1 00:00:00,012 --> 00:00:03,917 Hello. We will now start talking about contents 2 00:00:03,917 --> 00:00:09,235 of the universe and we'll begin with regular matter, the baryons. 3 00:00:09,235 --> 00:00:14,503 First, let us recall what we learned from the cosmological tests. 4 00:00:14,503 --> 00:00:19,013 What are the density parameters associated with different components we 5 00:00:19,013 --> 00:00:22,186 see in the universe? We know largely from the precision 6 00:00:22,186 --> 00:00:26,707 measurements of the cosmic microwave background that universe is very close to 7 00:00:26,707 --> 00:00:30,786 flat, if not absolutely flat. That is that omega total is equal to one, 8 00:00:30,786 --> 00:00:35,080 total matter energy density is exactly equal to critical, but with some very 9 00:00:35,080 --> 00:00:39,325 small deviation. We also know from dynamical measurements 10 00:00:39,325 --> 00:00:45,004 in microbe background, and to some extent supernovae, and other reasons that the 11 00:00:45,004 --> 00:00:50,658 density parameter of all matter in the universe that exercises gravity, is about 12 00:00:50,658 --> 00:00:53,782 0.27. And we know that the total density of 13 00:00:53,782 --> 00:00:59,238 baryons expressed in units of critical density is only about four and half or 5% 14 00:00:59,238 --> 00:01:03,854 of the critical density. As it turns out, luminous matter, stuff 15 00:01:03,854 --> 00:01:07,289 that we actually see in galaxies as stars or gas, 16 00:01:07,289 --> 00:01:12,932 only adds up to about half a percent. And so, therefore, looking at these 17 00:01:12,932 --> 00:01:16,747 simple inequalities, there are implications, 18 00:01:16,747 --> 00:01:22,723 because half a percent is less than 5%, there must be some sort of hidden or 19 00:01:22,723 --> 00:01:27,093 missing baryonic matter. Because 5% is less than 27%, 20 00:01:27,093 --> 00:01:31,397 there has to be some kind of non-baryonic dark matter, 21 00:01:31,397 --> 00:01:36,030 and because 27% is less than 1, there has to be a dark energy. 22 00:01:36,030 --> 00:01:39,088 So, let's look at the luminous bariums first, 23 00:01:39,088 --> 00:01:43,736 those would be mostly in galaxies, and the way to do this is to integrate 24 00:01:43,736 --> 00:01:47,445 them spatially. That is, what we need is a distribution 25 00:01:47,445 --> 00:01:52,412 function of galaxy luminosities, also known as the luminosity function. 26 00:01:52,412 --> 00:01:56,593 We obtained those from redshift surveys that measure distances to nearby 27 00:01:56,593 --> 00:02:00,685 galaxies, and then we simply add up the light in the volume in which those 28 00:02:00,685 --> 00:02:04,148 measurements that those are complete and average them over. 29 00:02:04,148 --> 00:02:08,417 The results from different redshift surveys are all in very good agreement 30 00:02:08,417 --> 00:02:12,195 and the amount of about couple 100 million solar luminosities per, per 31 00:02:13,335 --> 00:02:18,415 comoving cubic megaparsecs locally here now for Hubble constant of 70, which is 32 00:02:18,415 --> 00:02:22,094 close enough. But we want to find out how much mass is 33 00:02:22,094 --> 00:02:27,227 in those stars, and for that, we need the circled mass-to-light-ratios of stellar 34 00:02:27,227 --> 00:02:30,992 populations. Stars of different mass and different 35 00:02:30,992 --> 00:02:36,302 ages produce energy of different rates, and so, we have to integrate over all 36 00:02:36,302 --> 00:02:41,402 production of light for stellar population of certain age or mixture of 37 00:02:41,402 --> 00:02:44,687 ages and see. And so, here it is for a variety of 38 00:02:44,687 --> 00:02:50,092 galaxies of different Hubble types, and by and large, it doesn't change a lot 39 00:02:50,092 --> 00:02:54,242 for stellar populations we see in galaxies is of the order four, four or 40 00:02:54,242 --> 00:02:57,067 five. That is, on average, stellar populations 41 00:02:57,067 --> 00:03:02,242 are less efficient in producing light per unit mass, and the reason for that, is 42 00:03:02,242 --> 00:03:07,467 that there is a lot of mass in very low mass stars which are not very good light 43 00:03:07,467 --> 00:03:10,817 producers. You probably recall that more massive 44 00:03:10,817 --> 00:03:14,272 stars are fare more efficient in producing ash. 45 00:03:14,272 --> 00:03:19,049 Now, these measurements do include a little bit of dark matter, but it's not 46 00:03:19,049 --> 00:03:22,327 really important at the level we're talking about, 47 00:03:22,327 --> 00:03:27,033 because the more massive stars are far more luminous per unit of mass and 48 00:03:27,033 --> 00:03:32,266 because they live shorter, the age of stellar population plays an important 49 00:03:32,266 --> 00:03:37,908 role, and those that are dominated by younger stars will be more luminous per 50 00:03:37,908 --> 00:03:41,736 unit mass. Also, this will depend on the bandpass, 51 00:03:41,736 --> 00:03:45,657 because most of the light produced by luminous stars in ultraviolet, then bluer 52 00:03:45,657 --> 00:03:50,642 [UNKNOWN] will be more affected by star formation than the red ones. 53 00:03:50,642 --> 00:03:55,561 And the flip side of that, is that blue light is more susceptible to extinction 54 00:03:55,561 --> 00:03:58,534 by dust, so the corrections for that have to be 55 00:03:58,534 --> 00:04:01,319 made. So we added all up and we find out that 56 00:04:01,319 --> 00:04:06,167 the density of the material we actually see in galaxies is little less than a 57 00:04:06,167 --> 00:04:11,391 billion solar masses per comoving cubic megaparsec for favorite values of hubble 58 00:04:11,391 --> 00:04:16,569 constant. Converting that into grams per cubic centimeter and dividing it by the 59 00:04:16,569 --> 00:04:22,026 critical density gives us the omega in visible stellar populations and it's only 60 00:04:22,026 --> 00:04:26,610 about half a percent of the total. This is quite the remarkable result 61 00:04:26,610 --> 00:04:29,878 actually, that all the stuff that we see out there, 62 00:04:29,878 --> 00:04:34,927 adds up to less than a percent, only to half a percent of all the matter energy 63 00:04:34,927 --> 00:04:38,882 that there is in the universe that we know from [INAUDIBLE]. 64 00:04:38,882 --> 00:04:43,934 But how much should we see? And, the answer to that is in measuring of the 65 00:04:43,934 --> 00:04:48,886 total baryonic density, which you probably recall, is done in two very 66 00:04:48,886 --> 00:04:52,387 different ways. One is by measuring abundances of 67 00:04:52,387 --> 00:04:57,937 deuterium or other like nulcei, in the intergalactic clouds of high redshifts, 68 00:04:57,937 --> 00:05:02,785 the deterium is the most sensitive of those and that produces a result of the 69 00:05:02,785 --> 00:05:05,545 order of 4, 4 1/2% of the critical density. 70 00:05:05,545 --> 00:05:10,648 A completely different approach based on different measurements in different 71 00:05:10,648 --> 00:05:14,438 physics, is from [UNKNOWN] constellations in the early universe from cosmic 72 00:05:14,438 --> 00:05:19,274 microwave background, and that produces result which is in perfectly good 73 00:05:19,274 --> 00:05:22,163 agreement. So because of that, two completely 74 00:05:22,163 --> 00:05:27,218 different base of measuring yield the same result and is repeated again and 75 00:05:27,218 --> 00:05:30,254 again. We believe that, indeed, we do not, that 76 00:05:30,254 --> 00:05:34,956 the total baryonic density universe corresponds to density parameter or 77 00:05:36,162 --> 00:05:38,271 megavariance of about 5% or a little less. 78 00:05:38,271 --> 00:05:41,567 Where are the remaining 90% of the variance? 79 00:05:41,567 --> 00:05:46,324 Many suggestions have been made, but essentially bounce down to three 80 00:05:46,324 --> 00:05:50,558 different counts. One possibility is the varying form of 81 00:05:50,558 --> 00:05:56,502 some optically dark objects called MACHOs or Massive Compact Halo Objects, which 82 00:05:56,502 --> 00:06:02,282 could be of different physical nature, it could be ground words that is the 83 00:06:02,282 --> 00:06:07,507 substellar objects, could be planets, could be even gigantic comets, could be 84 00:06:07,507 --> 00:06:09,657 black holes. This is not known, 85 00:06:09,657 --> 00:06:12,632 but there are ways in which we can test that. 86 00:06:12,632 --> 00:06:17,332 As we will learn later in the class, this turns out not to be the viable 87 00:06:17,332 --> 00:06:20,957 possibility with that measurement and we know that. 88 00:06:20,957 --> 00:06:26,347 The second possibility is that these missing variants are in the form of very 89 00:06:26,347 --> 00:06:30,997 dense cold molecular clouds. This is a little bit ad hoc because we do 90 00:06:30,997 --> 00:06:36,197 know about molecular gas in the galaxy. This would have to be a completely new 91 00:06:36,197 --> 00:06:41,197 component, because these clouds would be very small, they'd be very hard to 92 00:06:41,197 --> 00:06:42,620 detect, it's cold, 93 00:06:42,620 --> 00:06:47,057 they will not emit much light. It's a legitimate possibility and people 94 00:06:47,057 --> 00:06:51,979 look for it, and so far, there doesn't seem to be any evidence for such a, the 95 00:06:51,979 --> 00:06:56,868 final possibility and most likely the correct one, is that these baryons are in 96 00:06:56,868 --> 00:07:01,867 form of hot gas bound to galaxy groups simply corresponding to the large scale 97 00:07:01,867 --> 00:07:06,532 structure, because, of the virial temperature equilibrium, it has to be 98 00:07:06,532 --> 00:07:10,302 balanced against whatever the gravitational potential is. 99 00:07:10,302 --> 00:07:15,437 The expected temperature of those gas is of the order of a million degrees Kelvin. 100 00:07:15,437 --> 00:07:19,077 So that is not as hot as x-ray gas in clusters of galaxies. 101 00:07:19,077 --> 00:07:24,082 The predictions are made by numerical simulation and structure formation, 102 00:07:24,082 --> 00:07:27,549 and, it is perfectly a reasonable thing to expect. 103 00:07:27,549 --> 00:07:33,217 Now, the problem is, that the gas, at that temperature emits primarily in hard 104 00:07:33,217 --> 00:07:39,090 ultraviolet to very soft x-rays and those wavelengths are very effectively absorbed 105 00:07:39,090 --> 00:07:42,988 by interstellar hydrogen. Our galaxy has an atmosphere, 106 00:07:42,988 --> 00:07:47,306 interstellar gas, mostly neutral hydrogen, and because that 107 00:07:47,306 --> 00:07:51,756 absorbs light in these wavelengths, we cannot see this gas. 108 00:07:51,756 --> 00:07:57,559 This is same as looking through earth's atmosphere that absorbs UV radiation 109 00:07:57,559 --> 00:08:02,590 through the ozone layer or in, infrared [UNKNOWN] bands of molecular water 110 00:08:02,590 --> 00:08:06,348 absorption. So we, since we cannot get outside milky 111 00:08:06,348 --> 00:08:09,934 way's atmosphere, we are doomed not to see this gas, 112 00:08:09,934 --> 00:08:15,364 which is a little bit of joke of nature that most of the barriers in the universe 113 00:08:15,364 --> 00:08:20,511 are hidden from us by hydrogen fog. However, there is a way to discover it 114 00:08:20,511 --> 00:08:24,827 and that is by absorption. Let's now take a quick look at some of 115 00:08:24,827 --> 00:08:29,361 these possibilities in turn. First, the possibility of MACHOs, Massive 116 00:08:29,361 --> 00:08:33,086 Compact Halo Objects. If we look at distribution of stellar 117 00:08:33,086 --> 00:08:37,927 masses, it is a very steep power load. There are many more low mass stars than 118 00:08:37,927 --> 00:08:43,038 there are high mass stars and because stellar luminosity scales roughly, as the 119 00:08:43,038 --> 00:08:47,829 fourth power of stellar mass, that means that the faint end of the luminosity 120 00:08:47,829 --> 00:08:52,557 function would contribute least amount of light, but most of the mass, 121 00:08:52,557 --> 00:08:57,327 and the critical issues is where is the cutoff if any, at the low mass end. 122 00:08:57,327 --> 00:09:02,382 We know that stars with masses less than about 8% of solar cannot fuse hydrogen 123 00:09:02,382 --> 00:09:06,372 into helium in their cores and those are the brown dwarfs. 124 00:09:06,372 --> 00:09:11,839 So you can pack a lot of mass in these substellar objects without them producing 125 00:09:11,839 --> 00:09:16,875 any observable physical signatures. You can go as far as you want down to 126 00:09:16,875 --> 00:09:20,292 mass function or there could be additional peaks, 127 00:09:20,292 --> 00:09:25,762 there may be interstellar planets or interstellar/balls and comets. 128 00:09:25,762 --> 00:09:30,436 They'd very hard to detect, but there are ways in which we can test some of these 129 00:09:30,436 --> 00:09:33,769 possibilities. Well, first of all, good thing is that we 130 00:09:33,769 --> 00:09:37,717 know that brown dwarfs do exist. They've been discovered almost two 131 00:09:37,717 --> 00:09:41,715 decades ago, and by now, we have thousands of them from various [UNKNOWN] 132 00:09:41,715 --> 00:09:45,588 surveys so we know fairly well what that population is like. 133 00:09:45,588 --> 00:09:50,477 And the answer is, there isn't enough of them to account for the missing baryonic 134 00:09:50,477 --> 00:09:53,390 matter let alone the rest of the dark matter. 135 00:09:53,390 --> 00:09:58,162 So the most viable possibility today, the one which most astronomers believe, 136 00:09:58,162 --> 00:10:01,542 is that most of the baryons are in the form of this hot, 137 00:10:01,542 --> 00:10:06,497 but not too hot gas, which emits radiation that's effectively 138 00:10:06,497 --> 00:10:11,812 absorbed by intergalactic by interstellar medium in our galaxy. 139 00:10:11,812 --> 00:10:18,357 But we can detect it in absorption, because of its physical state absorption 140 00:10:18,357 --> 00:10:25,028 lines of elements like oxygen or they're ionized multiple times, should be present 141 00:10:25,028 --> 00:10:29,694 and from the atomic physics, we can figure out if we observe some of these 142 00:10:29,694 --> 00:10:34,557 lines, and their strengths, and ratios, what is the temperature, and the density 143 00:10:34,557 --> 00:10:37,402 or Coulomb density of the [INAUDIBLE] gas. 144 00:10:37,402 --> 00:10:40,717 And that turns out to be exactly what was needed. 145 00:10:40,717 --> 00:10:46,362 these measurements have been on both with Hubble Space Telescope in ultraviolet and 146 00:10:46,362 --> 00:10:51,869 also with Chandra X-Ray emission, x-Rays, and this hot gas has been unambiguously 147 00:10:51,869 --> 00:10:56,729 detected in exactly the right amounts, and so, this is probably where all those 148 00:10:56,729 --> 00:11:00,686 missing bariums really are. But that doesn't tell us where the 149 00:11:00,686 --> 00:11:04,969 remaining matter density is, and so, next time we will talk about dark 150 00:11:04,969 --> 00:11:06,249 non-baryonic matter.