1 00:00:14,590 --> 00:00:18,910 Thank you very much. I have to make one correction to the introduction. 2 00:00:18,910 --> 00:00:25,840 He said I made seminal contributions to our understanding of dark matter, as you will understand from the start. 3 00:00:25,840 --> 00:00:34,150 We do not understand what dark matter is, and so no contributions can be viewed as seminal. 4 00:00:34,150 --> 00:00:39,550 I have to say it's a pleasure to be here. This is my first first visit to Oxford and the past three days. 5 00:00:39,550 --> 00:00:42,310 I've had a really enjoyable time walking around the town, 6 00:00:42,310 --> 00:00:47,350 talking with a variety of people in physics and astrophysics, and there's a long list of things I've learnt. 7 00:00:47,350 --> 00:00:52,960 I'm not too surprised because many of the things that I've learnt about astrophysics came from people here. 8 00:00:52,960 --> 00:00:56,800 For example, in James Bond, his books on Galactic Dynamics, 9 00:00:56,800 --> 00:01:03,040 which I would call the Rolls-Royce of theoretical astrophysics anyway, have had a great time visiting Oxford. 10 00:01:03,040 --> 00:01:12,790 I'd like to come back again sometime to visit. I should also tell you that we just hired a new faculty member at Johns Hopkins, 11 00:01:12,790 --> 00:01:18,700 away from the University of Mississippi and University of Mississippi in Oxford, Mississippi. 12 00:01:18,700 --> 00:01:25,510 And so for the past year, like all the time, I keep hearing, Oh, back in Oxford, we did this back in Oxford, back in Oxford. 13 00:01:25,510 --> 00:01:36,400 And so when I came here, I was thinking grits. So it's not quite what I expected, but I've been really having a blast. 14 00:01:36,400 --> 00:01:42,820 So in this talk, I will tell you about some spectacular science and about some lunacy. 15 00:01:42,820 --> 00:01:48,580 And what I'll try to tell you is that the lunacy is not completely crazy, 16 00:01:48,580 --> 00:01:54,070 and I'm going to unfortunately not give you a dramatic, definitive conclusion. 17 00:01:54,070 --> 00:01:58,730 I'm going to leave you with some uncertainty at the end, but that's the way it always is in science. 18 00:01:58,730 --> 00:02:04,330 There are things we know and things we don't know. 19 00:02:04,330 --> 00:02:09,590 So I also understand that Ray Rice was here giving this talk two years ago. 20 00:02:09,590 --> 00:02:15,260 Ray always got the Nobel prise. Since then, I'm not Ray Weiss. 21 00:02:15,260 --> 00:02:22,100 There is only one Ray Weiss. And thank goodness the number of Ray Rice, this is not zero. 22 00:02:22,100 --> 00:02:25,850 So back almost forty three plus a few months ago, 23 00:02:25,850 --> 00:02:34,460 there was a spectacular announcement from the logo and Virgo collaboration who were searching for gravitational waves. 24 00:02:34,460 --> 00:02:41,420 And after many years, they found them, and the gravitational waves they found were a signature, 25 00:02:41,420 --> 00:02:49,220 a signal from a pair of black holes that had merged in a burst of gravitational waves, as they did. 26 00:02:49,220 --> 00:02:56,060 So this was, I have to say, perhaps the most spectacular discovery I'd ever seen. 27 00:02:56,060 --> 00:03:00,650 Spectacular and definitive discovery I'd ever seen in science. Everybody agreed. 28 00:03:00,650 --> 00:03:07,700 There was a very quick Nobel prise went to Ray Rice, Barry Barish and Kip Thorne, the leaders and consumers of this project. 29 00:03:07,700 --> 00:03:14,180 And they also got every other prise if there's a prise for physics. They got it, and deservedly so. 30 00:03:14,180 --> 00:03:20,490 It was also a spectacular coincidence because it was the discovery was almost. 31 00:03:20,490 --> 00:03:23,400 Precisely, actually, as precisely as you can possibly imagine, 32 00:03:23,400 --> 00:03:31,050 one hundred years after Albert Einstein developed his theory of general relativity and Isaac Newton, 33 00:03:31,050 --> 00:03:36,120 sorry, Albert Einstein's theory of general relativity is a theory of gravity. 34 00:03:36,120 --> 00:03:41,730 We all know about gravity. Will demonstrate. 35 00:03:41,730 --> 00:03:49,110 That's gravity, things fall. It's not just that, it's also the Earth spinning around the sun, it's the moon spinning around the moon. 36 00:03:49,110 --> 00:03:54,480 It's the Solar System spinning around the centre of the Milky Way. It's the expansion of the universe. 37 00:03:54,480 --> 00:03:59,880 Everything that I just described can be everything that I just mentioned can be 38 00:03:59,880 --> 00:04:06,570 described as manifestations of Albert Einstein's theory of general relativity. 39 00:04:06,570 --> 00:04:12,750 And this is a picture of what general relativity says the universe looks like, and I'll describe momentarily. 40 00:04:12,750 --> 00:04:20,670 But before I do so, I'm going to show you an equation. Now, I'm always told that in popular talks, you're not supposed to solve equations, 41 00:04:20,670 --> 00:04:23,820 but I feel compelled to show an equation because I'm a theoretical physicist. 42 00:04:23,820 --> 00:04:30,660 That is all we do is write down equations, and I believe that you all know what an equation is. 43 00:04:30,660 --> 00:04:34,290 An equation has three things in the middle. 44 00:04:34,290 --> 00:04:38,940 There is an equal sign on the left hand side, there is something. 45 00:04:38,940 --> 00:04:40,500 And on the right hand side there something. 46 00:04:40,500 --> 00:04:48,120 And the point of the equation is to tell you that the thing on the left hand side is the same as the thing on the right hand side. 47 00:04:48,120 --> 00:04:55,920 So this is something that we call the Einstein Field Strength Tensor, and this is something that we call the stress energy tensor. 48 00:04:55,920 --> 00:05:06,690 So in words, the left hand side describes the curvature of space and the thing on the right hand side describes the matter of content. 49 00:05:06,690 --> 00:05:13,890 So this is an equation that describes the picture that Einstein had for gravity in the universe. 50 00:05:13,890 --> 00:05:19,080 So Einstein surmised that when you place a massive object somewhere in the universe, 51 00:05:19,080 --> 00:05:26,790 it curves the spacetime around it and therefore other objects will fall or move subject to that curvature. 52 00:05:26,790 --> 00:05:31,620 So there's an analogue that we can think of, which is not precise but is pretty good. 53 00:05:31,620 --> 00:05:36,450 Suppose I have a very large and very soft mattress, 54 00:05:36,450 --> 00:05:42,180 and I place a very heavy object right in the middle that will depress the mattress that looks something like this. 55 00:05:42,180 --> 00:05:47,490 So if I have that very heavy object depressing the mattress and then I put a smaller object over there, 56 00:05:47,490 --> 00:05:54,510 the object will fall into the very heavier object that is sort of roughly speaking, what Albert Einstein suggested. 57 00:05:54,510 --> 00:05:57,810 The only difference being that instead of being a two dimensional surface, 58 00:05:57,810 --> 00:06:03,240 he posited that the curvature is for all three spatial dimensions, as well as the time dimension. 59 00:06:03,240 --> 00:06:07,860 So that idea is encapsulated in this beautiful and elegant equation. 60 00:06:07,860 --> 00:06:14,370 G equals team. You know, I remember when I first learnt this in college, my senior year, I was taking general relativity. 61 00:06:14,370 --> 00:06:17,590 I saw one of my professors in the hallway, I said. 62 00:06:17,590 --> 00:06:27,160 General Tivoli, it's so elegant, know equals to me, and he said, yeah, but let's you know, that's what the rest of the year. 63 00:06:27,160 --> 00:06:31,960 So in words, this is what Einstein postulated. 64 00:06:31,960 --> 00:06:38,200 Spacetime tells matter how to move, and matter tells spacetime how to curve. 65 00:06:38,200 --> 00:06:48,860 So we place an object somewhere in space. And that curves the space, and then that curvature tells the other objects how to move. 66 00:06:48,860 --> 00:06:55,310 That's all there is to general relativity. Everything else is mathematics to make the statement quantitative. 67 00:06:55,310 --> 00:07:01,310 This was a remarkable idea because it had a number of observational consequences. 68 00:07:01,310 --> 00:07:08,090 First of all, it agreed and the limit of low mass objects and slowly moving objects with Einstein's. 69 00:07:08,090 --> 00:07:13,940 Sorry, Isaac Newton's theory of gravity from several hundred years before, but it said more. 70 00:07:13,940 --> 00:07:19,850 So one of the things that Albert Einstein realised almost immediately after developing this theory is that he could account for the perihelion, 71 00:07:19,850 --> 00:07:24,410 the advance of Mercury. So Mercury spins around the Sun. 72 00:07:24,410 --> 00:07:32,300 And as Isaac and Isaac Newton's gravity and Isaac Newton's theory, planets orbit around the sun, 73 00:07:32,300 --> 00:07:40,430 or pseudo orbit around the Sun on ellipses, the close in on themselves. But it was observed at the time that by the time that Einstein was around, 74 00:07:40,430 --> 00:07:47,840 that Mercury's orbit does not close in on itself, but it actually processes or advances by about two degrees per cent. 75 00:07:47,840 --> 00:07:55,940 And when Einstein described this system with this equation, he found that he could account for that perihelion advance. 76 00:07:55,940 --> 00:07:59,990 So that was the first indication that he was on the right track. 77 00:07:59,990 --> 00:08:04,490 The second indication was Eddington is famous eclipse experiment. 78 00:08:04,490 --> 00:08:10,610 So according to General Relativity, Light is affected by gravitational fields. 79 00:08:10,610 --> 00:08:15,020 So as a light ray passes by the sun, it gets deflected. 80 00:08:15,020 --> 00:08:17,270 It does not pass along a straight line. 81 00:08:17,270 --> 00:08:26,960 And so if we were to look at a star that is very close to the Sun and that star was over here, it would seem that the star was actually over here. 82 00:08:26,960 --> 00:08:34,700 All right. The other way around, if the star was over here, the light from that star would come along a trajectory that derives from this direction. 83 00:08:34,700 --> 00:08:48,310 And it appears as the star was over here. And this was verified very shortly after Einstein's prediction, actually 100 years ago by Arthur Eddington. 84 00:08:48,310 --> 00:08:52,150 So another prediction of Einstein's relativity that he realised fairly quickly 85 00:08:52,150 --> 00:08:56,950 after the development of the theory is the existence of gravitational waves, 86 00:08:56,950 --> 00:09:09,110 and this one took a hundred years to verify. So this is a picture of two stars that an audit orbit around each other. 87 00:09:09,110 --> 00:09:17,090 And this is the gravitational field due to the stars. And if you look at the stars from these two, this pair of objects from this direction, 88 00:09:17,090 --> 00:09:22,100 the gravitational field you'll see is slightly different than the stars that the gravitational field, 89 00:09:22,100 --> 00:09:26,030 if you're looking from this direction because here it seems like if you got just, you know, 90 00:09:26,030 --> 00:09:33,350 one to two objects along the given line of sight here, it seems you have two objects that are sort of alongside each other. 91 00:09:33,350 --> 00:09:42,080 And so as these as these massive objects spin around each other, the gravitational field that we will see will change in time. 92 00:09:42,080 --> 00:09:49,940 So these propagating perturbations in spacetime in the fabric of space are not too unfamiliar. 93 00:09:49,940 --> 00:09:56,420 So don't be too unfamiliar to you. All kinds of media can support waves. 94 00:09:56,420 --> 00:10:06,800 So for example, if I have a pond and I drop a pebble in the middle, that Pebble will induce perturbations to the surface of the water. 95 00:10:06,800 --> 00:10:16,220 And what will happen is those disturbances will then propagate out as waves in the in the in the in the heights of the water. 96 00:10:16,220 --> 00:10:23,060 So here the medium is the water. We disturb the water by shaking something and when we shake it or move it, 97 00:10:23,060 --> 00:10:27,850 those disturbances are propagated through the medium and then seen over here. 98 00:10:27,850 --> 00:10:33,890 So suppose I had a small toy boat far away from this point in the water? 99 00:10:33,890 --> 00:10:39,530 The way I could actually detect that wave is by seeing the toy boat bounce up and down. 100 00:10:39,530 --> 00:10:44,570 So if I were to move something over here up and down, that would cause a wave to propagate out, 101 00:10:44,570 --> 00:10:51,060 and I would detect that wave because a small boat over here would begin to shake up and down. 102 00:10:51,060 --> 00:10:57,570 Here's another example. Radio communications. Sorry, that's on radio communication. 103 00:10:57,570 --> 00:10:59,850 This is a string. Here's a second example. 104 00:10:59,850 --> 00:11:09,180 So suppose I have a string and I shake it up and down that disturbance and the string will propagate down the string as a wave. 105 00:11:09,180 --> 00:11:13,260 And then if I have some beads on the wave over here, those beads will bounce up and down. 106 00:11:13,260 --> 00:11:19,710 So again, there's a medium which is here the string, which can propagate disturbances, disturbance, the disturbances can be set, 107 00:11:19,710 --> 00:11:27,760 the waves can be sent out by shaking this thing up and down and then detected by the motions they induced in these beads over here. 108 00:11:27,760 --> 00:11:34,600 Here's the radio communications, the same thing happens with radio communications and a radio transmitter. 109 00:11:34,600 --> 00:11:40,030 I have an electron or a bunch of electrons, actually, and I shake those electrons up and down. 110 00:11:40,030 --> 00:11:43,750 Those electrons have associated with them electric fields. 111 00:11:43,750 --> 00:11:49,930 And as I shake the electron up and down, that disturbance gets propagated as a disturbance in the electric fields. 112 00:11:49,930 --> 00:11:55,310 The propagates over here. And then I detect that disturbance in electro that that wave like disturbance in 113 00:11:55,310 --> 00:12:00,220 the electromagnetic field through the motions that induces in the test electrons. 114 00:12:00,220 --> 00:12:07,360 And this in turn over here. So I shake electrons. It sends a wave out in the electromagnetic field and then I detect that electromagnetic 115 00:12:07,360 --> 00:12:12,190 waves through the motions and induces and electrons in the receiving antenna. 116 00:12:12,190 --> 00:12:23,740 The other example I'd like to give, which I first gave when I give this talk in California because it's fairly familiar, is is earthquakes. 117 00:12:23,740 --> 00:12:31,060 So there I have a geophysical wave and it can be propagated and actually don't even live in California. 118 00:12:31,060 --> 00:12:36,910 How many have you ever been sitting at home when a very large truck drove by? 119 00:12:36,910 --> 00:12:43,110 So they're the ground, which we think of is very, very stable and sturdy. 120 00:12:43,110 --> 00:12:51,330 There it is. It's not moving, we always think of the ground as very sturdy, but if a heavy truck comes by and a bumpy road and starts bouncing along, 121 00:12:51,330 --> 00:12:58,620 that can induce waves in the ground to propagate through the ground and then wind up shaking the house or building the trend and then shaking you. 122 00:12:58,620 --> 00:13:07,890 So again, a bouncing object propagate waves in a medium or very sturdy medium like the Earth, the surface of the Earth, and then shake the house. 123 00:13:07,890 --> 00:13:14,850 Now there it takes a very, very massive truck to induce a wave because the ground is very, very rigid. 124 00:13:14,850 --> 00:13:21,270 It is not, though perfectly rigid if it was perfectly rigid. It doesn't matter how heavy the truck is, nothing would ever happen. 125 00:13:21,270 --> 00:13:25,080 But since it's extremely rigid, you need a perfectly rigid. 126 00:13:25,080 --> 00:13:31,110 A very, very heavy truck will shake the ground and then you will feel that disturbance because it's going to then shake you. 127 00:13:31,110 --> 00:13:39,780 And it turns out that space time is no different. We always think of space time as rigid all the way, going back to Galileo. 128 00:13:39,780 --> 00:13:43,710 Any position in this room, I can specify in terms of three points. 129 00:13:43,710 --> 00:13:50,130 How far is it from that wall? How far is it from that wall and how far above the ground? 130 00:13:50,130 --> 00:13:55,380 And so we can all draw in our heads, an imaginary grid of lines? 131 00:13:55,380 --> 00:14:00,540 And to us, that grid is perfectly rigid. It turns out that that's not true. 132 00:14:00,540 --> 00:14:04,530 It is extremely rigid, but it is not perfectly rigid. 133 00:14:04,530 --> 00:14:14,310 And if I were to have extremely massive objects. Those are extremely massive objects could disturb the rigidity of the space. 134 00:14:14,310 --> 00:14:21,900 And so this is a movie that shows how this works. So we all know that the Earth spins around the sun. 135 00:14:21,900 --> 00:14:29,070 Jupiter spins around the sun. There are binary stars, which are two stars that are gravitationally bound that spin around each other. 136 00:14:29,070 --> 00:14:31,830 And as it turns out, as we now know, thanks to go, 137 00:14:31,830 --> 00:14:37,560 there are also binary black holes that are very massive objects and forget forget about the black holes. 138 00:14:37,560 --> 00:14:42,420 For now, just suppose I have two very massive objects that are in orbit around each other. 139 00:14:42,420 --> 00:14:44,880 The gravitational field due to that, 140 00:14:44,880 --> 00:14:52,650 pair of objects will change with time because these things are not steady and are not static in space, they're actually moving around. 141 00:14:52,650 --> 00:14:58,200 So if I were to look at the gravitational field due to these two massive objects at some distance over here, 142 00:14:58,200 --> 00:15:01,020 I would see the gravitational field changing with time. 143 00:15:01,020 --> 00:15:09,940 And those wavelength disturbances are exactly analogous to the wavelength disturbances in the ground on a very heavy truck passes by. 144 00:15:09,940 --> 00:15:15,340 So such gravitational waves were postulated by Einstein's general relativity 145 00:15:15,340 --> 00:15:22,730 simply as a consequence of his postulate that it's of that spacetime could curve. 146 00:15:22,730 --> 00:15:32,810 So this was 100 years ago, plus three years that I've thought about this, this gravitational waves propagate at the speed of light. 147 00:15:32,810 --> 00:15:41,030 They can carry energy just like electromagnetic waves, just like the wave in the ground when the truck drives by and then winds up shaking you. 148 00:15:41,030 --> 00:15:45,080 That motion has energy, so that wave has carried some energy. 149 00:15:45,080 --> 00:15:50,600 Likewise, gravitational waves can carry energy also like electromagnetic waves. 150 00:15:50,600 --> 00:15:58,010 These gravitational waves we've actually known or had indirect evidence exist as of 19 as of the early 70s. 151 00:15:58,010 --> 00:16:04,920 And actually, I think one of the core discoveries of that system is here. 152 00:16:04,920 --> 00:16:10,190 So not that system of pulsars, the discover of pulsars is here that the people did. 153 00:16:10,190 --> 00:16:14,850 This work were at Princeton, actually University of Massachusetts at the time. 154 00:16:14,850 --> 00:16:24,410 So it turns out that there are very, very massive complex stars called neutron stars, and some of them are actually in binary orbits. 155 00:16:24,410 --> 00:16:26,180 So a pair of them. 156 00:16:26,180 --> 00:16:35,970 And there was one system of two such neutron stars that were discovered, and it was seen that the orbital period was slowly increasing. 157 00:16:35,970 --> 00:16:42,990 Because this binary neutron star was emitting gravitational waves and those gravitational waves were carrying energy away, 158 00:16:42,990 --> 00:16:49,830 driving the neutron star to a higher period, sort of a longer a shorter period orbit, a higher frequency or so. 159 00:16:49,830 --> 00:16:54,460 As of nineteen ninety three, there was already a Nobel prise given for work 20 years earlier, 160 00:16:54,460 --> 00:17:01,230 actually 10 to 20 years earlier that demonstrated indirectly the existence of gravitational waves. 161 00:17:01,230 --> 00:17:07,210 But still? We had not yet detected gravitational waves directly. 162 00:17:07,210 --> 00:17:11,680 So how do you detect a gravitational wave? So I already told you it's analogous to everything else. 163 00:17:11,680 --> 00:17:16,810 We detect electromagnetic waves through the motion it induces in a test electron, 164 00:17:16,810 --> 00:17:24,010 we can detect a gravitational wave propagating through the ground through the motion of induces in the building that we're in. 165 00:17:24,010 --> 00:17:30,400 We detect a wave on the surface of a pond through the motion induces a toy boat. 166 00:17:30,400 --> 00:17:39,420 And likewise, gravitational waves can be detected through the characteristic motions that they induce in a set of test masses. 167 00:17:39,420 --> 00:17:45,060 Here is an overly dramatic representation of what might happen. 168 00:17:45,060 --> 00:17:49,500 So the Earth is held together by its own soft gravity. 169 00:17:49,500 --> 00:17:56,560 If a gravitational wave were to pass by, that would change the gravitational field that the Earth is immersed in and out, 170 00:17:56,560 --> 00:17:59,850 induce some motions and threaten the Earth. 171 00:17:59,850 --> 00:18:10,290 So this is a sort of cartoonish movie of what would happen if a gravitational wave passed by the Earth do not be alarmed. 172 00:18:10,290 --> 00:18:25,850 This effect is magnified by. A billion billion billion, more or less, a little less than a billion dollars billion. 173 00:18:25,850 --> 00:18:30,670 So well back in the early 70s, 174 00:18:30,670 --> 00:18:35,470 Ray Weiss and Kip Thorne and Ron Driver and others began to think about how you would 175 00:18:35,470 --> 00:18:41,830 actually detect a gravitational wave that was not quite so large and amplitude. 176 00:18:41,830 --> 00:18:44,800 And they came up with an idea called laser interferometry. 177 00:18:44,800 --> 00:18:51,280 And this is actually the mechanism that's used to detect these gravitational waves and for which the Nobel prise was awarded. 178 00:18:51,280 --> 00:19:01,580 So basically. What happens is you have a laser that emits a beam of light. 179 00:19:01,580 --> 00:19:09,500 This is a half silver mirror that then splits that light beam in two, so one beam goes in this direction, the other half of the beam goes over here. 180 00:19:09,500 --> 00:19:14,000 This mirror over here in this mirror then reflect the light back and then the light is combined. 181 00:19:14,000 --> 00:19:21,620 Now, if a gravitational wave propagates in a direction perpendicular to the plane of this, of these of this interferometer, 182 00:19:21,620 --> 00:19:30,380 what will happen is that this mirror will move towards move in this direction while this mirror moves away and then vice versa. 183 00:19:30,380 --> 00:19:40,590 So these will begin to shake, but the shake out of phase. And then this movie is going to show you how that phase is, how that motion is detected. 184 00:19:40,590 --> 00:19:47,520 So the light beam goes out in both directions and split when a gravitational wave passes, these things move. 185 00:19:47,520 --> 00:19:56,970 And then the last part I didn't tell you is that when the light? Combines light is a wave. 186 00:19:56,970 --> 00:20:02,040 And if that light then combines in phase. 187 00:20:02,040 --> 00:20:06,940 The beam over here will be bright, and if it's out of phase, it'll be faint. 188 00:20:06,940 --> 00:20:12,730 And what happens is when this what happens is when these things move the lights. 189 00:20:12,730 --> 00:20:21,720 Received at this detector oscillates. So this is the basic idea of the measurement. 190 00:20:21,720 --> 00:20:30,690 We take this interferometer, we send out a beam, and if these two mirrors change in distance, 191 00:20:30,690 --> 00:20:40,660 move and distance by an amount comparable to the wavelength of light, then the brightness of the signal over here will change. 192 00:20:40,660 --> 00:20:44,500 So this experiment was actually done. 193 00:20:44,500 --> 00:20:52,990 The detection was done by the local collaboration, Legault was working very closely with Virgo, which is a French and Italian government built laser. 194 00:20:52,990 --> 00:20:56,110 LIGO's an American and international American led project. 195 00:20:56,110 --> 00:21:03,640 They're also at least three other detectors that are being constructed for future measurements Geo, Tomah and Ajio. 196 00:21:03,640 --> 00:21:09,940 And there's also one being constructed in India LIGO India site. 197 00:21:09,940 --> 00:21:16,720 The signal, though, that there was initially detected was detected primarily by a legal action, exclusively biological. 198 00:21:16,720 --> 00:21:21,700 This is a US led project. There were two detectors built, not just one. 199 00:21:21,700 --> 00:21:25,600 One was built in the state of Washington and one was built in the state of Louisiana. 200 00:21:25,600 --> 00:21:29,860 And the reason is that this is such a ridiculous thing to look for, 201 00:21:29,860 --> 00:21:37,750 and the signal is so faint that if you saw such a signal with just one detector and claim to discover anything, no one would ever believe you. 202 00:21:37,750 --> 00:21:45,280 But if two detectors separated by a few thousand miles saw exactly the same gravitational wave signal exactly the same time, 203 00:21:45,280 --> 00:21:48,650 then you might believe that it was something more than just, 204 00:21:48,650 --> 00:21:57,520 you know, lightning striking in Seattle, Washington, or a heavy truck driving by one of the protective one of the two detectors. 205 00:21:57,520 --> 00:22:04,060 These detectors are four kilometres long, so here is the place where the laser is started. 206 00:22:04,060 --> 00:22:11,380 Here is one of the mirrors, and here's another one of the mirrors. So the latest sent out in this direction, this direction than recombined over here. 207 00:22:11,380 --> 00:22:18,010 This is the detector in Louisiana. And this is a picture of the detector along the Strip. 208 00:22:18,010 --> 00:22:25,650 It's four kilometres in that direction. This is the other detector in the state of Washington. 209 00:22:25,650 --> 00:22:31,680 And this is a movie provided by the logo collaboration disclaimer, I had absolutely nothing to do with this. 210 00:22:31,680 --> 00:22:36,120 I cannot be assigned any credit whatsoever for any of this. 211 00:22:36,120 --> 00:22:41,340 I like to show this because I think it's just absolutely mind boggling what they actually detected. 212 00:22:41,340 --> 00:22:46,050 So here is a video that shows what they detected. 213 00:22:46,050 --> 00:22:54,420 So this, I'm going to tell you, is an atom. That is the nucleus and this is the electron around an atom. 214 00:22:54,420 --> 00:23:05,660 And so here is what they actually detected. So there's the electron spinning around the nucleus now we zoom into the atomic nucleus. 215 00:23:05,660 --> 00:23:19,960 This is an atomic nucleus, a bunch of protons and neutrons. And the magnitude of the motion that they detect over those four kilometres is that. 216 00:23:19,960 --> 00:23:25,680 So I have to show you the slide. And a second. 217 00:23:25,680 --> 00:23:27,270 So this is not easy. 218 00:23:27,270 --> 00:23:36,420 And I have to tell you that I was at Caltech from 1999 to 2011 and my office was right down the hall way from the people working on. 219 00:23:36,420 --> 00:23:41,730 I mean, there were people working like all over the place, but a bunch of the local leadership was right down the hall for me. 220 00:23:41,730 --> 00:23:49,770 And so I'd hear about logo all the time. And any time I heard them talk about detecting gravitational waves, the following picture came into my mind. 221 00:23:49,770 --> 00:23:59,520 So this is a poster, a picture from Werner Herzog movie Fitzcarraldo, which is about a crazy guy in Brazil. 222 00:23:59,520 --> 00:24:05,880 In the eighteen hundreds who for some reason wants to take a ship over a mountain. 223 00:24:05,880 --> 00:24:09,300 And any time they talk to me, that's the first image that popped in my mind. 224 00:24:09,300 --> 00:24:16,380 And so I was completely stunned when they reported the detection reported in February 2016, 225 00:24:16,380 --> 00:24:23,190 the detection that they made in September two thousand fifteen of an actual gravitational wave signal. 226 00:24:23,190 --> 00:24:29,620 So this is a plot of the displacement. 227 00:24:29,620 --> 00:24:39,380 Of. The test masses of those mirrors as a function of time and one of these detectors in the state of Washington, 228 00:24:39,380 --> 00:24:44,840 another detection detector several thousand miles away. So this. 229 00:24:44,840 --> 00:24:52,200 How it. Since of show, yes. 230 00:24:52,200 --> 00:25:03,230 Now, that's actually not the movement to say this is what I want to show you. So this is actually a plot of the. 231 00:25:03,230 --> 00:25:09,760 Displacement of the Mirror. In one detector in the state of Washington, 232 00:25:09,760 --> 00:25:21,830 another detector several thousand miles away as a function of time on some particular time on September 14th, 2015, I think it was the evening. 233 00:25:21,830 --> 00:25:24,980 So the textures went up and down and up and down and up and down, 234 00:25:24,980 --> 00:25:30,110 up and down very more rapidly, very high amplitude, and then the signal sort of died away. 235 00:25:30,110 --> 00:25:42,100 And the same thing here is in Livingston. So one does not need to do any complicated statistical analysis to see that something is going on here. 236 00:25:42,100 --> 00:25:48,160 As it turns out, you do have to do some fairly complicated mathematical general relativity to show 237 00:25:48,160 --> 00:25:53,720 that this is exactly what you would expect from the merger of two black holes. 238 00:25:53,720 --> 00:26:00,530 So I've been saying black hole a few times, but I haven't yet told you what a black hole actually is. 239 00:26:00,530 --> 00:26:04,610 So this was another spectacular consequence of general relativity. 240 00:26:04,610 --> 00:26:10,820 I have to tell you, it's absolutely amazing that this silly little equation, Jimi, you know, equals team, you know? 241 00:26:10,820 --> 00:26:18,920 Could lead to so many novel and spectacular results and consequences. 242 00:26:18,920 --> 00:26:21,710 So this is another consequence of general relativity. 243 00:26:21,710 --> 00:26:29,260 It predicts the existence of black holes, a black hole you can think of as the densest possible object. 244 00:26:29,260 --> 00:26:38,920 So. The Earth is a massive object that is round, and it has some radius and we live on the surface of the Earth. 245 00:26:38,920 --> 00:26:48,070 And if I were to throw a ball in the air or any object this thing, it would probably go up and then come back down. 246 00:26:48,070 --> 00:26:55,580 But if I had a really good arm and I could throw a ball with a velocity greater than 11 kilometres per second. 247 00:26:55,580 --> 00:27:01,240 That ball would escape the gravitational field of the Earth and then run off to infinity. 248 00:27:01,240 --> 00:27:07,700 If I were to do this from the surface of the Sun, that escape velocity is 600 kilometres per second. 249 00:27:07,700 --> 00:27:14,420 If, however, I had an object that was so massive and so small, so dense. 250 00:27:14,420 --> 00:27:18,740 Well, sorry, I should say that the denser the object gets, the higher the escape velocity gets. 251 00:27:18,740 --> 00:27:24,050 So if I had a certain object that was so dense that the escape velocity was bigger than the speed of light. 252 00:27:24,050 --> 00:27:28,550 We know that nothing can travel faster than the speed of light. No light could ever be emitted from that object. 253 00:27:28,550 --> 00:27:33,260 In fact, nothing could ever escape from that object. So that's what a black hole is. 254 00:27:33,260 --> 00:27:40,750 It's an object that is so dense that its gravitational field is so strong that not even light can escape. 255 00:27:40,750 --> 00:27:47,950 These objects have been postulated for close to over 100 years to postulates, exist for over a hundred years. 256 00:27:47,950 --> 00:27:55,480 There have been pretty there's been pretty good evidence for the existence of astrophysical black holes for several decades. 257 00:27:55,480 --> 00:28:02,580 But no direct evidence. But it turns out that if you have a black hole. 258 00:28:02,580 --> 00:28:07,290 It's conceivable you might have pairs of black holes, I told you that many stars are in binaries. 259 00:28:07,290 --> 00:28:13,020 Our star is actually pretty unusual in that it has no gravitationally bound companion. 260 00:28:13,020 --> 00:28:19,560 Most stars are gravitationally and systems are binary systems that are gravitationally bound. 261 00:28:19,560 --> 00:28:23,010 And so it's conceivable that if black holes exist, they might also have. 262 00:28:23,010 --> 00:28:24,960 There might be black hole binaries. 263 00:28:24,960 --> 00:28:32,850 So here is another movie similar to the one that I showed you before of the gravitational field of two black holes. 264 00:28:32,850 --> 00:28:45,230 Let me see. So as this gravitational as these black holes spin around, they emit gravitational waves, those gravitational waves carry away energy. 265 00:28:45,230 --> 00:28:50,540 And as they carry away energy, the pair of binary black holes becomes more tightly bound, 266 00:28:50,540 --> 00:28:56,270 the distance becomes between them becomes smaller and the orbital frequency increases. 267 00:28:56,270 --> 00:29:02,750 They start to spin around each other more rapidly and the emission of gravitational waves becomes more intense. 268 00:29:02,750 --> 00:29:08,570 So I'll show that to you again. So you'll see that early times. 269 00:29:08,570 --> 00:29:18,650 The gravitational wave signal is fairly weak, but as these things spiral in and emit more energy and the orbital frequency increases, 270 00:29:18,650 --> 00:29:22,970 the intensity of the gravitational wave signal becomes greater. 271 00:29:22,970 --> 00:29:27,440 So it becomes brighter in gravitational waves and the frequency increases. 272 00:29:27,440 --> 00:29:31,670 So this is a calculation that you can do given me a new equals team, you know? 273 00:29:31,670 --> 00:29:44,040 And I have to say a lot of computational resources and the result of such a calculation is a gravitational wave signal that looks exactly like this. 274 00:29:44,040 --> 00:29:49,710 Interestingly enough, Lego had two ways to look for gravitational waves, 275 00:29:49,710 --> 00:29:55,590 and the simple way was just to wait for something to go bump in the night, and that's what happened. 276 00:29:55,590 --> 00:29:59,760 In this case, they didn't do any general relativity, they didn't do any black hole modelling. 277 00:29:59,760 --> 00:30:02,460 They just noticed that something happened. 278 00:30:02,460 --> 00:30:09,000 And then afterwards they went back to those gravitational wave templates that were obtained from mathematical relativity, 279 00:30:09,000 --> 00:30:14,040 and they found that the signal they saw agreed precisely with black holes. 280 00:30:14,040 --> 00:30:17,760 They were as stunned as the rest of the world, and they spent a long, 281 00:30:17,760 --> 00:30:25,500 long time all thousand members of the collaboration trying to figure out anything else that could mimic such a signal. 282 00:30:25,500 --> 00:30:28,320 And it turns out that this is a bona fide, 283 00:30:28,320 --> 00:30:38,100 genuine binary black hole merger the first direct evidence we've ever obtained for the existence of black holes and spectacular evidence of that. 284 00:30:38,100 --> 00:30:47,130 From the details of that gravitational wave signal, they were able to infer that the first black hole was thirty six times as much of the sun. 285 00:30:47,130 --> 00:30:55,770 The second one made twenty nine times as much as the Sun. The final black hole where it sixty two times the mass of the Sun. 286 00:30:55,770 --> 00:31:05,130 I'm assuming you can all add thirty six and twenty nine. Which turns out to be 65 three solar masses. 287 00:31:05,130 --> 00:31:14,520 Was released as gravitational wave energy. You remember another famous Einstein equation is e equals m c squared. 288 00:31:14,520 --> 00:31:20,100 An object of a given mass has a certain amount of energy. There is equivalent equivalence between mass and energy. 289 00:31:20,100 --> 00:31:29,040 So approximately five percent of the rest mass energy of the system was emitted in the form of gravitational waves. 290 00:31:29,040 --> 00:31:35,100 And they also were able to infer that this thing was about a billion light years away. 291 00:31:35,100 --> 00:31:42,150 This. Has no relevance to the talks. No direct relevance, but it's a great picture. 292 00:31:42,150 --> 00:31:50,910 This is actually a bona fide simulation of what the galaxy would look like. 293 00:31:50,910 --> 00:31:54,750 When we look out in the night sky, we see a bunch of stars from the southern hemisphere. 294 00:31:54,750 --> 00:31:56,100 We see a Milky Way. 295 00:31:56,100 --> 00:32:07,110 This is a bona fide simulation of what a night sky would look like if there was a pair of binary black holes pretty close by in the foreground. 296 00:32:07,110 --> 00:32:10,110 And actually, it's not a picture, it's a movie, 297 00:32:10,110 --> 00:32:25,770 and this is what that night sky would look like if you were fairly close to the binary black hole system near merger. 298 00:32:25,770 --> 00:32:31,000 So there you have it. And. 299 00:32:31,000 --> 00:32:42,510 The merger is about to happen. Uh, there it is. 300 00:32:42,510 --> 00:32:45,360 And you still see some residual emotions over here. 301 00:32:45,360 --> 00:32:54,660 Actually, the thing that surprised me when I first saw this movie is how quickly it once it emerges, it just turns into a spherical black hole. 302 00:32:54,660 --> 00:33:07,000 That ringtone is very, very quick. So everything I've told you until now is spectacular science, and now comes the lunacy. 303 00:33:07,000 --> 00:33:12,190 So I should say I wrote this paper three years ago. A lot of people paid attention. 304 00:33:12,190 --> 00:33:18,760 And ever since then, when I go to conferences and Rundle colleagues, I say, Do you really think primordial black holes are the dark matter? 305 00:33:18,760 --> 00:33:23,680 Really believe? So where do these black holes come from? 306 00:33:23,680 --> 00:33:32,740 So here is the most likely answer with very, very great certainty, although not absolute certainty. 307 00:33:32,740 --> 00:33:44,130 These black holes, or at least the majority of the now 15 binary black hole systems that they've observed, probably are the remnants of stars. 308 00:33:44,130 --> 00:33:48,600 So The Sun is a star. It's a very or fairly ordinary star. 309 00:33:48,600 --> 00:33:52,140 And over the past hundred years, especially over the past 70, 310 00:33:52,140 --> 00:33:59,340 I would say we've learnt a lot about how stars evolve and stars can be low mass and stars can be high mass. 311 00:33:59,340 --> 00:34:04,280 And roughly speaking, this is a picture that you would find in the astronomy class. 312 00:34:04,280 --> 00:34:11,330 Roughly speaking, small stars evolve and after they use up all the nuclear fuel that powers them, 313 00:34:11,330 --> 00:34:16,010 you know, the light from the Sun comes from nuclear reactions of this at the centre of the Sun. 314 00:34:16,010 --> 00:34:19,970 So star sign because they are burning nuclear fuel. 315 00:34:19,970 --> 00:34:22,280 When that nuclear fuel runs out, though, 316 00:34:22,280 --> 00:34:28,520 they can't support themselves against gravitational collapse and the lower mass stars turn into something called the white dwarf, 317 00:34:28,520 --> 00:34:32,960 which is pretty interesting, but not quite as interesting as what happens to huge stars. 318 00:34:32,960 --> 00:34:40,130 Those evolved to neutron stars if they're pretty massive and if the really massive they evolve to black holes. 319 00:34:40,130 --> 00:34:44,090 So we actually expect there to be a whole bunch of black holes out there in the 320 00:34:44,090 --> 00:34:49,500 universe because we see a whole bunch of stars in those stars don't live forever. 321 00:34:49,500 --> 00:34:56,340 So that is probably what these black holes are still stars like the Sun, 322 00:34:56,340 --> 00:35:03,210 the Sun weighs one solar mass by definition, and there are heavier stars out there, the stars. 323 00:35:03,210 --> 00:35:06,750 When you see stars, there are about 100 up to 100 solar masses. 324 00:35:06,750 --> 00:35:14,250 But the abundance of the more massive stars is hugely suppressed compared to the abundance of the lower mass stars. 325 00:35:14,250 --> 00:35:21,390 And so if you were to tell me, well, wow, we're going to see gravitational wave signal from a binary black hole merger, 326 00:35:21,390 --> 00:35:29,440 if you told me that back three and a half years ago, I would have said, Well, I'm guessing there's going to be five to 10 solar masses. 327 00:35:29,440 --> 00:35:34,120 But it wasn't. Both of these black holes were nearly 30 times the mass of the Sun. 328 00:35:34,120 --> 00:35:38,470 And so this was at the time, a little bit surprising. 329 00:35:38,470 --> 00:35:46,810 Why is it that the fees are still the remnants? There'll be so much heavier than a typical stellar mass. 330 00:35:46,810 --> 00:35:54,760 So I'm not a stellar astrophysicist. I'm a cosmologist, and much of my time is spent thinking about the dark matter, 331 00:35:54,760 --> 00:36:01,080 and I was sitting around the lunch table with a bunch of my post-docs and students. And they also think about dark matter. 332 00:36:01,080 --> 00:36:08,130 And so we start to wonder whether these black holes they saw might have something to do with the dark matter in the universe. 333 00:36:08,130 --> 00:36:13,060 So here's my rough, my quick primer on dark matter. 334 00:36:13,060 --> 00:36:21,790 We live in a galaxy called the Milky Way, and we can't actually take a picture of the galaxy that looks like this because we live in the middle of it. 335 00:36:21,790 --> 00:36:27,370 But there are plenty of other galaxies, like our own spiral galaxies to look sort of like this some blob of light. 336 00:36:27,370 --> 00:36:37,060 And these things are the light comes from stars. And the galaxy is a gravitationally bound system of, say, 10 billion stars. 337 00:36:37,060 --> 00:36:44,620 And the galaxy only extends out to about, you know, some 10000 light years in distance. 338 00:36:44,620 --> 00:36:53,530 But there are still satellites of this galaxy. There can be gas clouds or dwarf galaxies that orbit around this galaxy. 339 00:36:53,530 --> 00:37:00,040 And we can measure the velocities at which these satellites spin around these galaxies. 340 00:37:00,040 --> 00:37:05,860 And if Newton's laws were the entire story to gravity or general relativity was all there was to gravity. 341 00:37:05,860 --> 00:37:09,820 And if there was nothing in this galaxy beyond the stars that we see, 342 00:37:09,820 --> 00:37:18,090 we would expect the velocities at which satellite to spin to decrease that large radii for the same reason the Pluto. 343 00:37:18,090 --> 00:37:27,480 Which is not a planet anymore, spins around the sun far more slowly than does the Earth, and Mercury spins around the sun much more quickly. 344 00:37:27,480 --> 00:37:32,820 So things that are further away from the galaxy should spin around with a smaller velocity. 345 00:37:32,820 --> 00:37:35,580 It turns out, though, that's not the case. 346 00:37:35,580 --> 00:37:44,460 And this discovery in this form was made by Vera Rubin, who is an astrophysicist who made this discovery in the early 1970s and others. 347 00:37:44,460 --> 00:37:46,800 And I also, since their astrophysicist here, 348 00:37:46,800 --> 00:37:57,250 I should advertise there is going to be a workshop to honour Vera Rubin and her science at Georgetown University next month. 349 00:37:57,250 --> 00:38:02,050 If anybody's interested to attend, so this when it was discovered is a huge mystery, 350 00:38:02,050 --> 00:38:08,020 why is it that these things at large distance are spinning around so much more rapidly than they should? 351 00:38:08,020 --> 00:38:13,420 If this is all the matter in the galaxy and the inference that we make from this measurement, 352 00:38:13,420 --> 00:38:19,060 as well as from a huge array of other astronomical and cosmological observations, 353 00:38:19,060 --> 00:38:28,980 is that this galaxy and all galaxies, as far as we can tell, are immersed in a halo of some mysterious dark matter. 354 00:38:28,980 --> 00:38:36,670 And we have absolutely no idea what the stuff is. So all I can tell you about dark matter with certainty. 355 00:38:36,670 --> 00:38:44,500 Is we know how it is distributed. We know how to distribute in galaxies through detailed measurements like this. 356 00:38:44,500 --> 00:38:49,990 We know how it's distributed throughout the universe today. We know very well how it's distributed in the early universe. 357 00:38:49,990 --> 00:38:58,300 Its existence is not a question. It is there and we know very much where it is and where it isn't. 358 00:38:58,300 --> 00:39:02,860 It is dark. So by this, I mean, it emits no light. 359 00:39:02,860 --> 00:39:05,530 And it also absorbs no light. 360 00:39:05,530 --> 00:39:13,270 There's actually been no evidence for dark matter, apart from its gravitational effects on the stuff that we see for many years, 361 00:39:13,270 --> 00:39:20,090 and I would say they still are leading candidates involve new elementary particles. 362 00:39:20,090 --> 00:39:24,770 And 10 years ago, if I was here giving this talk, I would have been telling you that's going to be weakly interacting massive particle. 363 00:39:24,770 --> 00:39:29,540 It's got to be weakly interacting metaphorical. There's nothing else it can be. It's got to be a new elementary particle. 364 00:39:29,540 --> 00:39:38,450 And next week, you're going to hear a talk by one of the leading figures in the detection or attempts to detect particle dark matter. 365 00:39:38,450 --> 00:39:46,490 But we don't know that it's an elementary particle. We still have no empirical evidence to support this hypothesis, although we can't disprove it. 366 00:39:46,490 --> 00:39:48,710 We've been looking for a number of years and 10 years ago. 367 00:39:48,710 --> 00:39:56,120 When I gave this talk, I would have said as soon as the Large Hadron Collider turns on it, CERN, we are going to see this new elementary particle. 368 00:39:56,120 --> 00:40:03,560 Well, the Large Hadron Collider turned on about eight, seven or eight years ago, and they have not yet found anything. 369 00:40:03,560 --> 00:40:11,900 As you know, absence of evidence is not evidence of absence. However, it's starting to get disappointing that they haven't found anything yet. 370 00:40:11,900 --> 00:40:18,650 And so theorists over the past few years have started to wonder whether we should be thinking of alternative hypotheses. 371 00:40:18,650 --> 00:40:24,470 And that led us to consider the possibility that the black holes that likely detected are the dark matter. 372 00:40:24,470 --> 00:40:31,010 And in fact, Stephen Hawking wrote a paper in 1974 showing that the early universe, 373 00:40:31,010 --> 00:40:36,470 the Big Bang, could have plausibly produced some primordial black holes. 374 00:40:36,470 --> 00:40:40,700 Here are my collaborators. They are an assorted set of post-docs and students. 375 00:40:40,700 --> 00:40:47,300 And Adam Riess, who is a colleague at Johns Hopkins. So here's the question. 376 00:40:47,300 --> 00:40:48,980 We wrote this paper, 377 00:40:48,980 --> 00:40:56,180 we sat around for a while wondering whether we should actually post it online because it seemed kind of ridiculous and speculative, 378 00:40:56,180 --> 00:41:01,120 but not completely ridiculous. And there's a reason why we actually wrote the paper. 379 00:41:01,120 --> 00:41:08,740 It's easy enough to sit around the table and say, hey, maybe it's dark matter, but then we actually did a calculation, which is what we're paid to do. 380 00:41:08,740 --> 00:41:20,590 And we found a very, very surprising result. So we know from the laws of gravity that if we have two black holes that pass by each other, 381 00:41:20,590 --> 00:41:25,450 there's some chance that they will merge and form a, but they will form a binary and then merge. 382 00:41:25,450 --> 00:41:30,250 That's a straightforward calculation and straightforward to calculate how probable it is, 383 00:41:30,250 --> 00:41:35,020 the two black holes that pass by each other are going to form a binary merge. 384 00:41:35,020 --> 00:41:40,810 We also, thanks to the efforts of cosmologists over the past few decades, 385 00:41:40,810 --> 00:41:45,700 have pretty good ideas about how the dark matter on the galactic halo is distributed. 386 00:41:45,700 --> 00:41:53,440 So this is a picture not of a galaxy, but this is a picture of the distribution of dark matter within a galaxy. 387 00:41:53,440 --> 00:41:56,280 The what we believe to be the distribution of dark matter in the galaxy, 388 00:41:56,280 --> 00:42:01,650 given cosmological simulations and there are there's more dark matter in the son of the galaxy, 389 00:42:01,650 --> 00:42:07,560 less dark matter away from the centre of the galaxy, and the dark matter is also distributed in sub clumps. 390 00:42:07,560 --> 00:42:14,940 And so what we did is we took this distribution of dark matter. We passed with all the dark matter is made up of 30 solar mass black holes, 391 00:42:14,940 --> 00:42:18,390 and then we calculated the rate at which these black holes would merge and give you a 392 00:42:18,390 --> 00:42:23,340 gravitational wave signals if all the dark matter is made of 30 solar mass black holes. 393 00:42:23,340 --> 00:42:25,260 And when we did that calculation, 394 00:42:25,260 --> 00:42:34,800 we found this incredible coincidence that that number was done on the gravitational of the binary merger rate inferred from light. 395 00:42:34,800 --> 00:42:45,370 So that's why we wrote this paper to point out this very interesting coincidence. Since then, this paper has received a lot of attention. 396 00:42:45,370 --> 00:42:51,790 We've had people help us out, a number of theorists have shown said, Oh yeah, primordial black holes. 397 00:42:51,790 --> 00:42:54,260 It's very easy to produce primordial black holes in the early universe. 398 00:42:54,260 --> 00:42:58,630 In fact, 30 solar masses is just what we expect and just with the right abundance. 399 00:42:58,630 --> 00:43:05,290 This is a collection of words. You are not expected to understand the meanings of any of this meaning of any of these words. 400 00:43:05,290 --> 00:43:10,570 I do not understand the meaning of any of these words. Some of them, I understand, but not all of them. 401 00:43:10,570 --> 00:43:16,240 The point is, some theoretical physicists believe that this is actually a plausible. 402 00:43:16,240 --> 00:43:18,640 Candidate for the dark matter. 403 00:43:18,640 --> 00:43:26,740 Since then, though, a number of astrophysicists have ruled out of written paper saying that this is a ridiculous idea for dark matter. 404 00:43:26,740 --> 00:43:32,740 So some people show that using measurements of the cosmic microwave background you can rule this out is a dark matter candidate. 405 00:43:32,740 --> 00:43:37,630 Dwarf galaxy dynamics quasar lending x rays from galactic black hole supernova stars as pulsar timing. 406 00:43:37,630 --> 00:43:42,250 The list goes on and on. All these papers are really nice. 407 00:43:42,250 --> 00:43:45,970 The ideas are great. I think each one of them comes with a caveat, 408 00:43:45,970 --> 00:43:52,690 and I believe that there's no paper that I can point to that says that tells me I have to stop thinking about this idea. 409 00:43:52,690 --> 00:43:58,320 But all of them are very interesting astrophysics and could lead to novel ways to detect. 410 00:43:58,320 --> 00:44:07,410 Or test this idea, the primordial black holes are the dark matter. So I'm going to tell you briefly about one idea that my colleagues came up with to 411 00:44:07,410 --> 00:44:11,520 try to test the idea of whether these primordial black holes are the dark matter. 412 00:44:11,520 --> 00:44:22,300 So as you know, it's the job of theorists to propose theories and come up with hypotheses for answers to questions. 413 00:44:22,300 --> 00:44:26,620 But we're also supposed to provide mechanisms to test those hypotheses. 414 00:44:26,620 --> 00:44:32,440 And ultimately, the hypothesis the primordial black holes are the dark matter is not going to be determined by a bunch 415 00:44:32,440 --> 00:44:37,810 of theorists sitting around a room arguing it's going to be determined by experiments and observations. 416 00:44:37,810 --> 00:44:46,840 So one such observation experiment that we suggested you could do involves these very intriguing objects called fast radio bursts. 417 00:44:46,840 --> 00:44:56,800 So these fast radio bursts were detected roughly 20 years ago. Over the past few decades, about 20 something of them have been detected. 418 00:44:56,800 --> 00:45:03,010 Just last year, a new Canadian telescope that was designed in part to detect them has discovered about 419 00:45:03,010 --> 00:45:08,090 20 more of these and is expected to discover thousands more within the next few years. 420 00:45:08,090 --> 00:45:16,760 And the basic idea is that a fast radio burst is what it sounds like, it's a radio burst that is very, very fast. 421 00:45:16,760 --> 00:45:19,970 So you have a radio telescope. People are pointing radio telescopes in the sky, 422 00:45:19,970 --> 00:45:25,700 and they notice that there were just blips of radio frequency radiation coming from somewhere in the sky, 423 00:45:25,700 --> 00:45:33,430 and those blips lasted less than a millisecond. We do not know where these things come from. 424 00:45:33,430 --> 00:45:37,780 Plenty of theoretical astrophysics around the world have various ideas and hypotheses, 425 00:45:37,780 --> 00:45:42,670 but we do not know where they come from, but there's pretty good evidence that they come from far away. 426 00:45:42,670 --> 00:45:50,650 They come from other galaxies at cosmological distances. So if primordial black holes were the dark matter, 427 00:45:50,650 --> 00:45:56,650 there's some chance that the light from one of these fast radio bursts could pass near 428 00:45:56,650 --> 00:46:01,390 the trajectory of the light from one of these fast radio bursts as it travels to us. 429 00:46:01,390 --> 00:46:11,940 This is a telescope. It's not a peace sign. So if the introductory passes by a black hole, there's a chance that it could be gravitationally lensed. 430 00:46:11,940 --> 00:46:17,670 Remember, I told you about Ellington's experiment, where the light trajectory from a target gets bent, 431 00:46:17,670 --> 00:46:21,000 so it's introductory of light from a star can be burnt. 432 00:46:21,000 --> 00:46:27,180 If that trajectory passes by, a very massive object might get burnt in this direction and might actually get burnt in this direction. 433 00:46:27,180 --> 00:46:33,930 So if there's a black hole along the line of sight to a given fast radio burst, 434 00:46:33,930 --> 00:46:39,060 we might actually see not just one, but two images of the fast radio burst. 435 00:46:39,060 --> 00:46:44,430 It turns out that those two images are separated by an angular scale that's way too small to ever be resolved. 436 00:46:44,430 --> 00:46:52,320 But it turns out that the path length along this trajectory differs most generally from the path length along this trajectory. 437 00:46:52,320 --> 00:47:01,560 And so what you would see in this case is the fast radio burst from one trajectory, followed by a fainter echo from the other trajectory. 438 00:47:01,560 --> 00:47:08,220 And what we showed is that if if 30 solar mass primordial black holes are the dark matter. 439 00:47:08,220 --> 00:47:13,470 After this Canadian telescope has been observing for several years and discovered several thousand of these things, 440 00:47:13,470 --> 00:47:19,380 about 10 of them should exhibit this echo. So this is a straightforward test that is going to be carried out. 441 00:47:19,380 --> 00:47:22,170 I don't know what the answer is going to be, but within a few years, 442 00:47:22,170 --> 00:47:29,040 we'll be able to tell definitively whether dark, primordial black holes are the dark matter or not. 443 00:47:29,040 --> 00:47:34,590 Now, I should say that about a year ago. 444 00:47:34,590 --> 00:47:40,740 My collaborators and I did a calculation, so we've been thinking really hard about how to test the scenario, 445 00:47:40,740 --> 00:47:44,370 how to test the scenario, what can we do to figure out whether this is a real? 446 00:47:44,370 --> 00:47:50,910 This is really the dark matter or just a figment of our imagination. And we came across. 447 00:47:50,910 --> 00:47:57,780 Something that has sort of dampened my optimism about the scenario. 448 00:47:57,780 --> 00:48:04,080 So this is work that was done with the post up Yasin Ali, who is now on the faculty at NYU, 449 00:48:04,080 --> 00:48:09,240 and another postdoc, Eli Kovac's, is now on the faculty, Ben-Gurion University. 450 00:48:09,240 --> 00:48:13,770 So if primordial black holes are the dark matter and they're distributed throughout the universe, 451 00:48:13,770 --> 00:48:19,770 they will have would have been distributed randomly throughout the early universe. 452 00:48:19,770 --> 00:48:25,700 And if I have a bunch of black holes that are distributed randomly, there will be some places where I have two of them. 453 00:48:25,700 --> 00:48:33,800 There's just happen to be close by by chance. So if I have two black holes that are close by. 454 00:48:33,800 --> 00:48:39,390 Those black holes will form a binary that will form a gravitationally bound system. 455 00:48:39,390 --> 00:48:49,590 And so it is straightforward to calculate the fraction of primordial black holes that will actually be in primordial, gravitationally bound binaries. 456 00:48:49,590 --> 00:48:55,460 So we did this calculation. That's what we do for theoretical astrophysics, we calculate. 457 00:48:55,460 --> 00:49:01,820 And it turns out that only a very tiny fraction of the primordial black holes make up are to be found in binaries. 458 00:49:01,820 --> 00:49:07,640 But there are actually quite a few of them in terms of total numbers and binaries. 459 00:49:07,640 --> 00:49:12,960 If they survive, long enough, will merge and produce gravitational wave signals. 460 00:49:12,960 --> 00:49:20,490 And we did the calculation of how many gravitational wave signals we would have expected to see with Lego from these primordial miners. 461 00:49:20,490 --> 00:49:25,840 And it turns out to be a hundred times as many as Lego seen. 462 00:49:25,840 --> 00:49:28,570 So unless there's something that we're missing in the calculation, 463 00:49:28,570 --> 00:49:34,910 unless there's some unforeseen mechanism that disrupts these binaries after they're formed. 464 00:49:34,910 --> 00:49:40,040 My current belief is that this is a challenge, so challenging scenario to make work. 465 00:49:40,040 --> 00:49:44,690 My collaborator, Yacine, believes that there might be mechanisms that might disrupt this binary, so he doesn't. 466 00:49:44,690 --> 00:49:48,500 He knows he will not allow me to say that the scenario is ruled out. 467 00:49:48,500 --> 00:49:58,400 But the way I would put it is that when we wrote our first paper three years ago, this was a scenario that was innocent until proven guilty. 468 00:49:58,400 --> 00:50:05,510 And now I would say that this hypothesis is guilty until proven innocent. 469 00:50:05,510 --> 00:50:13,280 So one final update the first miner black hole was discovered on September 14th, 2015. 470 00:50:13,280 --> 00:50:21,370 It was announced to the world in February 2016. Legault then had a long run. 471 00:50:21,370 --> 00:50:24,100 And discovered several more black hole binaries. 472 00:50:24,100 --> 00:50:32,440 They've been shut down for about a year and they just turned back on started observing again a few weeks ago. 473 00:50:32,440 --> 00:50:37,990 And within the past few weeks, they've already detected like three or four binary black hole signals. 474 00:50:37,990 --> 00:50:43,990 So as of now, there are about 15 more black hole mergers. 475 00:50:43,990 --> 00:50:50,800 And if any of the legal people were here, if Ray Weiss were here, he would say, you should exhibit caution, don't over interpret the results. 476 00:50:50,800 --> 00:50:55,810 We don't understand all the systematics, we don't understand the distributions. 477 00:50:55,810 --> 00:51:03,680 But if you actually look. The masses of the black holes that they've discovered have a whole range of masses, 478 00:51:03,680 --> 00:51:11,820 but there actually seems to be a preponderance of black holes that have masses that are close to 30 solar masses. 479 00:51:11,820 --> 00:51:17,820 So I don't say that this is evidence the primordial black hole binaries are primordial, black holes are the dark matter. 480 00:51:17,820 --> 00:51:23,850 What I would say is that if we had not, if we had seen a bunch more binary black hole mergers and none of them, 481 00:51:23,850 --> 00:51:30,180 none of the rest of them had 30 solar masses, then I would say the scenario is highly unlikely. 482 00:51:30,180 --> 00:51:36,750 I don't want to say that this proves this scenario, but at least the scenario has not yet been killed. 483 00:51:36,750 --> 00:51:43,080 And it's an intriguing result regardless of whether the primordial black holes or stellar remnant black holes, 484 00:51:43,080 --> 00:51:45,540 because we really don't know where these things come from. 485 00:51:45,540 --> 00:51:54,100 And the fact that they seem to be more massive than what we would have guessed five to 10 ish or broad mass distribution, I think, is intriguing. 486 00:51:54,100 --> 00:52:02,870 So. I'd like to conclude. So gravitational waves were discovered that a spectacular science, 487 00:52:02,870 --> 00:52:08,660 we're doing all kinds of interesting things with these observations and measurements. And there's going to be plenty more to come. 488 00:52:08,660 --> 00:52:12,500 I'm curious about the dark matter. I'd like to know what it is. I don't know what it is. 489 00:52:12,500 --> 00:52:22,880 I have not made seminal contributions. I appreciate the compliment, but we're not in the winner's circle yet. 490 00:52:22,880 --> 00:52:26,750 So we've detected gravitational waves directly as of three years ago. 491 00:52:26,750 --> 00:52:31,820 We've seen several more. These are very bona fide astrophysical black hole signals. 492 00:52:31,820 --> 00:52:38,930 I forgot to mention you probably also saw also saw last month this image from the Event Horizon telescope of the of a black hole. 493 00:52:38,930 --> 00:52:43,610 The actual picture, the first picture of a black hole, which was pretty spectacular. 494 00:52:43,610 --> 00:52:51,580 As I said, these are most likely related to the endpoint of stellar evolution, but the details we really don't know a whole lot about. 495 00:52:51,580 --> 00:52:56,890 The jury's still out as to whether the primordial black holes can make up the dark matter, 496 00:52:56,890 --> 00:53:02,620 we should still be pursuing other dark matter candidates like you'll hear about next week from Elena Asprilla. 497 00:53:02,620 --> 00:53:10,690 And this is always a true statement at the end of any science talk, we will learn more with forthcoming observations and experiments. 498 00:53:10,690 --> 00:53:19,850 And I thank you very much for your interest and your attention.