1 00:00:00,012 --> 00:00:04,207 Let us now turn to the study of the early universe. 2 00:00:04,207 --> 00:00:07,617 First let me review a few of the key ideas. 3 00:00:07,617 --> 00:00:14,167 Obviously as already you were made to realize, as you turn clock backwards and 4 00:00:14,167 --> 00:00:19,221 go to a smaller universe, it was hotter and denser, 5 00:00:19,221 --> 00:00:23,531 in a fairly predictable way, since we know how matter behaves under high 6 00:00:23,531 --> 00:00:27,828 densities and temperatures. Although of course, some new interest in 7 00:00:27,828 --> 00:00:32,661 Physics can happen such as inflation. At any given time particles in universe 8 00:00:32,661 --> 00:00:37,449 have certain temperature, meaning certain energy and that means that particles of 9 00:00:37,449 --> 00:00:41,372 different duress mass energies will dominate at different times. 10 00:00:41,372 --> 00:00:46,993 Essentially universe acts as the ultimate accelerator, reaching energies of 11 00:00:46,993 --> 00:00:52,689 particles that will probably never be generated in the lab, and thus, providing 12 00:00:52,689 --> 00:00:57,070 a very interesting new window into particle physics itself. 13 00:00:57,070 --> 00:01:00,782 So as we go deeper in the past, the energies increase, 14 00:01:00,782 --> 00:01:06,113 and we get into regimes for different interactions and different particles 15 00:01:06,113 --> 00:01:11,228 become more dominant than others. However, the price of that is that the 16 00:01:11,228 --> 00:01:15,424 perigo, the higher the energy the less we actually know. 17 00:01:15,424 --> 00:01:20,726 We currently probe physics out to the energies of order of 100 teraelectron 18 00:01:20,726 --> 00:01:25,080 volts of rest mass. And we do have a fairly reliable theories 19 00:01:25,080 --> 00:01:28,016 what goes on up to the there or thereabouts. 20 00:01:28,016 --> 00:01:32,842 But beyond that things to get to be increasingly more speculative. 21 00:01:32,842 --> 00:01:36,695 And in any case, at some point this has to break down. 22 00:01:36,695 --> 00:01:42,362 Currently the best guess is that it breaks down around the Plack time, which 23 00:01:42,362 --> 00:01:47,214 is more less a guess. We will discuss Plack units later, but at 24 00:01:47,214 --> 00:01:51,127 any rate. 10 ^-43 seconds is certainly the boundary 25 00:01:51,127 --> 00:01:55,845 of what we can even guess. Here is just a schematic outline what 26 00:01:55,845 --> 00:02:01,176 history of the universe might look like. There is a very early period of 27 00:02:01,176 --> 00:02:05,641 inflationary expansion. Which is by now I think, fairly well 28 00:02:05,641 --> 00:02:10,424 established but is by no means proven. And then we go through the early 29 00:02:10,424 --> 00:02:16,017 universe, a hot soup of quarks and other particles until the atoms form, there are 30 00:02:16,017 --> 00:02:21,434 combine micro backgrounds released and that's where the astronomy as we know it 31 00:02:21,434 --> 00:02:25,154 really begins. So let's go quickly through thermal 32 00:02:25,154 --> 00:02:28,663 history of the universe. What are the key moments? 33 00:02:28,663 --> 00:02:34,197 At some point quantum gravity has to be important and dominate the physics of the 34 00:02:34,197 --> 00:02:39,779 universe. We do not know when that is, we do not have a quantum theory of gravity. 35 00:02:39,779 --> 00:02:43,614 And Planck time, 10^-43 seconds is about as good guess as any. 36 00:02:43,614 --> 00:02:49,962 The inflation, if it happened, happened when the universe was around 10^-33 37 00:02:49,962 --> 00:02:54,702 seconds old. And shortly thereafter, we, believe there 38 00:02:54,702 --> 00:03:01,177 was a grand unification or rather, splitting of the electroweak interaction 39 00:03:01,177 --> 00:03:06,351 from the strong interaction. They split from gravity sometime earlier, 40 00:03:06,351 --> 00:03:10,308 probably prior to the inflation. When the universe was around a 41 00:03:10,308 --> 00:03:15,231 microsecond old, baryogenesis happens. Quarks create things like protons and 42 00:03:15,231 --> 00:03:21,527 neutrons, particles that we know about. Between about 1 millisecond after the Big 43 00:03:21,527 --> 00:03:29,077 Bang and about 3 minutes is the time of the cosmic nucleus synthesis, where the 44 00:03:29,077 --> 00:03:33,762 light elm stabilize like [UNKNOWN] to helium 3, helium 4, lithium. 45 00:03:33,762 --> 00:03:38,950 Little bit of beryllium, maybe even some boron were formed and frozen. 46 00:03:38,950 --> 00:03:44,056 At that time, universe is still very much dominated by radiation. 47 00:03:44,056 --> 00:03:48,812 That changes around the time of the, hun, around 100,000 years, age. 48 00:03:48,812 --> 00:03:54,062 After which, matter becomes the dominant component of the dynamics of the universe 49 00:03:54,062 --> 00:03:58,822 and first structures begin to form. The seeds of the first structures begin 50 00:03:58,822 --> 00:04:03,607 to form from the dark matter itself. And when the universe was 380,000years 51 00:04:03,607 --> 00:04:08,462 old, and we know this number fairly well thanks to the procedure in cosmology 52 00:04:08,462 --> 00:04:11,692 measurements from cosmic microwave background. 53 00:04:11,692 --> 00:04:17,442 The gas becomes neutral. Electrons and protons recombine become 54 00:04:17,442 --> 00:04:23,017 atoms of hydrogen and helium and universe continues to expand. 55 00:04:23,017 --> 00:04:27,192 At that time there are no more sources of light. 56 00:04:27,192 --> 00:04:33,652 There are no stars in the galaxies. They, these being to, we think that they 57 00:04:33,652 --> 00:04:39,877 begin to form when the universe is few hundred million years old and that point 58 00:04:39,877 --> 00:04:45,627 they can reionize the neutral gas of the universe and that is pretty much 59 00:04:45,627 --> 00:04:51,110 completed by about age of 1 gigayear and that's where right now the frontier of 60 00:04:51,110 --> 00:04:55,253 our observational cosmology is. Physicists love to plot time history of 61 00:04:55,253 --> 00:04:59,748 the universe on a log axis because that gives lot more space at very short times 62 00:04:59,748 --> 00:05:04,193 in which they're interested, and all of the known astronomy happens just over 63 00:05:04,193 --> 00:05:09,129 there in the corner at the end Well, here is one from Mike Turner, and it's a handy 64 00:05:09,129 --> 00:05:13,546 little chart because shows you the temperature, meaning the energy of the 65 00:05:13,546 --> 00:05:18,154 particles, the density, as well as the time when it happened, which few of the 66 00:05:18,154 --> 00:05:23,438 key moments noted, just as I outlined a moment ago. In tabular format, 67 00:05:23,438 --> 00:05:28,389 here is a quick outline, again, of what are the key times and particles that 68 00:05:28,389 --> 00:05:32,047 dominate or, or processes that dominate at that time. 69 00:05:32,047 --> 00:05:35,048 Not to be to boring, but here is another one. 70 00:05:35,048 --> 00:05:38,608 All of them contain more or less the same information. 71 00:05:38,608 --> 00:05:43,755 I'm offering all of you, all of them to you because you may find one or the other 72 00:05:43,755 --> 00:05:47,349 more useful for, or, more clear than the others. 73 00:05:47,349 --> 00:05:52,460 Well, what is the empirical evidence for all this? The first and foremost is the 74 00:05:52,460 --> 00:05:56,878 Cosmic Microwave Background. Our direct probe of the early hot dense 75 00:05:56,878 --> 00:05:59,659 universe. So that reaches to the times when 76 00:05:59,659 --> 00:06:02,582 universe is a few hundred thousand years old. 77 00:06:02,582 --> 00:06:05,484 Or actually, it's a little in excess of 1000. 78 00:06:05,484 --> 00:06:10,002 And it's certainly based on a very well understood atomic physics. 79 00:06:10,002 --> 00:06:14,654 Next comes the cosmic nucleosynthesis, and that's based on well understood 80 00:06:14,654 --> 00:06:18,032 nuclear physics. The results of that are abundances of 81 00:06:18,032 --> 00:06:21,453 light elements that we can test observationally today. 82 00:06:21,453 --> 00:06:25,442 So, both experiments, in theory, seem to be fairly reliable, 83 00:06:25,442 --> 00:06:30,015 pushing in towards the big bang, all the way down to maybe, 1 millisecond. 84 00:06:30,015 --> 00:06:33,826 Beyond that, things get to be increasingly more speculative. 85 00:06:33,826 --> 00:06:38,478 We think that the a-symmetry between matter and anti-matter in the universe 86 00:06:38,478 --> 00:06:43,072 was set around the time when the universe was about a micro second old. 87 00:06:43,072 --> 00:06:48,849 This is based on a reasonably reliable physics of the standard model of particle 88 00:06:48,849 --> 00:06:52,588 physics. Nevertheless it's not necessarily firmly 89 00:06:52,588 --> 00:06:57,798 established, but and you, you can think of the actual observation that there is a 90 00:06:57,798 --> 00:07:03,170 lot more matter than anti-matter in the universe as an evidence of may have 91 00:07:03,170 --> 00:07:06,952 happened when the universe was a micro-second old. 92 00:07:06,952 --> 00:07:12,027 Then we reach the inflationary year, when universe was 10^-32 seconds old. 93 00:07:12,027 --> 00:07:17,227 Inflation makes, very testable predictions, and so far seems to be doing 94 00:07:17,227 --> 00:07:20,702 fairly well. A lot of people believe that something 95 00:07:20,702 --> 00:07:26,302 like inflation must have happened, which would be really remarkable giving us some 96 00:07:26,302 --> 00:07:31,879 insight into physics of the universe when it was only 10^-32 seconds old. 97 00:07:31,879 --> 00:07:35,175 But the upshot is that cosmological observations. 98 00:07:35,175 --> 00:07:40,662 Observations of the largest things we can think of, can constrain particle physics. 99 00:07:40,662 --> 00:07:44,659 Micro-physics, at the smallest scales, we can also think of. 100 00:07:44,659 --> 00:07:48,752 Well, let's first talk about the cosmic microwave background. 101 00:07:48,752 --> 00:07:53,291 In some sense, its existence is trivial prediction of big bang theory. 102 00:07:53,291 --> 00:07:58,189 If the universe was expanding from hot and dense state, then there has to be a 103 00:07:58,189 --> 00:08:03,058 thermal radiation relic from it. Initially, universe is all plasma, 104 00:08:03,058 --> 00:08:06,574 everything is completely ionized, there are no atoms. 105 00:08:06,574 --> 00:08:09,702 And when it gets cold enough, for atoms to form, 106 00:08:09,702 --> 00:08:13,095 this is where this thermal radiation gets released. 107 00:08:13,095 --> 00:08:18,006 So Alpher, Herman, Gamow and others made a prediction in 1940s and actually 108 00:08:18,006 --> 00:08:22,889 estimated that the temperature of the cosmic microwave background would be 109 00:08:22,889 --> 00:08:28,227 around 5 degrees kelvin. It was only 1960s that cosmologists began 110 00:08:28,227 --> 00:08:31,907 to try to detect this in a serious fashion. 111 00:08:31,907 --> 00:08:38,823 And there were upper limits until the actual discovery by Penzias and Wilson. 112 00:08:38,823 --> 00:08:45,299 However, there was an even earlier not quite discovery, as early as in 1941. 113 00:08:45,299 --> 00:08:51,937 And here is how There are molecules in space and their clouds such as sianogen, 114 00:08:51,937 --> 00:08:58,507 sianogen happens to have some energy transitions that are highly sensitive to 115 00:08:58,507 --> 00:09:03,095 temperature. And probe the regime around a few Kelvin. 116 00:09:03,095 --> 00:09:10,119 So Walter Adams, obtained observations, and McKellar had a theory inferring from 117 00:09:10,119 --> 00:09:16,842 these observations of these subtle little lines of sianogen that that gas was in a 118 00:09:16,842 --> 00:09:22,841 thermal bath of couple degrees Kelvin. They didn't make anything out of it at 119 00:09:22,841 --> 00:09:28,322 the time. You could think of other ways to heat up molecular clouds, like with 120 00:09:28,322 --> 00:09:31,736 starlight. And that stayed forgotten, certainly 121 00:09:31,736 --> 00:09:36,535 Gamow didn't know about it. Until Fred Hoyal, of all people, person 122 00:09:36,535 --> 00:09:41,912 who was against the Big Bang dug up this evidence and pointed it out that this 123 00:09:41,912 --> 00:09:47,574 could be in fact Consequence of this thermal background from the early years. 124 00:09:47,574 --> 00:09:52,834 But then again, it was a little too indirect and subtle for most people to 125 00:09:52,834 --> 00:09:56,031 believe. It took really direct discovery by 126 00:09:56,031 --> 00:10:00,803 Penzias and Wilson to make the solid case for the existence 127 00:10:00,803 --> 00:10:05,801 of cosmic micro background. Clearly this was a real milestone of 128 00:10:05,801 --> 00:10:09,556 cosmology. So as we know, the universe is filled 129 00:10:09,556 --> 00:10:15,188 with thermal radiation, temperature is 2.7 degrees Kelvin today, and is 130 00:10:15,188 --> 00:10:20,657 essentially a perfect black body. we have looked for deviations from the 131 00:10:20,657 --> 00:10:26,097 black body spectrum, and none were found. You could imagine such things happening, 132 00:10:26,097 --> 00:10:31,199 if some energetic processes in the early universe dumped extra energy into it and 133 00:10:31,199 --> 00:10:36,032 changed things away from the equilibrium thermal black body radiation but 134 00:10:36,032 --> 00:10:40,621 Evidently, this did not happen. So the good news is that physics is well 135 00:10:40,621 --> 00:10:43,402 understood. We know exactly how this went. 136 00:10:43,402 --> 00:10:48,325 The bad news is there were no surprises. There was no new physics to be found. 137 00:10:48,325 --> 00:10:53,098 The key discoveries there came from the COBE satellite, Cosmic Background 138 00:10:53,098 --> 00:10:55,642 Explorer. COBE was launched to probe. 139 00:10:55,642 --> 00:11:01,296 Cosmic microwave background in ways that would not be reachable from the ground, 140 00:11:01,296 --> 00:11:05,727 at least, not at the time. And here are pictures of cosmic micro 141 00:11:05,727 --> 00:11:08,654 sky. This is, the top one has the contrast 142 00:11:08,654 --> 00:11:12,982 knob turned up to 1,000. And you can see the dipole radiation, 143 00:11:12,982 --> 00:11:18,530 which was already known due to the motion of the milky way relative to the cosmic 144 00:11:18,530 --> 00:11:23,207 micro background. Subtracting that And turning the contrast 145 00:11:23,207 --> 00:11:28,927 knob to 100,000, you see a motley sky, and a big plane of galaxy with thermal 146 00:11:28,927 --> 00:11:33,427 emission from dust and synchrotron electrons and what not. 147 00:11:33,427 --> 00:11:39,397 And subtracting that galactic emission leaves just motley sky of fluctuations 148 00:11:39,397 --> 00:11:44,410 that were primordial in nature. Cosmologists long looked for those, 149 00:11:44,410 --> 00:11:48,532 but this was the first time they were actually really seen. 150 00:11:48,532 --> 00:11:53,846 And it was one of the reasons why John Mather and George Smoot got Nobel Prize 151 00:11:53,846 --> 00:11:58,642 for some of these discoveries. So let's estimate when, exactly, the 152 00:11:58,642 --> 00:12:03,092 recombination of primordial plasma into the atoms happens. 153 00:12:03,092 --> 00:12:08,870 Hydrogen becomes ionized if it absorbs photons with energies of 13.6 electron 154 00:12:08,870 --> 00:12:12,764 volts or more. And this is 3 * voltsman constant * the 155 00:12:12,764 --> 00:12:16,118 equivalent tempera, temperature of the gas. 156 00:12:16,118 --> 00:12:21,732 So that immediately implies temperature of around 50,000 degree kelvin. 157 00:12:21,732 --> 00:12:28,138 However, turns out, since there are many more photons than there are protons or 158 00:12:28,138 --> 00:12:33,932 electrons by, by a factor of billion. Even a tail of high energy photon, 159 00:12:33,932 --> 00:12:39,030 photons from a cooler gas or cooler temperature would suffice. 160 00:12:39,030 --> 00:12:45,154 And we can estimate that using the Boltzmann equation for equilibrium shown 161 00:12:45,154 --> 00:12:48,723 here. And it happens that given this over 162 00:12:48,723 --> 00:12:55,212 abundance of photons over protons and electrons the temperature could be as low 163 00:12:55,212 --> 00:12:59,694 as 2500 degrees and already one can get recombination. 164 00:12:59,694 --> 00:13:05,447 Since the observed temperature of the micro background is now about 2.7 165 00:13:05,447 --> 00:13:08,572 degrees, this immediately tells you that this 166 00:13:08,572 --> 00:13:13,647 happened around redshift of 1100. Well, obviously it doesn't happen all at 167 00:13:13,647 --> 00:13:16,572 once. There is a transition period as shown 168 00:13:16,572 --> 00:13:19,222 here. What, what is plotted here is the 169 00:13:19,222 --> 00:13:22,347 fraction of ionized hydrogen in the universe. 170 00:13:22,347 --> 00:13:25,897 Which begins at unity. It's all completely ionized. 171 00:13:25,897 --> 00:13:28,714 Then, slowly, the gas recombines, 172 00:13:28,714 --> 00:13:33,511 the electrons are bound into the atoms, and the process ends by about regid of 173 00:13:33,511 --> 00:13:37,656 about 1,100. So when we look at the cosmic micro 174 00:13:37,656 --> 00:13:43,942 background, what we're looking at is a photosphere of the hot, ionized universe, 175 00:13:43,942 --> 00:13:46,797 inside out. When we look at the star we see a 176 00:13:46,797 --> 00:13:49,942 photosphere of ionized ice from the outside in. 177 00:13:49,942 --> 00:13:54,916 So this is the inverted version thereof. You may recall when we talked about 178 00:13:54,916 --> 00:13:58,982 models that dominate, dominated by matter versus radiation. 179 00:13:58,982 --> 00:14:04,477 That because of the density, and it's density of photons changes in a steeper 180 00:14:04,477 --> 00:14:06,122 way, with expansion than it does of the matter. 181 00:14:06,122 --> 00:14:10,667 The two components must dominate with different times. 182 00:14:10,667 --> 00:14:15,792 The energy density of the radiation decreases as the fourth power of the 183 00:14:15,792 --> 00:14:20,227 expansion factor, or for the matter, it's only the third power simply the illusion 184 00:14:20,227 --> 00:14:24,261 evolving. And, so given today's densities of matter 185 00:14:24,261 --> 00:14:29,963 and microbacron radiation, which is well determined through Stefan Boltzmann 186 00:14:29,963 --> 00:14:33,933 formula, we can find at what point did these two cross. 187 00:14:33,933 --> 00:14:37,074 And the answer is around redshift of 5,000. 188 00:14:37,074 --> 00:14:42,907 So this was before the recombination, but well after cosmic nucleosynthesis was 189 00:14:42,907 --> 00:14:45,967 complete. Next, we will talk about cosmic 190 00:14:45,967 --> 00:14:47,118 nucleosynthesis.