1 00:00:03,780 --> 00:00:06,958 Hello. We now continue our quick overview of the 2 00:00:06,958 --> 00:00:10,600 history of cosmology. In the last module, we learned how 3 00:00:10,600 --> 00:00:15,699 expansion of the universe, observational discovery, and general activity which 4 00:00:15,699 --> 00:00:20,599 provided a theoretical framework for cosmology, really established it as a 5 00:00:20,599 --> 00:00:23,777 science. Something else however, happened in 1930 6 00:00:23,777 --> 00:00:28,744 that's of considerable importance. And that is the discovery of dark matter 7 00:00:28,744 --> 00:00:33,909 by Caltech physicist and astronomer Fritz Zwicky, who applied simple Newtonian 8 00:00:33,909 --> 00:00:37,220 mechanics to motions of galaxies and coma cluster. 9 00:00:37,220 --> 00:00:40,930 Their kinetic energy has to be balanced by the binding energy. 10 00:00:40,930 --> 00:00:45,358 And so he could deduce how much mass was needed in order to keep clusters 11 00:00:45,358 --> 00:00:48,290 together. So he found out that he needs 400 times 12 00:00:48,290 --> 00:00:52,957 as much invisible matter that does exert some gravitational influence that was 13 00:00:52,957 --> 00:00:56,428 actually seen in stars. Similar thing was found by another 14 00:00:56,428 --> 00:01:00,736 astronomer, Sinclair Smith, for Virgo cluster a few years later. 15 00:01:00,736 --> 00:01:03,310 But at that time, nobody took this seriously. 16 00:01:03,310 --> 00:01:08,207 Because this was just too outrageous statement and there was no other evidence 17 00:01:08,207 --> 00:01:11,245 for it. Until 1970's when sufficient evidence has 18 00:01:11,245 --> 00:01:14,716 accumulated that dark matter could no longer be ignored. 19 00:01:14,716 --> 00:01:19,614 And now, essentially, everybody believes in its existence for good reasons which 20 00:01:19,614 --> 00:01:23,174 we'll cover later. Dark matter is, today a key ingredient in 21 00:01:23,174 --> 00:01:27,870 the models of structure formation. But its exactly physical nature is not 22 00:01:27,870 --> 00:01:32,760 yet understood, and this is one of the outstanding problems of physics today. 23 00:01:32,760 --> 00:01:38,541 Meanwhile, observational astronomers following first, Hubble, but then his 24 00:01:38,541 --> 00:01:44,885 followers including Allan Sandage, tried to establish what kind of relativistic 25 00:01:44,885 --> 00:01:50,345 cosmological model we live in. Hubble designed a set of cosmological 26 00:01:50,345 --> 00:01:56,207 tests, how observations of distant galaxies can be used to determine local 27 00:01:56,207 --> 00:01:59,262 geometry. The foremost of those were so-called 28 00:01:59,262 --> 00:02:02,540 Hubble Diagram. Remember, that's the plot of relative 29 00:02:02,540 --> 00:02:07,583 distance versus velocity for galaxies far away from us. And near us, this is very 30 00:02:07,583 --> 00:02:11,440 close to a straight line. Now, as you go further deeper in space, 31 00:02:11,440 --> 00:02:16,180 relativistic effects start playing role and the line deviates from straight in 32 00:02:16,180 --> 00:02:19,660 way that depends on values of cosmological parameters. 33 00:02:19,660 --> 00:02:23,683 So, Sandage and his collaborators tried very hard to do this. 34 00:02:23,683 --> 00:02:29,181 Using brightest cluster galaxies is what they call standard candles, assuming that 35 00:02:29,181 --> 00:02:33,674 they're in general, luminosities constant. And that basically failed, 36 00:02:33,674 --> 00:02:38,837 because galaxies are not constant. They evolve, they're made of stars, stars of 37 00:02:38,837 --> 00:02:43,061 all the galaxies merge. And so until mid-90s, Hubble diagram was 38 00:02:43,061 --> 00:02:47,688 not considered as a serious cosmological test. But then, things change 39 00:02:47,688 --> 00:02:53,125 considerably. Galaxy evolution really became forefront 40 00:02:53,125 --> 00:02:56,528 of cosmology in 1970, or late 1970s and 1980s. 41 00:02:56,528 --> 00:03:02,805 And this was in part with the realization that we must understand how constituents 42 00:03:02,805 --> 00:03:08,174 of galaxies of all, and therefore galaxies themselves before we can use 43 00:03:08,174 --> 00:03:12,183 them for cosmology. But its also containing number of 44 00:03:12,183 --> 00:03:17,171 interesting question so on and so on. And, studies of this were enabled by 45 00:03:17,171 --> 00:03:22,587 development of modern instrumentation, ending up in charge capital devices. 46 00:03:22,587 --> 00:03:26,100 And both of that was done, Palomar Directory. 47 00:03:26,100 --> 00:03:31,955 Here are some of the famous astronomers who developed early instruments to study 48 00:03:31,955 --> 00:03:38,501 distant universe. Meanwhile, on the theoretical front, Fred 49 00:03:38,501 --> 00:03:43,790 Hoyle, Hermann Bondi and Thomas Gold came up with a new idea called the Steady 50 00:03:43,790 --> 00:03:47,654 State Cosmology. Remember, the cosmological principle says 51 00:03:47,654 --> 00:03:51,587 that universe is same at all places and all directions. 52 00:03:51,587 --> 00:03:57,418 Well, they said also at all times. But because it expands, new matter has to 53 00:03:57,418 --> 00:04:02,689 be created in order to fill up the gaps. And mechanism for this was not specific 54 00:04:02,689 --> 00:04:07,744 and that was clearly seen as a weakness. But in that sense, universe will always 55 00:04:07,744 --> 00:04:11,200 look the same, keep expanding but always look the same. 56 00:04:11,200 --> 00:04:15,610 And there are cosmological test that can distinguish between those two. 57 00:04:15,610 --> 00:04:21,811 In part, this was trying to respond to the extrapolation of the galactic cosmic 58 00:04:21,811 --> 00:04:28,091 expansion to what now called Big B,ang which at least Hoyle found distasteful 59 00:04:28,091 --> 00:04:34,223 and give it the name. But, Big Bang Theory actually made some 60 00:04:34,223 --> 00:04:39,688 important predictions. George Gamow and his colleagues, Alpher 61 00:04:39,688 --> 00:04:43,415 and Herman, actually considered what one might 62 00:04:43,415 --> 00:04:48,097 thought was primitive atom and ask what would be the physics if universe is so 63 00:04:48,097 --> 00:04:51,608 hot and dense. And so, what universe will do is what 64 00:04:51,608 --> 00:04:56,428 stars do in their course, which is convert hydrogen into helium, heavier 65 00:04:56,428 --> 00:04:59,526 elements. And did the develop that along with Hans 66 00:04:59,526 --> 00:05:04,759 Bethe and called Alpher, Bethe, and Gamow Theory. But, it turns out that they can 67 00:05:04,759 --> 00:05:09,922 predict formation of elements all the way up to helium, in other words, just 68 00:05:09,922 --> 00:05:14,280 hydrogen and helium. However, there's really been an afterglow 69 00:05:14,280 --> 00:05:19,521 of this cosmic thermonuclear explosion which takes to the red shift stretching 70 00:05:19,521 --> 00:05:24,099 the photons now will not be gamma rays, but will be really microwaves. 71 00:05:24,099 --> 00:05:28,080 So that black body temperature of five degrees Kelvin or so. 72 00:05:29,740 --> 00:05:35,795 Well, this was actually measured in 1965 by Penzias and Wilson, who deservedly got 73 00:05:35,795 --> 00:05:41,328 the Nobel Prize for this discovery. The cosmic micro background remains as 74 00:05:41,328 --> 00:05:47,793 one of the touchstones of cosmology. Today, we can measure it with space 75 00:05:47,793 --> 00:05:52,323 instruments, and its spectrum is as predicted by theory, pure black body 76 00:05:52,323 --> 00:05:56,280 radiation with which is now measured with exquisite precision. 77 00:05:56,280 --> 00:06:01,065 And might be actually the purest black body spectrum that we have measured 78 00:06:01,065 --> 00:06:06,719 anywhere. Another prediction of the Big Bang Theory 79 00:06:06,719 --> 00:06:11,130 is that the abundances of very liked elements. 80 00:06:11,130 --> 00:06:15,231 I said, you will only make helium, but that's not quite true. 81 00:06:15,231 --> 00:06:20,584 It will also make trace amounts of lithium and maybe a touch of baron and 82 00:06:20,584 --> 00:06:24,119 beryllium. But also, different isotopes of helium 83 00:06:24,119 --> 00:06:27,108 and hydrogen, deuterium, and helium three. 84 00:06:27,108 --> 00:06:32,862 So, here we have on the left plot of the helium mass fraction in star-forming 85 00:06:32,862 --> 00:06:36,449 galaxies, plotted against their oxygen abundance. 86 00:06:36,449 --> 00:06:41,754 Oxygen is only made in stars. So if both helium and oxygen were made in 87 00:06:41,754 --> 00:06:47,657 stars only, then this plot would be line going through the origin of zero point. 88 00:06:47,657 --> 00:06:50,796 But instead of that, there is an intercept. 89 00:06:50,796 --> 00:06:56,356 The zero point from which it starts is 0.24 mass fraction of helium. And 90 00:06:56,356 --> 00:07:02,052 therefore, stars must begin with that much helium, and the only place that 91 00:07:02,052 --> 00:07:07,460 helium can come from was from primordial nucleosynthesis. 92 00:07:07,460 --> 00:07:12,951 Models of primardial nucleosynthesis have been developed with great precision and 93 00:07:12,951 --> 00:07:18,310 now they're compared with observations. The plot on the right shows theoretical 94 00:07:18,310 --> 00:07:23,139 predictions as functions of the density, Baryon density of the universe. 95 00:07:23,139 --> 00:07:27,241 And the blue band shows where they the measured the value is. 96 00:07:27,241 --> 00:07:31,777 So, it all is consistent with what we now believe are the right cosmological 97 00:07:31,777 --> 00:07:35,132 parameters. And this is also seen as one the great 98 00:07:35,132 --> 00:07:38,756 pieces of evidence in favor of the Big Bang cosmology. 99 00:07:38,756 --> 00:07:42,380 In the meantime, another important discovery happened, 100 00:07:42,380 --> 00:07:46,877 or set of discoveries really. After the World War II, thanks to the 101 00:07:46,877 --> 00:07:49,380 radars, radio astronomy was born. 102 00:07:49,380 --> 00:07:54,520 And radio astronomers start mapping the sky in radial wavelengths, seeing their 103 00:07:54,520 --> 00:07:59,797 sources with nature was at first unknown. And astronomers like Walter Baade and 104 00:07:59,797 --> 00:08:04,458 Rudolph Minkowsky obtained optical counterparts, some of these sources. 105 00:08:04,458 --> 00:08:09,941 And discovered some of them are actually quite far away, implying that given the 106 00:08:09,941 --> 00:08:14,944 observe flux internal power luminosity of these objects must be enormous. 107 00:08:14,944 --> 00:08:19,020 This was a very surprising and important discovery. 108 00:08:19,020 --> 00:08:23,721 Given the optical, signs of something interesting were already there. 109 00:08:23,721 --> 00:08:26,764 In 1940's Carl Seyfert wrote his PhD thesis, 110 00:08:26,764 --> 00:08:32,157 observed bright nuclei nearby spiral galaxies and found to have these somewhat 111 00:08:32,157 --> 00:08:35,407 unusual spectra with very broad emission lines. 112 00:08:35,407 --> 00:08:38,726 And their nature was not understood at the time. 113 00:08:38,726 --> 00:08:44,395 But it took really combination of optical and radio astronomy with identification 114 00:08:44,395 --> 00:08:47,760 of quasars to really drive his point home. 115 00:08:47,760 --> 00:08:52,391 Cyril Hazard was one of the radio astronomers who obtained precise 116 00:08:52,391 --> 00:08:57,714 measurements of few quasi stellar sources and their positions. 117 00:08:57,714 --> 00:09:02,276 Allan Sandage and others at Palomar obtained their optical counterparts, 118 00:09:02,276 --> 00:09:07,046 and Maarten Schmidt and his collaborators figured out what's going on. 119 00:09:07,046 --> 00:09:10,365 Namely, from the shift of lines in this spectrum, 120 00:09:10,365 --> 00:09:13,337 they figure out they must be very far away. 121 00:09:13,337 --> 00:09:17,554 Remember, the faster objects we see, the further away they are. 122 00:09:17,554 --> 00:09:20,641 Well, this implied enormous distances to 123 00:09:20,641 --> 00:09:26,358 quasars, which then implied that the, their internal luminosities are huge. 124 00:09:26,358 --> 00:09:31,264 So they have an object that may be ten times or hundred times or thousand times 125 00:09:31,264 --> 00:09:34,104 more luminous that entire galaxy of stars. 126 00:09:34,104 --> 00:09:38,500 And that luminosity comes from regions smaller than solar system. 127 00:09:38,500 --> 00:09:41,932 Maarten Schmidt made it to the cover of Time Magazine. 128 00:09:41,932 --> 00:09:45,558 I think he was the second astronomer with that honor, 129 00:09:45,558 --> 00:09:50,026 the first one was Harlow Shapley. Meanwhile, another important line of 130 00:09:50,026 --> 00:09:53,652 study was happening. Namely, discovery of the large scale 131 00:09:53,652 --> 00:09:57,797 structure of the universe. It really began in 1930's with Harlow 132 00:09:57,797 --> 00:10:03,042 Shapley, Zwicky, and their collaborators starting to map how galaxies clump in 133 00:10:03,042 --> 00:10:05,826 space. It was clear that they're not purely 134 00:10:05,826 --> 00:10:09,596 randomly distributed. And through the 50s,' the 70s' more 135 00:10:09,596 --> 00:10:16,011 evidence was accumulated and was obvious that galaxies are clumped in clusters and 136 00:10:16,011 --> 00:10:22,127 less dense but more extensive structures. However, the really most important new 137 00:10:22,127 --> 00:10:25,334 development was measurements of red shifts, 138 00:10:25,334 --> 00:10:29,063 which imply distances to vast numbers of galaxies. 139 00:10:29,063 --> 00:10:33,762 First thousands, but now there are really hundreds of thousands. 140 00:10:33,762 --> 00:10:38,690 Gerard De Vaucouleurs, who was a famous informational astronomer, also pointed 141 00:10:38,690 --> 00:10:43,988 out that our immediate extra-galactic neighborhood forms what we call the local 142 00:10:43,988 --> 00:10:49,030 super cluster of which will one part, and virgo cluster in the center with some 143 00:10:49,030 --> 00:10:53,146 elongated structure in the sky as seen in the projection here. 144 00:10:53,146 --> 00:11:00,490 And this was an indication of structures that are larger than anything seen 145 00:11:00,490 --> 00:11:03,705 before. So today, of course, we know this with a 146 00:11:03,705 --> 00:11:08,758 much greater precision, and here is the projected map of density of galaxies from 147 00:11:08,758 --> 00:11:12,065 the Sloan Digital Sky Survey, a modern perchet survey. 148 00:11:12,065 --> 00:11:17,056 And shows these filaments and voids and bubbles which are important features of 149 00:11:17,056 --> 00:11:20,300 the galaxy distribution that we will study later on.