1 00:00:00,012 --> 00:00:05,074 Now, we have a whole collection of galaxies to compare to the Milky Way like 2 00:00:05,074 --> 00:00:08,777 the discovery excel planets only 80 years earlier. 3 00:00:08,777 --> 00:00:14,035 We have this whole sampling of galaxies ready made and the astronomers launched 4 00:00:14,035 --> 00:00:18,504 the study of these objects and comparision to the Milky Way thast is 5 00:00:18,504 --> 00:00:21,632 still on going and we are still learning a lot. 6 00:00:21,632 --> 00:00:26,834 as we'll see about how galaxies work and how they form and how they interact. 7 00:00:26,834 --> 00:00:32,493 We had a leftover question from the Milky Way about this crazy rotation curve, and, 8 00:00:32,493 --> 00:00:37,426 the person who pushed the study of rotation curve was an American astronomer 9 00:00:37,426 --> 00:00:42,693 called Vera Rubin, who developed a fantastic technology that made it very 10 00:00:42,693 --> 00:00:47,740 easy to measure the rotation curve of a galaxy with a single measurement, 11 00:00:47,740 --> 00:00:53,428 basically measuring the Doppler shift as a function of position in the galaxy, and 12 00:00:53,428 --> 00:00:58,422 produced large numbers of rotation curves of various spiral galaxies. 13 00:00:58,422 --> 00:01:03,562 And here are the kinds of results. This is actually from a paper by Rubin 14 00:01:03,562 --> 00:01:08,386 and what you see is basically the same thing we saw for the Milky Way. 15 00:01:08,386 --> 00:01:14,083 The rotational velocity rises rapidly in the center and then oscillates a little 16 00:01:14,083 --> 00:01:19,205 bit and then approaches a constant. What we do not see in any of these 17 00:01:19,205 --> 00:01:24,676 galaxies is the decline outside the radius where the effective mass of the 18 00:01:24,676 --> 00:01:29,126 galaxy is and so there's this large problem of missing mass. 19 00:01:29,126 --> 00:01:34,863 It's not a problem confined to the Milky Way, there is a whole component of the 20 00:01:34,863 --> 00:01:38,032 mass of a galaxy that we are missing, now. 21 00:01:38,032 --> 00:01:43,970 Once you realize this, you also realize that, like the Milky Way, many, galaxies, 22 00:01:43,970 --> 00:01:48,573 have orbiting satellites. So, Andromeda has several satellites that 23 00:01:48,573 --> 00:01:53,859 orbit it, and other galaxies have, or-, orbital satellites that we can see. 24 00:01:53,859 --> 00:01:58,606 And so, you can measure their orbital parameters to get a measure of the mass 25 00:01:58,606 --> 00:02:02,544 inside their radius. So, in a larger radius And the farther 26 00:02:02,544 --> 00:02:06,551 you look from a galaxy, the more the missing mass problem grows. 27 00:02:06,551 --> 00:02:11,260 It seems that once you get farther out, 95% of the mass of a galaxy is not 28 00:02:11,260 --> 00:02:16,427 accounted for by any of the components of a galaxy we have so far accounted for. 29 00:02:16,427 --> 00:02:21,105 So what have we accounted for? We've accounted for all the stars that we can 30 00:02:21,105 --> 00:02:23,402 see. Maybe off by some brown dwarfs. 31 00:02:23,402 --> 00:02:26,277 We've accounted for gas and dust that we can see. 32 00:02:26,277 --> 00:02:31,222 the Occam's, and, and, this, this extra matter is not clumped as tightly in the 33 00:02:31,222 --> 00:02:34,497 center of a galaxy. Remember, it extends, and as you go 34 00:02:34,497 --> 00:02:39,137 farther and farther away from the center, you're finding more and more missing 35 00:02:39,137 --> 00:02:42,442 mass. So this is some broadly distributed mass, 36 00:02:42,442 --> 00:02:47,702 a great big halo of mass around a galaxy. What's Occam's Razor? Well, the first 37 00:02:47,702 --> 00:02:51,085 guess would be, maybe there's a collection of gas. 38 00:02:51,085 --> 00:02:56,196 There's just a huge, we thought that maybe there were 60 billion solar masses 39 00:02:56,196 --> 00:02:59,975 of gas out there. Maybe there are, it turns out you need a 40 00:02:59,975 --> 00:03:03,610 factor of 100 more. It turns out that the visible mass of the 41 00:03:03,610 --> 00:03:06,228 galaxy in the case of the Milky Way is about 5%. 42 00:03:06,228 --> 00:03:10,632 We need 20 times more gas out there than we have, but if we have 20 times more gas 43 00:03:10,632 --> 00:03:14,165 then we would start to see its extinction and absorption lines. 44 00:03:14,165 --> 00:03:18,162 We don't see any absorption lines, whatever it is that's out there. 45 00:03:18,162 --> 00:03:23,421 Is not only not producing light, it's also not absorbing light, it's optically 46 00:03:23,421 --> 00:03:27,398 inactive. You can't hide, 100, 20 times the mass of 47 00:03:27,398 --> 00:03:31,670 the Milky Way in gas surrounding us without being able to see it. 48 00:03:31,670 --> 00:03:37,151 Something that, has no optical properties but is gravitationally active is an 49 00:03:37,151 --> 00:03:41,472 interesting thing. the name that's given to this weird stuff 50 00:03:41,472 --> 00:03:44,497 is Dark Matter. And, for example, for the Milky Way, our 51 00:03:44,497 --> 00:03:48,612 measurements of the orbits of satellite galaxies and so on, predict that there is 52 00:03:48,612 --> 00:03:51,252 a spherical, halo. The fact that it's spherical was 53 00:03:51,252 --> 00:03:55,157 initially conjectured just because otherwise it would be concentrated in the 54 00:03:55,157 --> 00:03:57,257 disk. But later measurements of orbiting 55 00:03:57,257 --> 00:04:00,322 satelites and other thing, measurements we'll discuss. 56 00:04:00,322 --> 00:04:06,422 Suggest a spherical halo, in fact, we know roughly the density profile of the 57 00:04:06,422 --> 00:04:11,412 star matter halo, that extends between 2 and 300 kiloparsecs. 58 00:04:11,412 --> 00:04:17,222 which means that if you give Andromeda a similar halo, the halo is essentially 59 00:04:17,222 --> 00:04:21,543 touched. And a mass noticed that is as I said, 20 60 00:04:21,543 --> 00:04:26,988 times larger than the mass of the entire, Milky Way galaxy, of all the components 61 00:04:26,988 --> 00:04:31,201 that we've added up so far. So, most of the mass of the galaxy is in 62 00:04:31,201 --> 00:04:36,408 this Dark Matter halo, that is what the sun is orbiting, certainly that is, since 63 00:04:36,408 --> 00:04:41,352 it's a diffused halo The error that we found for the sun was small, but as you 64 00:04:41,352 --> 00:04:45,667 get farther and farther out, the differences get bigger and bigger. 65 00:04:45,667 --> 00:04:51,022 what is this stuff? Well, it's certainly an important, thing to figure out. 66 00:04:51,022 --> 00:04:56,747 It dominates the galactic mass, therefore it is the thing that determines, things 67 00:04:56,747 --> 00:05:01,424 like gravitational collapse and gravitational orbits and Gravitational 68 00:05:01,424 --> 00:05:04,685 interactions between galaxies, and it's out there. 69 00:05:04,685 --> 00:05:08,372 What is it? Well, again we've ruled out an envelope of gas. 70 00:05:08,372 --> 00:05:13,238 How else could you stick matter in a big halo around it in a way that we wouldn't 71 00:05:13,238 --> 00:05:18,129 see it? Well, if you clump the matter into tight clumps, say, like brown dwarf 72 00:05:18,129 --> 00:05:21,095 stars, or white dwarf stars, or neutron stars. 73 00:05:21,095 --> 00:05:26,332 Well, a neutron star packs a lot of mass into a very tight region And so you would 74 00:05:26,332 --> 00:05:30,752 not see absorption lines because by and large you wouldn't be looking right 75 00:05:30,752 --> 00:05:34,537 through the neutron star. So packing matter into tight packages 76 00:05:34,537 --> 00:05:39,042 would be one way to hide a lot of mass in a large halo of the density of neutron 77 00:05:39,042 --> 00:05:43,702 stars doesn't need to be very high. Because there's a lot of volume there and 78 00:05:43,702 --> 00:05:47,057 neutron stars are very dense. So this is one suggestion. 79 00:05:47,057 --> 00:05:51,047 it goes under the name machos for massive compact halo objects. 80 00:05:51,047 --> 00:05:54,892 so the search goes on. How would you find white dwarfs way out 81 00:05:54,892 --> 00:05:59,697 there in the halo? Well we, we know that finding brown dwarfs is difficult even in 82 00:05:59,697 --> 00:06:05,668 the local surrounding of the thin disk and their numbers have osculated back and 83 00:06:05,668 --> 00:06:10,083 forth, over the past decade. How would you find your way out there? 84 00:06:10,083 --> 00:06:15,620 Well gravitational lensing is the idea. So, what we see here, is the light curve 85 00:06:15,620 --> 00:06:20,085 of a star, that is not a variable star. there is no periodisity. 86 00:06:20,085 --> 00:06:25,817 But it is a Brightness suddenly intensified, quite significantly for a 87 00:06:25,817 --> 00:06:28,972 period of about 30 days and then went back. 88 00:06:28,972 --> 00:06:35,315 We have, measurements here in black, and a theoretical model in red that explains 89 00:06:35,315 --> 00:06:40,388 what it is that was going on. What was happening to the star is that it 90 00:06:40,388 --> 00:06:48,154 was, as the earth moved and the sun moved, it was for those 30 days lined up, 91 00:06:48,154 --> 00:06:55,572 behind some dark object, like maybe a neutron star or a Brown Dwarf in between 92 00:06:55,572 --> 00:07:02,545 us and this star and when you have Massive object in between the star and 93 00:07:02,545 --> 00:07:09,522 the Earth, then what we see, we saw is that the deflection, the graviational 94 00:07:09,522 --> 00:07:15,696 deflection of light, will cause a focusing effect so that all of the 95 00:07:15,696 --> 00:07:21,412 starlight that would have gone in to some angle surrounding. 96 00:07:21,412 --> 00:07:27,327 The line, the direct line between the star and earth will in fact be focused by 97 00:07:27,327 --> 00:07:31,427 this gravitational lens and the star will appear brighter. 98 00:07:31,427 --> 00:07:34,399 So this lensing effect is well understood. 99 00:07:34,399 --> 00:07:39,392 It's a GR effect and we are I said we would apply relativity a little, so 100 00:07:39,392 --> 00:07:44,089 gravitational lensing, this is called microlensing because the object that's 101 00:07:44,089 --> 00:07:46,690 doing the lensing need not be very massive. 102 00:07:46,690 --> 00:07:49,778 the whole point is that you have precise alignment. 103 00:07:49,778 --> 00:07:54,518 And so we find these lensing events, very good we could use them to try to detect 104 00:07:54,518 --> 00:07:57,879 how often they occur. We have a sense for where stars and 105 00:07:57,879 --> 00:08:02,271 assume they're scattered randomly, scatter the neutron stars in the halo 106 00:08:02,271 --> 00:08:05,235 randomly. And look and make predictions for the 107 00:08:05,235 --> 00:08:09,393 frequency, with which, microlensing events should occur. 108 00:08:09,393 --> 00:08:13,204 We do that calculation. We measure microlensing events and we 109 00:08:13,204 --> 00:08:17,641 don't get nearly enough of them. In fact, no more than 10% of the missing 110 00:08:17,641 --> 00:08:21,892 mass of the Milky Way, say, could possibly be explained by a Macho. 111 00:08:21,892 --> 00:08:27,546 And so there may be some neutron stars, though it's not clear how they got out 112 00:08:27,546 --> 00:08:31,581 there into the halo. But there may be some compact halo 113 00:08:31,581 --> 00:08:37,517 objects, but certainly that is not the answer to the missing mass puzzle.So what 114 00:08:37,517 --> 00:08:41,412 is it? Well, Occam's Razor tells you not to add new. 115 00:08:41,412 --> 00:08:44,337 The ingredients to a theory until they're necessary. 116 00:08:44,337 --> 00:08:48,362 But, it seems that at this point, we are compelled to add a completely new 117 00:08:48,362 --> 00:08:52,312 ingredient to the theory, the new ingredient goes under the name WIMPS. 118 00:08:52,312 --> 00:08:56,952 you know that this was, a naming, convention by physicists when there's 119 00:08:56,952 --> 00:09:00,877 MACHOS and WIMPS and the WIMPS win. WIMPS stands for weakly interacting 120 00:09:00,877 --> 00:09:05,236 massive particles. What are these objects? Well, these are 121 00:09:05,236 --> 00:09:10,541 conjectured to be a completely new and yet undiscovered form of matter. 122 00:09:10,541 --> 00:09:16,311 We know about quarks and leptons and protons and neutrons and theres a wimpon, 123 00:09:16,311 --> 00:09:20,422 some new kind of particle that we have never detected. 124 00:09:20,422 --> 00:09:24,484 We have never detected it because it interacts only very weakly with matter. 125 00:09:24,484 --> 00:09:28,712 In fact, you can compute just as we did for neutrinos that hundreds of thousands 126 00:09:28,712 --> 00:09:33,396 of them are passing through your body, at any while we're having this conversation, 127 00:09:33,396 --> 00:09:36,142 and because they're weakly interacting nothing. 128 00:09:36,142 --> 00:09:38,752 Happens. Oh, wait, so why not just say that 129 00:09:38,752 --> 00:09:43,781 there's a halo full of neutrinos? Well, neutrinos are very light objects, and, 130 00:09:43,781 --> 00:09:48,876 light particles, and they, at typical, energies that you'd expect them to have, 131 00:09:48,876 --> 00:09:51,652 move relativistically, and would not clump. 132 00:09:51,652 --> 00:09:56,292 Remember, these things interact to gravitationally clump, to be bound to the 133 00:09:56,292 --> 00:10:01,164 Milky Way, or bound to themselves since their mass dominates, the mass of the 134 00:10:01,164 --> 00:10:05,499 Milky Way, into this spherical halo and not go wandering off into space. 135 00:10:05,499 --> 00:10:09,165 A halo made of neutrinos would not have the right properties. 136 00:10:09,165 --> 00:10:14,292 this is why we're talking about Weakly Interacting Massive Particles, so massive 137 00:10:14,292 --> 00:10:17,582 particles that interact with Other particles only weakly. 138 00:10:17,582 --> 00:10:21,268 That's why we haven't seen them, that's why they pass through bodies, etc. 139 00:10:21,268 --> 00:10:25,244 More over they have to interact with each other weakly, because if there were 140 00:10:25,244 --> 00:10:29,404 strong interactions say electromagnetic interactions or electromagnetic To some 141 00:10:29,404 --> 00:10:34,168 other electromagnetism interactions between these wimps, then they would rub 142 00:10:34,168 --> 00:10:37,684 against each other. They would be able to lose energy into 143 00:10:37,684 --> 00:10:42,289 efficiently convert gravitational potential energy into heat and then they 144 00:10:42,289 --> 00:10:46,709 would collapse, we have black stars and then we would be back to the macho 145 00:10:46,709 --> 00:10:48,822 scenario. We want them unlike. 146 00:10:48,822 --> 00:10:52,761 Real matter, normal matter, will just collapse to form galaxies in which there 147 00:10:52,761 --> 00:10:56,351 are stars and planets and people. We want these dark matter particles to 148 00:10:56,351 --> 00:10:59,729 stay very smoothly distributed in this big halo and never collapse. 149 00:10:59,729 --> 00:11:03,639 The way they never collapse is that they don't have a way to thermalize energy. 150 00:11:03,639 --> 00:11:07,762 That is because they weakly interact not only with us but also with each other. 151 00:11:07,762 --> 00:11:12,924 We do not know of a particle like this, so if they exist, they have not yet been 152 00:11:12,924 --> 00:11:17,763 discovered, and in fact, various theoretical models, ask your favorite 153 00:11:17,763 --> 00:11:23,319 theorist, they will have candidate, for what particle in some extension of 154 00:11:23,319 --> 00:11:27,272 current theories might be uh,. The candidate for dark matter. 155 00:11:27,272 --> 00:11:32,127 A popular candidate among many theorists, is, neutralinos that show up in super 156 00:11:32,127 --> 00:11:36,292 symmetric extensions of the standard model of particle physics, but this is 157 00:11:36,292 --> 00:11:40,252 not course on particle theory although you might be confused sometimes. 158 00:11:40,252 --> 00:11:44,647 So theorists have all kinds of ideas; experimentalists, on the other hand, want 159 00:11:44,647 --> 00:11:48,445 to actually detect this. Remember, 95% of the mass of the Milky 160 00:11:48,445 --> 00:11:53,148 Way has never been seen, is in the form of a particle we haven't discovered. 161 00:11:53,148 --> 00:11:57,983 Our standard model of particle physics is great for discovering the 5% we know 162 00:11:57,983 --> 00:12:01,250 about. Interesting, and humbling, and so these 163 00:12:01,250 --> 00:12:06,649 are 2 of the many ongoing experiments. These both work on a similar principle. 164 00:12:06,649 --> 00:12:10,142 There are other, Ideas that are being conducted. 165 00:12:10,142 --> 00:12:14,012 The idea here is that you have these very delicate detectors. 166 00:12:14,012 --> 00:12:19,197 once in a long while, one of the dark matter particles traversing the detector 167 00:12:19,197 --> 00:12:24,222 might interact with a proton in the detector, setting off a, sound wave 168 00:12:24,222 --> 00:12:28,887 because it will have transferred some momentum to that proton, which will 169 00:12:28,887 --> 00:12:33,150 exchange With the others in the crystal and that sound wave gets converted to an 170 00:12:33,150 --> 00:12:37,252 electronic signal which will be detected, so any time that somebody claps their 171 00:12:37,252 --> 00:12:41,591 hands within miles of this, the vibration is detected as a signal and of course, 172 00:12:41,591 --> 00:12:44,000 lots of things might interact with the proton. 173 00:12:44,000 --> 00:12:48,158 So you stick these things deep in mines underground, like neutrino detectors and 174 00:12:48,158 --> 00:12:51,392 there are dark matter searches going on all over the world. 175 00:12:51,392 --> 00:12:56,447 And at some point, hopefully, we will actually have dark matter detection and 176 00:12:56,447 --> 00:13:00,635 we will begin to understand the properties of whatever it is that 177 00:13:00,635 --> 00:13:05,405 constitutes most of the mass of our galaxy and, most of the mass of the 178 00:13:05,405 --> 00:13:09,282 universe, as we shall see. And so, nice mystery here, 179 00:13:09,282 --> 00:13:13,922 We don't know what it is. There are alternatives I should point out 180 00:13:13,922 --> 00:13:18,693 to positing dark matter. They are continually being pushed into 181 00:13:18,693 --> 00:13:25,152 less and less reasonable corners by increasing amounts of observational data. 182 00:13:25,152 --> 00:13:29,462 But, one of the leading one is to say that Newtonian gravity is an 183 00:13:29,462 --> 00:13:34,337 approximation, that there are corrections to it, not the Einsteinian corrections 184 00:13:34,337 --> 00:13:38,422 that are valid at large gravitational fields and high velocities. 185 00:13:38,422 --> 00:13:42,603 We're talking about corrections here that would become important at large 186 00:13:42,603 --> 00:13:47,716 distances, where the Halos lie, so maybe at large distances, gravity, operates a 187 00:13:47,716 --> 00:13:52,133 little differently, and so our Newtonian calculation of the mass enclosed was 188 00:13:52,133 --> 00:13:55,074 wrong. this was a reasonable theory, certainly 189 00:13:55,074 --> 00:13:59,417 no less reasonable than depositing a whole new kind of particle, but as we'll 190 00:13:59,417 --> 00:14:04,687 see in a few clips We think we have evidence to exclude that reasonable 191 00:14:04,687 --> 00:14:08,377 solution. There's still others in the creative 192 00:14:08,377 --> 00:14:14,212 minds of theorists but most of us think that cold dark matter, as this stuff is 193 00:14:14,212 --> 00:14:18,907 called, is what constitutes most of the mass of the universe. 194 00:14:18,907 --> 00:14:23,249 One dark thing, all around This halo of, wierd particles, 195 00:14:23,249 --> 00:14:27,714 We can look, into the core of other galaxies, just like we tried to peer into 196 00:14:27,714 --> 00:14:30,759 the core of ours, and see stars orbiting very rapidly. 197 00:14:30,759 --> 00:14:34,636 It's much easier to see the core of another galaxy, if you happen to be 198 00:14:34,636 --> 00:14:38,918 seeing it, head on, there is less of a disc of dust and gas between you and the 199 00:14:38,918 --> 00:14:41,181 core. And so, we can extend the rotation 200 00:14:41,181 --> 00:14:44,273 curves. All the way, very close to the center and 201 00:14:44,273 --> 00:14:48,626 we see that stars very close to the center in fact are moving at very high 202 00:14:48,626 --> 00:14:51,849 velocities. This tells us very close to the center is 203 00:14:51,849 --> 00:14:56,102 a very massive object, various other observations rule out most other 204 00:14:56,102 --> 00:15:00,815 candidates and at the end of the day, were lead to the conclusion that like the 205 00:15:00,815 --> 00:15:04,652 Milky Way. Most galaxies contain in their core, a 206 00:15:04,652 --> 00:15:09,992 super massive blackhole in fact the milkyway's black-hole is somewhat 207 00:15:09,992 --> 00:15:15,807 underwhelming masses of black-holes and other galaxies can be up to a billion 208 00:15:15,807 --> 00:15:21,267 solar masses which is as much as the masses of say all the gas, disc gas in 209 00:15:21,267 --> 00:15:26,392 the solar, in the milky way. it turns out that the properties of the 210 00:15:26,392 --> 00:15:31,152 black hole in the center of a galaxy are correlated in interesting ways with 211 00:15:31,152 --> 00:15:35,887 galactic parameters, the luminosity of the poles, the numbers of globular 212 00:15:35,887 --> 00:15:38,942 clusters. This, of course, is a great hint as to 213 00:15:38,942 --> 00:15:43,842 how to construct a theory of galactic evolution, where galaxies come from and 214 00:15:43,842 --> 00:15:47,082 how they form. We'll move to what little I'm able to 215 00:15:47,082 --> 00:15:52,072 confidently say about this in a bit. But, for now let's relish the fact that 216 00:15:52,072 --> 00:15:55,680 in the center of every galaxy is a super massive black hole. 217 00:15:55,680 --> 00:16:00,485 And at the outskirts of every galaxy is this huge halo of stuff and nobody has 218 00:16:00,485 --> 00:16:02,030 any idea what it's made of.