1 00:00:00,012 --> 00:00:03,103 If you're a string theorist, then maybe you're done. 2 00:00:03,103 --> 00:00:07,830 We have described the calculations and all of the answers and we know everything 3 00:00:07,830 --> 00:00:12,429 but if you're interested in Astronomy and have some interest in the real world, 4 00:00:12,429 --> 00:00:16,886 then now we'll discuss some of how we know about all these things and all the 5 00:00:16,886 --> 00:00:21,268 great phenomena that have taught us and some more about the consequences. 6 00:00:21,268 --> 00:00:26,589 So one of the most exciting discoveries in this field is the discovery of ancient 7 00:00:26,589 --> 00:00:29,863 light, essentially the farthest thing we can see. 8 00:00:29,863 --> 00:00:35,087 Where does this come from? Well remember, there was this time of ionization when 9 00:00:35,087 --> 00:00:40,142 the temperature was about 3000 Kelvin and that means that the radiation gas had an 10 00:00:40,142 --> 00:00:43,679 average energy corresponding to KB times 3000 Kelvin, 11 00:00:43,679 --> 00:00:48,972 that's exactly the ionization energy for ionizing the hydrogen atom. 12 00:00:48,972 --> 00:00:53,797 At temperatures above that, hydrogen atoms were unstable because the ambient 13 00:00:53,797 --> 00:00:58,792 photons in the photon gas could just knock electrons out of hydrogens so there 14 00:00:58,792 --> 00:01:02,877 were no hydrogen atoms. There was a dense plasma and at later 15 00:01:02,877 --> 00:01:08,914 times as the universe cooled, hydrogen formed atoms and we had neutral hydrogen 16 00:01:08,914 --> 00:01:15,667 atoms by in-large though they were later re-ionized in as I said, in the, the hot 17 00:01:15,667 --> 00:01:21,230 inter-cluster gas clouds. But after recombination radiation 18 00:01:21,230 --> 00:01:28,133 essentially decouples from matter. So, photons that existed at the ambient 19 00:01:28,133 --> 00:01:31,641 temperature, at the ambient energy, these were 20 00:01:31,641 --> 00:01:37,587 ultraviolet photons with a temperature corresponding to a temperature blackbody 21 00:01:37,587 --> 00:01:43,784 spectrum, with a temperature of 3000 Kelvin that light spectrum as a once 22 00:01:43,784 --> 00:01:49,090 neutral hydrogen atoms formed, those photons basically proceeded through the 23 00:01:49,090 --> 00:01:52,917 universe unimpeded. Some of them, of course, bumped into 24 00:01:52,917 --> 00:01:55,822 little fluctuations like stars and galaxies. 25 00:01:55,822 --> 00:02:00,187 Since most of space is empty, most of those photons are still around. 26 00:02:00,187 --> 00:02:03,892 And the energy in that radiation field has been conserved. 27 00:02:03,892 --> 00:02:07,947 Those photons are still here except they have been red-shifted. 28 00:02:07,947 --> 00:02:12,407 And so, we can compute, since this happened at Z of about 1,000/100, the 29 00:02:12,407 --> 00:02:18,292 temperature will have decreased by the scale growth of the scale factor by a 30 00:02:18,292 --> 00:02:22,857 factor of a thousand, we expect the universe to be imbued, to 31 00:02:22,857 --> 00:02:28,662 be bathed in a blackbody, in a gas of blackbody photons with a temperature of 32 00:02:28,662 --> 00:02:34,068 about 3 degrees Kelvin. And this was realized by a theorist at Princeton named 33 00:02:34,068 --> 00:02:38,860 Dicke in 1960, and this had been predicted by several other works that 34 00:02:38,860 --> 00:02:44,328 we'll turn to later. But Dicke in 1960 has the idea that he's a radio astronomer 35 00:02:44,328 --> 00:02:48,181 and he can build a Dicke radiometer and try to measure this. 36 00:02:48,181 --> 00:02:53,674 And indeed, he set some of the researchers in his group to try to design 37 00:02:53,674 --> 00:02:59,368 a Dicke radiometer to measure the this relic radiation from the Big Bang if you 38 00:02:59,368 --> 00:03:04,733 want direct evidence for the Big Bang. Now, independent of that, 30 miles from 39 00:03:04,733 --> 00:03:09,007 Princeton at AT&T Bell Labs, two engineers, Penzias and Wilson are 40 00:03:09,007 --> 00:03:13,627 building a Dicke radiometer. Their idea is they're trying to develop 41 00:03:13,627 --> 00:03:18,852 the technology that will become satellite communication so they build this great 42 00:03:18,852 --> 00:03:24,102 big huge horned radio antenna that's here seen on the right, and there's a noise. 43 00:03:24,102 --> 00:03:27,833 This always happens with experimenters with experiments. 44 00:03:27,833 --> 00:03:32,955 the experiment is not perfect and their antenna picks up some noise, and they try 45 00:03:32,955 --> 00:03:37,501 to clean the contacts, and they have this, this humorous episode where a 46 00:03:37,501 --> 00:03:40,752 family of pigeons had made their home in the antenna, 47 00:03:40,752 --> 00:03:44,873 look at the size of the thing, and they spent a few quality days scraping pigeon 48 00:03:44,873 --> 00:03:48,695 droppings from the antenna thinking maybe this is the cause of the noise. 49 00:03:48,695 --> 00:03:52,181 Nothing removes the noise usually when that happens in a radio 50 00:03:52,181 --> 00:03:56,451 experiment, you are probably receiving the BBC from somewhere, so they turn 51 00:03:56,451 --> 00:04:00,321 their antenna in different directions. It's a very directional antenna. 52 00:04:00,321 --> 00:04:03,298 And they find that the noise is the same from every direction. 53 00:04:03,298 --> 00:04:07,078 They ruled out all terrestrial sources. They try to imagine, maybe this is from 54 00:04:07,078 --> 00:04:09,589 some astronomical source, so they try to check 55 00:04:09,589 --> 00:04:13,367 whether when they aim it at the plane of the galaxy or away from the plane of the 56 00:04:13,367 --> 00:04:16,916 galaxy, they get a different signal. The signal doesn't care where they're 57 00:04:16,916 --> 00:04:19,221 aiming it. The signal is completely isotropic. 58 00:04:19,221 --> 00:04:23,272 It's not terrestrial, it's not galactic. In despair, they call an eminent 59 00:04:23,272 --> 00:04:27,547 theorist, luckily they called Dicke at Princeton and the, at least, legend has 60 00:04:27,547 --> 00:04:31,752 it that he is interrupted in the middle of a group meeting by a call from these 61 00:04:31,752 --> 00:04:36,162 engineers at AT&T Bell Labs who want to ask a question and Dicke comes back and I 62 00:04:36,162 --> 00:04:40,102 think it was just Bell Labs back then, and Dicke comes back and tells the 63 00:04:40,102 --> 00:04:44,122 research group, don't worry about designing that radiometer, we've been 64 00:04:44,122 --> 00:04:47,641 scooped. What these guys had discovered was indeed 65 00:04:47,641 --> 00:04:53,523 the 3 degree thermal radiation at microwave wavelengths that is the relic 66 00:04:53,523 --> 00:04:59,811 remnants of the 3000 degree photons that were around when hydrogen atoms first 67 00:04:59,811 --> 00:05:03,832 formed and since then, we learned a lot about this background 68 00:05:03,832 --> 00:05:06,590 radiation. In particular, it is a blackbody, 69 00:05:06,590 --> 00:05:11,246 remember that a red-shifted blackbody remains blackbody spectrum, except the 70 00:05:11,246 --> 00:05:16,125 temperature is has decreased and what you see here is experimental measurements 71 00:05:16,125 --> 00:05:21,361 from the COBE satellite in the 90s and fit to a blackbody spectrum and this is 72 00:05:21,361 --> 00:05:25,787 as beautiful a fit as you can get so we have measured the temperature of the 73 00:05:25,787 --> 00:05:29,019 universe. And the temperature of at least the 74 00:05:29,019 --> 00:05:34,283 photon gas in the universe, matter, of course, is essentially 0 temperature, 75 00:05:34,283 --> 00:05:39,154 dust is cooled down to 0. But the temperature of the radiation 76 00:05:39,154 --> 00:05:44,574 field in the universe is about 2.726 degrees, 2.726 Kelvin, and we know this 77 00:05:44,574 --> 00:05:49,527 with high precision. and this, the, the the other important 78 00:05:49,527 --> 00:05:56,432 point other than its a blackbody spectrum validating the fact that this comes from 79 00:05:56,432 --> 00:06:02,868 some promodial heat, hot dense universe is that it's isotropic to one factor to a 80 00:06:02,868 --> 00:06:06,441 degree of one part in a 100,000. So, this, if you want, is a great 81 00:06:06,441 --> 00:06:11,229 validation of our cosmological principle that the universe is homogeneous and 82 00:06:11,229 --> 00:06:16,142 isotropic because the blackbody radiation, that it, fills the universe is 83 00:06:16,142 --> 00:06:20,006 the same in all directions. For this discovery, of course, Penzias 84 00:06:20,006 --> 00:06:23,162 and Wilson received the Nobel Prize in 1978. 85 00:06:23,162 --> 00:06:27,333 Notice that this is the answer to the question, how far can you see? This is 86 00:06:27,333 --> 00:06:31,737 the oldest light you can ever see. Why is this the oldest light? this is the 87 00:06:31,737 --> 00:06:34,391 universe as it was when it was 380,00 years old. 88 00:06:34,391 --> 00:06:38,548 And, of course, there was 380,000 years of history before that, but we'll never 89 00:06:38,548 --> 00:06:42,935 see them because photons created then interacted strongly with the plasma were 90 00:06:42,935 --> 00:06:46,092 absorbed and re-emitted and absorbed and re-emitted. 91 00:06:46,092 --> 00:06:50,996 And then eventually, the photons that were emitted 390,000 years ago, those are 92 00:06:50,996 --> 00:06:55,862 the photons that are still around. So, this is the most distant and the most 93 00:06:55,862 --> 00:07:00,427 far into the past that we can ever see. We'll never, for example, see the Big 94 00:07:00,427 --> 00:07:02,703 Bang. So this a very good image. 95 00:07:02,703 --> 00:07:08,243 This describes the temperature fluxuation of the microwave background. 96 00:07:08,243 --> 00:07:11,273 Microwave background is extremely isotropic. 97 00:07:11,273 --> 00:07:16,763 If you were wanting some evidence for isotroppy of the universe this is about 98 00:07:16,763 --> 00:07:20,306 as good as it gets. But when in the 90s, COBE makes more 99 00:07:20,306 --> 00:07:25,085 precise measurements of the precise blackbody spectrum, 100 00:07:25,085 --> 00:07:28,821 you'll find that there is temperature variation. 101 00:07:28,821 --> 00:07:32,366 The temperature is not completely uniform. 102 00:07:32,366 --> 00:07:37,505 This is, in fact, the same map with higher precision, blue representing 103 00:07:37,505 --> 00:07:42,472 regions that are slightly hotter. Red representing regions that are 104 00:07:42,472 --> 00:07:47,332 slightly colder, obviously this was done by astronomers, and you see that there's 105 00:07:47,332 --> 00:07:51,787 a blue region in the sky and diametrically opposed, 180 degrees away, 106 00:07:51,787 --> 00:07:55,477 a red region in the sky. The blackbody spectrum of the universe is 107 00:07:55,477 --> 00:07:58,432 not isotropic. I know what that means, that means that 108 00:07:58,432 --> 00:08:00,949 we are not at risk relative to the Hubble flow. 109 00:08:00,949 --> 00:08:05,159 This is a measurement of the Earth's motion relative to the local Hubble flow. 110 00:08:05,159 --> 00:08:09,065 This is our peculiar motion, what we are seeing is that we are moving in a 111 00:08:09,065 --> 00:08:12,417 particular direction. In fact, subtracting the Earth's motion 112 00:08:12,417 --> 00:08:15,288 around the sun and sun's motion around the Milky Way, 113 00:08:15,288 --> 00:08:19,790 all of which will have an affect, you'll see, what we have really measured is that 114 00:08:19,790 --> 00:08:25,050 our local cluster, as a whole, is moving with a peculiar velocity of 600 km/s in 115 00:08:25,050 --> 00:08:30,161 the direction of the Virgo cluster. And again, that's not exactly our motion 116 00:08:30,161 --> 00:08:35,470 because you have to subtract all kinds of motion round the Milky Way to get that. 117 00:08:35,470 --> 00:08:40,922 But that we know, when we subtract that, we find 600 km/s towards Virgo as the 118 00:08:40,922 --> 00:08:44,754 motion of the local cluster, so there is a local rest frame. 119 00:08:44,754 --> 00:08:49,146 It's determined by the cosmic microwave background and we can measure our 120 00:08:49,146 --> 00:08:53,770 velocity relative to that, and the universe is really, really, really, very 121 00:08:53,770 --> 00:08:54,377 isotropic.