align:middle line:84% profession that really helped some of these new ideas 00:01:14.470 --> 00:01:17.530 align:middle line:84% take hold in a way that they might not have in other times 00:01:17.530 --> 00:01:18.640 align:middle line:90% or places. 00:01:18.640 --> 00:01:23.560 align:middle line:84% So today, we're going to focus on a kind of example 00:01:23.560 --> 00:01:27.940 align:middle line:84% of that new subfield, a relatively new subfield that's 00:01:27.940 --> 00:01:31.528 align:middle line:84% known as inflationary cosmology or simply cosmic inflation. 00:01:31.528 --> 00:01:33.070 align:middle line:84% So to make sense of that, we're going 00:01:33.070 --> 00:01:37.450 align:middle line:84% to first look at some of the work in cosmology 00:01:37.450 --> 00:01:41.740 align:middle line:84% before this merger of fields, before particle cosmology 00:01:41.740 --> 00:01:46.580 align:middle line:90% really in into its own. 00:01:46.580 --> 00:01:50.440 align:middle line:84% So we'll look at the coalescence of what 00:01:50.440 --> 00:01:52.390 align:middle line:84% came to be called the Big Bang model, 00:01:52.390 --> 00:01:55.420 align:middle line:84% still enormously successful framework for trying 00:01:55.420 --> 00:01:58.150 align:middle line:84% to make sense of very large scale changes in our universe 00:01:58.150 --> 00:02:00.070 align:middle line:90% over a long sweep of time. 00:02:00.070 --> 00:02:06.200 align:middle line:84% And then we'll see that already by the 1970s and '80s, 00:02:06.200 --> 00:02:09.789 align:middle line:84% there was some curiosities or maybe 00:02:09.789 --> 00:02:12.790 align:middle line:84% inconsistencies with that otherwise quite 00:02:12.790 --> 00:02:14.150 align:middle line:90% successful model. 00:02:14.150 --> 00:02:16.810 align:middle line:84% And so people began thinking about shortcomings of the Big 00:02:16.810 --> 00:02:19.900 align:middle line:84% Bang model around the time that this new subfield 00:02:19.900 --> 00:02:23.320 align:middle line:84% of particle cosmology began to come together. 00:02:23.320 --> 00:02:25.450 align:middle line:84% And then we'll see how cosmic inflation emerged 00:02:25.450 --> 00:02:28.060 align:middle line:84% from that particular moment to try 00:02:28.060 --> 00:02:30.010 align:middle line:84% to address some of the shortcomings 00:02:30.010 --> 00:02:31.640 align:middle line:84% while retaining some of the successes. 00:02:31.640 --> 00:02:33.700 align:middle line:84% So it's a framework for trying to understand 00:02:33.700 --> 00:02:39.520 align:middle line:84% the evolution of our universe over a huge expanse of time, 00:02:39.520 --> 00:02:42.220 align:middle line:84% increasingly using tools at the interface, 00:02:42.220 --> 00:02:44.740 align:middle line:84% not just of Einstein's general theory of relativity, 00:02:44.740 --> 00:02:47.800 align:middle line:84% but also ideas about particle physics and high energy 00:02:47.800 --> 00:02:48.370 align:middle line:90% phenomena. 00:02:48.370 --> 00:02:49.320 align:middle line:90% [BEEP] 00:02:49.320 --> 00:02:50.570 align:middle line:90% So that's we're heading today. 00:02:50.570 --> 00:02:54.230 align:middle line:84% And the asterisks are to remind you there's a set of strictly 00:02:54.230 --> 00:02:57.080 align:middle line:84% optional lecture notes on the Canvas site 00:02:57.080 --> 00:03:00.290 align:middle line:84% which go into a little bit more detail of some of these parts 00:03:00.290 --> 00:03:01.190 align:middle line:90% from the lecture. 00:03:01.190 --> 00:03:03.650 align:middle line:84% Again, strictly for your own interest as your interest 00:03:03.650 --> 00:03:04.520 align:middle line:90% and time allows. 00:03:04.520 --> 00:03:06.295 align:middle line:84% But if some of these things go by quickly, 00:03:06.295 --> 00:03:07.670 align:middle line:84% there's some more material there. 00:03:07.670 --> 00:03:09.620 align:middle line:84% And again, I'd be glad to chat more about this 00:03:09.620 --> 00:03:12.200 align:middle line:84% if questions come up beyond that. 00:03:12.200 --> 00:03:12.740 align:middle line:90% OK. 00:03:12.740 --> 00:03:17.600 align:middle line:84% So oftentimes, astronomers will describe 00:03:17.600 --> 00:03:22.250 align:middle line:84% the most salient features of our universe 00:03:22.250 --> 00:03:25.070 align:middle line:84% in terms of what they call large scale structure. 00:03:25.070 --> 00:03:26.840 align:middle line:90% It's really quite remarkable. 00:03:26.840 --> 00:03:30.890 align:middle line:84% And this [CLEARS THROAT] picture's been emerging really 00:03:30.890 --> 00:03:34.760 align:middle line:84% over a century, for 100 years or even more, 00:03:34.760 --> 00:03:39.560 align:middle line:84% that when astronomers turn their telescopes to the sky and look 00:03:39.560 --> 00:03:42.530 align:middle line:84% at many different length scales, many different characteristic 00:03:42.530 --> 00:03:46.850 align:middle line:84% lengths, either very, very, very large length scales-- 00:03:46.850 --> 00:03:50.810 align:middle line:84% this is a modern example from the Hubble telescope deep field 00:03:50.810 --> 00:03:51.500 align:middle line:90% survey-- 00:03:51.500 --> 00:03:54.410 align:middle line:84% where you can look at clusters and even super clusters 00:03:54.410 --> 00:04:02.420 align:middle line:84% of galaxies on enormous scales, tens or even thousands of-- 00:04:02.420 --> 00:04:02.990 align:middle line:90% let's see. 00:04:02.990 --> 00:04:04.790 align:middle line:90% I want to get my units right. 00:04:04.790 --> 00:04:07.330 align:middle line:84% Basically billions of light years across. 00:04:07.330 --> 00:04:08.510 align:middle line:90% It's a huge scale. 00:04:08.510 --> 00:04:11.420 align:middle line:84% Or we can zoom in to the size of single galaxies 00:04:11.420 --> 00:04:14.300 align:middle line:84% like the Andromeda galaxy or our own Milky Way galaxy. 00:04:14.300 --> 00:04:17.750 align:middle line:84% Or if we zoom in even closer to home with the solar system 00:04:17.750 --> 00:04:20.060 align:middle line:84% or even really in human terms, there 00:04:20.060 --> 00:04:22.580 align:middle line:84% are concentrations of enormous matter 00:04:22.580 --> 00:04:25.912 align:middle line:84% and energy and activity separated by huge voids. 00:04:25.912 --> 00:04:27.620 align:middle line:84% I like to make the joke with my apologies 00:04:27.620 --> 00:04:29.520 align:middle line:84% to Tiffany that in the Cambridge example, 00:04:29.520 --> 00:04:31.520 align:middle line:84% we have all this stuff happening like by this data center, 00:04:31.520 --> 00:04:33.260 align:middle line:84% whereas there's nothing at all happening at Harvard. 00:04:33.260 --> 00:04:34.218 align:middle line:90% Is that right, Tiffany? 00:04:34.218 --> 00:04:35.390 align:middle line:90% Just a simple nod yes or no. 00:04:35.390 --> 00:04:36.080 align:middle line:90% Yeah, OK. 00:04:36.080 --> 00:04:37.010 align:middle line:90% Sorry, Tiffany. 00:04:37.010 --> 00:04:37.670 align:middle line:90% I'm teasing. 00:04:37.670 --> 00:04:40.970 align:middle line:84% The point is on scales from kilometers out 00:04:40.970 --> 00:04:45.110 align:middle line:84% to billions of light years and everything in between, 00:04:45.110 --> 00:04:48.560 align:middle line:84% we find this lumpiness, that there's a pattern to it. 00:04:48.560 --> 00:04:51.523 align:middle line:84% Matter is not uniformly smushed out in space. 00:04:51.523 --> 00:04:52.440 align:middle line:90% Thank goodness, right? 00:04:52.440 --> 00:04:54.750 align:middle line:84% There is actually teeming pockets 00:04:54.750 --> 00:04:58.590 align:middle line:84% of activity separated by large voids 00:04:58.590 --> 00:05:03.220 align:middle line:84% where very little matter or energy is located. 00:05:03.220 --> 00:05:04.830 align:middle line:84% And so one of the questions is, what 00:05:04.830 --> 00:05:07.470 align:middle line:84% could account for that structure across these scales 00:05:07.470 --> 00:05:10.590 align:middle line:84% from meters or kilometers up to tens of billions 00:05:10.590 --> 00:05:12.390 align:middle line:90% of light years? 00:05:12.390 --> 00:05:14.460 align:middle line:84% It turns out that ordinary gravity-- 00:05:14.460 --> 00:05:17.160 align:middle line:84% even Newtonian gravity, let alone Einstein's fancier 00:05:17.160 --> 00:05:19.230 align:middle line:84% version that we looked at in class, 00:05:19.230 --> 00:05:22.530 align:middle line:84% general theory of relativity-- that these gravitational 00:05:22.530 --> 00:05:25.560 align:middle line:84% frameworks are sufficient to help us make sense 00:05:25.560 --> 00:05:30.810 align:middle line:84% of this hierarchy of scales, of structure across large distance 00:05:30.810 --> 00:05:32.106 align:middle line:90% scales. 00:05:32.106 --> 00:05:35.610 align:middle line:84% [CLEARS THROAT] If we assume, to start, 00:05:35.610 --> 00:05:38.580 align:middle line:84% there's some initial, very tiny lumpiness 00:05:38.580 --> 00:05:41.880 align:middle line:84% to begin with, if we assume some very tiny inhomogeneity, 00:05:41.880 --> 00:05:44.940 align:middle line:84% a little bit of unevenness in the distribution of matter 00:05:44.940 --> 00:05:48.210 align:middle line:84% and energy at early times, then gravity will do the rest. 00:05:48.210 --> 00:05:49.770 align:middle line:84% Gravity will make those regions that 00:05:49.770 --> 00:05:53.070 align:middle line:84% happen to have slightly more matter or energy per unit 00:05:53.070 --> 00:05:57.600 align:middle line:84% volume than average-- the gravitational force will then 00:05:57.600 --> 00:06:00.610 align:middle line:84% attract more and more matter and energy to those local regions. 00:06:00.610 --> 00:06:04.440 align:middle line:84% So it'll become more and more dense, more pockets 00:06:04.440 --> 00:06:05.430 align:middle line:90% of activity. 00:06:05.430 --> 00:06:07.470 align:middle line:84% And meanwhile, the areas that happen 00:06:07.470 --> 00:06:10.440 align:middle line:84% to start out with slightly less than average matter 00:06:10.440 --> 00:06:13.380 align:middle line:84% and energy per volume, slightly under dense regions, 00:06:13.380 --> 00:06:16.080 align:middle line:84% will become more and more further evacuated. 00:06:16.080 --> 00:06:20.460 align:middle line:84% And so even in much more quantitative precision, much 00:06:20.460 --> 00:06:24.840 align:middle line:84% more detail, we can account for this array of structure 00:06:24.840 --> 00:06:28.680 align:middle line:84% from human size scales out to the super galactic using really 00:06:28.680 --> 00:06:33.090 align:middle line:84% only gravity as long as we start with some initial otherwise 00:06:33.090 --> 00:06:34.740 align:middle line:90% unaccounted for lumpiness. 00:06:34.740 --> 00:06:37.800 align:middle line:84% A tiny amount of unevenness in the distribution 00:06:37.800 --> 00:06:42.480 align:middle line:84% of matter and energy will grow more and more uneven over time. 00:06:42.480 --> 00:06:45.950 align:middle line:84% So a challenge for astronomers for a century or so 00:06:45.950 --> 00:06:49.088 align:middle line:84% has been to try to make that account more precise and more 00:06:49.088 --> 00:06:50.630 align:middle line:84% quantitative and compare it with more 00:06:50.630 --> 00:06:52.460 align:middle line:90% and more kinds of observations. 00:06:52.460 --> 00:06:54.960 align:middle line:84% And there have been two main conceptual ingredients, 00:06:54.960 --> 00:06:56.750 align:middle line:84% especially as we'll come to in the more 00:06:56.750 --> 00:07:00.410 align:middle line:84% recent versions of this in the era of particle cosmology. 00:07:00.410 --> 00:07:03.230 align:middle line:84% One of the sets of tools, the conceptual ingredients, 00:07:03.230 --> 00:07:06.230 align:middle line:84% not surprisingly, is some theory of gravity. 00:07:06.230 --> 00:07:08.810 align:middle line:84% And since [CLEARS THROAT] the early years 00:07:08.810 --> 00:07:11.960 align:middle line:84% of the 20th century, as we've seen many times in this class, 00:07:11.960 --> 00:07:15.230 align:middle line:84% the framework of choice has been Einstein's general theory 00:07:15.230 --> 00:07:17.840 align:middle line:84% of relativity, which, as we've seen, 00:07:17.840 --> 00:07:21.980 align:middle line:84% describes the phenomena that we associate with gravitation 00:07:21.980 --> 00:07:24.050 align:middle line:84% as really being nothing but geometry, 00:07:24.050 --> 00:07:27.170 align:middle line:84% being local deformations, local curvature 00:07:27.170 --> 00:07:29.540 align:middle line:84% in this almost physical type fabric 00:07:29.540 --> 00:07:32.630 align:middle line:84% of space time in response to the distribution 00:07:32.630 --> 00:07:33.740 align:middle line:90% of matter and energy. 00:07:33.740 --> 00:07:37.010 align:middle line:84% And the other main ingredient especially, as I say, 00:07:37.010 --> 00:07:40.310 align:middle line:84% refined in recent years with insights from high energy 00:07:40.310 --> 00:07:43.070 align:middle line:84% nuclear and particle physics has been some prevailing 00:07:43.070 --> 00:07:44.780 align:middle line:84% understanding of matter, especially 00:07:44.780 --> 00:07:47.510 align:middle line:84% matter at very high energies and temperatures, matter 00:07:47.510 --> 00:07:51.860 align:middle line:84% that consists of things like electrons and photons, 00:07:51.860 --> 00:07:54.740 align:middle line:84% matter that nowadays people are pretty well convinced 00:07:54.740 --> 00:07:56.660 align:middle line:84% consists of things like quarks and gluons 00:07:56.660 --> 00:07:59.210 align:middle line:84% and even some more exotic particles like the Higgs 00:07:59.210 --> 00:08:03.300 align:middle line:84% particle that we looked at least briefly in the previous class. 00:08:03.300 --> 00:08:06.260 align:middle line:84% So we have these two ingredients of the structure and behavior 00:08:06.260 --> 00:08:08.720 align:middle line:84% of space time as governed presumably 00:08:08.720 --> 00:08:12.680 align:middle line:84% by Einstein's theory or something perhaps similar to it 00:08:12.680 --> 00:08:14.990 align:middle line:84% and then the stuff that's filling that space time, 00:08:14.990 --> 00:08:17.900 align:middle line:84% an idea about matter, especially how matter behaves at very 00:08:17.900 --> 00:08:20.530 align:middle line:90% high energies and densities. 00:08:20.530 --> 00:08:22.260 align:middle line:84% So with those two ingredients, the goal 00:08:22.260 --> 00:08:25.560 align:middle line:84% has been, again, for many decades to account 00:08:25.560 --> 00:08:28.050 align:middle line:84% for the observational features of our universe 00:08:28.050 --> 00:08:29.680 align:middle line:90% even on very large scales. 00:08:29.680 --> 00:08:33.150 align:middle line:84% So as a reminder, the so-called field equations 00:08:33.150 --> 00:08:34.950 align:middle line:84% of Einstein's general theory of relativity 00:08:34.950 --> 00:08:37.710 align:middle line:84% take this deceptively simple looking form. 00:08:37.710 --> 00:08:40.169 align:middle line:84% This side is what he had to learn from his friend Marcel 00:08:40.169 --> 00:08:40.679 align:middle line:90% Grossmann. 00:08:40.679 --> 00:08:43.620 align:middle line:84% This is the geometry of a warping spacetime, the way 00:08:43.620 --> 00:08:46.050 align:middle line:84% to quantify things like gradients, rates 00:08:46.050 --> 00:08:48.400 align:middle line:90% of change of space and time. 00:08:48.400 --> 00:08:50.010 align:middle line:84% And this tells you where the stuff is. 00:08:50.010 --> 00:08:52.830 align:middle line:84% This is the distribution of matter and energy. 00:08:52.830 --> 00:08:56.580 align:middle line:84% Pretty soon after Einstein arrived 00:08:56.580 --> 00:08:58.410 align:middle line:84% at this form of his field equations 00:08:58.410 --> 00:09:01.740 align:middle line:84% in November of 1915, within a few years, 00:09:01.740 --> 00:09:05.490 align:middle line:84% Einstein himself and then soon several other colleagues began 00:09:05.490 --> 00:09:07.350 align:middle line:84% applying these equations not only 00:09:07.350 --> 00:09:10.170 align:middle line:84% to local phenomena like, say, the warping of space 00:09:10.170 --> 00:09:12.810 align:middle line:90% outside the sun. 00:09:12.810 --> 00:09:19.050 align:middle line:84% His friend Karl Schwarzschild first found an exact solution 00:09:19.050 --> 00:09:23.603 align:middle line:84% for very early in this work not only for local phenomena, 00:09:23.603 --> 00:09:25.020 align:middle line:84% but actually for global phenomena. 00:09:25.020 --> 00:09:27.270 align:middle line:84% Could one build at least a toy model 00:09:27.270 --> 00:09:31.020 align:middle line:84% of an entire universe that might satisfy or might be governed 00:09:31.020 --> 00:09:32.760 align:middle line:90% by Einstein's field equations? 00:09:32.760 --> 00:09:34.200 align:middle line:84% And actually some other colleagues 00:09:34.200 --> 00:09:37.500 align:middle line:84% were very quick to find that there were some exact solutions 00:09:37.500 --> 00:09:41.940 align:middle line:84% even on this global or universal cosmic scale that would satisfy 00:09:41.940 --> 00:09:42.810 align:middle line:90% Einstein's equation. 00:09:42.810 --> 00:09:46.590 align:middle line:84% They took three particularly simple forms. 00:09:46.590 --> 00:09:48.870 align:middle line:84% Depending on the amount of stuff, 00:09:48.870 --> 00:09:51.798 align:middle line:84% depending on the distribution of matter and energy, 00:09:51.798 --> 00:09:54.090 align:middle line:84% if you assume that the matter and energy was spread out 00:09:54.090 --> 00:10:00.000 align:middle line:84% perfectly uniformly as a toy model, a uniform density 00:10:00.000 --> 00:10:03.330 align:middle line:84% of stuff per volume, then depending on whether there was 00:10:03.330 --> 00:10:07.980 align:middle line:84% more than some critical value, less than some critical value, 00:10:07.980 --> 00:10:10.230 align:middle line:84% or exactly equal to some critical value, 00:10:10.230 --> 00:10:14.340 align:middle line:84% these Goldilocks situations for the global shape 00:10:14.340 --> 00:10:17.760 align:middle line:84% of space in response to how much stuff per volume 00:10:17.760 --> 00:10:19.620 align:middle line:90% was filling that toy universe. 00:10:19.620 --> 00:10:21.760 align:middle line:84% If you had more than some critical value, 00:10:21.760 --> 00:10:24.700 align:middle line:84% a critical value that came from the equations themselves 00:10:24.700 --> 00:10:28.930 align:middle line:84% above that, an overdense region, space itself would warp back 00:10:28.930 --> 00:10:32.200 align:middle line:84% onto itself like a closed sphere, the surface 00:10:32.200 --> 00:10:34.480 align:middle line:84% of a closed sphere that would be a positively 00:10:34.480 --> 00:10:36.580 align:middle line:90% curved geometry globally. 00:10:36.580 --> 00:10:38.680 align:middle line:84% If you had less stuff per volume, 00:10:38.680 --> 00:10:40.630 align:middle line:84% if the universe were underdense compared 00:10:40.630 --> 00:10:42.970 align:middle line:84% to that critical value, then, in fact, the universe 00:10:42.970 --> 00:10:44.860 align:middle line:90% would open up away from itself. 00:10:44.860 --> 00:10:48.130 align:middle line:84% You'd have a hyperbolic solution or an open geometry 00:10:48.130 --> 00:10:49.850 align:middle line:90% with negative curvature. 00:10:49.850 --> 00:10:51.820 align:middle line:84% And only if the amount of stuff per volume 00:10:51.820 --> 00:10:54.340 align:middle line:84% were exactly equal to that critical value 00:10:54.340 --> 00:10:58.810 align:middle line:84% would the sections of space be flat, 00:10:58.810 --> 00:11:02.390 align:middle line:84% obey the geometry of ordinary Euclidean geometry. 00:11:02.390 --> 00:11:05.440 align:middle line:84% And so you can have these global features, for example, 00:11:05.440 --> 00:11:10.480 align:middle line:84% on a positively curved geometrical surface. 00:11:10.480 --> 00:11:12.460 align:middle line:84% Parallel lines, lines that are parallel 00:11:12.460 --> 00:11:14.920 align:middle line:84% at the equator like these here, will actually 00:11:14.920 --> 00:11:16.130 align:middle line:90% converge at the poles. 00:11:16.130 --> 00:11:18.820 align:middle line:84% So lines that are parallel in some part of the space 00:11:18.820 --> 00:11:20.320 align:middle line:84% will not remain parallel forever. 00:11:20.320 --> 00:11:23.470 align:middle line:84% That breaks the Euclidean assumption, the fifth postulate 00:11:23.470 --> 00:11:24.670 align:middle line:90% about parallel lines. 00:11:24.670 --> 00:11:27.190 align:middle line:84% Likewise, on a positively curved surface, 00:11:27.190 --> 00:11:30.730 align:middle line:84% you can draw a triangle and add up 00:11:30.730 --> 00:11:33.790 align:middle line:84% the sum of the angles contained within that triangle 00:11:33.790 --> 00:11:37.060 align:middle line:84% and add up to more than 180 degrees, 00:11:37.060 --> 00:11:40.900 align:middle line:84% whereas Euclidean triangles have to have exactly 180 degrees. 00:11:40.900 --> 00:11:43.900 align:middle line:84% Likewise for open geometry, parallel lines 00:11:43.900 --> 00:11:44.848 align:middle line:90% will actually diverge. 00:11:44.848 --> 00:11:46.390 align:middle line:84% They'll get further and further apart 00:11:46.390 --> 00:11:50.110 align:middle line:84% from each other over distance instead of remaining parallel. 00:11:50.110 --> 00:11:53.620 align:middle line:84% And triangles will sum up to have less than 180 degrees 00:11:53.620 --> 00:11:55.150 align:middle line:90% of the internal angle. 00:11:55.150 --> 00:11:57.790 align:middle line:84% So there are these self-consistent, non-Euclidean 00:11:57.790 --> 00:11:58.750 align:middle line:90% geometries. 00:11:58.750 --> 00:12:02.710 align:middle line:84% And they could apply not only to local physics like the warping 00:12:02.710 --> 00:12:05.800 align:middle line:84% of spacetime outside a massive object like the sun, 00:12:05.800 --> 00:12:08.290 align:middle line:84% but even to these toy universes, these otherwise very 00:12:08.290 --> 00:12:12.800 align:middle line:84% simple models of a universe as a whole. 00:12:12.800 --> 00:12:15.722 align:middle line:84% Well, very soon after that, people-- not Einstein himself. 00:12:15.722 --> 00:12:16.930 align:middle line:90% He thought this was horrible. 00:12:16.930 --> 00:12:18.610 align:middle line:84% But some of his colleagues who began 00:12:18.610 --> 00:12:21.610 align:middle line:84% pursuing these cosmological solutions 00:12:21.610 --> 00:12:24.430 align:middle line:84% began to realize that the universe could not only 00:12:24.430 --> 00:12:26.800 align:middle line:84% have a shape at a given moment in time. 00:12:26.800 --> 00:12:29.320 align:middle line:84% But the shape could change over time, 00:12:29.320 --> 00:12:31.330 align:middle line:84% or the size could change over time. 00:12:31.330 --> 00:12:33.550 align:middle line:84% You could have expanding or collapsing 00:12:33.550 --> 00:12:35.650 align:middle line:84% solutions, also strictly consistent 00:12:35.650 --> 00:12:37.780 align:middle line:90% with Einstein's equations. 00:12:37.780 --> 00:12:39.520 align:middle line:84% Einstein thought that was horrible. 00:12:39.520 --> 00:12:43.540 align:middle line:84% He had a very strong aesthetic and philosophical preference 00:12:43.540 --> 00:12:46.870 align:middle line:84% for a universe that had no beginning that was simply 00:12:46.870 --> 00:12:49.240 align:middle line:84% static, that would look the same for any observer 00:12:49.240 --> 00:12:53.380 align:middle line:84% at any moment for an infinite expanse of time. 00:12:53.380 --> 00:12:55.390 align:middle line:84% But other colleagues showed at least 00:12:55.390 --> 00:12:57.280 align:middle line:84% it was consistent with his own equations 00:12:57.280 --> 00:13:00.100 align:middle line:84% to have universes that would change over time, that could 00:13:00.100 --> 00:13:02.193 align:middle line:90% either expand or contract. 00:13:02.193 --> 00:13:04.360 align:middle line:84% That was actually a prediction made by some of these 00:13:04.360 --> 00:13:06.910 align:middle line:84% colleagues even before some empirical evidence began 00:13:06.910 --> 00:13:11.450 align:middle line:84% to come in starting in the late 1920s with some at the time 00:13:11.450 --> 00:13:14.290 align:middle line:84% absolutely enormous telescopes-- now they're pipsqueaks compared 00:13:14.290 --> 00:13:15.880 align:middle line:90% to what astronomers have today-- 00:13:15.880 --> 00:13:19.300 align:middle line:84% but what was at the time some of the largest telescopes 00:13:19.300 --> 00:13:20.960 align:middle line:90% of available on the planet. 00:13:20.960 --> 00:13:24.050 align:middle line:84% Astronomers like Edwin Hubble in Southern California 00:13:24.050 --> 00:13:29.240 align:middle line:84% were able to collect information about not just the distribution 00:13:29.240 --> 00:13:32.360 align:middle line:84% of distant galaxies, but could also measure how rapidly they 00:13:32.360 --> 00:13:35.810 align:middle line:84% were moving with respect to us by measuring a Doppler shift, 00:13:35.810 --> 00:13:39.200 align:middle line:84% slight shifts in the spectral lines 00:13:39.200 --> 00:13:40.790 align:middle line:90% associated with those galaxies. 00:13:40.790 --> 00:13:43.220 align:middle line:84% And Hubble found this remarkable trend 00:13:43.220 --> 00:13:46.340 align:middle line:84% that the further away from us a given galaxy was, 00:13:46.340 --> 00:13:49.460 align:middle line:84% the faster it tended to be moving away from us further 00:13:49.460 --> 00:13:50.060 align:middle line:90% still. 00:13:50.060 --> 00:13:52.550 align:middle line:84% So the objects that were relatively close to us 00:13:52.550 --> 00:13:55.280 align:middle line:84% were moving away from us at one average speed. 00:13:55.280 --> 00:13:57.980 align:middle line:84% Objects that were moving further away from us 00:13:57.980 --> 00:14:01.540 align:middle line:84% now were moving even further away from us at a faster speed. 00:14:01.540 --> 00:14:04.970 align:middle line:84% So there's a remarkably close to linear relationship 00:14:04.970 --> 00:14:06.770 align:middle line:84% between the object's distance from us 00:14:06.770 --> 00:14:08.600 align:middle line:84% today and the rate at which they're 00:14:08.600 --> 00:14:10.610 align:middle line:84% moving further away from us that became known 00:14:10.610 --> 00:14:13.610 align:middle line:84% as Hubble's law, more recently amended 00:14:13.610 --> 00:14:16.250 align:middle line:84% to be the Hubble Lemaitre law because it actually predicted 00:14:16.250 --> 00:14:18.800 align:middle line:84% first by a theoretical physicist even before Hubble 00:14:18.800 --> 00:14:20.222 align:middle line:90% had found that data. 00:14:20.222 --> 00:14:22.430 align:middle line:84% Now we have, of course, as you know, the Hubble Space 00:14:22.430 --> 00:14:24.860 align:middle line:84% Telescope named in honor of Edwin Hubble, which 00:14:24.860 --> 00:14:28.430 align:middle line:84% has been able to extend this to extraordinarily far distances, 00:14:28.430 --> 00:14:29.900 align:middle line:84% not just the ones that Hubble could 00:14:29.900 --> 00:14:32.180 align:middle line:84% access with his ground based telescope. 00:14:32.180 --> 00:14:34.250 align:middle line:90% And the basic trend holds. 00:14:34.250 --> 00:14:36.320 align:middle line:84% There's some interesting deviations. 00:14:36.320 --> 00:14:38.690 align:middle line:84% But the idea is nonetheless evidence 00:14:38.690 --> 00:14:41.840 align:middle line:84% consistent with our universe expanding, not just having 00:14:41.840 --> 00:14:44.420 align:middle line:84% a shape to it, but actually stretching and getting larger 00:14:44.420 --> 00:14:45.690 align:middle line:90% over time. 00:14:45.690 --> 00:14:47.420 align:middle line:84% So you can actually then work backwards 00:14:47.420 --> 00:14:51.770 align:middle line:84% and say for how long has our observable universe been 00:14:51.770 --> 00:14:52.280 align:middle line:90% stretching? 00:14:52.280 --> 00:14:55.580 align:middle line:84% When did this stretching or expanding phase begin? 00:14:55.580 --> 00:14:57.590 align:middle line:84% You can work it backwards and say 00:14:57.590 --> 00:15:00.440 align:middle line:84% given the rate of expansion that can be measured today, 00:15:00.440 --> 00:15:02.870 align:middle line:84% whether with Hubble's techniques or now with the more 00:15:02.870 --> 00:15:06.740 align:middle line:84% modern ones with space based telescopes, work it back, 00:15:06.740 --> 00:15:08.390 align:middle line:84% and it's consistent with the beginning 00:15:08.390 --> 00:15:12.750 align:middle line:84% of that expansion being not quite 14 billion years ago, 00:15:12.750 --> 00:15:14.930 align:middle line:90% billions of years ago. 00:15:14.930 --> 00:15:16.910 align:middle line:84% Our own universe seems to have been stretching 00:15:16.910 --> 00:15:19.007 align:middle line:90% and getting larger and larger. 00:15:19.007 --> 00:15:20.840 align:middle line:84% So this gentleman here, whose name I already 00:15:20.840 --> 00:15:22.820 align:middle line:84% mentioned briefly, Georges Lemaitre, 00:15:22.820 --> 00:15:24.920 align:middle line:84% was really at the forefront of this work, 00:15:24.920 --> 00:15:28.700 align:middle line:84% starting in the 1920s and throughout the 1930s. 00:15:28.700 --> 00:15:30.260 align:middle line:84% You might notice in this photograph 00:15:30.260 --> 00:15:32.990 align:middle line:84% he's wearing a Catholic priest's clerical collar. 00:15:32.990 --> 00:15:35.900 align:middle line:84% George Lemaitre is I think a fascinating scholar. 00:15:35.900 --> 00:15:38.990 align:middle line:84% He was indeed an ordained Catholic priest. 00:15:38.990 --> 00:15:44.105 align:middle line:84% He was also an MIT trained PhD astrophysicist. 00:15:44.105 --> 00:15:45.740 align:middle line:90% And he was originally Belgian. 00:15:45.740 --> 00:15:48.050 align:middle line:84% He studied briefly in Cambridge, England 00:15:48.050 --> 00:15:50.540 align:middle line:84% with one of the first converge to general relativity, 00:15:50.540 --> 00:15:51.560 align:middle line:90% Arthur Eddington. 00:15:51.560 --> 00:15:53.960 align:middle line:84% Then he came to MIT to finish his PhD 00:15:53.960 --> 00:15:56.960 align:middle line:84% and then was finding many of these solutions 00:15:56.960 --> 00:15:59.430 align:middle line:84% to Einstein's field equations even before Einstein did. 00:15:59.430 --> 00:16:01.250 align:middle line:84% And, in fact, Einstein came thinking he must be wrong, 00:16:01.250 --> 00:16:02.708 align:middle line:84% and then Lemaitre kept being right. 00:16:02.708 --> 00:16:05.180 align:middle line:84% So they became very nice colleagues. 00:16:05.180 --> 00:16:08.690 align:middle line:84% But Einstein started off by always being frustrated 00:16:08.690 --> 00:16:10.820 align:middle line:84% that Lemaitre found solutions that Einstein 00:16:10.820 --> 00:16:12.770 align:middle line:90% found abhorrent or disgusting. 00:16:12.770 --> 00:16:14.510 align:middle line:84% [LAUGHS] Yet Lemaitre showed they 00:16:14.510 --> 00:16:16.760 align:middle line:84% were at least mathematically self-consistent 00:16:16.760 --> 00:16:19.670 align:middle line:84% and gradually became more and more relevant in the light 00:16:19.670 --> 00:16:22.880 align:middle line:84% of data like Edwin Hubble's about the expanding universe. 00:16:22.880 --> 00:16:24.620 align:middle line:84% Lemaitre was one of the first to start 00:16:24.620 --> 00:16:27.710 align:middle line:84% thinking about playing that filmstrip backwards to say 00:16:27.710 --> 00:16:30.380 align:middle line:84% if things are moving further apart from each other 00:16:30.380 --> 00:16:33.230 align:middle line:84% on average today, and if the universe in general 00:16:33.230 --> 00:16:35.450 align:middle line:84% is expanding today, then was it, in fact, 00:16:35.450 --> 00:16:37.130 align:middle line:90% smaller at earlier times? 00:16:37.130 --> 00:16:40.250 align:middle line:84% You can imagine playing a filmstrip backwards 00:16:40.250 --> 00:16:41.750 align:middle line:84% and watching these galaxies actually 00:16:41.750 --> 00:16:45.320 align:middle line:84% approach each other as you look at earlier and earlier 00:16:45.320 --> 00:16:49.100 align:middle line:84% times, heading back toward that roughly 14 billion year old 00:16:49.100 --> 00:16:50.280 align:middle line:90% starting point. 00:16:50.280 --> 00:16:52.160 align:middle line:84% So it was really Lemaitre who began 00:16:52.160 --> 00:16:54.320 align:middle line:84% writing about this both in technical papers 00:16:54.320 --> 00:16:57.470 align:middle line:84% and soon in some very charming, more popular books 00:16:57.470 --> 00:16:59.660 align:middle line:84% that if the universe is getting bigger today, 00:16:59.660 --> 00:17:01.610 align:middle line:84% it must have been smaller in the past. 00:17:01.610 --> 00:17:04.099 align:middle line:84% And what if you ride that all the way back? 00:17:04.099 --> 00:17:07.895 align:middle line:84% Was there a primeval moment, was there a single moment when 00:17:07.895 --> 00:17:10.520 align:middle line:84% all the matter of the universe, at least all the matter that we 00:17:10.520 --> 00:17:13.520 align:middle line:84% can see, was actually on top of each other, 00:17:13.520 --> 00:17:16.490 align:middle line:84% that the universe should've started in a very, very hot 00:17:16.490 --> 00:17:19.700 align:middle line:84% and dense state and been expanding ever since, maybe 00:17:19.700 --> 00:17:20.983 align:middle line:90% infinitely dense? 00:17:20.983 --> 00:17:22.400 align:middle line:84% But either way, there was a moment 00:17:22.400 --> 00:17:24.628 align:middle line:84% when all the stuff that we see in the sky 00:17:24.628 --> 00:17:26.420 align:middle line:84% should've been closer and closer and closer 00:17:26.420 --> 00:17:29.940 align:middle line:84% together and been stretching and expanding ever since. 00:17:29.940 --> 00:17:32.300 align:middle line:84% So it's Lemaitre who begins thinking about what comes 00:17:32.300 --> 00:17:34.280 align:middle line:90% to be known the Big Bang model. 00:17:34.280 --> 00:17:36.170 align:middle line:84% He was calling it a primeval atom, 00:17:36.170 --> 00:17:39.620 align:middle line:84% that there was this initial fireball from a very, very 00:17:39.620 --> 00:17:40.670 align:middle line:90% hot, dense state. 00:17:40.670 --> 00:17:43.100 align:middle line:84% And he was very eager to understand the early stages 00:17:43.100 --> 00:17:45.400 align:middle line:90% of that expansion. 00:17:45.400 --> 00:17:47.400 align:middle line:84% [CLEARS THROAT] That's where things stood really 00:17:47.400 --> 00:17:48.750 align:middle line:90% through the 1930s. 00:17:48.750 --> 00:17:51.690 align:middle line:84% As we saw, there were a number of disruptions 00:17:51.690 --> 00:17:56.100 align:middle line:84% when much of the world descended into the Second World War. 00:17:56.100 --> 00:17:58.200 align:middle line:84% And then soon after the Second World War, 00:17:58.200 --> 00:18:01.800 align:middle line:84% new groups began coming back to these somewhat old questions. 00:18:01.800 --> 00:18:03.930 align:middle line:84% Some of the newer groups had experience with things 00:18:03.930 --> 00:18:06.420 align:middle line:84% like the Manhattan Project and in general 00:18:06.420 --> 00:18:09.300 align:middle line:84% were much better versed in things like nuclear physics 00:18:09.300 --> 00:18:12.120 align:middle line:84% than had been known even in Lemaitre's day. 00:18:12.120 --> 00:18:15.035 align:middle line:84% So the field had expanded, and some of these folks 00:18:15.035 --> 00:18:16.410 align:middle line:84% had direct experience from things 00:18:16.410 --> 00:18:18.510 align:middle line:84% like the Manhattan Project, one of whom 00:18:18.510 --> 00:18:20.340 align:middle line:90% was actually George Gamow. 00:18:20.340 --> 00:18:22.140 align:middle line:84% So one of the most active groups soon 00:18:22.140 --> 00:18:26.018 align:middle line:84% after the Second World War was based at the advanced-- 00:18:26.018 --> 00:18:27.810 align:middle line:84% excuse me-- the Applied Physics Laboratory. 00:18:27.810 --> 00:18:29.880 align:middle line:84% We looked briefly at this when we talked 00:18:29.880 --> 00:18:31.230 align:middle line:90% about the Second World War. 00:18:31.230 --> 00:18:36.000 align:middle line:84% That was another one of these US based defense laboratories 00:18:36.000 --> 00:18:38.100 align:middle line:84% built in a hurry to try to advance 00:18:38.100 --> 00:18:40.320 align:middle line:84% a bunch of wartime defense projects. 00:18:40.320 --> 00:18:42.480 align:middle line:84% It was much like the MIT Rad Lab. 00:18:42.480 --> 00:18:45.070 align:middle line:84% At the Applied Physics Lab, they worked on things 00:18:45.070 --> 00:18:46.750 align:middle line:90% like proximity fuses and so on. 00:18:46.750 --> 00:18:49.240 align:middle line:84% Starting very soon after the war, 00:18:49.240 --> 00:18:52.480 align:middle line:84% there was some unclassified research going on 00:18:52.480 --> 00:18:55.030 align:middle line:90% at that research lab as well. 00:18:55.030 --> 00:18:58.330 align:middle line:84% And George Gamow was advising two younger physicists, 00:18:58.330 --> 00:19:00.490 align:middle line:90% Robert Herman and Ralph Alpher. 00:19:00.490 --> 00:19:03.010 align:middle line:84% Here's a famous composite photograph. 00:19:03.010 --> 00:19:06.460 align:middle line:84% They're making a not so subtle gesture to the fact 00:19:06.460 --> 00:19:10.360 align:middle line:84% that Gamow was widely rumored at least to enjoy his drink. 00:19:10.360 --> 00:19:13.930 align:middle line:84% So his head is emerging from the vapors of Cointreau, 00:19:13.930 --> 00:19:15.130 align:middle line:90% of a liqueur. 00:19:15.130 --> 00:19:17.867 align:middle line:84% So this was a trio that began coming back 00:19:17.867 --> 00:19:20.200 align:middle line:84% to some of these questions about the very early universe 00:19:20.200 --> 00:19:22.960 align:middle line:84% inspired by the writings of Georges Lemaitre 00:19:22.960 --> 00:19:26.140 align:middle line:84% but now with a lot more knowledge about high energy 00:19:26.140 --> 00:19:29.230 align:middle line:84% interactions among elementary particles as well. 00:19:29.230 --> 00:19:32.620 align:middle line:84% And in a series of really quite ahead of its time 00:19:32.620 --> 00:19:35.480 align:middle line:84% farsighted work starting in the late 1940s, 00:19:35.480 --> 00:19:38.170 align:middle line:84% this trio and a small number of other colleagues 00:19:38.170 --> 00:19:40.360 align:middle line:84% around the world began trying to fill 00:19:40.360 --> 00:19:43.130 align:middle line:84% in this picture, this primeval fireball picture. 00:19:43.130 --> 00:19:45.550 align:middle line:84% And in fact, it soon became known simply 00:19:45.550 --> 00:19:47.020 align:middle line:90% as the Big Bang model. 00:19:47.020 --> 00:19:49.780 align:middle line:84% They realized that if the universe was very hot and dense 00:19:49.780 --> 00:19:52.120 align:middle line:84% at early times, then the conditions 00:19:52.120 --> 00:19:54.820 align:middle line:84% in which these elementary particles would find themselves 00:19:54.820 --> 00:19:58.870 align:middle line:84% should be quite different than what we find commonly 00:19:58.870 --> 00:20:00.550 align:middle line:90% around ourselves today. 00:20:00.550 --> 00:20:03.880 align:middle line:84% In particular, at early times, the ambient energy, 00:20:03.880 --> 00:20:07.000 align:middle line:84% the interaction that any random elementary particle 00:20:07.000 --> 00:20:10.092 align:middle line:84% would likely carry, should be very, very high. 00:20:10.092 --> 00:20:11.800 align:middle line:84% Temperature, after all, is just a measure 00:20:11.800 --> 00:20:13.520 align:middle line:90% of kinetic energy of motion. 00:20:13.520 --> 00:20:15.020 align:middle line:84% So you have a very high temperature. 00:20:15.020 --> 00:20:17.103 align:middle line:84% That's like saying that the average kinetic energy 00:20:17.103 --> 00:20:19.780 align:middle line:84% of each constituent, each elementary particle, 00:20:19.780 --> 00:20:21.200 align:middle line:90% was very high. 00:20:21.200 --> 00:20:23.050 align:middle line:84% It could've been, for example, higher 00:20:23.050 --> 00:20:26.420 align:middle line:84% than the binding energy of stable hydrogen atoms. 00:20:26.420 --> 00:20:28.930 align:middle line:84% So if that were the case, then every time some 00:20:28.930 --> 00:20:30.700 align:middle line:84% positively charged nuclear particle 00:20:30.700 --> 00:20:34.510 align:middle line:84% like even just a single proton would approach 00:20:34.510 --> 00:20:37.630 align:middle line:84% or be in proximity to a negatively charged electron, 00:20:37.630 --> 00:20:41.568 align:middle line:84% they might begin to form a stable electrically neutral 00:20:41.568 --> 00:20:44.110 align:middle line:84% hydrogen atom, which, of course, is just a bound state of one 00:20:44.110 --> 00:20:45.580 align:middle line:90% electron and one proton. 00:20:45.580 --> 00:20:48.790 align:middle line:84% But before they could form that single stable atom, 00:20:48.790 --> 00:20:52.100 align:middle line:84% some ambient particles from the environment 00:20:52.100 --> 00:20:55.390 align:middle line:84% like a single photon would have such high energy, 00:20:55.390 --> 00:20:57.940 align:middle line:84% would come and zap them apart because the average energy 00:20:57.940 --> 00:21:00.640 align:middle line:84% of everything was higher than that binding 00:21:00.640 --> 00:21:04.550 align:middle line:84% energy of the Coulomb attraction of a hydrogen atom. 00:21:04.550 --> 00:21:07.840 align:middle line:84% So at early times, you would have an electrically charged 00:21:07.840 --> 00:21:10.030 align:middle line:84% plasma, that the universe would not 00:21:10.030 --> 00:21:11.890 align:middle line:84% be filled with electrically neutral atoms 00:21:11.890 --> 00:21:14.890 align:middle line:84% because they literally couldn't form yet because they were 00:21:14.890 --> 00:21:18.220 align:middle line:84% blasted apart every time a single putative atom got 00:21:18.220 --> 00:21:22.090 align:middle line:84% close enough to begin to form the average jostling of all 00:21:22.090 --> 00:21:25.960 align:middle line:84% this high temperature junk in its environment 00:21:25.960 --> 00:21:27.220 align:middle line:90% would blast it apart. 00:21:27.220 --> 00:21:31.720 align:middle line:84% So photons then become trapped between charged particles. 00:21:31.720 --> 00:21:35.050 align:middle line:84% They begin to piece all this together. 00:21:35.050 --> 00:21:37.120 align:middle line:84% At early times in cosmic history, 00:21:37.120 --> 00:21:38.980 align:middle line:84% the universe should've been opaque. 00:21:38.980 --> 00:21:41.410 align:middle line:84% You literally wouldn't have been able to see anything 00:21:41.410 --> 00:21:45.010 align:middle line:84% because the mean free path of any given photon 00:21:45.010 --> 00:21:46.600 align:middle line:90% would be very, very short. 00:21:46.600 --> 00:21:48.910 align:middle line:84% The photons would each be trapped, 00:21:48.910 --> 00:21:52.480 align:middle line:84% kicked like soccer balls between all 00:21:52.480 --> 00:21:54.460 align:middle line:90% these loose electric charges. 00:21:54.460 --> 00:21:57.010 align:middle line:90% 00:21:57.010 --> 00:22:00.070 align:middle line:84% Light can't propagate in a charged plasma 00:22:00.070 --> 00:22:02.620 align:middle line:84% because it's always bouncing between these very 00:22:02.620 --> 00:22:04.780 align:middle line:90% nearby free electric charges. 00:22:04.780 --> 00:22:09.280 align:middle line:84% So they could calculate and say when would that effect go away? 00:22:09.280 --> 00:22:11.710 align:middle line:84% Well, when the average or ambient temperature 00:22:11.710 --> 00:22:16.360 align:middle line:84% fell below the average binding energy of a single hydrogen 00:22:16.360 --> 00:22:18.730 align:middle line:84% atom-- and that would happen at a distinct moment 00:22:18.730 --> 00:22:20.060 align:middle line:90% in cosmic history. 00:22:20.060 --> 00:22:23.470 align:middle line:84% So again, they were trying to flesh out Lemaitre's picture 00:22:23.470 --> 00:22:24.940 align:middle line:90% of an evolving universe. 00:22:24.940 --> 00:22:26.800 align:middle line:84% It wasn't just hot and dense at one time. 00:22:26.800 --> 00:22:29.230 align:middle line:84% But that would mean that certain kinds of interactions 00:22:29.230 --> 00:22:32.950 align:middle line:84% among elementary particles would dominate. 00:22:32.950 --> 00:22:35.860 align:middle line:84% And then those would change over time. 00:22:35.860 --> 00:22:38.710 align:middle line:84% In particular, as this entire hot ball 00:22:38.710 --> 00:22:42.280 align:middle line:84% of gas of charged plasma is expanding, 00:22:42.280 --> 00:22:44.970 align:middle line:84% the average temperature should decrease in size, 00:22:44.970 --> 00:22:47.900 align:middle line:84% much like the temperature of a gas inside a balloon 00:22:47.900 --> 00:22:50.420 align:middle line:84% will fall as the balloon expands. 00:22:50.420 --> 00:22:53.150 align:middle line:84% So as the volume of space stretches, 00:22:53.150 --> 00:22:56.270 align:middle line:84% as you have an expanding universe as Le Maitre showed 00:22:56.270 --> 00:22:59.030 align:middle line:84% what could be possible, the average temperature 00:22:59.030 --> 00:23:01.230 align:middle line:84% of all the stuff inside it should fall. 00:23:01.230 --> 00:23:03.980 align:middle line:84% It should fall in a quantitatively calculable way, 00:23:03.980 --> 00:23:06.060 align:middle line:84% again, using Einstein's equations. 00:23:06.060 --> 00:23:08.660 align:middle line:84% So again, they put numbers to that and say, well, 00:23:08.660 --> 00:23:13.130 align:middle line:84% at a particular moment in time, now using the modern values-- 00:23:13.130 --> 00:23:15.380 align:middle line:84% they had the right idea that different values for some 00:23:15.380 --> 00:23:16.710 align:middle line:90% measurements-- 00:23:16.710 --> 00:23:20.540 align:middle line:84% we now calculate at around 380,000 years, 00:23:20.540 --> 00:23:23.570 align:middle line:84% after the start of that stretch after that primeval atom 00:23:23.570 --> 00:23:27.200 align:middle line:84% begins to expand, the ambient or average temperature 00:23:27.200 --> 00:23:29.150 align:middle line:84% of all the junk inside that universe 00:23:29.150 --> 00:23:32.750 align:middle line:84% should've fallen below this Coulomb 00:23:32.750 --> 00:23:35.960 align:middle line:84% attractive energy for neutral hydrogen. 00:23:35.960 --> 00:23:41.120 align:middle line:84% So whereas at earlier times, earlier than 380,000 years, 00:23:41.120 --> 00:23:44.960 align:middle line:84% you would have a charged plasma in an opaque universe. 00:23:44.960 --> 00:23:47.300 align:middle line:84% At that moment, the average energy 00:23:47.300 --> 00:23:49.610 align:middle line:84% per photon or per elementary particle 00:23:49.610 --> 00:23:51.710 align:middle line:84% would fall so that you could actually 00:23:51.710 --> 00:23:56.030 align:middle line:84% begin to form stable atoms of hydrogen. 00:23:56.030 --> 00:24:00.170 align:middle line:84% So only at that time, a new phase in the universe 00:24:00.170 --> 00:24:01.850 align:middle line:90% would begin to unfold. 00:24:01.850 --> 00:24:05.030 align:middle line:84% The universe would be filled with neutral atoms of hydrogen. 00:24:05.030 --> 00:24:08.420 align:middle line:84% And now you have a mean free path for light 00:24:08.420 --> 00:24:10.520 align:middle line:90% that's arbitrarily long. 00:24:10.520 --> 00:24:12.870 align:middle line:84% Light can pass through electrically neutral matter. 00:24:12.870 --> 00:24:14.480 align:middle line:84% It does so in our own atmosphere, 00:24:14.480 --> 00:24:15.930 align:middle line:90% let alone in empty space. 00:24:15.930 --> 00:24:19.520 align:middle line:84% So once you can have stable, electrically neutral atoms 00:24:19.520 --> 00:24:21.890 align:middle line:84% like hydrogen atoms at a particular moment 00:24:21.890 --> 00:24:25.130 align:middle line:84% in the cooling evolution history of our universe, 00:24:25.130 --> 00:24:28.730 align:middle line:84% only then would you have things like photons traveling 00:24:28.730 --> 00:24:30.420 align:middle line:90% macroscopic distances. 00:24:30.420 --> 00:24:32.750 align:middle line:84% So after that time, when the temperature 00:24:32.750 --> 00:24:36.260 align:middle line:84% has fallen below about 10,000 degrees Kelvin, 00:24:36.260 --> 00:24:38.090 align:middle line:90% photons are free. 00:24:38.090 --> 00:24:41.480 align:middle line:84% And then they can now travel large distances. 00:24:41.480 --> 00:24:45.390 align:middle line:84% And their energy continues to redshift. 00:24:45.390 --> 00:24:48.970 align:middle line:84% They lose energy as the universe continues to expand. 00:24:48.970 --> 00:24:50.940 align:middle line:84% So the energy of those photons would've 00:24:50.940 --> 00:24:54.130 align:middle line:84% started at the equivalent of around 10,000 degrees Kelvin 00:24:54.130 --> 00:24:57.690 align:middle line:84% and now today would be much, much, much lower than that 00:24:57.690 --> 00:25:00.060 align:middle line:84% because the universe has been expanding and draining 00:25:00.060 --> 00:25:02.850 align:middle line:84% that average energy per particle over time [CLEARS THROAT] 00:25:02.850 --> 00:25:04.350 align:middle line:84% so that today the universe should be 00:25:04.350 --> 00:25:05.940 align:middle line:90% filled with this remnant glow. 00:25:05.940 --> 00:25:09.720 align:middle line:84% This is all work that they predict around 1948, '49, 00:25:09.720 --> 00:25:12.750 align:middle line:90% '50, Gamov, Herman, and Alpher. 00:25:12.750 --> 00:25:17.190 align:middle line:84% So they argue that today, this bath of remnant radiation 00:25:17.190 --> 00:25:20.010 align:middle line:84% from that early hot, dense state should be filling 00:25:20.010 --> 00:25:21.570 align:middle line:90% the sky in every direction. 00:25:21.570 --> 00:25:26.040 align:middle line:84% It should be more or less even distribution, a uniform glow. 00:25:26.040 --> 00:25:29.640 align:middle line:84% But instead of it being a very, very high energy X-ray or gamma 00:25:29.640 --> 00:25:32.040 align:middle line:84% ray radiation, it should be redshifted 00:25:32.040 --> 00:25:35.860 align:middle line:84% all the way down into the low energy microwave band. 00:25:35.860 --> 00:25:39.480 align:middle line:84% So this became known as the cosmic microwave background 00:25:39.480 --> 00:25:40.320 align:middle line:90% radiation. 00:25:40.320 --> 00:25:42.570 align:middle line:84% And they say this should be filling the sky. 00:25:42.570 --> 00:25:45.790 align:middle line:84% It should be everywhere in a uniform pattern. 00:25:45.790 --> 00:25:46.635 align:middle line:90% Here's an aside. 00:25:46.635 --> 00:25:47.760 align:middle line:90% I like to think about this. 00:25:47.760 --> 00:25:49.080 align:middle line:90% That's very abstract. 00:25:49.080 --> 00:25:52.650 align:middle line:84% I think about it as the evolution of a dance party. 00:25:52.650 --> 00:25:55.227 align:middle line:84% It turns out I don't attend dance parties very often. 00:25:55.227 --> 00:25:57.310 align:middle line:84% This is what the internet tells me they look like. 00:25:57.310 --> 00:25:59.032 align:middle line:90% So this, I'm sure, is accurate. 00:25:59.032 --> 00:26:00.990 align:middle line:84% If you just Google "dance party" and throw away 00:26:00.990 --> 00:26:02.640 align:middle line:90% the bad pictures-- anyway. 00:26:02.640 --> 00:26:05.738 align:middle line:84% So at early times, the DJ's playing some raucous house 00:26:05.738 --> 00:26:07.530 align:middle line:84% music, and everyone's just jostling around. 00:26:07.530 --> 00:26:09.790 align:middle line:84% The average energy per dancer is very high. 00:26:09.790 --> 00:26:10.530 align:middle line:90% So I'm told. 00:26:10.530 --> 00:26:13.050 align:middle line:84% That's like the charged particles 00:26:13.050 --> 00:26:15.450 align:middle line:84% where the mean free path is effectively zero. 00:26:15.450 --> 00:26:17.490 align:middle line:84% No one could cross that dance floor. 00:26:17.490 --> 00:26:19.950 align:middle line:84% And then at a calculable moment, if the DJ 00:26:19.950 --> 00:26:22.560 align:middle line:84% knows what she's doing, she'll put on some slow music. 00:26:22.560 --> 00:26:24.740 align:middle line:84% And you start having couples form like in Harry 00:26:24.740 --> 00:26:25.740 align:middle line:90% Potter at the Yule Ball. 00:26:25.740 --> 00:26:28.020 align:middle line:84% Again, that's what I assume school dances are like. 00:26:28.020 --> 00:26:28.930 align:middle line:90% I don't know. 00:26:28.930 --> 00:26:31.560 align:middle line:84% So at some later, calculable moment, 00:26:31.560 --> 00:26:34.500 align:middle line:84% the average energy in the room begins to fall. 00:26:34.500 --> 00:26:36.060 align:middle line:90% The DJ reads the room. 00:26:36.060 --> 00:26:39.750 align:middle line:84% And now you start having couples so that you actually 00:26:39.750 --> 00:26:42.420 align:middle line:84% could cross the floor because now you have a mean free path 00:26:42.420 --> 00:26:45.510 align:middle line:84% to cross the dance floor, unlike that very 00:26:45.510 --> 00:26:48.000 align:middle line:84% exciting early universe phase when everything's just 00:26:48.000 --> 00:26:49.650 align:middle line:90% a mash or a mush. 00:26:49.650 --> 00:26:51.345 align:middle line:90% This makes perfect sense to me. 00:26:51.345 --> 00:26:53.220 align:middle line:84% You can tell me whether it's accurate or not. 00:26:53.220 --> 00:26:56.760 align:middle line:84% Anyway, that's the analogy for what Gamov, Alpher, and Herman 00:26:56.760 --> 00:26:58.950 align:middle line:84% were putting real numbers to to try 00:26:58.950 --> 00:27:01.140 align:middle line:84% to make sense of these different phases of the very 00:27:01.140 --> 00:27:02.040 align:middle line:90% early universe. 00:27:02.040 --> 00:27:06.720 align:middle line:84% Very high temperature, early dense state 00:27:06.720 --> 00:27:08.970 align:middle line:84% should be qualitatively different in its behavior 00:27:08.970 --> 00:27:11.950 align:middle line:90% than a later lower energy state. 00:27:11.950 --> 00:27:14.795 align:middle line:84% So they predicted as early as 1948 00:27:14.795 --> 00:27:16.920 align:middle line:84% that there should be this remnant glow from the Big 00:27:16.920 --> 00:27:21.090 align:middle line:84% Bang, all those photons that only then at 380,000 years 00:27:21.090 --> 00:27:24.450 align:middle line:84% after the Big Bang were able to start streaming freely. 00:27:24.450 --> 00:27:26.830 align:middle line:84% And the question was, where is it? 00:27:26.830 --> 00:27:30.450 align:middle line:84% Well, almost 20 years later, 15 years later, 00:27:30.450 --> 00:27:32.610 align:middle line:84% these two radial physicists working 00:27:32.610 --> 00:27:35.520 align:middle line:84% at Bell Labs, Robert Wilson and Arno Penzias, 00:27:35.520 --> 00:27:40.920 align:middle line:84% were using a new horn antenna sensitive to radial microwave 00:27:40.920 --> 00:27:42.540 align:middle line:90% and radial band frequencies. 00:27:42.540 --> 00:27:44.660 align:middle line:84% This was basically left over from the early 00:27:44.660 --> 00:27:46.850 align:middle line:90% telecommunications age. 00:27:46.850 --> 00:27:48.500 align:middle line:84% Soon after the launch of Sputnik, 00:27:48.500 --> 00:27:50.210 align:middle line:84% lots of folks like private companies 00:27:50.210 --> 00:27:52.880 align:middle line:84% wanted to get into the satellite communications business. 00:27:52.880 --> 00:27:55.130 align:middle line:84% Often it was just bouncing radial waves off 00:27:55.130 --> 00:27:57.410 align:middle line:84% of reflectors in the sky in low Earth orbit 00:27:57.410 --> 00:27:59.222 align:middle line:84% and then bouncing signal back to Earth. 00:27:59.222 --> 00:28:01.680 align:middle line:84% Of course, it became more and more sophisticated than that. 00:28:01.680 --> 00:28:03.980 align:middle line:84% And they were actually given time on this telescope 00:28:03.980 --> 00:28:06.590 align:middle line:84% not only to fine tune the corporate program 00:28:06.590 --> 00:28:09.650 align:middle line:84% for telephonics, but also to conduct 00:28:09.650 --> 00:28:11.210 align:middle line:90% actual radial astronomy. 00:28:11.210 --> 00:28:14.450 align:middle line:84% They were interested in the evolution of nearby galaxies. 00:28:14.450 --> 00:28:15.830 align:middle line:90% They were not doing cosmology. 00:28:15.830 --> 00:28:18.050 align:middle line:84% They were interested in radial signatures 00:28:18.050 --> 00:28:21.050 align:middle line:90% of astronomically nearby things. 00:28:21.050 --> 00:28:24.980 align:middle line:84% But they found this remnant hum in their electronics. 00:28:24.980 --> 00:28:27.230 align:middle line:84% This should've been among the most precise instruments 00:28:27.230 --> 00:28:31.070 align:middle line:84% available on the planet for that band of the spectrum. 00:28:31.070 --> 00:28:33.200 align:middle line:84% And they couldn't get rid of a residual hum. 00:28:33.200 --> 00:28:35.750 align:middle line:84% At one point, they climbed inside that huge horn antenna 00:28:35.750 --> 00:28:38.570 align:middle line:84% on their hands and knees to scrub out what they graciously 00:28:38.570 --> 00:28:42.560 align:middle line:84% called special dielectric materials from pigeons 00:28:42.560 --> 00:28:45.060 align:middle line:84% who had made a nest in there-- that, of course, meant pigeon 00:28:45.060 --> 00:28:47.100 align:middle line:84% droppings-- because they figured that might 00:28:47.100 --> 00:28:51.540 align:middle line:84% be messing with the electronics of some extra insulating layer. 00:28:51.540 --> 00:28:52.930 align:middle line:90% That didn't make any difference. 00:28:52.930 --> 00:28:55.860 align:middle line:84% Finally, they were put in touch with a group at Princeton 00:28:55.860 --> 00:28:59.460 align:middle line:84% that was independently rediscovering many of the ideas 00:28:59.460 --> 00:29:03.570 align:middle line:84% from George Gamow and his group, actually at the time unaware 00:29:03.570 --> 00:29:05.670 align:middle line:84% that Gamow had even done these calculations. 00:29:05.670 --> 00:29:07.290 align:middle line:84% And they likewise convinced themselves 00:29:07.290 --> 00:29:10.380 align:middle line:84% about this cosmic microwave background remnant radiation. 00:29:10.380 --> 00:29:14.242 align:middle line:84% So now the group here, these folks are in Southern Jersey. 00:29:14.242 --> 00:29:15.450 align:middle line:90% They were close to Princeton. 00:29:15.450 --> 00:29:16.750 align:middle line:90% They all got together. 00:29:16.750 --> 00:29:19.470 align:middle line:84% They said, oh, what you found is actually the remnant glow 00:29:19.470 --> 00:29:20.220 align:middle line:90% from the Big Bang. 00:29:20.220 --> 00:29:22.680 align:middle line:84% This residual hum in your receiver 00:29:22.680 --> 00:29:26.370 align:middle line:84% consistent with an energy of about three degrees above zero, 00:29:26.370 --> 00:29:31.080 align:middle line:84% three degrees Kelvin, was really the leftover photons, 00:29:31.080 --> 00:29:33.420 align:middle line:84% that remnant hot radiation from the Big 00:29:33.420 --> 00:29:35.370 align:middle line:84% Bang that had been streaming freely 00:29:35.370 --> 00:29:37.020 align:middle line:90% for the next 14 billion years. 00:29:37.020 --> 00:29:38.760 align:middle line:84% And the average energy per photon 00:29:38.760 --> 00:29:41.040 align:middle line:84% had fallen steadily since the time 00:29:41.040 --> 00:29:44.160 align:middle line:84% when they were first released in that early dance party. 00:29:44.160 --> 00:29:46.650 align:middle line:84% They were very soon afterwards awarded the Nobel Prize 00:29:46.650 --> 00:29:49.480 align:middle line:84% for actually detecting evidence of the Big Bang. 00:29:49.480 --> 00:29:51.720 align:middle line:84% So let me pause there and ask any questions. 00:29:51.720 --> 00:29:53.460 align:middle line:84% Any questions on that stuff so far? 00:29:53.460 --> 00:30:00.800 align:middle line:90% 00:30:00.800 --> 00:30:01.830 align:middle line:90% So far so good? 00:30:01.830 --> 00:30:03.590 align:middle line:84% AUDIENCE: This might be a dumb question. 00:30:03.590 --> 00:30:04.590 align:middle line:90% DAVID KAISER: Go for it. 00:30:04.590 --> 00:30:07.820 align:middle line:84% AUDIENCE: But guess I'm just wondering how 00:30:07.820 --> 00:30:11.590 align:middle line:84% if you have photons that are like-- 00:30:11.590 --> 00:30:15.920 align:middle line:84% I understand that the photons from the CMB, 00:30:15.920 --> 00:30:18.530 align:middle line:84% they're from us looking in the past. 00:30:18.530 --> 00:30:23.040 align:middle line:84% But I don't understand why they're still there, 00:30:23.040 --> 00:30:26.440 align:middle line:84% why they haven't passed this already. 00:30:26.440 --> 00:30:30.270 align:middle line:84% I guess are they constantly being emitted? 00:30:30.270 --> 00:30:34.008 align:middle line:84% Or why is it still there basically? 00:30:34.008 --> 00:30:34.800 align:middle line:90% DAVID KAISER: Yeah. 00:30:34.800 --> 00:30:35.370 align:middle line:90% Thank you. 00:30:35.370 --> 00:30:36.780 align:middle line:84% That's actually a really good question. 00:30:36.780 --> 00:30:38.530 align:middle line:84% Basically, the idea is that they should've 00:30:38.530 --> 00:30:40.200 align:middle line:90% been everywhere at once. 00:30:40.200 --> 00:30:42.330 align:middle line:84% So the idea was the whole universe 00:30:42.330 --> 00:30:45.510 align:middle line:84% is filled in the early times with very high energy particles 00:30:45.510 --> 00:30:48.150 align:middle line:84% that are at early, early times too high energy 00:30:48.150 --> 00:30:50.453 align:middle line:84% to form stable, electrically neutral atoms. 00:30:50.453 --> 00:30:52.120 align:middle line:84% So you have this huge plasma everywhere, 00:30:52.120 --> 00:30:54.990 align:middle line:84% not just in one corner, not just over there in the sky, 00:30:54.990 --> 00:30:56.980 align:middle line:90% but everywhere. 00:30:56.980 --> 00:30:59.860 align:middle line:84% And likewise, there are photons everywhere. 00:30:59.860 --> 00:31:02.400 align:middle line:84% It was at least the idea of more or less uniformly 00:31:02.400 --> 00:31:04.600 align:middle line:84% distributed with no real pattern to it. 00:31:04.600 --> 00:31:08.910 align:middle line:84% And so from every part of space, from every single direction 00:31:08.910 --> 00:31:11.880 align:middle line:84% in the sky, those photons began to move freely 00:31:11.880 --> 00:31:14.850 align:middle line:84% at this single moment in time or a very short lived moment 00:31:14.850 --> 00:31:15.900 align:middle line:90% in time. 00:31:15.900 --> 00:31:17.400 align:middle line:84% So basically, the universe should've 00:31:17.400 --> 00:31:21.310 align:middle line:84% been filled with light, originally a very high energy. 00:31:21.310 --> 00:31:23.680 align:middle line:84% And then actually the energy of that light 00:31:23.680 --> 00:31:28.000 align:middle line:84% should be falling as the container expands, 00:31:28.000 --> 00:31:31.580 align:middle line:84% so as the average energy inside that balloon goes down. 00:31:31.580 --> 00:31:34.790 align:middle line:84% So we're basically just moving through a bath of light. 00:31:34.790 --> 00:31:37.335 align:middle line:84% So the photons aren't coming from that direction 00:31:37.335 --> 00:31:39.460 align:middle line:84% of sky the way we think of with point-like sources. 00:31:39.460 --> 00:31:41.350 align:middle line:84% There's a galaxy there, a quasar, 00:31:41.350 --> 00:31:43.510 align:middle line:84% a particular bright star in our neighborhood. 00:31:43.510 --> 00:31:45.920 align:middle line:90% The photons were everywhere. 00:31:45.920 --> 00:31:48.640 align:middle line:84% And it's like sitting in a bathtub full of these photons. 00:31:48.640 --> 00:31:50.920 align:middle line:84% And they're just losing their energy 00:31:50.920 --> 00:31:56.385 align:middle line:84% as the overall size of space continues to grow. 00:31:56.385 --> 00:31:57.760 align:middle line:84% So the idea was that there should 00:31:57.760 --> 00:31:59.080 align:middle line:84% be-- so it's not just that they should 00:31:59.080 --> 00:32:00.330 align:middle line:90% have a particular temperature. 00:32:00.330 --> 00:32:02.530 align:middle line:84% They should be everywhere in the sky 00:32:02.530 --> 00:32:04.910 align:middle line:90% was the idea, a uniform pattern. 00:32:04.910 --> 00:32:07.360 align:middle line:84% So if they could point that radial telescope 00:32:07.360 --> 00:32:10.480 align:middle line:84% in any direction, and they should find more or less 00:32:10.480 --> 00:32:13.840 align:middle line:84% the same signal, which is indeed what they were finding. 00:32:13.840 --> 00:32:17.110 align:middle line:84% So think about sitting in a bathtub that at first is 00:32:17.110 --> 00:32:18.760 align:middle line:90% like boiling hot water. 00:32:18.760 --> 00:32:21.550 align:middle line:84% And then you're sitting there while the water continues 00:32:21.550 --> 00:32:24.040 align:middle line:84% to cool down, but you're still immersed in that. 00:32:24.040 --> 00:32:28.000 align:middle line:84% The photons are coming from every single region of space. 00:32:28.000 --> 00:32:32.230 align:middle line:84% And all that's changing is their average temperature per photon 00:32:32.230 --> 00:32:34.910 align:middle line:90% is the main idea. 00:32:34.910 --> 00:32:36.955 align:middle line:84% Another way of saying it is it's-- 00:32:36.955 --> 00:32:38.080 align:middle line:90% it's hard to get-- 00:32:38.080 --> 00:32:39.070 align:middle line:84% [LAUGHS] hard to get one's head around. 00:32:39.070 --> 00:32:39.790 align:middle line:90% Believe me. 00:32:39.790 --> 00:32:42.130 align:middle line:84% But instead of saying the Big Bang happened over there 00:32:42.130 --> 00:32:43.713 align:middle line:84% and things are stretching out from it, 00:32:43.713 --> 00:32:45.550 align:middle line:90% the Big Bang happens everywhere. 00:32:45.550 --> 00:32:50.620 align:middle line:84% So every part of space that we could see today once 00:32:50.620 --> 00:32:52.210 align:middle line:84% had been at the Big Bang, so to speak. 00:32:52.210 --> 00:32:53.410 align:middle line:90% The Big Bang happened there. 00:32:53.410 --> 00:32:56.080 align:middle line:84% It happened at x equals 0, and x equals 1, equals 2. 00:32:56.080 --> 00:32:59.800 align:middle line:84% Any place we could put spatial coordinates to on this model, 00:32:59.800 --> 00:33:03.173 align:middle line:84% those all experienced the Big Bang at the same time. 00:33:03.173 --> 00:33:05.590 align:middle line:84% So it wasn't the Bang happened there and stuff was flowing 00:33:05.590 --> 00:33:07.180 align:middle line:90% outward from it towards us. 00:33:07.180 --> 00:33:09.520 align:middle line:84% Again, think about being inside a balloon 00:33:09.520 --> 00:33:11.430 align:middle line:90% or let's say a bathtub. 00:33:11.430 --> 00:33:15.010 align:middle line:84% So there should be a uniform set of properties 00:33:15.010 --> 00:33:16.600 align:middle line:90% filling that space. 00:33:16.600 --> 00:33:18.820 align:middle line:84% And we're just immersed in it trying to measure it 00:33:18.820 --> 00:33:21.010 align:middle line:90% as we flow through. 00:33:21.010 --> 00:33:22.510 align:middle line:84% I'd be glad to chat more about that. 00:33:22.510 --> 00:33:25.930 align:middle line:84% But I say all that as if that's obvious. 00:33:25.930 --> 00:33:26.700 align:middle line:90% It's not obvious. 00:33:26.700 --> 00:33:28.117 align:middle line:84% I'd be glad to chat more about it. 00:33:28.117 --> 00:33:31.150 align:middle line:84% But that's the kind of reckoning that people like Lemaitre 00:33:31.150 --> 00:33:33.748 align:middle line:84% got very comfortable with starting in the '20s and '30s. 00:33:33.748 --> 00:33:35.290 align:middle line:84% And it took other people a lot longer 00:33:35.290 --> 00:33:37.420 align:middle line:84% to try to get their heads around. 00:33:37.420 --> 00:33:41.410 align:middle line:84% Alex rightly puts in the chat that Edwin Hubble was actually 00:33:41.410 --> 00:33:42.820 align:middle line:90% pretty lucky. 00:33:42.820 --> 00:33:43.520 align:middle line:90% It's true. 00:33:43.520 --> 00:33:45.850 align:middle line:84% There's been a tremendous amount of controversy right 00:33:45.850 --> 00:33:50.050 align:middle line:84% to this day or let's just say earnest disagreement 00:33:50.050 --> 00:33:52.750 align:middle line:84% over what's called the cosmic distance ladder, which 00:33:52.750 --> 00:33:56.380 align:middle line:84% is to say it's actually pretty easy now, relatively easy 00:33:56.380 --> 00:33:59.140 align:middle line:84% to measure the speeds with which objects are moving away 00:33:59.140 --> 00:34:01.510 align:middle line:84% from us because that has to do with spectroscopy. 00:34:01.510 --> 00:34:03.880 align:middle line:84% We can measure these atomic transition lines 00:34:03.880 --> 00:34:05.293 align:middle line:90% with great, great accuracy. 00:34:05.293 --> 00:34:07.210 align:middle line:84% And so you can just do a Doppler shift to say, 00:34:07.210 --> 00:34:09.219 align:middle line:84% oh, I would expect that line to be here. 00:34:09.219 --> 00:34:10.159 align:middle line:90% It's over here. 00:34:10.159 --> 00:34:14.110 align:middle line:84% And the difference is a direct measure of the relative speed, 00:34:14.110 --> 00:34:15.639 align:middle line:90% the recession speed. 00:34:15.639 --> 00:34:19.120 align:middle line:84% What's not so easy is to figure out how far away from us 00:34:19.120 --> 00:34:21.770 align:middle line:84% that thing is right now, the actual distance right now. 00:34:21.770 --> 00:34:24.590 align:middle line:84% We can get the velocity from these very fancy 00:34:24.590 --> 00:34:25.699 align:middle line:90% spectroscopic-- 00:34:25.699 --> 00:34:26.570 align:middle line:90% I say we. 00:34:26.570 --> 00:34:28.620 align:middle line:84% The people who do it are actually astronomers. 00:34:28.620 --> 00:34:30.237 align:middle line:84% They kindly share their information. 00:34:30.237 --> 00:34:31.320 align:middle line:90% I don't know how to do it. 00:34:31.320 --> 00:34:32.630 align:middle line:90% But one can do it well. 00:34:32.630 --> 00:34:34.429 align:middle line:84% Now, whole teams can do it very, very well. 00:34:34.429 --> 00:34:36.050 align:middle line:84% What's hard to do is to calibrate what's 00:34:36.050 --> 00:34:37.175 align:middle line:90% called the distance ladder. 00:34:37.175 --> 00:34:39.590 align:middle line:84% How far away is that object right now, let alone 00:34:39.590 --> 00:34:41.610 align:middle line:84% how quickly is it continuing to move away? 00:34:41.610 --> 00:34:45.860 align:middle line:84% And so this was especially off compared to modern day values 00:34:45.860 --> 00:34:46.830 align:middle line:90% in Hubble's time. 00:34:46.830 --> 00:34:49.760 align:middle line:90% In fact, his value was-- 00:34:49.760 --> 00:34:50.630 align:middle line:90% well, let's see. 00:34:50.630 --> 00:34:52.290 align:middle line:90% 500 versus 70. 00:34:52.290 --> 00:34:56.510 align:middle line:84% So what we now call Hubble's parameter is roughly 70 00:34:56.510 --> 00:34:57.740 align:middle line:90% in appropriate units. 00:34:57.740 --> 00:34:59.420 align:middle line:90% And he measured 500. 00:34:59.420 --> 00:35:02.520 align:middle line:84% So he measured a much quicker average rate of expansion 00:35:02.520 --> 00:35:05.990 align:middle line:84% than what we have mostly settled on today. 00:35:05.990 --> 00:35:07.820 align:middle line:90% But the basic picture was there. 00:35:07.820 --> 00:35:11.810 align:middle line:84% The picture was enough to get a small number of people 00:35:11.810 --> 00:35:15.260 align:middle line:84% to pay attention to Georges Lemaitre's otherwise quite 00:35:15.260 --> 00:35:17.180 align:middle line:90% obscure mathematical solutions. 00:35:17.180 --> 00:35:18.110 align:middle line:90% So Lemaitre. 00:35:18.110 --> 00:35:21.710 align:middle line:84% There's also a Russian and by then Soviet physicist Aleksandr 00:35:21.710 --> 00:35:24.380 align:middle line:84% Friedmann, who was doing very similar work. 00:35:24.380 --> 00:35:26.270 align:middle line:84% A few of these mathematical physicists 00:35:26.270 --> 00:35:28.250 align:middle line:84% were using Einstein's equations in ways 00:35:28.250 --> 00:35:30.060 align:middle line:84% that Einstein thought was awful, as I say. 00:35:30.060 --> 00:35:31.970 align:middle line:84% He was not at all a fan of this early on. 00:35:31.970 --> 00:35:36.420 align:middle line:84% But to the few people who paid any attention at all, 00:35:36.420 --> 00:35:39.830 align:middle line:84% it looked like just a mathematical curiosity. 00:35:39.830 --> 00:35:43.790 align:middle line:84% And it was only when paired with observations most famously 00:35:43.790 --> 00:35:44.360 align:middle line:90% from Hubble-- 00:35:44.360 --> 00:35:46.152 align:middle line:84% Hubble actually had a number of assistants, 00:35:46.152 --> 00:35:48.440 align:middle line:84% and other groups began contributing as well. 00:35:48.440 --> 00:35:52.220 align:middle line:84% But it was Hubble's data that got the biggest splash, that 00:35:52.220 --> 00:35:54.690 align:middle line:84% made the biggest impact on the community at the time. 00:35:54.690 --> 00:35:56.930 align:middle line:84% And regardless of whether we agree 00:35:56.930 --> 00:35:59.600 align:middle line:84% with the number he inferred of the actual rate, 00:35:59.600 --> 00:36:02.630 align:middle line:84% it seemed pretty clear to many people at the time 00:36:02.630 --> 00:36:06.380 align:middle line:84% that this was consistent with an actual overall expansion, 00:36:06.380 --> 00:36:08.090 align:middle line:90% with the change over time. 00:36:08.090 --> 00:36:11.072 align:middle line:84% And that made these seemingly pure mathematical solutions 00:36:11.072 --> 00:36:12.530 align:middle line:84% like Friedmann's and especially the 00:36:12.530 --> 00:36:14.630 align:middle line:84% follow up work by Georges Lemaitre, that 00:36:14.630 --> 00:36:16.700 align:middle line:84% made those mathematical solutions look much, much 00:36:16.700 --> 00:36:19.190 align:middle line:84% more curious and interesting than they 00:36:19.190 --> 00:36:21.980 align:middle line:90% had prior to Hubble's data. 00:36:21.980 --> 00:36:24.410 align:middle line:84% Although, Alex, you're absolutely right. 00:36:24.410 --> 00:36:26.630 align:middle line:84% If one took his value literally it 00:36:26.630 --> 00:36:28.220 align:middle line:84% looked like the universe would be 00:36:28.220 --> 00:36:29.760 align:middle line:90% younger than things inside it. 00:36:29.760 --> 00:36:31.880 align:middle line:90% And that's not good, right? 00:36:31.880 --> 00:36:34.820 align:middle line:84% So how could the Milky Way galaxy or even the planet Earth 00:36:34.820 --> 00:36:36.940 align:middle line:84% be older than the universe in which it resides? 00:36:36.940 --> 00:36:38.648 align:middle line:84% So it did lead to these kinds of puzzles. 00:36:38.648 --> 00:36:41.510 align:middle line:84% And that was eventually smoothed over or made 00:36:41.510 --> 00:36:43.280 align:middle line:90% more consistent by the 1950s. 00:36:43.280 --> 00:36:44.060 align:middle line:90% That took a while. 00:36:44.060 --> 00:36:45.000 align:middle line:90% It's an excellent point. 00:36:45.000 --> 00:36:46.190 align:middle line:90% Any other questions on that? 00:36:46.190 --> 00:36:48.730 align:middle line:90% 00:36:48.730 --> 00:36:49.360 align:middle line:90% OK. 00:36:49.360 --> 00:36:52.600 align:middle line:84% So this is a, I think, safe to say, 00:36:52.600 --> 00:36:56.170 align:middle line:84% remarkably successful set of ideas that eventually becomes 00:36:56.170 --> 00:36:59.410 align:middle line:84% called the Big Bang model, going all the way back 00:36:59.410 --> 00:37:01.000 align:middle line:84% to really Einstein, but especially 00:37:01.000 --> 00:37:03.190 align:middle line:84% people like George Lemaitre, a huge boost 00:37:03.190 --> 00:37:07.960 align:middle line:84% to try to give real quantitative teeth to these internal phases 00:37:07.960 --> 00:37:11.560 align:middle line:84% or intermediate phases by people like George Gamow 00:37:11.560 --> 00:37:13.960 align:middle line:90% and his younger assistants. 00:37:13.960 --> 00:37:18.630 align:middle line:84% And it starts to match observations really quite well. 00:37:18.630 --> 00:37:20.720 align:middle line:90% And yet we're not done. 00:37:20.720 --> 00:37:22.470 align:middle line:84% And people began to get worried about some 00:37:22.470 --> 00:37:24.240 align:middle line:84% of these features of the Big Bang model 00:37:24.240 --> 00:37:26.370 align:middle line:84% not only to cherish its successes. 00:37:26.370 --> 00:37:28.500 align:middle line:84% So for this next part, it's actually really helpful 00:37:28.500 --> 00:37:30.810 align:middle line:90% to adopt convenient coordinates. 00:37:30.810 --> 00:37:34.530 align:middle line:84% If we accept the notion that there is a universal stretching 00:37:34.530 --> 00:37:39.630 align:middle line:84% of space, then it's actually helpful to adopt 00:37:39.630 --> 00:37:42.460 align:middle line:84% coordinates that take that into account. 00:37:42.460 --> 00:37:48.000 align:middle line:84% So what astronomers have done really for generations 00:37:48.000 --> 00:37:52.400 align:middle line:84% is to adopt what are called comoving coordinates. 00:37:52.400 --> 00:37:55.600 align:middle line:84% And then a physical coordinate at any given moment in time 00:37:55.600 --> 00:37:57.970 align:middle line:84% would be scaled by this universal stretching, 00:37:57.970 --> 00:38:00.260 align:middle line:90% this so-called scale factor. 00:38:00.260 --> 00:38:05.800 align:middle line:84% So Hubble's data was consistent with galaxies sitting still 00:38:05.800 --> 00:38:08.680 align:middle line:90% at some fixed comoving location. 00:38:08.680 --> 00:38:12.530 align:middle line:84% You could plot down the Andromeda galaxy at r equals 7. 00:38:12.530 --> 00:38:15.230 align:middle line:84% And then it's moving away from us because all of space 00:38:15.230 --> 00:38:16.770 align:middle line:90% is stretching in between. 00:38:16.770 --> 00:38:19.760 align:middle line:84% So you can have galaxies that are more or less sitting still 00:38:19.760 --> 00:38:22.610 align:middle line:84% locally but are receding from us just as we 00:38:22.610 --> 00:38:25.297 align:middle line:84% are receding from them because the space in between 00:38:25.297 --> 00:38:25.880 align:middle line:90% is stretching. 00:38:25.880 --> 00:38:28.370 align:middle line:84% And you can accommodate that by inserting 00:38:28.370 --> 00:38:31.790 align:middle line:84% this one universal stretching function called the scale 00:38:31.790 --> 00:38:33.150 align:middle line:90% factor. 00:38:33.150 --> 00:38:36.140 align:middle line:84% So the distance between, say, the Milky Way and the Andromeda 00:38:36.140 --> 00:38:38.330 align:middle line:84% galaxy at any given time, the physical distance 00:38:38.330 --> 00:38:39.290 align:middle line:90% really would change. 00:38:39.290 --> 00:38:41.420 align:middle line:84% It is getting more distant over time 00:38:41.420 --> 00:38:44.270 align:middle line:84% because the space in between is stretching. 00:38:44.270 --> 00:38:46.190 align:middle line:84% So if you plot things in terms of what's 00:38:46.190 --> 00:38:48.860 align:middle line:84% called comoving distance, you scale out, 00:38:48.860 --> 00:38:51.320 align:middle line:84% you take into account that universal scaling, 00:38:51.320 --> 00:38:54.230 align:middle line:84% then it would just look like Milky Way at r equals 0, 00:38:54.230 --> 00:38:57.050 align:middle line:84% and Andromeda is stuck at r equals 7 00:38:57.050 --> 00:38:58.585 align:middle line:90% in these convenient coordinates. 00:38:58.585 --> 00:38:59.960 align:middle line:84% Then you have to be a little more 00:38:59.960 --> 00:39:03.320 align:middle line:84% clever to adopt your clock to make things, again, actually 00:39:03.320 --> 00:39:04.320 align:middle line:90% really simple. 00:39:04.320 --> 00:39:06.560 align:middle line:84% This is actually called conformal time. 00:39:06.560 --> 00:39:09.230 align:middle line:84% That might remind you of our beloved friends, 00:39:09.230 --> 00:39:10.940 align:middle line:84% the 19th century Cambridge Wranglers. 00:39:10.940 --> 00:39:12.300 align:middle line:90% At least my beloved friends. 00:39:12.300 --> 00:39:14.490 align:middle line:84% We looked briefly at the Wrangler stuff 00:39:14.490 --> 00:39:16.650 align:middle line:90% at these conformal mappings. 00:39:16.650 --> 00:39:19.440 align:middle line:84% We're really doing a Wrangler-ish thing here, 00:39:19.440 --> 00:39:23.277 align:middle line:84% very similar idea, to adopt coordinates for the time, 00:39:23.277 --> 00:39:25.110 align:middle line:84% for the rate at which we think clocks should 00:39:25.110 --> 00:39:27.420 align:middle line:84% tick to be convenient that also takes 00:39:27.420 --> 00:39:30.940 align:middle line:84% into account that changing stretching rates over time. 00:39:30.940 --> 00:39:33.900 align:middle line:84% And so what we call conformal time, often labeled 00:39:33.900 --> 00:39:37.560 align:middle line:84% by the Greek letter tau, is basically a variable tick 00:39:37.560 --> 00:39:41.190 align:middle line:84% rate that is really convenient because then we 00:39:41.190 --> 00:39:45.720 align:middle line:84% can start making a dynamical changing spacetime look just 00:39:45.720 --> 00:39:48.270 align:middle line:84% like the spacetime of special relativity, 00:39:48.270 --> 00:39:50.160 align:middle line:84% look just like a Minkowski diagram. 00:39:50.160 --> 00:39:51.980 align:middle line:84% Why do we do all this funny stuff 00:39:51.980 --> 00:39:53.730 align:middle line:84% I had to wave my hands around for comoving 00:39:53.730 --> 00:39:55.590 align:middle line:90% distance and conformal time? 00:39:55.590 --> 00:39:58.800 align:middle line:84% Because when we make spacetime plots where 00:39:58.800 --> 00:40:01.830 align:middle line:84% we use comoving distance rather than physical distance 00:40:01.830 --> 00:40:04.560 align:middle line:84% for the spatial part and conformal time, 00:40:04.560 --> 00:40:07.590 align:middle line:84% this variable clock rate rather than what's called cosmic time 00:40:07.590 --> 00:40:09.700 align:middle line:84% or sometimes simply called physical time, 00:40:09.700 --> 00:40:12.270 align:middle line:84% then actually we get back to some simple looking 00:40:12.270 --> 00:40:14.370 align:middle line:84% arrangements like light rays, once again, 00:40:14.370 --> 00:40:17.550 align:middle line:90% travel on 45 degree diagonals. 00:40:17.550 --> 00:40:19.890 align:middle line:84% If we were to try to take into account the changing 00:40:19.890 --> 00:40:22.860 align:middle line:84% stretching rate of space, these paths of light rays 00:40:22.860 --> 00:40:26.040 align:middle line:84% would become these very complicated, bent, twisted 00:40:26.040 --> 00:40:26.760 align:middle line:90% paths. 00:40:26.760 --> 00:40:29.310 align:middle line:84% But in these special, very simple fine coordinates, 00:40:29.310 --> 00:40:31.980 align:middle line:84% light just travels on 45 degree diagonals. 00:40:31.980 --> 00:40:35.640 align:middle line:84% We sit still at a fixed value of comoving location. 00:40:35.640 --> 00:40:37.860 align:middle line:84% And then light comes to us at 145 degrees. 00:40:37.860 --> 00:40:39.120 align:middle line:90% That sounds very abstract. 00:40:39.120 --> 00:40:40.200 align:middle line:90% We do that all the time. 00:40:40.200 --> 00:40:43.830 align:middle line:84% This is just an example in space time of a conformal mapping 00:40:43.830 --> 00:40:48.340 align:middle line:84% of a sort that we all use every day like a Mercator projection. 00:40:48.340 --> 00:40:54.300 align:middle line:84% So what this does is it inserts certain kinds of location 00:40:54.300 --> 00:40:59.370 align:middle line:84% dependent artifacts like Antarctica 00:40:59.370 --> 00:41:01.170 align:middle line:84% looks huge on a Mercator projection 00:41:01.170 --> 00:41:02.520 align:middle line:90% or Greenland for that matter. 00:41:02.520 --> 00:41:04.935 align:middle line:84% The surface area of Antarctica is nowhere near 00:41:04.935 --> 00:41:07.560 align:middle line:84% the surface area of Africa even though it looks so much larger. 00:41:07.560 --> 00:41:09.570 align:middle line:84% But that's because we've had a stretching. 00:41:09.570 --> 00:41:12.000 align:middle line:84% And the amount of stretching increases toward the poles 00:41:12.000 --> 00:41:15.687 align:middle line:84% if we do a two dimensional spatial conformal map. 00:41:15.687 --> 00:41:17.020 align:middle line:90% We're doing the same thing here. 00:41:17.020 --> 00:41:18.940 align:middle line:84% We're stretching our time coordinate 00:41:18.940 --> 00:41:20.910 align:middle line:84% so time gets more and more stretched out 00:41:20.910 --> 00:41:22.680 align:middle line:90% towards earlier times. 00:41:22.680 --> 00:41:24.330 align:middle line:90% We can take that into account. 00:41:24.330 --> 00:41:26.160 align:middle line:84% We know how to use a Mercator projection. 00:41:26.160 --> 00:41:28.860 align:middle line:84% And it makes other relationships remarkably easy. 00:41:28.860 --> 00:41:30.810 align:middle line:84% Likewise our conformal maps here make 00:41:30.810 --> 00:41:36.090 align:middle line:84% the paths of light, for example, very easy to follow. 00:41:36.090 --> 00:41:39.840 align:middle line:84% When people begin using these convenient coordinates, 00:41:39.840 --> 00:41:42.090 align:middle line:84% they also go back to some questions about 00:41:42.090 --> 00:41:44.040 align:middle line:84% or features of the Big Bang model, 00:41:44.040 --> 00:41:45.900 align:middle line:84% and they start having new questions. 00:41:45.900 --> 00:41:48.870 align:middle line:84% So we talked briefly last time about Robert Dicke. 00:41:48.870 --> 00:41:51.540 align:middle line:84% He was, again, a veteran of some of the wartime projects. 00:41:51.540 --> 00:41:55.380 align:middle line:84% He was an expert in microwave electronics, a radar Rad Lab 00:41:55.380 --> 00:41:56.130 align:middle line:90% veteran. 00:41:56.130 --> 00:41:59.910 align:middle line:84% And after the war, he went back to Princeton and about 10 years 00:41:59.910 --> 00:42:01.830 align:middle line:84% after that became very interested 00:42:01.830 --> 00:42:03.630 align:middle line:84% in general relativity and cosmology. 00:42:03.630 --> 00:42:06.210 align:middle line:84% And he began retooling his whole research group 00:42:06.210 --> 00:42:08.550 align:middle line:84% around questions related to things like the Big Bang. 00:42:08.550 --> 00:42:12.480 align:middle line:84% We saw that in 1961, he published this alternate 00:42:12.480 --> 00:42:14.130 align:middle line:84% to Einstein's own theory of gravity, 00:42:14.130 --> 00:42:15.940 align:middle line:84% the Brans-Dicke theory of gravity, 00:42:15.940 --> 00:42:17.148 align:middle line:90% which we looked at last time. 00:42:17.148 --> 00:42:18.720 align:middle line:90% It's the same Robert Dicke. 00:42:18.720 --> 00:42:23.070 align:middle line:84% So he introduced this conundrum in 1969, 00:42:23.070 --> 00:42:26.920 align:middle line:84% so soon after the discovery of the cosmic microwave background 00:42:26.920 --> 00:42:28.920 align:middle line:84% radiation when people began to take the Big Bang 00:42:28.920 --> 00:42:31.860 align:middle line:84% model more and more seriously, including Robert Dicke. 00:42:31.860 --> 00:42:34.740 align:middle line:84% He goes back to what I mentioned briefly before, that according 00:42:34.740 --> 00:42:38.160 align:middle line:84% to Einstein's equations, as clarified by people 00:42:38.160 --> 00:42:40.920 align:middle line:84% like Alexander Friedmann and Georges Lemaitre, 00:42:40.920 --> 00:42:43.350 align:middle line:84% you can have these very simple geometries. 00:42:43.350 --> 00:42:46.830 align:middle line:84% At any given moment in time, the shape of space 00:42:46.830 --> 00:42:49.410 align:middle line:84% could either have a positive geometry where it closes back 00:42:49.410 --> 00:42:53.460 align:middle line:84% on itself like the surface of a sphere, an open or negatively 00:42:53.460 --> 00:42:56.460 align:middle line:84% curved geometry, or a flat geometry. 00:42:56.460 --> 00:42:58.740 align:middle line:84% What controls which geometry you have 00:42:58.740 --> 00:43:01.800 align:middle line:84% is this ratio of the actual amount of stuff per volume, 00:43:01.800 --> 00:43:05.100 align:middle line:84% the actual density of matter and energy per volume, 00:43:05.100 --> 00:43:07.390 align:middle line:90% compared to some critical value. 00:43:07.390 --> 00:43:10.200 align:middle line:84% So it became common after Dicke to just 00:43:10.200 --> 00:43:13.100 align:middle line:84% introduce the Greek letter capital omega simply 00:43:13.100 --> 00:43:15.630 align:middle line:90% to refer to that ratio. 00:43:15.630 --> 00:43:17.960 align:middle line:84% What's the ratio of the actual stuff 00:43:17.960 --> 00:43:19.910 align:middle line:84% per volume in our universe compared 00:43:19.910 --> 00:43:21.140 align:middle line:90% to that critical value? 00:43:21.140 --> 00:43:23.410 align:middle line:84% And only for that Goldilocks solution, 00:43:23.410 --> 00:43:26.510 align:middle line:84% we have exactly the balanced amount of stuff per volume 00:43:26.510 --> 00:43:29.660 align:middle line:84% would you expect to have space obey Euclidean 00:43:29.660 --> 00:43:31.340 align:middle line:90% geometry on large scales. 00:43:31.340 --> 00:43:34.730 align:middle line:84% If omega is larger than 1, you have more stuff for volume. 00:43:34.730 --> 00:43:36.500 align:middle line:90% You have a positive curvature. 00:43:36.500 --> 00:43:38.930 align:middle line:84% If omega is smaller than 1, you have 00:43:38.930 --> 00:43:41.360 align:middle line:84% less stuff per volume than the critical value. 00:43:41.360 --> 00:43:44.090 align:middle line:84% You expect it's open or hyperbolic geometry. 00:43:44.090 --> 00:43:45.320 align:middle line:90% So far so good. 00:43:45.320 --> 00:43:49.710 align:middle line:84% Then Dicke plugged this quantity into Einstein's own equations. 00:43:49.710 --> 00:43:51.890 align:middle line:84% So this is a time dependent quantity. 00:43:51.890 --> 00:43:53.870 align:middle line:84% After all, we're talking about densities. 00:43:53.870 --> 00:43:55.670 align:middle line:84% The density should depend on the volume. 00:43:55.670 --> 00:43:57.200 align:middle line:90% It's stuff per volume. 00:43:57.200 --> 00:43:59.240 align:middle line:84% If you have a universe expanding over time, 00:43:59.240 --> 00:44:01.500 align:middle line:84% the volume should be changing over time. 00:44:01.500 --> 00:44:04.070 align:middle line:84% So the density should presumably go down. 00:44:04.070 --> 00:44:05.270 align:middle line:90% The density should fall. 00:44:05.270 --> 00:44:08.060 align:middle line:84% If you have a fixed amount of stuff in a space time 00:44:08.060 --> 00:44:11.570 align:middle line:84% and you stretch that space, the density falls. 00:44:11.570 --> 00:44:15.000 align:middle line:84% If you throw four marbles into a bucket, 00:44:15.000 --> 00:44:19.290 align:middle line:84% you have a density of four per cubic liter, 00:44:19.290 --> 00:44:21.720 align:middle line:90% let's say, a cubic liter bucket. 00:44:21.720 --> 00:44:23.317 align:middle line:84% If you double the size of the bucket, 00:44:23.317 --> 00:44:24.900 align:middle line:84% then you have still only four marbles, 00:44:24.900 --> 00:44:26.100 align:middle line:90% but now a larger volume. 00:44:26.100 --> 00:44:27.730 align:middle line:90% The density's gone down. 00:44:27.730 --> 00:44:30.540 align:middle line:84% And so what Dicke demonstrated is 00:44:30.540 --> 00:44:33.210 align:middle line:84% that according to Einstein's equations, this solution 00:44:33.210 --> 00:44:35.070 align:middle line:84% that looks like the Goldilocks solution, 00:44:35.070 --> 00:44:36.875 align:middle line:84% the spatially flat solution where 00:44:36.875 --> 00:44:39.000 align:middle line:84% you have just the right amount of stuff per volume, 00:44:39.000 --> 00:44:41.140 align:middle line:84% is actually an unstable solution. 00:44:41.140 --> 00:44:44.940 align:middle line:84% It's an unstable equilibrium point of Einstein's equations. 00:44:44.940 --> 00:44:47.610 align:middle line:84% A universe should generically become more and more 00:44:47.610 --> 00:44:50.200 align:middle line:90% different from flat over time. 00:44:50.200 --> 00:44:51.750 align:middle line:84% And if you just plug in the notion 00:44:51.750 --> 00:44:54.360 align:middle line:84% that the density is falling 1 over the volume, 00:44:54.360 --> 00:44:59.235 align:middle line:84% the volume should go the cube of the spatial dimensions, 00:44:59.235 --> 00:45:01.750 align:middle line:90% so go a cubed. 00:45:01.750 --> 00:45:05.470 align:middle line:84% So in fact, this part comes just from Einstein's equations, 00:45:05.470 --> 00:45:08.190 align:middle line:84% 1 over a squared, a being that scale factor, 00:45:08.190 --> 00:45:10.780 align:middle line:84% like the radius at any given moment in time, 00:45:10.780 --> 00:45:13.310 align:middle line:84% and rho is the density of stuff per volume. 00:45:13.310 --> 00:45:16.490 align:middle line:84% Well, if the rho is going 1 over a cubed, 00:45:16.490 --> 00:45:18.260 align:middle line:84% then this whole quantity here should 00:45:18.260 --> 00:45:20.460 align:middle line:90% grow with a scale factor. 00:45:20.460 --> 00:45:22.940 align:middle line:84% So the difference from a flat universe, 00:45:22.940 --> 00:45:26.540 align:middle line:84% the deviation from spatial flatness Dicke shows, 00:45:26.540 --> 00:45:29.060 align:middle line:84% generically should grow over time. 00:45:29.060 --> 00:45:31.700 align:middle line:84% If a universe started out being close to but not 00:45:31.700 --> 00:45:34.250 align:middle line:84% identically equal to flat at early times, 00:45:34.250 --> 00:45:39.140 align:middle line:84% it should look nothing like spatially flat at later times. 00:45:39.140 --> 00:45:41.000 align:middle line:84% Depending on the sign, it could either 00:45:41.000 --> 00:45:44.250 align:middle line:84% become more and more like a hyperbolic saddle or more 00:45:44.250 --> 00:45:45.500 align:middle line:90% and more like a closed sphere. 00:45:45.500 --> 00:45:47.750 align:middle line:84% What it should not do is say looking anything 00:45:47.750 --> 00:45:50.610 align:middle line:84% like the flat Euclidean solution. 00:45:50.610 --> 00:45:53.990 align:middle line:84% So if you extrapolate this backwards to early times, 00:45:53.990 --> 00:45:56.520 align:middle line:84% the time of nucleosynthesis, for example, 00:45:56.520 --> 00:46:01.250 align:middle line:84% or even to the times of when the cosmic microwave background 00:46:01.250 --> 00:46:04.745 align:middle line:84% radiation was released, you find that you have to fine tune, 00:46:04.745 --> 00:46:08.330 align:middle line:84% you have to have some reason why the amount of stuff per volume 00:46:08.330 --> 00:46:11.810 align:middle line:84% was not just in the neighborhood of that critical value, 00:46:11.810 --> 00:46:15.120 align:middle line:84% but, in fact, was exponentially close to it you. 00:46:15.120 --> 00:46:18.930 align:middle line:84% As Dicke points out, you have these exponential fine tunings 00:46:18.930 --> 00:46:22.230 align:middle line:84% for the universe to be even remotely close to a spatially 00:46:22.230 --> 00:46:25.050 align:middle line:84% flat or Euclidean-like behavior today, which 00:46:25.050 --> 00:46:27.480 align:middle line:84% is looking more and more consistent with observations 00:46:27.480 --> 00:46:29.400 align:middle line:90% by the '60s and '70s and '80s. 00:46:29.400 --> 00:46:33.210 align:middle line:84% Even if it wasn't compellingly equal to flat, 00:46:33.210 --> 00:46:35.820 align:middle line:84% this parameter omega was, say, 0.3. 00:46:35.820 --> 00:46:37.860 align:middle line:90% It wasn't 10 to the minus 70. 00:46:37.860 --> 00:46:39.600 align:middle line:90% It wasn't 5. 00:46:39.600 --> 00:46:42.060 align:middle line:90% It was 0.3 or 1.1. 00:46:42.060 --> 00:46:44.820 align:middle line:84% It was in the rough vicinity of 1 to the extent 00:46:44.820 --> 00:46:48.000 align:middle line:84% that the measurements could converge the observations. 00:46:48.000 --> 00:46:51.990 align:middle line:84% And yet a measurement of anywhere near 1 today 00:46:51.990 --> 00:46:55.110 align:middle line:84% suggested that it had to have been exponentially close to 1 00:46:55.110 --> 00:46:56.430 align:middle line:90% at early times. 00:46:56.430 --> 00:46:59.350 align:middle line:84% And that seems like this very strange or unexplained fine 00:46:59.350 --> 00:46:59.850 align:middle line:90% tuning. 00:46:59.850 --> 00:47:01.308 align:middle line:84% If the universe has been stretching 00:47:01.308 --> 00:47:05.490 align:middle line:84% for 14 billion years, what set it to be so exponentially 00:47:05.490 --> 00:47:08.580 align:middle line:84% arbitrarily close to spatially flat given that is 00:47:08.580 --> 00:47:10.710 align:middle line:90% an unstable equilibrium point? 00:47:10.710 --> 00:47:13.290 align:middle line:84% That became known as a flatness problem. 00:47:13.290 --> 00:47:17.142 align:middle line:84% That was introduced by Bob Dicke in 1969. 00:47:17.142 --> 00:47:18.600 align:middle line:84% Dicke actually was really thorough. 00:47:18.600 --> 00:47:20.225 align:middle line:84% He was thinking about other things too. 00:47:20.225 --> 00:47:23.640 align:middle line:84% So 10 years later, he introduced the next big real conundrum 00:47:23.640 --> 00:47:25.110 align:middle line:90% for the Big Bang model. 00:47:25.110 --> 00:47:27.210 align:middle line:84% And this one he did with his younger student, 00:47:27.210 --> 00:47:30.030 align:middle line:84% by that point, his collaborator, James Peebles. 00:47:30.030 --> 00:47:31.420 align:middle line:90% You might know Peebles' name. 00:47:31.420 --> 00:47:33.900 align:middle line:84% He actually just received the Nobel Prize in physics 00:47:33.900 --> 00:47:37.260 align:middle line:84% a little over a year ago for much of this work. 00:47:37.260 --> 00:47:39.730 align:middle line:84% Peebles had done his PhD with Dicke at Princeton. 00:47:39.730 --> 00:47:41.760 align:middle line:84% So 10 years later, Dicke and Peebles 00:47:41.760 --> 00:47:43.890 align:middle line:84% introduced the second big conundrum. 00:47:43.890 --> 00:47:45.940 align:middle line:84% And this one's called the horizon problem. 00:47:45.940 --> 00:47:48.240 align:middle line:84% So now let's go back to this very convenient, 00:47:48.240 --> 00:47:49.660 align:middle line:90% conformal diagram. 00:47:49.660 --> 00:47:51.720 align:middle line:84% So I'm going to use the same funny coordinates 00:47:51.720 --> 00:47:52.620 align:middle line:90% I mentioned before. 00:47:52.620 --> 00:47:55.410 align:middle line:84% I'm mapping the history of the universe using 00:47:55.410 --> 00:47:56.920 align:middle line:90% comoving distances. 00:47:56.920 --> 00:47:59.910 align:middle line:84% So I've taken into account that universal stretching of space 00:47:59.910 --> 00:48:04.720 align:middle line:84% and that variable clock rate, that conformal time. 00:48:04.720 --> 00:48:07.620 align:middle line:84% So now, according to this very lovely picture 00:48:07.620 --> 00:48:12.420 align:middle line:84% that people like George Gamow and Ralph Alpher and Robert 00:48:12.420 --> 00:48:16.050 align:middle line:84% Herman put together, we should be receiving these microwave 00:48:16.050 --> 00:48:18.670 align:middle line:84% photons today from literally every direction in the sky. 00:48:18.670 --> 00:48:20.170 align:middle line:84% This goes back to Steven's question. 00:48:20.170 --> 00:48:23.110 align:middle line:84% Imagine you have a three dimensional version of this. 00:48:23.110 --> 00:48:25.150 align:middle line:84% Everywhere in the sky, you see these photons 00:48:25.150 --> 00:48:25.900 align:middle line:90% heading toward us. 00:48:25.900 --> 00:48:26.980 align:middle line:84% Some of them are heading away from us. 00:48:26.980 --> 00:48:28.540 align:middle line:84% But we're just immersed in the bath. 00:48:28.540 --> 00:48:31.480 align:middle line:84% The ones that we see have been heading on trajectories 00:48:31.480 --> 00:48:34.360 align:middle line:84% toward us since they were first released, 00:48:34.360 --> 00:48:38.320 align:middle line:84% since that moment 380,000 years after the Big Bang 00:48:38.320 --> 00:48:40.780 align:middle line:84% when photons could begin streaming freely, 00:48:40.780 --> 00:48:43.010 align:middle line:84% when they could travel macroscopic distances 00:48:43.010 --> 00:48:45.010 align:middle line:84% because the universe is filled with electrically 00:48:45.010 --> 00:48:46.210 align:middle line:90% neutral matter. 00:48:46.210 --> 00:48:48.760 align:middle line:84% There's some moment after which the photons begin 00:48:48.760 --> 00:48:50.170 align:middle line:90% traveling large distances. 00:48:50.170 --> 00:48:52.690 align:middle line:84% They've been traveling that whole time 00:48:52.690 --> 00:48:54.910 align:middle line:84% until some of them enter our antennas 00:48:54.910 --> 00:48:55.983 align:middle line:90% and our satellites today. 00:48:55.983 --> 00:48:57.400 align:middle line:84% So remember, in these coordinates, 00:48:57.400 --> 00:49:03.138 align:middle line:84% we sit still at some fixed value of comoving location, r equals 00:49:03.138 --> 00:49:04.180 align:middle line:90% 7 or whatever you'd like. 00:49:04.180 --> 00:49:05.770 align:middle line:90% You can call it r equals 0. 00:49:05.770 --> 00:49:09.280 align:middle line:84% And then light travels on these lovely, convenient 45 degree 00:49:09.280 --> 00:49:10.120 align:middle line:90% diagonals. 00:49:10.120 --> 00:49:11.980 align:middle line:84% So from this corner of the sky, it's 00:49:11.980 --> 00:49:13.720 align:middle line:84% like pointing your telescope over there. 00:49:13.720 --> 00:49:15.370 align:middle line:84% From this corner of the sky, looking 00:49:15.370 --> 00:49:18.112 align:middle line:84% in the opposite direction, we receive this uniform bath 00:49:18.112 --> 00:49:19.570 align:middle line:84% of photons that have been traveling 00:49:19.570 --> 00:49:21.310 align:middle line:90% towards us this whole time. 00:49:21.310 --> 00:49:23.170 align:middle line:84% Well, here's where Dicke and Peebles 00:49:23.170 --> 00:49:25.240 align:middle line:90% start raising some questions. 00:49:25.240 --> 00:49:29.590 align:middle line:84% We received this remarkably uniform signal on the sky today 00:49:29.590 --> 00:49:32.450 align:middle line:84% from opposite sides-- matter of fact from any direction 00:49:32.450 --> 00:49:35.710 align:middle line:84% that we point our radial telescopes both on Earth or now 00:49:35.710 --> 00:49:36.850 align:middle line:90% from satellites. 00:49:36.850 --> 00:49:40.240 align:middle line:84% There's a comoving distance delta 00:49:40.240 --> 00:49:44.500 align:middle line:84% r across which we receive these photons. 00:49:44.500 --> 00:49:46.280 align:middle line:84% And they look remarkably uniform. 00:49:46.280 --> 00:49:48.700 align:middle line:84% However, those photons were emitted 00:49:48.700 --> 00:49:51.100 align:middle line:84% at a finite age when the universe was 00:49:51.100 --> 00:49:53.980 align:middle line:84% only a short portion of its current age. 00:49:53.980 --> 00:49:58.030 align:middle line:84% It was only 380,000 years old as opposed to nearly 14 billion 00:49:58.030 --> 00:49:59.680 align:middle line:90% years old. 00:49:59.680 --> 00:50:02.500 align:middle line:84% Light can only but travel at a fixed speed at least 00:50:02.500 --> 00:50:04.190 align:middle line:90% according to Einstein's theory. 00:50:04.190 --> 00:50:07.510 align:middle line:84% So if the universe has only been around for so long, 00:50:07.510 --> 00:50:10.060 align:middle line:84% then light could only have traveled so far. 00:50:10.060 --> 00:50:12.010 align:middle line:84% That's called the horizon distance. 00:50:12.010 --> 00:50:14.740 align:middle line:84% What's the furthest possible distance 00:50:14.740 --> 00:50:16.360 align:middle line:84% that a light beam could've traveled 00:50:16.360 --> 00:50:18.700 align:middle line:84% traveling at that constant speed of light for as long 00:50:18.700 --> 00:50:19.960 align:middle line:90% as it was able to? 00:50:19.960 --> 00:50:22.385 align:middle line:84% So even though an actual physical light beam 00:50:22.385 --> 00:50:24.760 align:middle line:84% couldn't have traveled because the universe was optically 00:50:24.760 --> 00:50:28.960 align:middle line:84% opaque, any information, any physical signal, any force, 00:50:28.960 --> 00:50:32.800 align:middle line:84% anything that is limited by Einstein's speed limit 00:50:32.800 --> 00:50:35.530 align:middle line:84% should only be able to travel up to and 00:50:35.530 --> 00:50:37.250 align:middle line:90% limited by the speed of light. 00:50:37.250 --> 00:50:39.250 align:middle line:84% That means there's a furthest distance according 00:50:39.250 --> 00:50:42.370 align:middle line:84% to which any information or influence or physical force 00:50:42.370 --> 00:50:45.730 align:middle line:84% or anything could've traveled since the Big Bang 00:50:45.730 --> 00:50:48.700 align:middle line:84% up to the time when that radiation was first emitted. 00:50:48.700 --> 00:50:50.410 align:middle line:84% That's called the horizon distance. 00:50:50.410 --> 00:50:53.650 align:middle line:84% And as they show, for any finite age, 00:50:53.650 --> 00:50:57.040 align:middle line:84% for any universe that has this beginning a finite time ago, 00:50:57.040 --> 00:50:59.785 align:middle line:84% at any moment in time, there's a furthest distance 00:50:59.785 --> 00:51:01.660 align:middle line:84% that anything traveling at the speed of light 00:51:01.660 --> 00:51:03.092 align:middle line:84% could possibly have gotten to yet. 00:51:03.092 --> 00:51:04.550 align:middle line:84% That's called the horizon distance. 00:51:04.550 --> 00:51:07.390 align:middle line:84% So what they show is that at the time 00:51:07.390 --> 00:51:11.110 align:middle line:84% that the microwave background photons began their journey 00:51:11.110 --> 00:51:13.330 align:middle line:84% when they first began to free stream, 00:51:13.330 --> 00:51:15.070 align:middle line:84% the universe was still so young that 00:51:15.070 --> 00:51:18.460 align:middle line:84% the furthest possible distance that any causal influence 00:51:18.460 --> 00:51:21.880 align:middle line:84% should've been able to travel was a tiny fraction 00:51:21.880 --> 00:51:24.580 align:middle line:84% of the distance across which we actually 00:51:24.580 --> 00:51:27.860 align:middle line:84% measure remarkably uniform signals on the sky today. 00:51:27.860 --> 00:51:31.300 align:middle line:84% So the horizon distance was actually a factor of 100 00:51:31.300 --> 00:51:34.270 align:middle line:84% shorter than the smoothness scale 00:51:34.270 --> 00:51:37.900 align:middle line:84% across which we receive remarkably uniform information. 00:51:37.900 --> 00:51:39.340 align:middle line:90% How could that be? 00:51:39.340 --> 00:51:42.160 align:middle line:84% If this portion of the sky never had a chance 00:51:42.160 --> 00:51:45.010 align:middle line:84% to become in any kind of physical equilibrium 00:51:45.010 --> 00:51:48.100 align:middle line:84% or even exchange a single tweet, to have absolutely 00:51:48.100 --> 00:51:50.800 align:middle line:84% no information of this part of the sky about what 00:51:50.800 --> 00:51:52.990 align:middle line:84% the average conditions are in this part of the sky, 00:51:52.990 --> 00:51:56.127 align:middle line:84% how could they have become indistinguishable 00:51:56.127 --> 00:51:57.460 align:middle line:90% in the signals we receive today? 00:51:57.460 --> 00:52:00.020 align:middle line:84% That became known as the horizon problem. 00:52:00.020 --> 00:52:04.270 align:middle line:84% And this was heightened as more and more data came in, 00:52:04.270 --> 00:52:06.700 align:middle line:84% more and more careful observations of the microwave 00:52:06.700 --> 00:52:08.050 align:middle line:90% background radiation. 00:52:08.050 --> 00:52:10.180 align:middle line:84% It became clear that signal really is 00:52:10.180 --> 00:52:12.940 align:middle line:90% uniform to one part in 100,000. 00:52:12.940 --> 00:52:17.412 align:middle line:84% It's remarkably uniform to a tiny fraction of a percent. 00:52:17.412 --> 00:52:19.890 align:middle line:90% It was at 1,000th of 1%. 00:52:19.890 --> 00:52:22.260 align:middle line:84% That signal is uniform across every direction 00:52:22.260 --> 00:52:24.060 align:middle line:84% we look in the sky today, even though it's 00:52:24.060 --> 00:52:26.310 align:middle line:84% coming from all these regions that when that light was 00:52:26.310 --> 00:52:29.490 align:middle line:84% emitted couldn't have possibly had any physical interaction 00:52:29.490 --> 00:52:32.843 align:middle line:84% with each other or had even a single status update saying, 00:52:32.843 --> 00:52:35.010 align:middle line:84% I'm going to release my photons at this temperature. 00:52:35.010 --> 00:52:36.130 align:middle line:90% You should do the same. 00:52:36.130 --> 00:52:37.890 align:middle line:84% So the time that the light was emitted, 00:52:37.890 --> 00:52:42.120 align:middle line:84% the smoothness scale was much, much larger 00:52:42.120 --> 00:52:45.450 align:middle line:84% than the causally self-connected scale. 00:52:45.450 --> 00:52:47.230 align:middle line:84% That becomes known as the horizon problem. 00:52:47.230 --> 00:52:50.790 align:middle line:84% So why on Earth would the CMB be so uniform today 00:52:50.790 --> 00:52:53.040 align:middle line:84% to this exponential accuracy from regions of sky 00:52:53.040 --> 00:52:56.130 align:middle line:84% that were never ever in causal contact? 00:52:56.130 --> 00:52:59.970 align:middle line:84% Plus, why on Earth do we have this distribution of scales 00:52:59.970 --> 00:53:01.470 align:middle line:84% that I started off in the beginning? 00:53:01.470 --> 00:53:03.617 align:middle line:84% Where does large scale structure come from? 00:53:03.617 --> 00:53:04.200 align:middle line:90% Oh, I'm sorry. 00:53:04.200 --> 00:53:05.070 align:middle line:90% I skipped ahead. 00:53:05.070 --> 00:53:05.580 align:middle line:90% Sorry. 00:53:05.580 --> 00:53:08.850 align:middle line:84% This is my anthropomorphic analogy 00:53:08.850 --> 00:53:12.150 align:middle line:84% for why the horizon problem should give you pause. 00:53:12.150 --> 00:53:13.650 align:middle line:84% This usually works better when we're 00:53:13.650 --> 00:53:15.900 align:middle line:84% meeting in person in an actual classroom. 00:53:15.900 --> 00:53:19.680 align:middle line:84% But imagine we're all sitting in a nice big comfortable room 00:53:19.680 --> 00:53:20.970 align:middle line:90% socially distanced. 00:53:20.970 --> 00:53:23.700 align:middle line:84% And everyone has come in the room, 00:53:23.700 --> 00:53:26.020 align:middle line:84% and I've stolen your cell phones. 00:53:26.020 --> 00:53:26.520 align:middle line:90% Sorry. 00:53:26.520 --> 00:53:27.395 align:middle line:90% You'll get them back. 00:53:27.395 --> 00:53:30.660 align:middle line:84% I've temporarily taken your cell phones and your laptops. 00:53:30.660 --> 00:53:34.680 align:middle line:84% I've blindfolded you all and put in earplugs 00:53:34.680 --> 00:53:37.510 align:middle line:84% and handed every single one of you a ping pong ball. 00:53:37.510 --> 00:53:39.670 align:middle line:84% I won't actually do this, but just imagine. 00:53:39.670 --> 00:53:41.830 align:middle line:84% And then without any prior coordination, 00:53:41.830 --> 00:53:43.420 align:middle line:84% I say, please throw your ping pong 00:53:43.420 --> 00:53:47.200 align:middle line:84% ball at the same speed at the same time to one part in 10 00:53:47.200 --> 00:53:50.470 align:middle line:84% to the 5 without any chance to coordinate, without anyone 00:53:50.470 --> 00:53:52.540 align:middle line:84% saying, ready, set, go or you being 00:53:52.540 --> 00:53:54.730 align:middle line:84% able to say to your neighbor, here's my plan. 00:53:54.730 --> 00:53:55.840 align:middle line:90% Let's coordinate. 00:53:55.840 --> 00:53:59.170 align:middle line:84% That's what it's like to have these causally disjoint regions 00:53:59.170 --> 00:54:02.225 align:middle line:84% emitting these photons not just with the same energy, 00:54:02.225 --> 00:54:03.850 align:middle line:84% but with the exact same energy released 00:54:03.850 --> 00:54:04.840 align:middle line:90% at the exact same time. 00:54:04.840 --> 00:54:07.900 align:middle line:84% That's the point of this series of ping pong balls distributed 00:54:07.900 --> 00:54:08.830 align:middle line:90% through space. 00:54:08.830 --> 00:54:11.350 align:middle line:84% Now, as I was saying, what about the lumps? 00:54:11.350 --> 00:54:14.260 align:middle line:84% This entire Big Bang model has still 00:54:14.260 --> 00:54:17.590 align:middle line:84% had to assume by fiat with no real explanation 00:54:17.590 --> 00:54:20.740 align:middle line:84% that there was some initial lumpiness, 00:54:20.740 --> 00:54:23.890 align:middle line:84% there's some inhomogeneity that over time could then 00:54:23.890 --> 00:54:27.280 align:middle line:84% grow to become this cascading hierarchy of scales, which 00:54:27.280 --> 00:54:30.228 align:middle line:84% is why we'd have super clusters of galaxies separated 00:54:30.228 --> 00:54:31.520 align:middle line:90% by huge voids and all the rest. 00:54:31.520 --> 00:54:34.300 align:middle line:84% So the Big Bang model had some amazing successes 00:54:34.300 --> 00:54:37.070 align:middle line:84% but some pretty stubborn quandaries as well. 00:54:37.070 --> 00:54:39.540 align:middle line:84% So I'll pause there again and ask the questions about that. 00:54:39.540 --> 00:54:41.710 align:middle line:84% Any questions on the shortcomings 00:54:41.710 --> 00:54:43.970 align:middle line:84% of the Big Bang as people began articulating them 00:54:43.970 --> 00:54:45.215 align:middle line:90% throughout the '60s and '70s? 00:54:45.215 --> 00:54:51.020 align:middle line:90% 00:54:51.020 --> 00:54:53.420 align:middle line:84% Feel free to jump in or use the chat or either way. 00:54:53.420 --> 00:54:56.930 align:middle line:90% 00:54:56.930 --> 00:55:00.590 align:middle line:84% And again, there's more on the quantitative details 00:55:00.590 --> 00:55:02.900 align:middle line:84% of that in that optional primer you 00:55:02.900 --> 00:55:06.200 align:middle line:90% can find on the Canvas site. 00:55:06.200 --> 00:55:07.880 align:middle line:84% So Fisher asks, is it useful to think 00:55:07.880 --> 00:55:10.670 align:middle line:84% of the universe as spherical still? 00:55:10.670 --> 00:55:11.690 align:middle line:90% Yeah. 00:55:11.690 --> 00:55:13.230 align:middle line:90% These pictures get pretty hard. 00:55:13.230 --> 00:55:21.710 align:middle line:84% So basically, we can imagine choosing some point of interest 00:55:21.710 --> 00:55:23.720 align:middle line:84% and drawing some sphere around it 00:55:23.720 --> 00:55:26.900 align:middle line:84% and asking what's happening in that region? 00:55:26.900 --> 00:55:29.840 align:middle line:84% Is that region itself will grow over time? 00:55:29.840 --> 00:55:32.510 align:middle line:84% And is that region representative of some larger 00:55:32.510 --> 00:55:33.950 align:middle line:90% sample from which it's taken? 00:55:33.950 --> 00:55:37.730 align:middle line:84% So we can still ask about the behavior of some randomly drawn 00:55:37.730 --> 00:55:40.190 align:middle line:84% sphere even if the global shape of space 00:55:40.190 --> 00:55:42.050 align:middle line:90% might not be spherical. 00:55:42.050 --> 00:55:45.230 align:middle line:84% As you can imagine, filling a perfectly rectilinear Euclidean 00:55:45.230 --> 00:55:48.080 align:middle line:84% space with a bunch of representative 00:55:48.080 --> 00:55:51.230 align:middle line:84% spheres whose behavior we could study. 00:55:51.230 --> 00:55:53.180 align:middle line:84% Now, it could be that the entire universe 00:55:53.180 --> 00:55:55.010 align:middle line:90% has a global shape to it. 00:55:55.010 --> 00:55:57.800 align:middle line:84% We could be living in a closed universe 00:55:57.800 --> 00:56:00.525 align:middle line:84% where on the largest scales it actually looks like a sphere. 00:56:00.525 --> 00:56:02.150 align:middle line:84% But we could, again, do the same trick. 00:56:02.150 --> 00:56:03.470 align:middle line:84% We could still ask about locally, 00:56:03.470 --> 00:56:05.303 align:middle line:84% let's fill it with some representative shape 00:56:05.303 --> 00:56:08.310 align:middle line:84% and ask about the average behavior within that shape. 00:56:08.310 --> 00:56:11.330 align:middle line:84% So what we mean by what spherical, 00:56:11.330 --> 00:56:14.150 align:middle line:84% we can continue to use spheres usefully 00:56:14.150 --> 00:56:15.980 align:middle line:84% even if we live in a flat universe 00:56:15.980 --> 00:56:19.770 align:middle line:84% as long as we're careful to distinguish a sample volume 00:56:19.770 --> 00:56:22.120 align:middle line:90% versus the global properties. 00:56:22.120 --> 00:56:23.080 align:middle line:90% It's a good question. 00:56:23.080 --> 00:56:24.747 align:middle line:84% Alex asks, what about the model problem? 00:56:24.747 --> 00:56:25.390 align:middle line:90% Yes, very good. 00:56:25.390 --> 00:56:26.370 align:middle line:90% So I left that one out. 00:56:26.370 --> 00:56:28.320 align:middle line:84% That was something that exercised Alan Guth 00:56:28.320 --> 00:56:30.375 align:middle line:90% in particular because-- 00:56:30.375 --> 00:56:31.500 align:middle line:90% maybe that's getting ahead. 00:56:31.500 --> 00:56:33.208 align:middle line:84% Maybe I'll talk a bit about that actually 00:56:33.208 --> 00:56:34.672 align:middle line:90% in the last part of the class. 00:56:34.672 --> 00:56:35.880 align:middle line:90% That's a good question, Alex. 00:56:35.880 --> 00:56:38.610 align:middle line:84% Any other questions about that, about the shortcomings 00:56:38.610 --> 00:56:39.270 align:middle line:90% of Big Bang? 00:56:39.270 --> 00:56:41.225 align:middle line:90% 00:56:41.225 --> 00:56:43.600 align:middle line:84% OK, let me press on because these are actually questions. 00:56:43.600 --> 00:56:45.460 align:middle line:84% Alex is already giving us a segue 00:56:45.460 --> 00:56:46.970 align:middle line:84% to the next part, which is great. 00:56:46.970 --> 00:56:51.350 align:middle line:84% So let's go to that last part for class today. 00:56:51.350 --> 00:56:52.390 align:middle line:90% I love this photograph. 00:56:52.390 --> 00:56:53.840 align:middle line:90% This is to me priceless. 00:56:53.840 --> 00:56:56.195 align:middle line:84% This is what Alan Guth looked like circa 1980. 00:56:56.195 --> 00:56:57.820 align:middle line:84% I think MIT used to have a law that you 00:56:57.820 --> 00:57:00.460 align:middle line:84% had to dress so as to match your own blackboard. 00:57:00.460 --> 00:57:01.740 align:middle line:90% I think we relaxed that rule. 00:57:01.740 --> 00:57:04.120 align:middle line:84% But anyway, he was blending into his surroundings. 00:57:04.120 --> 00:57:05.770 align:middle line:84% His room, like the universe, should've 00:57:05.770 --> 00:57:08.140 align:middle line:84% been perfectly homogeneous and isotropic. 00:57:08.140 --> 00:57:10.690 align:middle line:84% Anyway, here's a very young smiling Alan 00:57:10.690 --> 00:57:12.220 align:middle line:90% approximately 40 years ago. 00:57:12.220 --> 00:57:13.900 align:middle line:84% He was wondering about these questions 00:57:13.900 --> 00:57:16.120 align:middle line:84% as well as we'll see in a moment. 00:57:16.120 --> 00:57:18.880 align:middle line:84% He was, however, coming at this having been trained 00:57:18.880 --> 00:57:20.720 align:middle line:90% at MIT in particle theory. 00:57:20.720 --> 00:57:22.870 align:middle line:84% He was not trained in relativity or cosmology. 00:57:22.870 --> 00:57:25.030 align:middle line:84% He was much more the same generation 00:57:25.030 --> 00:57:27.730 align:middle line:84% as Tony Zee, whose work we talked about briefly 00:57:27.730 --> 00:57:29.050 align:middle line:90% in the previous lecture. 00:57:29.050 --> 00:57:31.300 align:middle line:84% When Alan was in graduate school, 00:57:31.300 --> 00:57:35.620 align:middle line:84% he was studying high energy physics and therefore not 00:57:35.620 --> 00:57:37.420 align:middle line:90% gravitational cosmology. 00:57:37.420 --> 00:57:40.120 align:middle line:84% He wound up doing a series of postdoctoral studies. 00:57:40.120 --> 00:57:44.150 align:middle line:84% He liked Tony Zee, accidentally heard some talks 00:57:44.150 --> 00:57:47.570 align:middle line:84% about some of his early work in gravitation. 00:57:47.570 --> 00:57:50.630 align:middle line:84% In particular, he heard some lectures by Robert Dicke when 00:57:50.630 --> 00:57:54.050 align:middle line:84% Dicke was on the lecture circuit talking about these curiosities 00:57:54.050 --> 00:57:55.950 align:middle line:84% or shortcomings of the Big Bang model. 00:57:55.950 --> 00:57:57.800 align:middle line:84% And that really stuck in Alan's mind. 00:57:57.800 --> 00:58:00.600 align:middle line:84% He was not originally asking questions about the cosmos, 00:58:00.600 --> 00:58:03.680 align:middle line:84% but he was haphazardly encountering 00:58:03.680 --> 00:58:05.900 align:middle line:84% some of those questions, again, very much like Tony 00:58:05.900 --> 00:58:06.950 align:middle line:90% Zee around the same time. 00:58:06.950 --> 00:58:08.780 align:middle line:84% What Alan was interested in was in things 00:58:08.780 --> 00:58:11.030 align:middle line:84% like spontaneous symmetry breaking and the Higgs 00:58:11.030 --> 00:58:11.660 align:middle line:90% mechanism. 00:58:11.660 --> 00:58:15.170 align:middle line:84% That was all the rage for a lot of particle theorists 00:58:15.170 --> 00:58:17.420 align:middle line:84% in the early and mid '70s by then. 00:58:17.420 --> 00:58:20.600 align:middle line:84% And he was wondering about shapes for the potential energy 00:58:20.600 --> 00:58:22.460 align:middle line:84% function of that Higgs field that 00:58:22.460 --> 00:58:24.440 align:middle line:90% might have a extra structure. 00:58:24.440 --> 00:58:28.200 align:middle line:84% There might be a kind of dimple to that energy function. 00:58:28.200 --> 00:58:30.530 align:middle line:84% We could imagine the Higgs field getting temporarily 00:58:30.530 --> 00:58:35.000 align:middle line:84% stuck at some metastable state at the origin 00:58:35.000 --> 00:58:36.770 align:middle line:84% of its own potential energy function 00:58:36.770 --> 00:58:39.290 align:middle line:84% where there's a barrier in any direction, 00:58:39.290 --> 00:58:41.580 align:middle line:84% but it's not an infinitely high barrier. 00:58:41.580 --> 00:58:43.580 align:middle line:84% So according to quantum theory, that Higgs field 00:58:43.580 --> 00:58:47.240 align:middle line:84% should eventually decay to the genuine global state 00:58:47.240 --> 00:58:50.810 align:middle line:84% of lowest energy, anywhere along this so-called vacuum circle. 00:58:50.810 --> 00:58:54.020 align:middle line:84% And Alan was realizing upon hearing Bob Dicke's lecture 00:58:54.020 --> 00:58:57.470 align:middle line:84% that that could have remarkable cosmological implications. 00:58:57.470 --> 00:59:00.860 align:middle line:84% If there were a time, even a short time during which 00:59:00.860 --> 00:59:03.050 align:middle line:84% the matter that's filling the universe 00:59:03.050 --> 00:59:07.280 align:middle line:84% could be temporarily stuck in a metastable state in which it 00:59:07.280 --> 00:59:09.500 align:middle line:84% had some non-zero potential energy, 00:59:09.500 --> 00:59:12.920 align:middle line:84% but it couldn't release or relax that energy arbitrarily quickly 00:59:12.920 --> 00:59:16.610 align:middle line:84% because it's stuck in this metastable so-called false 00:59:16.610 --> 00:59:19.220 align:middle line:84% vacuum, then that could have implications 00:59:19.220 --> 00:59:21.290 align:middle line:84% for the global shape of space and not just 00:59:21.290 --> 00:59:23.300 align:middle line:84% for the behavior of elementary particles. 00:59:23.300 --> 00:59:24.740 align:middle line:90% I highlight the date. 00:59:24.740 --> 00:59:26.600 align:middle line:90% Thank goodness for historians. 00:59:26.600 --> 00:59:30.380 align:middle line:84% Alan is unbelievably anal retentive and writes everything 00:59:30.380 --> 00:59:32.228 align:middle line:84% down and has pretty neat handwriting. 00:59:32.228 --> 00:59:33.770 align:middle line:84% A lot of people who write things down 00:59:33.770 --> 00:59:35.870 align:middle line:84% and have egregious handwriting, and there's more people 00:59:35.870 --> 00:59:37.037 align:middle line:90% who don't write things down. 00:59:37.037 --> 00:59:39.500 align:middle line:84% Alan, although he likes to blend in with his blackboard, 00:59:39.500 --> 00:59:41.208 align:middle line:84% writes things down with neat handwriting. 00:59:41.208 --> 00:59:42.860 align:middle line:90% And so I note the date. 00:59:42.860 --> 00:59:45.320 align:middle line:90% 41 years ago to the day-- 00:59:45.320 --> 00:59:47.300 align:middle line:84% today is December 7-- to the day, 00:59:47.300 --> 00:59:50.510 align:middle line:84% he was up very late as is his wont piecing together 00:59:50.510 --> 00:59:53.090 align:middle line:84% his ideas about these Higgs like functions 00:59:53.090 --> 00:59:55.503 align:middle line:84% with these funny metastable states 00:59:55.503 --> 00:59:57.170 align:middle line:84% where the energy density of the universe 00:59:57.170 --> 01:00:00.980 align:middle line:84% could get trapped temporarily at some large non-zero value. 01:00:00.980 --> 01:00:04.280 align:middle line:84% And he was putting that together in his mind with the lectures 01:00:04.280 --> 01:00:07.220 align:middle line:84% he had literally just heard from Robert Dicke not long before. 01:00:07.220 --> 01:00:09.920 align:middle line:84% And he calls this a spectacular realization. 01:00:09.920 --> 01:00:13.820 align:middle line:84% So my request number 3 to you scientists 01:00:13.820 --> 01:00:16.070 align:middle line:84% is both write things down, use neat handwriting. 01:00:16.070 --> 01:00:18.680 align:middle line:84% And when you do something cool, tell us it. 01:00:18.680 --> 01:00:21.020 align:middle line:84% Tell us that you're excited and put it in a box 01:00:21.020 --> 01:00:22.550 align:middle line:84% because when we're going through your notebooks, honestly, 01:00:22.550 --> 01:00:23.360 align:middle line:90% most of it's just garbage. 01:00:23.360 --> 01:00:24.200 align:middle line:90% We just don't care. 01:00:24.200 --> 01:00:26.325 align:middle line:84% But he actually did us a favor and put it in a box. 01:00:26.325 --> 01:00:27.410 align:middle line:90% Pay attention to this. 01:00:27.410 --> 01:00:29.090 align:middle line:84% His notebook's actually now on display 01:00:29.090 --> 01:00:31.820 align:middle line:84% in the Adler Planetarium in Chicago, literally 01:00:31.820 --> 01:00:33.350 align:middle line:90% this page of notes. 01:00:33.350 --> 01:00:36.080 align:middle line:84% He realized that this kind of feature 01:00:36.080 --> 01:00:39.380 align:middle line:84% could actually lead to a cosmologically distinct kind 01:00:39.380 --> 01:00:42.660 align:middle line:84% of evolution, that if you have a period of time 01:00:42.660 --> 01:00:44.900 align:middle line:84% even briefly during which the energy 01:00:44.900 --> 01:00:47.510 align:middle line:84% density, the amount of stuff per volume, gets 01:00:47.510 --> 01:00:50.810 align:middle line:84% stuck, gets stuck at some non-zero value 01:00:50.810 --> 01:00:53.960 align:middle line:84% and can't change quickly, then the energy density 01:00:53.960 --> 01:00:55.460 align:middle line:90% could remain constant. 01:00:55.460 --> 01:00:58.130 align:middle line:84% If the energy density, the stuff per volume, 01:00:58.130 --> 01:01:01.430 align:middle line:84% remains constant, then very counterintuitively, 01:01:01.430 --> 01:01:04.790 align:middle line:84% you have a runaway growth in the size of space. 01:01:04.790 --> 01:01:07.160 align:middle line:84% That stretch function, the scale factor 01:01:07.160 --> 01:01:09.380 align:middle line:84% going back just to Einstein's equations, 01:01:09.380 --> 01:01:11.270 align:middle line:84% will grow exponentially quickly, will 01:01:11.270 --> 01:01:13.860 align:middle line:84% have a period of accelerated expansion 01:01:13.860 --> 01:01:15.860 align:middle line:84% during which the universe won't just get bigger. 01:01:15.860 --> 01:01:20.240 align:middle line:84% It'll get bigger faster if you have this counterintuitive even 01:01:20.240 --> 01:01:23.360 align:middle line:84% temporary phase during which the stuff per volume 01:01:23.360 --> 01:01:26.270 align:middle line:84% stays constant even as the volume grows exponentially. 01:01:26.270 --> 01:01:27.290 align:middle line:90% That could happen. 01:01:27.290 --> 01:01:31.730 align:middle line:84% Alan began wondering if you have this weird state of matter that 01:01:31.730 --> 01:01:34.640 align:middle line:84% was at least hypothetical and of right interest 01:01:34.640 --> 01:01:37.280 align:middle line:84% to particle physicists because they were worried about things 01:01:37.280 --> 01:01:40.610 align:middle line:84% like symmetry breaking and the Higgs mechanism, that does not 01:01:40.610 --> 01:01:43.090 align:middle line:90% happen with marbles in a bucket. 01:01:43.090 --> 01:01:45.730 align:middle line:84% It does not happen with electrons or quarks or protons. 01:01:45.730 --> 01:01:48.708 align:middle line:84% It happens for certain kinds of elementary particles, 01:01:48.708 --> 01:01:51.250 align:middle line:84% including things like these very simple fields like the Higgs 01:01:51.250 --> 01:01:54.610 align:middle line:84% field or Higgs particle, which for other reasons 01:01:54.610 --> 01:01:56.890 align:middle line:84% could have some funny shape to their potential energy 01:01:56.890 --> 01:01:57.763 align:middle line:90% function. 01:01:57.763 --> 01:01:59.680 align:middle line:84% Alex, I'm going to skip the monopolar problem, 01:01:59.680 --> 01:02:01.010 align:middle line:84% but it comes from this discussion as well. 01:02:01.010 --> 01:02:02.500 align:middle line:84% And I'd be delighted to chat more about that if you'd 01:02:02.500 --> 01:02:03.167 align:middle line:90% like afterwards. 01:02:03.167 --> 01:02:05.458 align:middle line:84% But in the interest of time, Alan 01:02:05.458 --> 01:02:07.750 align:middle line:84% was worried about some exotic features from these Higgs 01:02:07.750 --> 01:02:11.278 align:middle line:84% fields that can get twisted up in some topological shape. 01:02:11.278 --> 01:02:12.820 align:middle line:84% But he was really just wondering what 01:02:12.820 --> 01:02:16.450 align:middle line:84% happens if the universe gets stuck even temporarily 01:02:16.450 --> 01:02:19.660 align:middle line:84% such that the matter that dominates it, fills it, 01:02:19.660 --> 01:02:23.150 align:middle line:84% can't release or relax its potential energy arbitrarily 01:02:23.150 --> 01:02:23.650 align:middle line:90% quickly. 01:02:23.650 --> 01:02:25.300 align:middle line:84% That's called a metastable state. 01:02:25.300 --> 01:02:28.000 align:middle line:84% And if you go back to Einstein's equations exactly in the form 01:02:28.000 --> 01:02:29.740 align:middle line:84% that he began learning from Bob Dicke 01:02:29.740 --> 01:02:31.720 align:middle line:84% from that series of lectures, then you 01:02:31.720 --> 01:02:33.400 align:middle line:84% have these very different solutions 01:02:33.400 --> 01:02:35.500 align:middle line:90% for the average size of space. 01:02:35.500 --> 01:02:37.060 align:middle line:90% It grows exponentially quickly. 01:02:37.060 --> 01:02:39.580 align:middle line:84% And as Alan and others were quick to confirm, 01:02:39.580 --> 01:02:43.880 align:middle line:84% this happens very naturally, or at least it's a kind of feature 01:02:43.880 --> 01:02:47.000 align:middle line:84% that one stumbles upon readily, if when 01:02:47.000 --> 01:02:50.510 align:middle line:84% studying these exotic Higgs-like fields from particle physics. 01:02:50.510 --> 01:02:53.300 align:middle line:84% It does not happen with spin one half particles, for example. 01:02:53.300 --> 01:02:55.400 align:middle line:84% It does not very easily happen even 01:02:55.400 --> 01:02:57.620 align:middle line:84% with photons or gluons or things that 01:02:57.620 --> 01:03:00.020 align:middle line:84% happens with these Higgs-like scalar particles 01:03:00.020 --> 01:03:01.610 align:middle line:90% most naturally. 01:03:01.610 --> 01:03:04.287 align:middle line:84% Soon after that, actually within a few months, 01:03:04.287 --> 01:03:06.620 align:middle line:84% a number of other colleagues, some in the United States, 01:03:06.620 --> 01:03:08.900 align:middle line:84% some in what was still then the Soviet Union, 01:03:08.900 --> 01:03:13.310 align:middle line:84% were finding similar behaviors and even more generic or simple 01:03:13.310 --> 01:03:14.390 align:middle line:90% arrangements. 01:03:14.390 --> 01:03:18.510 align:middle line:84% So Paul Steinhardt was working with his then PhD student Andy 01:03:18.510 --> 01:03:19.010 align:middle line:90% Albrecht. 01:03:19.010 --> 01:03:21.690 align:middle line:84% They were at the time at Penn, University of Pennsylvania. 01:03:21.690 --> 01:03:25.070 align:middle line:84% Meanwhile, in Moscow, Alexander Starobinsky and Andrei Linde 01:03:25.070 --> 01:03:28.790 align:middle line:84% were working quite independently of Steinhardt and Albrecht. 01:03:28.790 --> 01:03:31.490 align:middle line:84% And then again, realizing that if you study the dynamics, 01:03:31.490 --> 01:03:33.410 align:middle line:84% the behavior of these exotic quantum 01:03:33.410 --> 01:03:36.830 align:middle line:84% fields like a Higgs field in a stretching space time, 01:03:36.830 --> 01:03:39.242 align:middle line:84% if you take that stretching of space seriously, 01:03:39.242 --> 01:03:40.700 align:middle line:84% then you don't even need to cook up 01:03:40.700 --> 01:03:42.560 align:middle line:84% those exotic Higgs-like potentials 01:03:42.560 --> 01:03:44.390 align:middle line:84% that Alan was first thinking about. 01:03:44.390 --> 01:03:48.765 align:middle line:84% Quite generically, you'll have a damped oscillator behavior. 01:03:48.765 --> 01:03:51.140 align:middle line:84% If you look at the evolution of some field like the Higgs 01:03:51.140 --> 01:03:54.470 align:middle line:84% field, its self-consistent change over time, 01:03:54.470 --> 01:03:57.800 align:middle line:84% its equation of motion, includes a damping factor 01:03:57.800 --> 01:03:59.330 align:middle line:90% like a damped oscillator. 01:03:59.330 --> 01:04:02.360 align:middle line:84% This comes from the fact that space itself is stretching. 01:04:02.360 --> 01:04:05.150 align:middle line:84% And that alone it turns out is enough to find 01:04:05.150 --> 01:04:08.120 align:middle line:84% these self-consistent solutions in which the field moves 01:04:08.120 --> 01:04:10.290 align:middle line:90% very slowly. 01:04:10.290 --> 01:04:12.720 align:middle line:84% You can imagine it rolling down this hill, 01:04:12.720 --> 01:04:15.140 align:middle line:84% rolling down-- sorry-- rolling down slowly 01:04:15.140 --> 01:04:17.270 align:middle line:84% as a function of time because it's 01:04:17.270 --> 01:04:19.522 align:middle line:84% like a frictional, overdamped oscillator. 01:04:19.522 --> 01:04:21.230 align:middle line:84% Again, there's more of that in the primer 01:04:21.230 --> 01:04:22.970 align:middle line:90% if you're curious to see more. 01:04:22.970 --> 01:04:26.420 align:middle line:84% Even that not literally fixed behavior-- 01:04:26.420 --> 01:04:29.210 align:middle line:84% it's literally changing, just changing slowly enough. 01:04:29.210 --> 01:04:31.670 align:middle line:84% That will lead to a slowly enough changing 01:04:31.670 --> 01:04:33.800 align:middle line:84% potential energy trapped in that field 01:04:33.800 --> 01:04:35.240 align:middle line:84% that you'll still get these nearly 01:04:35.240 --> 01:04:36.920 align:middle line:90% exponential-like solutions. 01:04:36.920 --> 01:04:39.860 align:middle line:84% So you can have inflation happening even more generically 01:04:39.860 --> 01:04:43.450 align:middle line:84% as these folks began to find very soon after Alan 01:04:43.450 --> 01:04:45.915 align:middle line:84% even without worrying about a very particular shape 01:04:45.915 --> 01:04:47.290 align:middle line:84% for the potential energy function 01:04:47.290 --> 01:04:50.950 align:middle line:84% just when you think about these fields like a Higgs-like field 01:04:50.950 --> 01:04:53.060 align:middle line:90% in the early universe. 01:04:53.060 --> 01:04:55.060 align:middle line:84% So then you come back to those quandaries 01:04:55.060 --> 01:04:57.430 align:middle line:84% that Alan had first heard about from Bob Dicke. 01:04:57.430 --> 01:05:00.730 align:middle line:84% And you ask, how would these things 01:05:00.730 --> 01:05:04.450 align:middle line:84% look if you now take into account this very early, very 01:05:04.450 --> 01:05:09.005 align:middle line:84% brief phase of exponentially fast stretching of space? 01:05:09.005 --> 01:05:11.380 align:middle line:84% Go back to the equation that Dicke had first written down 01:05:11.380 --> 01:05:12.880 align:middle line:90% for the flatness problem. 01:05:12.880 --> 01:05:16.120 align:middle line:84% But now we have a phase during which the scale factor grows 01:05:16.120 --> 01:05:19.060 align:middle line:84% exponentially-- e to the something times t, 01:05:19.060 --> 01:05:21.220 align:middle line:90% so it grows very fast in time-- 01:05:21.220 --> 01:05:23.825 align:middle line:84% while the energy density remains nearly constant. 01:05:23.825 --> 01:05:25.450 align:middle line:84% So instead of that falling with volume, 01:05:25.450 --> 01:05:27.790 align:middle line:84% it temporarily remains nearly constant. 01:05:27.790 --> 01:05:32.170 align:middle line:84% Now you'd see this expression, the deviation of the universe 01:05:32.170 --> 01:05:33.430 align:middle line:90% from spatially flat. 01:05:33.430 --> 01:05:36.640 align:middle line:84% That deviation should rapidly fall to 0. 01:05:36.640 --> 01:05:40.030 align:middle line:84% The universe today should look indistinguishable 01:05:40.030 --> 01:05:43.240 align:middle line:84% from a flat universe because the difference from flatness 01:05:43.240 --> 01:05:45.470 align:middle line:90% was driven to 0 dynamically. 01:05:45.470 --> 01:05:47.660 align:middle line:84% By having even a very brief phase 01:05:47.660 --> 01:05:50.990 align:middle line:84% of exponentially rapid accelerating expansion, 01:05:50.990 --> 01:05:54.320 align:middle line:84% you drive the universe towards a flat shape 01:05:54.320 --> 01:05:57.767 align:middle line:84% rather than having it flow away from a flat shape. 01:05:57.767 --> 01:06:00.100 align:middle line:84% And again, I go through that in more quantitative detail 01:06:00.100 --> 01:06:01.010 align:middle line:90% in the primer. 01:06:01.010 --> 01:06:03.340 align:middle line:84% So the latest measurement from the Planck collaboration 01:06:03.340 --> 01:06:07.360 align:middle line:84% using a satellite is that this parameter 01:06:07.360 --> 01:06:10.090 align:middle line:84% in our actual universe today is 1 to better 01:06:10.090 --> 01:06:11.440 align:middle line:90% than a percent level accuracy. 01:06:11.440 --> 01:06:12.190 align:middle line:90% Now, let me pause. 01:06:12.190 --> 01:06:13.300 align:middle line:84% I know I'm going to run long today, 01:06:13.300 --> 01:06:14.560 align:middle line:90% but I just can't help myself. 01:06:14.560 --> 01:06:17.020 align:middle line:84% When I was in graduate school not super long ago-- 01:06:17.020 --> 01:06:18.040 align:middle line:90% kind of long ago-- 01:06:18.040 --> 01:06:21.970 align:middle line:84% I was friends with a bunch of observational astronomy grad 01:06:21.970 --> 01:06:23.110 align:middle line:90% students in the dorm. 01:06:23.110 --> 01:06:24.257 align:middle line:90% And they were basically me. 01:06:24.257 --> 01:06:25.090 align:middle line:90% They would tease me. 01:06:25.090 --> 01:06:28.030 align:middle line:84% Like, you work on inflation, but we know that omega is 0.3. 01:06:28.030 --> 01:06:29.710 align:middle line:90% So you're a loser. 01:06:29.710 --> 01:06:31.300 align:middle line:84% Why do you waste your time on this? 01:06:31.300 --> 01:06:32.410 align:middle line:84% Some were nice, but a lot of them 01:06:32.410 --> 01:06:33.550 align:middle line:90% were actually kind of mean. 01:06:33.550 --> 01:06:34.360 align:middle line:90% I was like, oh, no. 01:06:34.360 --> 01:06:35.318 align:middle line:90% Go look for more stuff. 01:06:35.318 --> 01:06:37.360 align:middle line:84% You're missing two thirds of the stuff out there. 01:06:37.360 --> 01:06:38.120 align:middle line:90% Just try again. 01:06:38.120 --> 01:06:39.250 align:middle line:90% So maybe I was mean too. 01:06:39.250 --> 01:06:40.360 align:middle line:90% But they were meaner. 01:06:40.360 --> 01:06:41.420 align:middle line:90% They were more of them. 01:06:41.420 --> 01:06:43.462 align:middle line:84% So when I was in grad school, it looked very much 01:06:43.462 --> 01:06:44.950 align:middle line:90% like omega was 0.3. 01:06:44.950 --> 01:06:48.280 align:middle line:84% And if you squinted at it, you could maybe make it 0.35. 01:06:48.280 --> 01:06:50.830 align:middle line:84% It was not 1 according to the best observations 01:06:50.830 --> 01:06:51.640 align:middle line:90% around the world. 01:06:51.640 --> 01:06:56.705 align:middle line:84% Today, [LAUGHS] it's 1 to better than a percent level accuracy. 01:06:56.705 --> 01:06:57.830 align:middle line:90% And they can just stick it. 01:06:57.830 --> 01:07:00.740 align:middle line:84% So I like sending them holiday cards saying, thinking of you. 01:07:00.740 --> 01:07:01.700 align:middle line:90% Omega's 1. 01:07:01.700 --> 01:07:02.450 align:middle line:90% See you next year. 01:07:02.450 --> 01:07:04.130 align:middle line:84% Anyway, this is a remarkable shift 01:07:04.130 --> 01:07:06.390 align:middle line:84% even over the course of 20 to 25 years, 01:07:06.390 --> 01:07:09.230 align:middle line:84% let alone since the days of Lemaitre and Hubble. 01:07:09.230 --> 01:07:09.800 align:middle line:90% OK. 01:07:09.800 --> 01:07:12.440 align:middle line:84% What about the more subtle one that Bob Dicke and James 01:07:12.440 --> 01:07:15.140 align:middle line:84% Peebles worked out called the so-called horizon problem? 01:07:15.140 --> 01:07:18.440 align:middle line:84% Go back to our funny map, our conformal map. 01:07:18.440 --> 01:07:20.510 align:middle line:84% Now, remember, the horizon problem 01:07:20.510 --> 01:07:22.550 align:middle line:84% was originally phrased because we 01:07:22.550 --> 01:07:25.760 align:middle line:84% thought there was an origin to all of time, this Big Bang 01:07:25.760 --> 01:07:28.670 align:middle line:90% surface, at tau equals 0. 01:07:28.670 --> 01:07:31.520 align:middle line:84% And if you add up the time between tau equals 0 01:07:31.520 --> 01:07:34.490 align:middle line:84% and when those photons begin to travel freely, 01:07:34.490 --> 01:07:36.470 align:middle line:84% there was only a fixed horizon distance. 01:07:36.470 --> 01:07:39.590 align:middle line:84% It was much smaller than the smoothness scale 01:07:39.590 --> 01:07:41.300 align:middle line:84% that we could measure empirically. 01:07:41.300 --> 01:07:43.280 align:middle line:84% Well, if inflation happened, there 01:07:43.280 --> 01:07:44.900 align:middle line:84% should've been a very brief period 01:07:44.900 --> 01:07:48.140 align:middle line:84% before what had previously been called the Big Bang. 01:07:48.140 --> 01:07:51.770 align:middle line:84% So we're adding more real estate along our time axis. 01:07:51.770 --> 01:07:54.080 align:middle line:84% We're unfurling a little bit extra time 01:07:54.080 --> 01:07:56.420 align:middle line:84% that hadn't been taken into account in the standard Big 01:07:56.420 --> 01:07:59.460 align:middle line:90% Bang model. 01:07:59.460 --> 01:08:01.770 align:middle line:84% So if you allow for more time before what 01:08:01.770 --> 01:08:03.660 align:middle line:84% you would call the Big Bang, you can 01:08:03.660 --> 01:08:06.420 align:middle line:84% continue tracing those past light cones further and further 01:08:06.420 --> 01:08:09.600 align:middle line:84% back and say there should be some time earlier than what 01:08:09.600 --> 01:08:11.670 align:middle line:84% we were starting from during which 01:08:11.670 --> 01:08:14.640 align:middle line:84% all the past light cones from the entire region we see today 01:08:14.640 --> 01:08:16.052 align:middle line:90% would indeed have overlapped. 01:08:16.052 --> 01:08:18.510 align:middle line:84% So then it would at least be plausible there's at least now 01:08:18.510 --> 01:08:21.807 align:middle line:84% a causally self-consistent mechanism by means 01:08:21.807 --> 01:08:24.390 align:middle line:84% of which the universe could have similar conditions everywhere 01:08:24.390 --> 01:08:27.180 align:middle line:84% because they actually were causally connected at a time 01:08:27.180 --> 01:08:31.020 align:middle line:84% before we had previously taken into account. 01:08:31.020 --> 01:08:33.890 align:middle line:84% So therefore, you could have the horizon distance, 01:08:33.890 --> 01:08:36.410 align:middle line:84% the maximum causal distance, becomes 01:08:36.410 --> 01:08:38.479 align:middle line:84% actually much larger than the smoothness scale 01:08:38.479 --> 01:08:39.479 align:middle line:90% that we measure. 01:08:39.479 --> 01:08:41.390 align:middle line:84% So now the horizon distance is larger 01:08:41.390 --> 01:08:43.520 align:middle line:84% because there was more time that we hadn't yet 01:08:43.520 --> 01:08:44.720 align:middle line:90% taken into account. 01:08:44.720 --> 01:08:46.729 align:middle line:84% Any kind of causal influence would've 01:08:46.729 --> 01:08:49.460 align:middle line:84% had more time to propagate than we had previously 01:08:49.460 --> 01:08:50.640 align:middle line:90% accounted for. 01:08:50.640 --> 01:08:53.090 align:middle line:84% So now you get the ratio at least in the right order. 01:08:53.090 --> 01:08:55.279 align:middle line:84% You can have a horizon distance that is larger. 01:08:55.279 --> 01:08:58.010 align:middle line:84% In fact, it could be much, much larger, exponentially larger 01:08:58.010 --> 01:09:00.330 align:middle line:84% than the smoothness scale we observed. 01:09:00.330 --> 01:09:02.870 align:middle line:84% Now, remember, this is a funny coordinate. 01:09:02.870 --> 01:09:05.479 align:middle line:84% It takes an unbelievably short amount 01:09:05.479 --> 01:09:07.279 align:middle line:84% of physical or cosmic time, the time 01:09:07.279 --> 01:09:09.260 align:middle line:84% that we measure on our wristwatches, 01:09:09.260 --> 01:09:10.310 align:middle line:90% to accomplish that. 01:09:10.310 --> 01:09:13.250 align:middle line:84% In fact, it takes about 10 to the minus 36th 01:09:13.250 --> 01:09:14.720 align:middle line:90% of a single second. 01:09:14.720 --> 01:09:16.760 align:middle line:84% That's all it takes for this inflation. 01:09:16.760 --> 01:09:19.700 align:middle line:84% If the universe expanded exponentially just 01:09:19.700 --> 01:09:23.000 align:middle line:84% for that sub, sub, sub blink of an eye, then all of a sudden, 01:09:23.000 --> 01:09:25.490 align:middle line:84% the causal structure of the entire observable universe 01:09:25.490 --> 01:09:27.229 align:middle line:90% is turned upside down. 01:09:27.229 --> 01:09:29.810 align:middle line:84% You basically erase the horizon problem 01:09:29.810 --> 01:09:33.870 align:middle line:84% because there actually was a time when all the stuff we see 01:09:33.870 --> 01:09:36.240 align:middle line:84% would've been in causal contact, very comfortably 01:09:36.240 --> 01:09:37.649 align:middle line:90% in causal contact. 01:09:37.649 --> 01:09:39.600 align:middle line:84% And the universe in that tiny blink, 01:09:39.600 --> 01:09:42.120 align:middle line:84% a billion, billion, billion billionth of a second, 01:09:42.120 --> 01:09:44.830 align:middle line:84% grew by about 30 orders of magnitude. 01:09:44.830 --> 01:09:46.050 align:middle line:90% It didn't keep doing that. 01:09:46.050 --> 01:09:48.840 align:middle line:84% It wasn't doing that during the rest of this Big Bang 01:09:48.840 --> 01:09:49.590 align:middle line:90% evolution. 01:09:49.590 --> 01:09:51.450 align:middle line:90% There's this tiny blip. 01:09:51.450 --> 01:09:54.480 align:middle line:84% And taking that into account suddenly rearranges 01:09:54.480 --> 01:09:58.590 align:middle line:84% the causal ordering and basically addresses these out 01:09:58.590 --> 01:10:00.810 align:middle line:90% of order causal conundra. 01:10:00.810 --> 01:10:04.710 align:middle line:84% So here's a plot in more familiar coordinates, 01:10:04.710 --> 01:10:08.070 align:middle line:84% going back now to time measured in seconds and space measured 01:10:08.070 --> 01:10:12.060 align:middle line:84% in meters rather than comoving distance and conformal time. 01:10:12.060 --> 01:10:15.150 align:middle line:84% You can see that as you trace backwards from today, 01:10:15.150 --> 01:10:17.790 align:middle line:84% instead of going back to saying the universe at early times 01:10:17.790 --> 01:10:20.060 align:middle line:84% should've been on the order of 1 meter, 01:10:20.060 --> 01:10:21.810 align:middle line:84% you say at those early times, the universe 01:10:21.810 --> 01:10:24.420 align:middle line:84% was actually exponentially tinier than you had thought. 01:10:24.420 --> 01:10:29.860 align:middle line:84% It grew exponentially quickly to map onto where we see today. 01:10:29.860 --> 01:10:33.240 align:middle line:84% And so the universe was so tiny, it could very easily 01:10:33.240 --> 01:10:35.160 align:middle line:84% have been in a kind of equilibrium 01:10:35.160 --> 01:10:37.410 align:middle line:84% or at least a causally self-connected state. 01:10:37.410 --> 01:10:39.690 align:middle line:84% So during this tiny blink of an eye, 01:10:39.690 --> 01:10:41.670 align:middle line:84% the universe grew exponentially quickly 01:10:41.670 --> 01:10:44.790 align:middle line:84% and then mapped onto the standard Big Bang evolution. 01:10:44.790 --> 01:10:47.700 align:middle line:84% And that alone is enough to address the flatness 01:10:47.700 --> 01:10:50.067 align:middle line:90% and horizon problems. 01:10:50.067 --> 01:10:51.900 align:middle line:84% It turns out it does more than that as well. 01:10:51.900 --> 01:10:53.733 align:middle line:84% This is what gets I think even more exciting 01:10:53.733 --> 01:10:56.600 align:middle line:84% and what has occupied much of the community ever since. 01:10:56.600 --> 01:11:00.350 align:middle line:84% That Higgs-like field that was driving inflation, 01:11:00.350 --> 01:11:03.875 align:middle line:84% that was very slowly evolving in its potential 01:11:03.875 --> 01:11:06.000 align:middle line:84% should've been subject to the uncertainty principle 01:11:06.000 --> 01:11:07.490 align:middle line:90% just like all matters should be. 01:11:07.490 --> 01:11:10.220 align:middle line:84% And this became clear to people about a year or so 01:11:10.220 --> 01:11:13.190 align:middle line:84% after Alan and Paul and Andy and all those folks 01:11:13.190 --> 01:11:15.290 align:middle line:84% began writing the first papers on inflation. 01:11:15.290 --> 01:11:18.758 align:middle line:84% By 1982, '83, pretty early on, people 01:11:18.758 --> 01:11:20.300 align:middle line:84% realized that not only would you have 01:11:20.300 --> 01:11:24.140 align:middle line:84% a gross feature of the evolution of those exotic particle 01:11:24.140 --> 01:11:25.250 align:middle line:90% physics-like fields. 01:11:25.250 --> 01:11:26.840 align:middle line:84% They should also have quantum wiggles 01:11:26.840 --> 01:11:28.550 align:middle line:84% because how could they not because they 01:11:28.550 --> 01:11:30.508 align:middle line:84% should be subject to the uncertainty principle. 01:11:30.508 --> 01:11:31.910 align:middle line:84% These are quantum fields evolving 01:11:31.910 --> 01:11:34.580 align:middle line:90% in a dynamical spacetime. 01:11:34.580 --> 01:11:36.110 align:middle line:90% I still can't believe it. 01:11:36.110 --> 01:11:39.050 align:middle line:84% But it is the case that you can study 01:11:39.050 --> 01:11:41.660 align:middle line:84% the evolution of those quantum fluctuations 01:11:41.660 --> 01:11:44.927 align:middle line:84% with a remarkably simple looking oscillator equation. 01:11:44.927 --> 01:11:46.760 align:middle line:84% I've hidden all the hard stuff in this term. 01:11:46.760 --> 01:11:47.870 align:middle line:90% I made them look easy. 01:11:47.870 --> 01:11:49.453 align:middle line:84% But you can actually take into account 01:11:49.453 --> 01:11:52.440 align:middle line:84% that frictional damping, the stretching of space, 01:11:52.440 --> 01:11:56.340 align:middle line:84% and the reaction of that jittering trampoline 01:11:56.340 --> 01:11:57.960 align:middle line:90% back on the evolution of matter. 01:11:57.960 --> 01:12:01.600 align:middle line:84% We can solve these equations to unbelievable accuracy 01:12:01.600 --> 01:12:03.720 align:middle line:84% and realize that we should have a prediction 01:12:03.720 --> 01:12:07.590 align:middle line:84% today for tiny seeds, tiny unevenness, 01:12:07.590 --> 01:12:10.860 align:middle line:84% in a very early distribution of matter and energy 01:12:10.860 --> 01:12:14.100 align:middle line:84% because the universe was filled with quantum fields. 01:12:14.100 --> 01:12:15.860 align:middle line:84% And as we've seen a number of times now, 01:12:15.860 --> 01:12:17.610 align:middle line:84% quantum jitter, the uncertainty principle, 01:12:17.610 --> 01:12:20.280 align:middle line:84% means that we can never specify the energy of that field 01:12:20.280 --> 01:12:22.560 align:middle line:84% to arbitrary precision at any time. 01:12:22.560 --> 01:12:24.660 align:middle line:84% At any given moment, that field would 01:12:24.660 --> 01:12:26.790 align:middle line:84% be subject to slight, slight quantum 01:12:26.790 --> 01:12:30.570 align:middle line:84% fluctuations in the distribution of energy across space. 01:12:30.570 --> 01:12:33.630 align:middle line:84% That starts to yield this tiny little fluctuation 01:12:33.630 --> 01:12:35.580 align:middle line:84% in why there's slightly more matter 01:12:35.580 --> 01:12:39.570 align:middle line:84% and energy in this region of space than the other one. 01:12:39.570 --> 01:12:43.290 align:middle line:84% So now those very tiny quantum scale fluctuations 01:12:43.290 --> 01:12:46.240 align:middle line:84% get stretched as the whole universe stretches. 01:12:46.240 --> 01:12:48.430 align:middle line:84% As the scale factor grows exponentially, 01:12:48.430 --> 01:12:52.990 align:middle line:84% you have the average length between the distance 01:12:52.990 --> 01:12:54.880 align:middle line:84% between crests of those tiny wiggles 01:12:54.880 --> 01:12:58.120 align:middle line:84% get stretched to galactic and even super galactic scales 01:12:58.120 --> 01:12:59.710 align:middle line:90% all within that blink of an eye. 01:12:59.710 --> 01:13:01.270 align:middle line:84% So now you have a reason why there's 01:13:01.270 --> 01:13:03.112 align:middle line:90% a primordial inhomogeneity. 01:13:03.112 --> 01:13:04.570 align:middle line:84% And you also have a reason why it's 01:13:04.570 --> 01:13:05.830 align:middle line:90% on the right length scales. 01:13:05.830 --> 01:13:08.770 align:middle line:84% It's going to seed galaxy formation, not mess 01:13:08.770 --> 01:13:11.230 align:middle line:84% around with your atoms like a Lamb shift 01:13:11.230 --> 01:13:14.890 align:middle line:84% because you have matter in an early quantum state 01:13:14.890 --> 01:13:16.880 align:middle line:84% as the universe is stretching exponentially. 01:13:16.880 --> 01:13:19.270 align:middle line:84% So you can go back to now a much more modern picture 01:13:19.270 --> 01:13:22.535 align:middle line:84% of a very tiny lumpiness captured in that microwave 01:13:22.535 --> 01:13:23.410 align:middle line:90% background radiation. 01:13:23.410 --> 01:13:25.270 align:middle line:84% This is from the Planck satellite team. 01:13:25.270 --> 01:13:27.640 align:middle line:84% It's exaggerating with false color imaging 01:13:27.640 --> 01:13:31.930 align:middle line:84% the slight one part in 100,000 offsets between the regions 01:13:31.930 --> 01:13:34.510 align:middle line:84% of sky that are slightly higher energy 01:13:34.510 --> 01:13:38.260 align:middle line:84% photons in the CMB and slightly lower energy photons. 01:13:38.260 --> 01:13:40.150 align:middle line:84% And the idea now is that the regions 01:13:40.150 --> 01:13:43.510 align:middle line:84% of the sky from which these photons were emitted 01:13:43.510 --> 01:13:47.180 align:middle line:84% are telling us about the very, very tiny unevenness 01:13:47.180 --> 01:13:49.040 align:middle line:84% in the distribution of matter and energy 01:13:49.040 --> 01:13:51.410 align:middle line:84% at the moment those photons were emitted. 01:13:51.410 --> 01:13:55.310 align:middle line:84% There's a tiny, tiny little excess gravitational potential. 01:13:55.310 --> 01:13:58.440 align:middle line:84% There was a little more stuff per volume at that region. 01:13:58.440 --> 01:14:00.650 align:middle line:84% So the photon then had to spend a little more energy 01:14:00.650 --> 01:14:02.930 align:middle line:84% climbing out of that gravitational well. 01:14:02.930 --> 01:14:04.760 align:middle line:84% We should receive it today as being 01:14:04.760 --> 01:14:08.755 align:middle line:84% a little less energy than average, very slightly less. 01:14:08.755 --> 01:14:10.130 align:middle line:84% Meanwhile, other photons would've 01:14:10.130 --> 01:14:12.710 align:middle line:84% come from regions that were slightly evacuated, a little 01:14:12.710 --> 01:14:14.850 align:middle line:90% less dense than average. 01:14:14.850 --> 01:14:17.900 align:middle line:84% So the photons we receive today had to spend less energy 01:14:17.900 --> 01:14:20.270 align:middle line:84% gravitationally to overcome that very tiny 01:14:20.270 --> 01:14:22.130 align:middle line:90% gravitational potential. 01:14:22.130 --> 01:14:24.680 align:middle line:84% They should have slightly more energy on average today 01:14:24.680 --> 01:14:25.920 align:middle line:90% than the average. 01:14:25.920 --> 01:14:28.370 align:middle line:84% So we can actually map the quantum fluctuations 01:14:28.370 --> 01:14:32.900 align:middle line:84% which leave an imprint in this dynamical fabric of space 01:14:32.900 --> 01:14:36.170 align:middle line:84% and time that then maps to this distribution of these very 01:14:36.170 --> 01:14:38.750 align:middle line:90% tiny unevenness in the CMB. 01:14:38.750 --> 01:14:41.480 align:middle line:84% They have been mapped by three generations of satellites 01:14:41.480 --> 01:14:44.480 align:middle line:84% above the ground with increasingly precise ground 01:14:44.480 --> 01:14:45.900 align:middle line:90% based measurements as well. 01:14:45.900 --> 01:14:49.970 align:middle line:84% And each of these came out 10 years apart with an increase 01:14:49.970 --> 01:14:54.470 align:middle line:84% of about a factor of 30 in the angular resolution of the sky. 01:14:54.470 --> 01:14:57.930 align:middle line:84% I was a senior in college a long time ago 01:14:57.930 --> 01:15:02.310 align:middle line:84% when the first of these released their data in September 1992. 01:15:02.310 --> 01:15:03.900 align:middle line:90% I was a senior that year. 01:15:03.900 --> 01:15:06.990 align:middle line:84% And the [INAUDIBLE] team led actually in part by our own Rai 01:15:06.990 --> 01:15:09.810 align:middle line:84% Weiss, who later became very famous for his work 01:15:09.810 --> 01:15:11.023 align:middle line:90% on gravitational waves-- 01:15:11.023 --> 01:15:13.440 align:middle line:84% Rai was one of the science leaders for this early mission, 01:15:13.440 --> 01:15:14.412 align:middle line:90% a NASA mission. 01:15:14.412 --> 01:15:15.870 align:middle line:84% They were the first ones to measure 01:15:15.870 --> 01:15:19.200 align:middle line:84% these tiny, tiny fluctuations on the order of about one 01:15:19.200 --> 01:15:21.990 align:middle line:84% part in 100,000 but over huge scales. 01:15:21.990 --> 01:15:23.910 align:middle line:84% It was like they had very poor eyeglasses. 01:15:23.910 --> 01:15:26.310 align:middle line:84% It was very fuzzy, very poor resolution. 01:15:26.310 --> 01:15:28.950 align:middle line:84% Roughly 10 years later, another NASA mission 01:15:28.950 --> 01:15:32.220 align:middle line:84% called WMAP was able to increase the resolution 01:15:32.220 --> 01:15:33.660 align:middle line:90% by a factor of 30. 01:15:33.660 --> 01:15:36.030 align:middle line:84% And they released their data in 2003. 01:15:36.030 --> 01:15:38.670 align:middle line:84% Then the European Space Agency collaboration 01:15:38.670 --> 01:15:41.220 align:middle line:84% called the Planck satellite released their data 01:15:41.220 --> 01:15:46.110 align:middle line:84% 10 years after that starting in 2013 with another factor of 30 01:15:46.110 --> 01:15:47.850 align:middle line:90% in the spatial resolution. 01:15:47.850 --> 01:15:50.880 align:middle line:84% And so we can now make plots like this. 01:15:50.880 --> 01:15:53.400 align:middle line:84% The solid green line is the generic prediction 01:15:53.400 --> 01:15:56.130 align:middle line:84% from the simplest models of inflation, what's 01:15:56.130 --> 01:15:58.840 align:middle line:84% the pattern of bumps and wiggles on the sky you should see 01:15:58.840 --> 01:16:01.180 align:middle line:84% today-- it's basically fancy Fourier transform 01:16:01.180 --> 01:16:04.600 align:middle line:84% more or less-- what's the power on different angular scales 01:16:04.600 --> 01:16:06.100 align:middle line:90% that you should see today. 01:16:06.100 --> 01:16:08.950 align:middle line:84% And you can actually measure many, many quantities, 01:16:08.950 --> 01:16:11.660 align:middle line:84% many features of that distribution. 01:16:11.660 --> 01:16:14.650 align:middle line:84% The red dots are the actual observations from Planck team. 01:16:14.650 --> 01:16:16.210 align:middle line:84% And in many cases, the error bars 01:16:16.210 --> 01:16:19.060 align:middle line:84% are expanded so we can see them with our naked eye. 01:16:19.060 --> 01:16:22.670 align:middle line:84% This is such a precise set of measurements that, in fact, 01:16:22.670 --> 01:16:24.830 align:middle line:84% sometimes we have to make the error bars larger. 01:16:24.830 --> 01:16:27.220 align:middle line:84% So now not only do we know do we live in a universe that 01:16:27.220 --> 01:16:30.160 align:middle line:84% is indistinguishable from flat as inflation suggests 01:16:30.160 --> 01:16:32.140 align:middle line:84% we should, but the actual pattern 01:16:32.140 --> 01:16:35.050 align:middle line:84% of those wiggles, the pattern of the very slight 01:16:35.050 --> 01:16:37.910 align:middle line:84% early unevenness in the sky matches predictions 01:16:37.910 --> 01:16:39.910 align:middle line:84% to, again, better than a percent level accuracy. 01:16:39.910 --> 01:16:42.278 align:middle line:90% I find that astonishing. 01:16:42.278 --> 01:16:43.570 align:middle line:90% Let me take a few more minutes. 01:16:43.570 --> 01:16:46.540 align:middle line:84% There's one more set of things that were found 01:16:46.540 --> 01:16:48.310 align:middle line:84% or I should say that were predicted. 01:16:48.310 --> 01:16:49.930 align:middle line:84% So inflation should not only make 01:16:49.930 --> 01:16:52.720 align:middle line:84% these early primordial density perturbations 01:16:52.720 --> 01:16:56.170 align:middle line:84% where the photons should be slightly more or less energetic 01:16:56.170 --> 01:16:58.330 align:middle line:84% depending on the quantum fluctuations 01:16:58.330 --> 01:16:59.710 align:middle line:90% of that Higgs-like field. 01:16:59.710 --> 01:17:03.170 align:middle line:84% There should be primordial gravitational waves as well. 01:17:03.170 --> 01:17:06.670 align:middle line:84% This is now much like the waves that Rai and his huge team 01:17:06.670 --> 01:17:10.510 align:middle line:84% found locally from the collision of, say, black holes. 01:17:10.510 --> 01:17:12.850 align:middle line:84% There should be primordial gravitational waves 01:17:12.850 --> 01:17:14.452 align:middle line:90% excited by inflation as well. 01:17:14.452 --> 01:17:16.660 align:middle line:84% And these are waves that actually stretch and squeeze 01:17:16.660 --> 01:17:19.310 align:middle line:84% space in a two dimensional pattern. 01:17:19.310 --> 01:17:22.270 align:middle line:84% So these are mathematically more complicated structure. 01:17:22.270 --> 01:17:26.350 align:middle line:84% You should have this periodic squeezing, stretching, and then 01:17:26.350 --> 01:17:28.370 align:middle line:90% inverted by 90 degrees. 01:17:28.370 --> 01:17:32.590 align:middle line:84% So a version of these were found by the LIGO collaboration 01:17:32.590 --> 01:17:35.080 align:middle line:90% and announced early in 2016. 01:17:35.080 --> 01:17:36.310 align:middle line:90% These are not primordial. 01:17:36.310 --> 01:17:38.290 align:middle line:84% These are from local effects like the collision 01:17:38.290 --> 01:17:40.960 align:middle line:84% of black holes in our own galaxy. 01:17:40.960 --> 01:17:43.030 align:middle line:84% Inflation says similar kinds of things 01:17:43.030 --> 01:17:45.430 align:middle line:84% should've been happening in the earliest moments 01:17:45.430 --> 01:17:48.940 align:middle line:84% everywhere in space through this very violent, rapid stretching 01:17:48.940 --> 01:17:49.940 align:middle line:90% of space. 01:17:49.940 --> 01:17:53.740 align:middle line:84% So go back to that dance party I was telling you about before. 01:17:53.740 --> 01:17:57.340 align:middle line:84% At the moment when the electrically neutral atoms 01:17:57.340 --> 01:18:01.660 align:middle line:84% start to form, if there were this sea or bath 01:18:01.660 --> 01:18:05.260 align:middle line:84% of gravitational waves, then the high school auditorium-- 01:18:05.260 --> 01:18:07.510 align:middle line:84% imagine where this dance is happening-- should've 01:18:07.510 --> 01:18:11.320 align:middle line:84% been subjected to this periodic, very particular pattern 01:18:11.320 --> 01:18:12.880 align:middle line:90% of squeezing and stretching. 01:18:12.880 --> 01:18:16.330 align:middle line:84% So while the atoms are forming, gravity waves 01:18:16.330 --> 01:18:17.980 align:middle line:90% would be rippling through them. 01:18:17.980 --> 01:18:20.500 align:middle line:84% That should yield a characteristic twisting 01:18:20.500 --> 01:18:24.725 align:middle line:84% or curl pattern of polarization in that cosmic microwave 01:18:24.725 --> 01:18:25.600 align:middle line:90% background radiation. 01:18:25.600 --> 01:18:27.225 align:middle line:84% Not only should they be slightly hotter 01:18:27.225 --> 01:18:28.900 align:middle line:90% and colder spots in the sky. 01:18:28.900 --> 01:18:36.280 align:middle line:84% If you zoom in by a factor of another 20, 01:18:36.280 --> 01:18:38.950 align:middle line:84% you should actually see a corkscrew pattern, 01:18:38.950 --> 01:18:41.320 align:middle line:84% that the hotter and colder regions actually 01:18:41.320 --> 01:18:43.030 align:middle line:84% have this twisting pattern, which really 01:18:43.030 --> 01:18:45.040 align:middle line:84% is like that container of spacetime 01:18:45.040 --> 01:18:47.200 align:middle line:84% being stretched and squeezed as the gravity 01:18:47.200 --> 01:18:50.170 align:middle line:90% waves ran through it. 01:18:50.170 --> 01:18:54.670 align:middle line:84% In March of 2014, a team using the BICEP satellite 01:18:54.670 --> 01:18:56.950 align:middle line:84% at the South Pole announced they had actually measured 01:18:56.950 --> 01:18:58.370 align:middle line:90% exactly that corkscrew pattern. 01:18:58.370 --> 01:19:01.800 align:middle line:84% This is from their now famous or infamous paper. 01:19:01.800 --> 01:19:03.570 align:middle line:84% We had a celebration here at MIT. 01:19:03.570 --> 01:19:07.260 align:middle line:84% This is me [LAUGHS] cheering on Andrei Linde and members 01:19:07.260 --> 01:19:09.000 align:middle line:90% of the experimental team. 01:19:09.000 --> 01:19:11.087 align:middle line:84% We had a toast with non-alcoholic cider. 01:19:11.087 --> 01:19:13.170 align:middle line:84% I want to be clear it was middle of the afternoon. 01:19:13.170 --> 01:19:16.830 align:middle line:84% We were getting drunk on the ideas, but not on hard cider. 01:19:16.830 --> 01:19:18.780 align:middle line:84% Unfortunately, pretty soon after that, 01:19:18.780 --> 01:19:21.690 align:middle line:84% it turned out the BICEP team had measured data 01:19:21.690 --> 01:19:25.450 align:middle line:90% consistent with local noise. 01:19:25.450 --> 01:19:27.960 align:middle line:84% Many of my friends on the BICEP team managed 01:19:27.960 --> 01:19:31.950 align:middle line:84% to find out the Milky Way galaxy is dusty, which we knew. 01:19:31.950 --> 01:19:34.740 align:middle line:84% [LAUGHS] So basically, the signal 01:19:34.740 --> 01:19:38.520 align:middle line:84% they had hoped to measure was actually swamped by foregrounds 01:19:38.520 --> 01:19:40.110 align:middle line:84% they had not yet been able to control. 01:19:40.110 --> 01:19:42.652 align:middle line:84% And this was found by a number of very sophisticated analyzes 01:19:42.652 --> 01:19:43.540 align:middle line:90% soon afterwards. 01:19:43.540 --> 01:19:45.960 align:middle line:84% So it remains an open question to this day 01:19:45.960 --> 01:19:49.980 align:middle line:84% whether these primordial curling, twisting patterns 01:19:49.980 --> 01:19:51.570 align:middle line:90% really can be detected. 01:19:51.570 --> 01:19:53.170 align:middle line:84% Maybe there's such small magnitude, 01:19:53.170 --> 01:19:55.110 align:middle line:90% it'll evade our detection. 01:19:55.110 --> 01:19:55.740 align:middle line:90% We don't know. 01:19:55.740 --> 01:19:57.990 align:middle line:84% There are ongoing efforts to this day. 01:19:57.990 --> 01:20:01.200 align:middle line:84% BICEP is now souped up, and they have a much more sophisticated 01:20:01.200 --> 01:20:02.550 align:middle line:90% series of telescopes. 01:20:02.550 --> 01:20:06.070 align:middle line:84% There's another team also at the South Pole, the Planck 01:20:06.070 --> 01:20:06.570 align:middle line:90% satellite. 01:20:06.570 --> 01:20:09.330 align:middle line:84% There's new efforts being built on the Atacama Desert, 01:20:09.330 --> 01:20:10.990 align:middle line:90% very high altitude in Chile. 01:20:10.990 --> 01:20:11.760 align:middle line:90% So stay tuned. 01:20:11.760 --> 01:20:14.010 align:middle line:84% We'll hopefully learn more about that final prediction 01:20:14.010 --> 01:20:16.240 align:middle line:90% from inflation before too long. 01:20:16.240 --> 01:20:17.010 align:middle line:90% Let me wrap up. 01:20:17.010 --> 01:20:20.700 align:middle line:84% Cosmic inflation arises from types of matter in interactions 01:20:20.700 --> 01:20:24.480 align:middle line:84% that we now know exist that are heart and soul of this particle 01:20:24.480 --> 01:20:27.840 align:middle line:84% cosmology community, things like the Higgs boson. 01:20:27.840 --> 01:20:29.940 align:middle line:84% And it addresses several of these long standing 01:20:29.940 --> 01:20:32.670 align:middle line:84% conundra about the standard Big Bang model 01:20:32.670 --> 01:20:34.470 align:middle line:84% and makes specific predictions for what 01:20:34.470 --> 01:20:36.390 align:middle line:84% we should see on the sky today, including 01:20:36.390 --> 01:20:38.640 align:middle line:84% very minute statistical predictions for things 01:20:38.640 --> 01:20:41.340 align:middle line:84% like the cosmic microwave background radiation. 01:20:41.340 --> 01:20:44.400 align:middle line:84% And the simplest models fit to unbelievable accuracy 01:20:44.400 --> 01:20:47.610 align:middle line:84% despite what my mean dormmates used to say in the mid '90s. 01:20:47.610 --> 01:20:49.440 align:middle line:84% Now we have extraordinary agreement 01:20:49.440 --> 01:20:51.120 align:middle line:84% with many, many of these predictions, 01:20:51.120 --> 01:20:52.720 align:middle line:90% albeit not the final one. 01:20:52.720 --> 01:20:54.300 align:middle line:90% So why is the universe lumpy? 01:20:54.300 --> 01:20:56.760 align:middle line:90% Why is this cascade of scales? 01:20:56.760 --> 01:21:00.510 align:middle line:84% Because space time is wiggly, and matter is jiggly. 01:21:00.510 --> 01:21:02.460 align:middle line:84% Now, there's an alternate hypothesis, 01:21:02.460 --> 01:21:04.800 align:middle line:90% my final set of slides. 01:21:04.800 --> 01:21:05.980 align:middle line:90% I mentioned this last time. 01:21:05.980 --> 01:21:07.645 align:middle line:84% And I just want to make it clear. 01:21:07.645 --> 01:21:09.270 align:middle line:84% I mentioned that Alan Guth been working 01:21:09.270 --> 01:21:13.480 align:middle line:84% on this since around 1980, since December 7, 1979, in fact. 01:21:13.480 --> 01:21:15.720 align:middle line:84% As you also know, he's won many awards, including 01:21:15.720 --> 01:21:19.140 align:middle line:84% the award from The Boston Globe for the messiest office 01:21:19.140 --> 01:21:19.920 align:middle line:90% in Boston. 01:21:19.920 --> 01:21:21.720 align:middle line:84% This was published in The Globe at the time 01:21:21.720 --> 01:21:24.220 align:middle line:90% that he won first place. 01:21:24.220 --> 01:21:26.050 align:middle line:84% They also published these photographs. 01:21:26.050 --> 01:21:28.440 align:middle line:84% These are all shots from his office at the time. 01:21:28.440 --> 01:21:30.180 align:middle line:84% I used to have to walk through that just 01:21:30.180 --> 01:21:31.890 align:middle line:84% to try to meet with my PhD advisor. 01:21:31.890 --> 01:21:33.450 align:middle line:90% It was not OSHA certified. 01:21:33.450 --> 01:21:36.810 align:middle line:84% So an alternate hypothesis for why the universe is so messy 01:21:36.810 --> 01:21:38.730 align:middle line:84% is actually because Alan's been generating 01:21:38.730 --> 01:21:41.800 align:middle line:84% the mess in his own office, and it's expanded to cosmic scales. 01:21:41.800 --> 01:21:43.540 align:middle line:90% So I'm going to close with that. 01:21:43.540 --> 01:21:45.815 align:middle line:84% And I'll be glad to stay a bit longer if people 01:21:45.815 --> 01:21:46.440 align:middle line:90% have questions. 01:21:46.440 --> 01:21:47.460 align:middle line:84% Again, I'm sorry for running late. 01:21:47.460 --> 01:21:48.750 align:middle line:84% Feel free to drop off if you need. 01:21:48.750 --> 01:21:49.740 align:middle line:90% Any questions on that? 01:21:49.740 --> 01:21:56.560 align:middle line:90% 01:21:56.560 --> 01:21:58.392 align:middle line:84% The photos in Alan's office are on Canvas. 01:21:58.392 --> 01:22:00.600 align:middle line:84% So if you want to study that part of today's lecture, 01:22:00.600 --> 01:22:03.310 align:middle line:84% it's probably the most important lesson, you'll ever take away. 01:22:03.310 --> 01:22:06.630 align:middle line:84% You can study those at your leisure as well. 01:22:06.630 --> 01:22:24.000 align:middle line:90%