1 00:00:03,280 --> 00:00:08,557 Hello, again. Let us now review briefly the history of 2 00:00:08,557 --> 00:00:13,315 modern cosmology. Before we could study the universe as a 3 00:00:13,315 --> 00:00:19,369 whole in scientific sense, we actually discovered and understood galaxies which 4 00:00:19,369 --> 00:00:24,350 are its major constituents. At first, people didn't know what they 5 00:00:24,350 --> 00:00:27,798 were. They were just as much in sky, they were 6 00:00:27,798 --> 00:00:31,783 called nebulae, and famous catalog by Chales Messier. 7 00:00:31,783 --> 00:00:38,067 And then, William Herschel also cataloged many thousands of them, and followed by 8 00:00:38,067 --> 00:00:39,140 others. Now 9 00:00:39,140 --> 00:00:45,669 even before that was really properly understood, philosophers tried to address 10 00:00:45,669 --> 00:00:51,568 the question. And they talked about island universes, or what today we would 11 00:00:51,568 --> 00:00:55,997 call a galaxy. They had no scientific reason to believe 12 00:00:55,997 --> 00:01:00,459 one way or the other but it was an interesting speculation. 13 00:01:00,459 --> 00:01:07,038 Now, in 19th and 20th century, more catalogs or galaxies were produced but 14 00:01:07,038 --> 00:01:12,558 there was still no understanding. For example, the very basic question was, 15 00:01:12,558 --> 00:01:16,889 are these nebulae, Galaxies like the Milky Way or are there 16 00:01:16,889 --> 00:01:22,755 just some smudges within our own galaxy, the Milky Way, which is the universe as a 17 00:01:22,755 --> 00:01:26,014 whole? So, up until 1920s, the early 1920s, this 18 00:01:26,014 --> 00:01:31,222 was still an unsolved question. And there was a famous great debate 19 00:01:31,222 --> 00:01:37,889 between two astronomers, Harlow Shapley from Harvard, and Heber Curtis from Lick 20 00:01:37,889 --> 00:01:42,004 Observatory, whether or not galaxies or a nebula, I 21 00:01:42,004 --> 00:01:48,095 should say, are island universes just like the Milky Way, or are there just 22 00:01:48,095 --> 00:01:54,780 some smudges of gas inside the Milky Way? The debate was inconclusive. 23 00:01:54,780 --> 00:02:00,205 Shapley had wrong answer, that Milky Way is all of it. 24 00:02:00,205 --> 00:02:06,366 Curtis was advocating that galaxies are many like the Milky Way. 25 00:02:06,366 --> 00:02:12,526 And arguments were not really based on any solid experimental data. 26 00:02:12,526 --> 00:02:17,743 This all changed with Edwin Humble in 1923 on using Mt. 27 00:02:17,743 --> 00:02:22,353 Wilson Observatory. He obtained photographs of Andromeda 28 00:02:22,353 --> 00:02:27,660 galaxy and found a variable star, so-called surface in Andromeda. 29 00:02:27,660 --> 00:02:33,937 Comparing its brightness to brightness of surface in our galaxy, immediately told 30 00:02:33,937 --> 00:02:39,254 us that Andromeda is much, much further away than anything in our galaxy. 31 00:02:39,254 --> 00:02:44,350 Within our galaxy, people knew about stars that are clusters that are 32 00:02:44,350 --> 00:02:48,264 kiloparsecs away. Andromeda turned out to be several 33 00:02:48,264 --> 00:02:52,935 hundred kiloparsecs away. Therefore, it was galaxy just like the 34 00:02:52,935 --> 00:02:58,706 Milky Way, and then all the others which are much fainter, presumably even further 35 00:02:58,706 --> 00:03:01,769 out. So, all of a sudden, the picture changed 36 00:03:01,769 --> 00:03:07,070 from Milky Way being the entire universe to a much, much bigger universe, 37 00:03:07,070 --> 00:03:10,910 populated with galaxies like Milky Way and others. 38 00:03:10,910 --> 00:03:15,580 This is how Hubble really became famous. He did something else later on, 39 00:03:15,580 --> 00:03:21,684 discovering expansion of the universe. But, before we get into this, theory 40 00:03:21,684 --> 00:03:28,189 actually made some important advances. If you want to understand universe at 41 00:03:28,189 --> 00:03:33,967 large from physical terms, the only interaction that actually matters is 42 00:03:33,967 --> 00:03:37,395 gravity. Because all other forces are short range 43 00:03:37,395 --> 00:03:42,286 except for electromagnetic force, but charges are mixed so well, positive 44 00:03:42,286 --> 00:03:46,974 and negative ones, so that net electromagnetic field is pretty much a 45 00:03:46,974 --> 00:03:50,099 zero in any average sense. Not so with gravity. 46 00:03:50,099 --> 00:03:53,020 Gravity cannot be cancelled or compensated. 47 00:03:53,020 --> 00:03:58,387 and so, if you want to understand how things happen on large scales, you need 48 00:03:58,387 --> 00:04:02,259 the theory of gravity. This came in the form of theory of 49 00:04:02,259 --> 00:04:06,960 relativity, and in particular general theory of relativity. 50 00:04:06,960 --> 00:04:11,204 So, Einstein came up with the way that describes that, 51 00:04:11,204 --> 00:04:16,410 and we'll talk a little more about it a couple lectures from now. 52 00:04:16,410 --> 00:04:21,953 But he immediately understood that this can be actually applied to study of the 53 00:04:21,953 --> 00:04:27,081 universe as a whole and even taught a class about that from 1919, this was 54 00:04:27,081 --> 00:04:31,100 before it was actually proven that the theory was correct. 55 00:04:31,100 --> 00:04:36,783 He assumed that it was just normal matter like stars and so on. But it's finite in 56 00:04:36,783 --> 00:04:40,040 spatial size, and we'll talk about how that's possible. 57 00:04:40,040 --> 00:04:44,005 In 1917, Einstein made first cosmological models. 58 00:04:44,005 --> 00:04:50,366 He believed in his theory, of course, even though it was experimentally proven 59 00:04:50,366 --> 00:04:54,166 only in 1919. And, he had something interesting. 60 00:04:54,166 --> 00:05:01,106 He, he found out that universe has to either collapse on itself under its own 61 00:05:01,106 --> 00:05:06,429 gravity, or maybe expand forever. Collapse under its own gravity seemed 62 00:05:06,429 --> 00:05:11,717 like a natural thing, and in order to balance that, he introduced a force 63 00:05:11,717 --> 00:05:16,441 called cosmological constant. This turned out to be a very unstable 64 00:05:16,441 --> 00:05:19,332 cosmological model, and it's wrong anyway. 65 00:05:19,332 --> 00:05:25,396 at the same time, Dutch physicist Willem De Sitter developed similar model and ob, 66 00:05:25,396 --> 00:05:28,980 obtained equations for expanding universe. 67 00:05:28,980 --> 00:05:33,542 Now, later on, two of them came up with the different model in the beers names 68 00:05:33,542 --> 00:05:38,690 but that is a different story. Back to observations, discovery of the 69 00:05:38,690 --> 00:05:44,460 expanding universe was probably one of the greatest scientific discoveries of 70 00:05:44,460 --> 00:05:49,443 all times. Two people need special credit here 71 00:05:49,443 --> 00:05:55,255 especially Vesto Melvin Slipher who was an astronomer at Lowell Observatory in 72 00:05:55,255 --> 00:05:59,029 Arizona who measured velocities, radial velocities, 73 00:05:59,029 --> 00:06:02,727 of galaxies. And obtained the data that were much 74 00:06:02,727 --> 00:06:08,197 later that were later used by Hubble. That's roughly same time Knut Lundmark 75 00:06:08,197 --> 00:06:14,088 and Carl Wirtz in Sweden and Germany obtained similar results. But, their plot 76 00:06:14,088 --> 00:06:19,826 of velocity versus estimated distance to the galaxies didn't show anything. 77 00:06:19,826 --> 00:06:23,462 A little bit later, Edwin Hubble plotted distances to 78 00:06:23,462 --> 00:06:28,790 galaxies which are measured little better using relative brightness as a measure of 79 00:06:28,790 --> 00:06:33,283 distance, against velocities most of which are obtained by Slipher, 80 00:06:33,283 --> 00:06:38,610 and found this remarkable trend that the further away galaxy was the faster it was 81 00:06:38,610 --> 00:06:42,398 going away from us. This is now known as the Hubble diagram 82 00:06:42,398 --> 00:06:46,160 and was immediate evidence for an expanding universe. 83 00:06:46,160 --> 00:06:51,480 Here is a brief explanation of the expansion of the universe. 84 00:06:51,480 --> 00:06:57,102 Imagine just a piece of the volume of the universe populated with galaxies, more or 85 00:06:57,102 --> 00:07:01,370 less uniformally distributed. If the space expands in itself, it 86 00:07:01,370 --> 00:07:06,137 carries the galaxies apart. And, the further apart from, from each 87 00:07:06,137 --> 00:07:11,023 other they are to begin with, the larger the distances there will be next time. 88 00:07:11,023 --> 00:07:16,097 So, since the, the growth in the distance between any two galaxies is proportional 89 00:07:16,097 --> 00:07:20,043 to the distance itself, and its time derivative is the velocity. 90 00:07:20,043 --> 00:07:24,992 So therefore, you get the velocities of recession from galaxies from each other 91 00:07:24,992 --> 00:07:28,500 is proportional to their distance which is Hubble's Law. 92 00:07:28,500 --> 00:07:33,828 Now, Einstein go to see this for himself. He visited Pasadena numerous of times in 93 00:07:33,828 --> 00:07:35,518 1930s. Here, he is on Mt. 94 00:07:35,518 --> 00:07:40,457 Wilson with Edwin Hubble and Walter Adams, another famous astronomer, looking 95 00:07:40,457 --> 00:07:43,381 though an eyepiece of the 100 inch telescope. 96 00:07:43,381 --> 00:07:47,735 This was completely fake and posed picture because that's not what 97 00:07:47,735 --> 00:07:51,330 astronomers do. Allegedly, Einstein declared failure to 98 00:07:51,330 --> 00:07:56,705 expan, the, the to predict expanding universe as the greatest mistake of his 99 00:07:56,705 --> 00:07:59,661 career. And remember, he was, invented 100 00:07:59,661 --> 00:08:04,633 cosmological constant as a means of preventing the expan, collapse of the 101 00:08:04,633 --> 00:08:09,807 universe for that wasn't really needed. He failed to predict expansion of the 102 00:08:09,807 --> 00:08:13,234 universe, wven though that was implicitly contained 103 00:08:13,234 --> 00:08:16,727 in his equations. And that was probably the greatest 104 00:08:16,727 --> 00:08:21,700 scientific prediction anyone could make. So, this is why he wasn't so happy. 105 00:08:24,120 --> 00:08:29,317 In the meantime, two theorists really developed modern relativistic 106 00:08:29,317 --> 00:08:33,661 cosmological models. Alexander Friedmann in Soviet Union 107 00:08:33,661 --> 00:08:37,773 developed relativity-based expanding universe models. 108 00:08:37,773 --> 00:08:43,669 Then, he came up with the equation that bears his name that we will use very 109 00:08:43,669 --> 00:08:47,243 heavily. Then, roughly at the same time or few 110 00:08:47,243 --> 00:08:53,477 years later, George Lemaitre in Belgium developed similar cosmological models. 111 00:08:53,477 --> 00:08:59,876 And also, he said, okay, if the universe is expanding now what if we just look 112 00:08:59,876 --> 00:09:02,993 back. It all must have started with very dense 113 00:09:02,993 --> 00:09:08,325 state which is probably very hot and he called us the Cosmic Egg. 114 00:09:08,325 --> 00:09:11,395 And so, that was essentially a first notion of 115 00:09:11,395 --> 00:09:16,514 the Big Bang, but it was not taken very seriously because he was little too much 116 00:09:16,514 --> 00:09:21,633 of a extrapolation and he was a Jesuit priest and you know that sounds a little 117 00:09:21,633 --> 00:09:26,176 suspiciously like biblical creation. So, that was forgotten for a little 118 00:09:26,176 --> 00:09:29,129 while. Then in 1930's, relativistic cosmology 119 00:09:29,129 --> 00:09:33,658 really started to flourish. And here are some of the more important 120 00:09:33,658 --> 00:09:37,782 contributors to it. Milne developed a model that is based on 121 00:09:37,782 --> 00:09:40,216 special relativity. It includes no matter. 122 00:09:40,216 --> 00:09:43,461 It's just the dynamics of space and time itself. 123 00:09:43,461 --> 00:09:48,059 And Eddington who was the person who'd actually proven that general relativity 124 00:09:48,059 --> 00:09:53,805 is right promoted their own models and tried to think about the interfaces, 125 00:09:53,805 --> 00:09:57,930 interfaces between quantum theory and relativistic cosmology. 126 00:09:57,930 --> 00:10:02,949 Robertson and Walker are two mathematicians who developed a 127 00:10:02,949 --> 00:10:09,307 mathematical description that will actual, that's actually still being used 128 00:10:09,307 --> 00:10:13,240 in explaining or describing expanding universe.