1 00:00:16,990 --> 00:00:25,030 It's a real pleasure to introduce today's speaker, Professor Elliott Dart, 2 00:00:25,450 --> 00:00:34,090 who holds the Thomas and Alison Schneider chair in the physics department at the University of California at Berkeley. 3 00:00:35,020 --> 00:00:43,060 I have known Elliott personally for many years and watched him grow from a student to, 4 00:00:43,360 --> 00:00:48,480 in my view, one of the world's leading theoretical astrophysicists. 5 00:00:48,760 --> 00:00:58,430 And it's a real pleasure to have him here for this lecture. His interests, I have to I have to have an index card for his interests alone. 6 00:00:58,450 --> 00:01:01,660 You know, usually I put the career on, but this is just his interest. 7 00:01:01,690 --> 00:01:11,560 He's on his pay web page. He lists black holes, stellar physics, plasma astrophysics, cosmology and galaxy formation. 8 00:01:11,710 --> 00:01:16,840 And he's made seminal contributions in all of these areas. 9 00:01:17,440 --> 00:01:23,679 So Elliott did his Ph.D. in 1999 at Harvard University with Professor Ramesh Narayan 10 00:01:23,680 --> 00:01:28,930 and then a two year postdoc at the Institute for Advanced Study in Princeton. 11 00:01:29,440 --> 00:01:39,830 And he was quickly snapped up at the University of California at Berkeley and rose rapidly through the ranks. 12 00:01:40,270 --> 00:01:45,340 And at the moment, in addition to his position in the physics department, 13 00:01:45,730 --> 00:01:54,970 he is also the head of the Theoretical Astrophysics Centre in the Astronomy Department and has led it to become, 14 00:01:55,420 --> 00:02:00,490 I would say, one of the most important schools of theoretical astrophysics in the world. 15 00:02:00,910 --> 00:02:03,430 So it's a real pleasure once again to have him here. 16 00:02:03,580 --> 00:02:13,290 And he is going to tell us about how the universe evolved from smooth to that looks like pretty small scale lumpy going. 17 00:02:14,140 --> 00:02:23,650 Thanks, Steve. So thanks very much. 18 00:02:23,660 --> 00:02:34,820 It's a real pleasure to be here. So what I'd like to do today is give you an overview of, I think, what one of the. 19 00:02:36,050 --> 00:02:44,090 Yeah, right. One of the key astronomical parts of our understanding of our origins. 20 00:02:44,540 --> 00:02:52,609 It's one of, I think the great contributions to of astronomy to the broader culture is the impact 21 00:02:52,610 --> 00:02:57,679 that the field has had on understanding our place in the universe and in particular, 22 00:02:57,680 --> 00:03:06,950 our origins. And I'm going to do that by first giving you some context about our understanding of the history of the universe, 23 00:03:07,280 --> 00:03:18,349 and then focusing in on some of the cutting edge areas where we're still struggling to understand how we went from the smooth beginnings that we 24 00:03:18,350 --> 00:03:30,080 see in the cosmic microwave background to the very rich and complicated population of galaxies and stars and black holes that we see in the sky. 25 00:03:31,380 --> 00:03:38,640 And so I want to start just by telling you a little bit about some of the objects that we see, since I know there is a diverse background. 26 00:03:38,640 --> 00:03:41,930 So this is actually a picture of our own galaxy. 27 00:03:41,940 --> 00:03:47,730 This is a picture of the Milky Way galaxy taken by satellite above the atmosphere of the earth. 28 00:03:48,210 --> 00:03:54,000 To give you a sense of scale, our galaxy is about 100,000 light years across. 29 00:03:54,600 --> 00:04:03,960 That's something like 10 million times bigger than the distance between the Sun and Pluto or Neptune, roughly the size of the solar system. 30 00:04:05,550 --> 00:04:14,910 The galaxy contains something like 100 billion stars, a total mass of a trillion times the mass of the sun. 31 00:04:15,210 --> 00:04:20,640 And most of the mass in our galaxy is actually not in anything that you can see in this picture, 32 00:04:21,000 --> 00:04:28,620 but it's in the form of dark matter or some kind of other fundamental particle not gas, not stars. 33 00:04:29,010 --> 00:04:36,390 And dark matter plays a very important role in the story of the evolution of the history of the universe. 34 00:04:38,220 --> 00:04:43,650 This is our kind of nearest big neighbour, the Andromeda Galaxy. 35 00:04:43,980 --> 00:04:49,680 That's about 3 million light years away, actually about 4 billion years from now. 36 00:04:49,680 --> 00:04:55,470 Our Galaxy and Andromeda are going to slam into each other by that point. 37 00:04:55,530 --> 00:05:04,110 That really won't matter much to the Earth because the Earth will have already boiled away as the sun expands to become a red giant late in its life. 38 00:05:05,550 --> 00:05:12,240 But this fact that there are two galaxies near each other in this case that will collide in the age of the universe. 39 00:05:12,240 --> 00:05:14,280 That's actually not that uncommon. 40 00:05:14,610 --> 00:05:24,210 That's a kind of an insight into how structure forms in the universe that you have galaxies orbiting around each other, 41 00:05:24,420 --> 00:05:28,680 just like planets orbit around stars and stars orbit around each other. 42 00:05:29,270 --> 00:05:35,760 In fact, galaxies are very often found in collections held together by gravity, 43 00:05:36,000 --> 00:05:47,070 such as this example of a cluster of galaxies where you have many different galaxies all orbiting around the gravity of their mutual dark matter. 44 00:05:52,170 --> 00:05:59,760 This picture is one of the most famous pictures in astronomy of galaxies in the distant universe. 45 00:05:59,790 --> 00:06:04,440 Actually, the famous part is right there. This just gives you a sense of scale. 46 00:06:04,890 --> 00:06:10,140 So this is an image that I'll show on the next slide is an image taken with the Hubble Space Telescope. 47 00:06:10,560 --> 00:06:18,480 It'll zoom in to this little region here, which for scale is small compared to the moon on the sky. 48 00:06:18,720 --> 00:06:25,680 And in that tiny little patch of sky, when we stare for a very long time with the Hubble Space Telescope, 49 00:06:26,010 --> 00:06:31,500 we see something like 10,000 galaxies in this image here. 50 00:06:32,010 --> 00:06:44,610 And then this region zooms in yet further to show in more detail some of the particular galaxies observed in this one little patch of sky. 51 00:06:45,420 --> 00:06:56,040 And because these the Hubble Space Telescope is showing us galaxies that are very faint, those are galaxies that are very far away. 52 00:06:56,220 --> 00:06:59,790 And it's taken light a very long time to travel to us. 53 00:07:00,090 --> 00:07:10,980 And so we're actually seeing the galaxies as they were something like 12 billion years ago when the universe was much younger than it is today. 54 00:07:11,490 --> 00:07:17,420 And that's, of course, one of the great wonders of astronomy that's sitting here on Earth. 55 00:07:17,430 --> 00:07:24,090 We can use the fact that it takes light to travel a certain amount of time to reach us, 56 00:07:24,420 --> 00:07:28,350 to figure out the entire history of the universe without going anywhere. 57 00:07:29,490 --> 00:07:38,729 And that was really started pioneered by Hubble in the early 1920s by observing distant 58 00:07:38,730 --> 00:07:44,700 galaxies and measuring how they're moving relative to our own Milky Way galaxy. 59 00:07:45,210 --> 00:07:55,230 And what Hubble discovered was the expansion of the universe, the fact that if we sit on our galaxy here looking up at the sky, 60 00:07:55,530 --> 00:08:00,660 we measure all the galaxies around us seeming to move away from us. 61 00:08:01,110 --> 00:08:04,450 And that's not something special about our galaxy. 62 00:08:04,470 --> 00:08:06,240 Everything's not running away from us. 63 00:08:06,600 --> 00:08:17,159 In fact, we think this is a property of space itself, that all galaxies are moving away from each other as shown in this kind of schematic image. 64 00:08:17,160 --> 00:08:26,040 If you sit on any one of these galaxies and watch all of the other ones, you'll see all of the galaxies moving away from each other. 65 00:08:27,570 --> 00:08:36,180 And in fact, one of the great goals in astronomy since the discovery of the expansion of the universe has been to understand 66 00:08:36,180 --> 00:08:46,830 really the history of that expansion was the distance between galaxies always expanding at the same rate, 67 00:08:47,130 --> 00:08:51,090 where things going faster in the past, slower in the past, etc. 68 00:08:51,600 --> 00:09:01,650 And the real breakthrough actually came only relatively recently, certainly in the history of astronomy in the late 1990s, 69 00:09:01,920 --> 00:09:08,100 with the recognition that in fact, the expansion of the universe is currently accelerating. 70 00:09:08,130 --> 00:09:12,330 So things are starting to move apart faster and faster and faster. 71 00:09:12,900 --> 00:09:20,970 And we've kind of given that a name, although we don't really understand why that is, why the expansion is accelerating. 72 00:09:21,210 --> 00:09:29,910 But the name we've given it is that most of the energy in the universe has to be in something that we call dark energy. 73 00:09:30,720 --> 00:09:39,210 And this really is a label for a form of energy that produces the acceleration that's observed. 74 00:09:39,660 --> 00:09:49,020 And this dark energy is completely different from the dark matter that we think makes up most of the mass of our galaxy. 75 00:09:49,320 --> 00:09:56,940 And in fact, that leaves very little room. Only about 4% of the stuff in the universe is the stuff that's in this room, 76 00:09:57,420 --> 00:10:03,660 the iron in our blood, the oxygen we breathe, or in fact, mostly the hydrogen in water. 77 00:10:05,260 --> 00:10:15,010 So the next big clue to thinking about our astronomical origins came from what might seem to be a slightly unexpected source. 78 00:10:15,310 --> 00:10:20,260 It came basically from taking pictures like this one, pictures of the sky. 79 00:10:20,290 --> 00:10:28,810 So I already said this is a picture of the night sky taken with an infrared telescope orbiting around the earth. 80 00:10:29,290 --> 00:10:38,110 You can take pictures, though, of the sky with different types of telescopes that observe different parts of the electromagnetic spectrum. 81 00:10:38,560 --> 00:10:42,340 This is a picture of the sky taken with an X-ray telescope. 82 00:10:42,880 --> 00:10:49,210 What you see with an X-ray telescope is very different than what you see with an infrared telescope. 83 00:10:49,600 --> 00:10:57,520 You see these kind of circles of X-ray emission, some point sources of X-ray radiation. 84 00:10:57,820 --> 00:11:07,600 Those point sources of X-ray radiation in some cases are, in fact, gas swirling on to black holes as the gas spirals on to the black hole. 85 00:11:07,630 --> 00:11:12,490 It produces huge amounts of radiation. That's something I'll come back to a little bit later. 86 00:11:13,060 --> 00:11:21,550 But in fact, the picture of the sky that turns out to be the most important for thinking about our origins is actually a really boring picture. 87 00:11:22,450 --> 00:11:30,939 It's this. This is a picture of this night sky taken with a radio telescope that observes in the microwave 88 00:11:30,940 --> 00:11:37,270 part of the electromagnetic spectrum the same microwaves that you use for heating up your food. 89 00:11:38,170 --> 00:11:47,499 And actually, this picture of the sky corresponds to a source of radiation, a source of light microwaves. 90 00:11:47,500 --> 00:11:57,280 In this case, that has a very particular temperature, about 2.7 and Kelvin, or -454 Fahrenheit. 91 00:11:57,280 --> 00:12:03,430 Or now I realise I should have put it up in centigrade and can't do the conversion immediately. 92 00:12:04,630 --> 00:12:12,810 So this is in fact a signature of the expansion of the universe. 93 00:12:12,820 --> 00:12:21,850 This is I as a theorist. I'm particularly proud that this is an example where the theorists made a prediction before the observations were made. 94 00:12:22,240 --> 00:12:31,360 And what Gamma and others predicted was that if you take this expansion seriously and you win back the clock, 95 00:12:31,810 --> 00:12:36,100 things in the distant past were much closer together, much denser. 96 00:12:36,400 --> 00:12:38,350 The universe was much hotter. 97 00:12:38,620 --> 00:12:48,880 And at some earlier phase, all of the gas in the universe itself should produce a bath of radiation that now fills the universe. 98 00:12:49,270 --> 00:12:59,170 And we think that this cosmic microwave background is indeed the glow of an early phase in the universe's history, 99 00:12:59,440 --> 00:13:03,250 when things were much denser and much hotter than they are today. 100 00:13:03,550 --> 00:13:16,270 And over time, as the universe has expanded, that very hot early universe has cooled to produce the microwave glow that we see around us now. 101 00:13:16,480 --> 00:13:26,680 When we look up at the sky and this was first detected with one of the early radio telescopes by Penzias and Wilson in the 1960s. 102 00:13:27,040 --> 00:13:35,320 And since then, really the study of the microwave sky has been one of the most important areas of astronomy. 103 00:13:35,770 --> 00:13:44,050 It's provided much of our detailed quantitative information about the energy and mass content of the universe, 104 00:13:44,320 --> 00:13:50,380 about the expansion history of the universe, and about the properties of the universe when it was much younger. 105 00:13:51,400 --> 00:14:04,030 And our kind of best picture now of what the infant universe looks like was taken by the European Planck telescope just a few years ago, 106 00:14:04,300 --> 00:14:13,000 and it's illustrated here. And so the difference between these two pictures is that in this original picture, 107 00:14:13,420 --> 00:14:20,800 which is really just a PowerPoint oval that I drew myself, but in this in this original picture, 108 00:14:21,130 --> 00:14:27,459 the telescope wasn't good enough to see that there are actually tiny differences 109 00:14:27,460 --> 00:14:32,230 in the amount of microwave light coming from one part of the sky to another part. 110 00:14:32,500 --> 00:14:37,149 And it took the development of technology better telescopes, 111 00:14:37,150 --> 00:14:47,710 better instrumentation to actually be able to see that different parts of the sky produce very slightly different amounts of microwave light. 112 00:14:48,190 --> 00:14:51,490 And in fact, the differences are really small. 113 00:14:51,910 --> 00:14:57,370 The difference in amount of microwave light from one part of the sky to another is something like 114 00:14:57,370 --> 00:15:04,840 .01 percent differences in the amount of light that we get from one part of the sky to the other. 115 00:15:05,260 --> 00:15:16,090 And we know how to interpret differences in the amount of this kind of thermal radiation produced by hot objects. 116 00:15:16,360 --> 00:15:23,650 And what it tells us is that in the early universe at this time when this radiation was produced, 117 00:15:24,880 --> 00:15:31,450 there were only tiny differences at this level of 0.01%, 118 00:15:31,750 --> 00:15:45,040 one part in 100,000 differences in the temperature and density of the matter that fills the universe at every place that we can see on the sky. 119 00:15:45,280 --> 00:15:52,150 So that part of the universe and that part of the universe were almost exactly the same in their density and temperature, 120 00:15:52,450 --> 00:15:56,620 with differences that amount to only about 0.01%. 121 00:15:57,190 --> 00:16:02,620 And this time is about 380,000 years after the Big Bang. 122 00:16:03,160 --> 00:16:07,150 The Big Bang is sometimes what we call T equals zero. 123 00:16:07,510 --> 00:16:12,730 But that's really just the time when you rewind the clock, 124 00:16:13,000 --> 00:16:19,570 when things get so dense and so hot that the laws of physics as we know them, start to break down. 125 00:16:19,780 --> 00:16:23,140 And we don't really know what happened before that time. 126 00:16:23,910 --> 00:16:29,130 And so in that sense, when we talk about the universe being 13.8 billion years old, 127 00:16:29,340 --> 00:16:35,430 what we really mean is the universe is 13.8 billion years old since we don't know what happened. 128 00:16:37,300 --> 00:16:46,870 So this is the universe then. And that's an incredibly different universe than what we see around us today in this room. 129 00:16:47,200 --> 00:16:52,720 There are enormous differences in the property of matter from one place to another. 130 00:16:53,050 --> 00:16:58,180 The difference in my density and temperature from the difference of the density and 131 00:16:58,180 --> 00:17:02,979 temperature of the air to the density and temperature of the centre of the earth, 132 00:17:02,980 --> 00:17:15,040 of the centre of the sun is many, many, many orders of magnitude, much larger than these tiny differences of 0.01% in the early universe. 133 00:17:15,460 --> 00:17:17,500 So how did we go then, 134 00:17:17,800 --> 00:17:31,720 from this early phase to this incredibly rich diversity of structure that we now see around us with billions of planets in our own galaxy, 135 00:17:31,720 --> 00:17:35,380 billions of galaxies in the visible universe. 136 00:17:36,100 --> 00:17:39,520 And so that's really the rest of the story that I want to tell you. 137 00:17:39,760 --> 00:17:48,729 This is in some sense been just trying to explain the meaning of the title Smooth to Lumpy is this transition 138 00:17:48,730 --> 00:17:56,920 from the very early universe we see in the microwave background to all of the complicated structure we see today? 139 00:17:57,690 --> 00:18:04,080 And there is actually a single, simple one word answer for how this happened. 140 00:18:04,970 --> 00:18:08,420 And so that one word answer is gravity. 141 00:18:09,260 --> 00:18:16,730 Gravity is the force that dominates the behaviour of the universe on the largest scales. 142 00:18:17,060 --> 00:18:26,930 The other forces that we know of in nature, electromagnetism, the weak and strong nuclear forces pale in comparison to gravity. 143 00:18:27,170 --> 00:18:40,550 When considering the large scales of the universe as a whole, and in particular what gravity does, is it takes very small differences. 144 00:18:41,240 --> 00:18:48,740 Those very small differences that you see in the microwave background, and it makes them bigger and bigger as time goes on. 145 00:18:49,430 --> 00:18:52,620 And the kind of general argument goes like this. 146 00:18:52,910 --> 00:19:02,090 We see in the microwave background that if you look in different places, the position is meant to be different points along this axis. 147 00:19:02,630 --> 00:19:08,090 Different places in the universe have slightly different densities and regions 148 00:19:08,090 --> 00:19:13,880 where there happens to be more stuff are regions where gravity is stronger. 149 00:19:14,270 --> 00:19:19,130 Those regions pull in more surrounding material, 150 00:19:19,430 --> 00:19:28,730 and so those regions grow in mass more effectively than regions where there isn't initially very much matter. 151 00:19:29,030 --> 00:19:35,690 And so gravity takes small differences and makes them bigger and bigger as time goes on. 152 00:19:36,380 --> 00:19:45,380 This is a form of instability called the gravitational instability studied by genes in the early 1900s. 153 00:19:45,680 --> 00:19:57,110 And it's really the critical mechanism for taking these tiny initial differences and making them bigger and bigger and bigger as time goes on. 154 00:19:57,590 --> 00:20:05,900 And this is where in detail the fact that the universe is dominated by dark matter becomes important. 155 00:20:06,800 --> 00:20:11,780 Most of the mass in the universe is not the normal matter in this room. 156 00:20:11,780 --> 00:20:23,840 It's dark matter. And so most of the mass producing the gravity that causes these kind of lumps to grow with time is dark matter. 157 00:20:24,440 --> 00:20:24,979 And in fact, 158 00:20:24,980 --> 00:20:34,940 this is the fundamental difference between dark matter and dark energy is that dark energy doesn't really participate in this clumping process. 159 00:20:35,300 --> 00:20:39,440 It's uniform. It's relatively uniformly spread throughout space. 160 00:20:39,860 --> 00:20:45,590 So dark energy is relatively smooth, while dark matter is relatively inhomogeneous, 161 00:20:45,860 --> 00:20:50,330 and it dominates over normal matter in producing the growth of structure. 162 00:20:51,020 --> 00:20:55,280 So this is kind of a cartoon to give you a physical picture of what this looks like. 163 00:20:55,580 --> 00:21:02,090 I want to now show you some examples from more realistic numerical simulations of what this looks like. 164 00:21:02,850 --> 00:21:12,510 So this is a movie that starts when the universe was quite young and goes forward in time following just the dark matter. 165 00:21:12,870 --> 00:21:20,640 And the box here is about the size of a region that will make a galaxy like our own Milky Way galaxy. 166 00:21:21,360 --> 00:21:25,740 And the kind of little bumps represent the dark matter. 167 00:21:26,100 --> 00:21:28,410 And so initially, I'm going to start over again. 168 00:21:28,620 --> 00:21:36,810 Initially, you see things are moving out and then gravity kind of winds and causes things to collapse back down. 169 00:21:37,140 --> 00:21:42,000 And so initially, the expansion of the universe drives things that things apart. 170 00:21:42,330 --> 00:21:49,590 But then gravity is strong enough to overcome the expansion of the universe and cause our galaxy to collapse in on itself. 171 00:21:50,160 --> 00:21:56,940 And this then is the formation of the dark matter backbone to our galaxy. 172 00:21:57,330 --> 00:22:04,770 And this basic process of gravity winning out over the expansion of the universe and causing things 173 00:22:04,770 --> 00:22:12,090 to collapse in on themselves is really the kind of central process that gets galaxy formation going. 174 00:22:12,930 --> 00:22:19,379 And you'll also notice one of the things you see visually in this movie that's quite important to how we think 175 00:22:19,380 --> 00:22:29,070 galaxies form is that initially the little small lumps that represent the first galaxies are quite small. 176 00:22:29,520 --> 00:22:32,880 And then as time goes on, they get bigger and bigger and bigger. 177 00:22:33,210 --> 00:22:41,850 So we think, in fact, that galaxies started off relatively small, maybe only consisting of a few stars, actually. 178 00:22:42,120 --> 00:22:48,930 And as time goes on, you build up bigger and bigger galaxies as illustrated by this movie. 179 00:22:50,280 --> 00:23:00,690 Now, to give you a sense of scale, I'm going to add to this movie the actual size of the stars in our galaxy, 180 00:23:01,230 --> 00:23:05,640 and that's just this tiny little region at the centre. 181 00:23:06,330 --> 00:23:14,730 So most of what we call our galaxy actually consists of this large region that's dominated by the dark matter. 182 00:23:15,060 --> 00:23:22,590 The region where the star is in the gas and the planets are is confined really just to the central part. 183 00:23:25,930 --> 00:23:28,690 So this is a few million light years big. 184 00:23:29,620 --> 00:23:38,620 This now shows the distribution of dark matter spread out over a region that something like a billion times bigger. 185 00:23:38,980 --> 00:23:42,620 So this is about a few billion light years across now. 186 00:23:42,940 --> 00:23:50,500 And this shows the distribution of dark matter in a computer simulation in the present day. 187 00:23:50,770 --> 00:23:56,560 And again, the kind of whitish regions are regions where there's lots of dark matter. 188 00:23:56,860 --> 00:24:00,460 And the dark regions are regions where there is very little dark matter. 189 00:24:00,970 --> 00:24:08,380 And this really is the dark matter backbone that dictates where galaxies form. 190 00:24:08,740 --> 00:24:20,590 Regions that have lots of dark matter have enough gravitational pull to bring in gas and stars from their surroundings and build up galaxies. 191 00:24:20,890 --> 00:24:23,200 And so it's the gravity of the dark matter, 192 00:24:23,200 --> 00:24:35,170 pulling in the gas and stars that leads the galaxies that we can actually see in stars, in gas to form where the dark matter is. 193 00:24:35,830 --> 00:24:43,210 And so, in fact, in a very real sense, when we look up at the distribution of galaxies on the sky. 194 00:24:43,510 --> 00:24:51,130 So this is now an image of where galaxies are positioned, observed on the sky. 195 00:24:51,520 --> 00:24:59,050 So we're sitting at the centre and galaxies out here further away from us, ones in here closer to us. 196 00:24:59,500 --> 00:25:02,590 And this shows the angle on the sky. 197 00:25:02,860 --> 00:25:08,410 And so you see this kind of lumpy, inhomogeneous pattern of galaxies on the sky. 198 00:25:08,770 --> 00:25:16,840 And that really is, to first approximation, that distribution of galaxies is tracing the distribution of dark matter. 199 00:25:18,130 --> 00:25:23,950 Because the dark matter dominates the gravity. And so it tells the gas in the stars where to go. 200 00:25:26,030 --> 00:25:36,380 If that were it. If that were the whole story, our understanding of how galaxies form would really be a solved problem. 201 00:25:36,920 --> 00:25:41,510 But it's not. And an indication of that is that. 202 00:25:42,960 --> 00:25:48,240 When we look at the properties that galaxies actually have, how big are they? 203 00:25:48,480 --> 00:25:50,130 How massive are they? 204 00:25:50,730 --> 00:26:01,070 It turns out that the properties of galaxies are related in a rather complicated way to the properties of the dark matter around them. 205 00:26:01,650 --> 00:26:15,060 And so a famous way that astronomers like to illustrate this is to show how many objects are there versus the mass of the object. 206 00:26:15,360 --> 00:26:20,610 So really massive objects are over here, low mass objects are here. 207 00:26:20,970 --> 00:26:29,910 And the solid line represents the distribution of these dark matter clumps that you see in this image. 208 00:26:30,450 --> 00:26:36,270 So the regions like this one are lots of dark matter and those are rare. 209 00:26:36,660 --> 00:26:40,090 And so those are objects out here. 210 00:26:40,090 --> 00:26:50,910 Rare, very massive objects. The solid circles represent observed galaxies, the stellar mass of observed galaxies. 211 00:26:51,270 --> 00:27:01,140 And the point of this is just to drive home that when you actually look at the masses you see in stars and galaxies, 212 00:27:01,410 --> 00:27:07,950 it's not the case that the mass of stars is just equal to the mass of dark matter, 213 00:27:07,950 --> 00:27:15,390 or it's not equal to the mass of dark matter times point one or some simple number like that. 214 00:27:15,740 --> 00:27:26,670 There's a complicated relationship between the distribution of galaxies and the properties of galaxies that we see and the properties of dark matter. 215 00:27:27,000 --> 00:27:33,450 And in some ways, this is the challenge that the modern theory of galaxy formation is trying to get 216 00:27:33,450 --> 00:27:41,610 at what drives these variations in the sizes and shapes of galaxies that we see, 217 00:27:41,880 --> 00:27:47,610 what sets the masses of galaxies relative to the masses of the dark matter around. 218 00:27:48,810 --> 00:27:57,600 And so what I want to do is try to describe to you a couple of the areas of forefront research 219 00:27:57,600 --> 00:28:03,150 trying to get at this question and just give you some of the big ideas that we're grappling with. 220 00:28:03,970 --> 00:28:08,680 And I'm going to emphasise really two key questions. 221 00:28:09,070 --> 00:28:18,970 So one is trying to understand a little bit more about how gas actually gets into galaxies, 222 00:28:19,300 --> 00:28:29,140 because that determines then how the mass of normal matter, the stuff that we can actually see grows in time. 223 00:28:29,680 --> 00:28:39,970 And then the second question that I'll address is what happens once you start actually forming stars and galaxies? 224 00:28:40,270 --> 00:28:46,120 What happens once you start actually forming black holes at the centres of galaxies? 225 00:28:46,450 --> 00:28:52,170 How does that actually change this picture of gravity? 226 00:28:52,180 --> 00:28:55,540 The gravity of dark matter dominates. It pulls stuff in. 227 00:28:55,780 --> 00:29:00,519 That's the end of the story. And so these are the questions that I want to address. 228 00:29:00,520 --> 00:29:08,380 And these really are questions that get at the heart of a lot of research that's being done today. 229 00:29:10,000 --> 00:29:16,870 So there are really two major ways that galaxies grow in mass with tall. 230 00:29:17,900 --> 00:29:24,950 One is that just like you saw how dark matter lumps start out small and then they 231 00:29:24,950 --> 00:29:28,880 collide with each other and they build up bigger and bigger dark matter lumps. 232 00:29:29,270 --> 00:29:33,500 The same thing happens with the stars and gas in galaxies. 233 00:29:33,830 --> 00:29:41,150 So galaxies slam into each other. The Milky Way in Andromeda, as I said, will collide with each other in 4 billion years. 234 00:29:41,390 --> 00:29:49,760 And that's not an accident. That's actually a property of things start out small and they build up bigger and bigger as time goes on. 235 00:29:50,300 --> 00:29:59,840 And this, in fact, is a series of images taken with the Hubble Space Telescope showing galaxies in the act of colliding with each other. 236 00:30:00,230 --> 00:30:05,090 And what they look like is kind of train wrecks, right, when galaxies slam together. 237 00:30:05,330 --> 00:30:12,420 You get this very complicated, disordered shape. The other way that we think galaxies grow. 238 00:30:12,420 --> 00:30:20,640 And that may actually be the dominant way that galaxies grow, is that they actually just pull in surrounding gas. 239 00:30:20,970 --> 00:30:23,700 And that's shown in this computer simulation. 240 00:30:24,030 --> 00:30:32,310 The dashed line represents about the size of that dark matter region I showed you in the earlier simulation. 241 00:30:32,640 --> 00:30:39,570 And the red shows gas that's being pulled in by the gravity of the dark matter. 242 00:30:40,020 --> 00:30:44,700 And that gas gets pulled in to the central part of the galaxy. 243 00:30:45,000 --> 00:30:47,730 And as it gets pulled into the centre, 244 00:30:48,030 --> 00:30:56,970 eventually that gas can turn into stars forming the stars that we see in these images with the Hubble Space Telescope. 245 00:30:57,600 --> 00:31:03,720 And understanding which of these is actually the dominant way that galaxies grow. 246 00:31:03,990 --> 00:31:10,470 This has been a major set of questions that people have been working on over the last 5 to 10 years. 247 00:31:10,740 --> 00:31:21,660 And in particular, one of the real forefront areas today is trying to directly observe this gas as it flows into galaxies. 248 00:31:21,990 --> 00:31:26,420 And that turns out to be quite challenging for a variety of observational reasons. 249 00:31:28,490 --> 00:31:32,690 So this again is our picture of how galaxies assemble. 250 00:31:32,900 --> 00:31:42,810 They start out as these very small lumps and as time marches on, they become bigger and bigger structures. 251 00:31:44,470 --> 00:31:48,340 As a theorist, I like to simplify things. 252 00:31:48,670 --> 00:31:51,790 This is a little complicated. It's very messy. 253 00:31:51,790 --> 00:32:00,340 It's not symmetric. I like circles. So this is kind of a simpler theoretical picture of how a galaxy grows. 254 00:32:00,760 --> 00:32:11,800 The white region shows the gas that has actually fallen into the gravitational clutches of the dark matter associated with that galaxy. 255 00:32:12,160 --> 00:32:21,220 Outside this blue stuff is the gas that's getting pulled into the galaxy by the gravity of the dark matter. 256 00:32:21,460 --> 00:32:28,450 As time goes on, you can see that the size of the galaxy is actually growing in time. 257 00:32:28,810 --> 00:32:39,440 And again, as I already illustrated in one of the earlier pictures, we think that where the stars actually sit is at the very, very centre here. 258 00:32:39,460 --> 00:32:52,990 That's where things are dense enough that this gas that fills the space in the dark matter halo, the gas can actually cool to form new stars. 259 00:32:53,530 --> 00:33:04,210 And so one of the big questions is trying to understand what is the behaviour of this gas that fills up the halo of dark matter? 260 00:33:04,540 --> 00:33:09,940 How does it behave? How does the gas actually get to the centre of the galaxy? 261 00:33:10,300 --> 00:33:20,980 How does it cool to form stars? And so one of the discoveries that's come out really over the last five or ten years, work done by Steve, 262 00:33:21,310 --> 00:33:38,040 this here and then myself has been demonstrating that gas in galaxy, just like water on your stove undergoes a very vigorous kind of boiling motion. 263 00:33:38,350 --> 00:33:51,309 It's not this simple static picture that you might have from this kind of image, but it's a much more dynamic process than we earlier thought. 264 00:33:51,310 --> 00:33:55,840 The gas is actually churning around, boiling very much. 265 00:33:55,840 --> 00:34:01,540 The physics here is in fact, quite analogous to what happens in a pot of water. 266 00:34:01,810 --> 00:34:10,180 And we're, in fact, still struggling to understand how does this change our picture of what determines 267 00:34:10,180 --> 00:34:16,509 the properties of gas in the very central regions where the stars actually form, 268 00:34:16,510 --> 00:34:26,050 where the galaxy actually grows? And so this is one area that my group is doing a lot of work, is trying to understand this particular question. 269 00:34:28,640 --> 00:34:39,490 Okay. So I want to turn now to what is really I think probably the most important question in our modern understanding of 270 00:34:39,490 --> 00:34:50,649 galaxy formation is trying to understand observationally and theoretically what happens once stars start forming, 271 00:34:50,650 --> 00:34:59,770 once black holes start forming? What effect does that have on the rest of the process of forming galaxies? 272 00:35:00,130 --> 00:35:05,020 And this goes under the general name of feedback. 273 00:35:05,020 --> 00:35:12,010 And the idea behind that name is that once you start forming stars and black holes, 274 00:35:12,310 --> 00:35:22,030 that the process of star formation and black hole growth feeds back on or impacts the rest of the gas in galaxies, 275 00:35:22,030 --> 00:35:26,410 the rest of the process of galaxy formation. 276 00:35:26,740 --> 00:35:34,540 And the kind of simple moral of the story is that once gas gets into galaxies, 277 00:35:34,900 --> 00:35:45,460 it starts being affected by processes and forces in addition to the gravity of the dark matter that brought it into the galaxy in the first place. 278 00:35:45,850 --> 00:35:54,160 So on very large scales in the universe, the gravity of dark matter is the dominant force and it dictates what happens. 279 00:35:54,460 --> 00:36:01,330 But once you get inside galaxies, the gravity of dark matter is actually no longer the dominant force. 280 00:36:01,630 --> 00:36:08,740 There are other processes like stellar explosions, the effects of black holes on their environment, 281 00:36:08,980 --> 00:36:17,510 which can be much more important than the gravity of dark matter in dictating how the gas behaves. 282 00:36:18,040 --> 00:36:25,419 And the kind of dramatic illustration of this is that we have this picture of gas 283 00:36:25,420 --> 00:36:31,330 flowing into galaxies as illustrated in this computer simulation shown here. 284 00:36:31,720 --> 00:36:41,710 But we actually think that essentially 90% of the gas that at any one time came into a galaxy 285 00:36:41,950 --> 00:36:50,469 is later blown back out of that galaxy by the combined effects of stellar explosions, 286 00:36:50,470 --> 00:36:54,400 black holes, etc. And that's actually illustrated. 287 00:36:54,400 --> 00:36:59,200 This is not just something theorists make up. That's illustrated in these two images. 288 00:36:59,200 --> 00:37:01,000 This is a Hubble space telescope image. 289 00:37:01,390 --> 00:37:11,650 This is an image with the Chandra X-ray telescope which actually show these dramatic outflows of gas driven by star formation. 290 00:37:12,370 --> 00:37:24,730 And so what I want to do is try to give you a simple feel physically for why is it that once stars start forming, once black holes start growing, 291 00:37:25,060 --> 00:37:30,610 why is it that that has such a dramatic effect that it can actually blow out 292 00:37:30,880 --> 00:37:35,440 most of the stuff that at any time made it into the galaxy in the first place? 293 00:37:36,190 --> 00:37:49,030 And the way we like to think about this as physicists is in thinking about the forces on gas or the energetics of gas. 294 00:37:49,420 --> 00:37:53,980 And so the way I'm going to explain this to you is really in both ways, forces in energy. 295 00:37:54,870 --> 00:38:02,070 And so if you turn if you have a certain amount of mass in a galaxy, a certain amount of gas, 296 00:38:02,490 --> 00:38:12,570 and it turns into stars, the process of forming stars actually dumps energy back into its surroundings. 297 00:38:12,900 --> 00:38:16,200 And the way that happens is a couple of different ways. 298 00:38:16,470 --> 00:38:20,760 One is star shone. They produce light. 299 00:38:21,120 --> 00:38:30,990 Nuclear fusion in stars produces enormous amounts of light, and that heats up and pushes on the surrounding gas. 300 00:38:31,350 --> 00:38:37,830 This is a picture of one of the regions in our own galaxy where massive stars are forming. 301 00:38:38,130 --> 00:38:44,460 And you see these regions lit up by the newly formed stars within. 302 00:38:45,480 --> 00:38:50,280 The other thing that happens is that massive stars at the end of their lives 303 00:38:50,580 --> 00:38:57,630 actually explode after they collapse to form either a neutron star or a black hole. 304 00:38:57,930 --> 00:39:01,560 And those explosions release enormous amounts of energy. 305 00:39:02,430 --> 00:39:10,140 These are an example of this. This is an X-ray image of the explosion of a massive star. 306 00:39:10,440 --> 00:39:18,060 So these processes of the radiation produced by nuclear fusion in stars or the explosion 307 00:39:18,270 --> 00:39:23,130 associated with the formation of neutron stars and black holes at the ends of their lives. 308 00:39:23,490 --> 00:39:32,370 Take some of the energy associated with forming stars and it dumps it back into the surroundings. 309 00:39:32,700 --> 00:39:39,240 And that amount of energy is about 0.1 percent of M.C. Square. 310 00:39:39,270 --> 00:39:43,470 So I take a certain amount of gas and I turn it into stars. 311 00:39:43,740 --> 00:39:52,380 I get about point 1% of EMC squared back out in the form of radiation and stellar explosions. 312 00:39:52,680 --> 00:39:58,620 And this is a nice astronomical application of equals MC squared. 313 00:39:59,920 --> 00:40:08,320 This amount of energy actually pales in comparison to the amount of energy we get out of black holes. 314 00:40:08,740 --> 00:40:16,809 So we think of nuclear fusion in stars or nuclear fusion here on the earth as being this very efficient source of energy. 315 00:40:16,810 --> 00:40:21,190 It's the source of energy that keeps the earth hot and allows life to proceed. 316 00:40:21,490 --> 00:40:27,910 It's the source of energy that we think we hope one day will solve the energy needs here on Earth. 317 00:40:28,300 --> 00:40:29,290 But in fact, 318 00:40:29,890 --> 00:40:41,960 turning a mass of gas into a black hole is a far more efficient way of getting energy out than nuclear fusion in stars up to 100 times more efficient. 319 00:40:42,930 --> 00:40:51,360 And the way this happens is that as gas falls in to form a black hole, 320 00:40:51,570 --> 00:40:57,780 it orbits around, it gets hotter and hotter and hotter due to friction in that gas. 321 00:40:58,110 --> 00:41:05,640 Friction mediated by magnetic fields. And that friction actually releases enormous amounts of energy. 322 00:41:05,970 --> 00:41:17,880 And something like 10% of M.c squared comes out in the form of energy when you form black holes in galaxies. 323 00:41:18,240 --> 00:41:21,780 And that energy, again, it comes out in different forms. 324 00:41:22,110 --> 00:41:25,770 Some of it comes out in the form of radiation light. 325 00:41:26,460 --> 00:41:33,000 This is an image of a accreting black hole at the centre of a galaxy, a quasar, 326 00:41:33,000 --> 00:41:41,790 where this single region at the centre of the galaxy with a massive black hole that maybe weighs a billion times the mass of the sun. 327 00:41:42,180 --> 00:41:52,650 This tiny little region, no bigger than our solar system, actually produces more light than the entire billions of stars in the galaxy. 328 00:41:52,920 --> 00:41:59,610 And the only way you can even see the stars in this galaxy is if you use the exquisite resolution 329 00:41:59,610 --> 00:42:05,100 of the Hubble Space Telescope to subtract off the light from the accreting black hole, 330 00:42:05,430 --> 00:42:12,330 revealing the faint distribution of stars making up the galaxy around the black hole. 331 00:42:13,410 --> 00:42:19,979 Another way that the energy released forming black holes comes out is in the form of 332 00:42:19,980 --> 00:42:28,380 powerful outflows that carry away energy and energy of motion of relativistic particles, 333 00:42:28,710 --> 00:42:33,810 much like the stellar explosions at the ends of the life of massive stars. 334 00:42:34,050 --> 00:42:42,120 Forming black holes releases energy in the form of powerful outflows shown here in this radio image. 335 00:42:42,120 --> 00:42:48,660 The pink stuff is material moving at nearly the speed of light ejected from the vicinity of the black hole. 336 00:42:49,890 --> 00:42:54,990 This is actually one of the real surprises in our thinking about how galaxies form. 337 00:42:55,800 --> 00:43:06,960 Over the last ten years in particular, has been the recognition that black holes at the centres of galaxies are actually important for that process. 338 00:43:07,320 --> 00:43:11,850 And at first glance, that sounds like a kind of crazy idea. 339 00:43:12,180 --> 00:43:16,170 And so let me first tell you why it sounds like a crazy idea and then tell you why it's actually right. 340 00:43:16,740 --> 00:43:22,170 So the reason it sounds a little bit crazy is that the mass of the black hole at the centre 341 00:43:22,170 --> 00:43:30,210 of a galaxy is something like 100,000 times smaller than the mass of its surrounding galaxy. 342 00:43:30,540 --> 00:43:37,410 So the gravity of the black hole on the scale of the galaxy as a whole is completely irrelevant. 343 00:43:37,680 --> 00:43:41,790 All that matters is the gravity of the dark matter and the gravity of the star. 344 00:43:43,540 --> 00:43:54,040 Moreover, the size of the black hole, the radius of the event horizon, the Schwarzschild radius is really just 100 times or so bigger than the sun. 345 00:43:54,250 --> 00:43:56,350 That's about the distance between the earth and the sun. 346 00:43:56,890 --> 00:44:04,540 And so that's again, that's a region that's 10 billion times smaller than the size of the galaxy. 347 00:44:04,930 --> 00:44:13,300 So how can this tiny little region that is completely negligible in mass and completely negligible in size? 348 00:44:13,540 --> 00:44:19,119 How can it affect the galaxy as a whole? And the explanation for that is, again, 349 00:44:19,120 --> 00:44:28,960 this fact that throwing stuff into a black hole is the most efficient way of getting energy out that we know of in astronomy. 350 00:44:29,530 --> 00:44:42,700 And so, in fact, black holes at the centres of galaxies are these extraordinarily powerful sources of light that impact their surroundings. 351 00:44:42,910 --> 00:44:54,280 When the light and outflows produced by the black hole travels out from the black hole and runs into surrounding material. 352 00:44:54,700 --> 00:45:05,290 And we think, in fact, that the amount of energy and the amount of radiation produced by gas spiralling on to a black hole at the centre of a galaxy 353 00:45:05,590 --> 00:45:19,390 can be enough that the force produced as gas in a galaxy gets hit by the radiation produced in the vicinity of the black hole. 354 00:45:20,080 --> 00:45:29,590 The force produced on that surrounding gas can be stronger than the gravity of everything else in the galaxy. 355 00:45:29,620 --> 00:45:32,980 All of the dark matter. All of the stars, all of the gas. 356 00:45:33,490 --> 00:45:43,990 So black holes are indeed irrelevant in terms of the store, their size, their gravitational influence on the galaxy as a whole. 357 00:45:44,320 --> 00:45:54,400 But they, in fact, have a can have a really outsized impact because of their extraordinarily efficient production of energy. 358 00:45:56,020 --> 00:46:02,020 And so another way to kind of frame this issue is that if you, again, 359 00:46:02,020 --> 00:46:07,920 take a certain amount of mass and ask how much energy do you get out turning it into stars 360 00:46:07,930 --> 00:46:13,450 or turning it into a black hole where you got about a 10th of a percent of the energy out? 361 00:46:13,450 --> 00:46:18,640 If you turn some mass into stars, you get about 10% out. 362 00:46:18,970 --> 00:46:31,510 If you turn that mass into a black hole, but it only takes about a million years or 10 billion years, depending on the size of the galaxy. 363 00:46:31,810 --> 00:46:44,440 It only takes a tiny amount of energy being dumped into the gas orbiting around in the galaxy to completely push the gas out of the galaxy. 364 00:46:45,540 --> 00:46:52,350 And that essential difference between the amount of energy generated by nuclear fusion, 365 00:46:52,620 --> 00:46:56,940 the amount of energy generated by accretion of gas on to black holes, 366 00:46:57,300 --> 00:47:04,770 and the comparatively small amount of energy needed to blow gas out of the galaxy. 367 00:47:05,070 --> 00:47:13,590 That's really the essential reason why the process of forming stars and the process of growing black 368 00:47:13,590 --> 00:47:21,480 holes at the centres of galaxies has such a big impact on the evolution and life of galaxies, 369 00:47:21,810 --> 00:47:27,209 because these processes release enormous amounts of energy and substantially 370 00:47:27,210 --> 00:47:34,290 affect how gas how gas actually behaves on the scales of galaxies as a whole. 371 00:47:35,130 --> 00:47:48,660 And so our modern picture of kind of the life cycle of stuff in galaxies is one in which gravity pulls matter into galaxies. 372 00:47:49,080 --> 00:47:54,060 Gravity causes matter to collapse and form stars. 373 00:47:54,420 --> 00:47:59,310 Stars then shine by nuclear fusion for millions or billions of years. 374 00:47:59,640 --> 00:48:05,160 They eventually end their life as white dwarfs, neutron stars or black holes. 375 00:48:05,430 --> 00:48:11,250 And then, in fact, most of the gas in galaxies gets blown back out again, 376 00:48:11,580 --> 00:48:18,180 only to later get pulled back into galaxies and be part of this large scale cycle. 377 00:48:18,660 --> 00:48:30,030 And so we have this picture right from the time of Carl Sagan that were all formed from the kind of debris of stars. 378 00:48:30,360 --> 00:48:39,570 And that picture is right. But it really operates not on the scales of stars, but really on the scales of galaxies as a whole. 379 00:48:39,870 --> 00:48:50,010 Stuff gets blown out of galaxies, then falls back in, turns into stars, gets black, blown out again, and participates in this large scale cycle. 380 00:48:50,580 --> 00:49:01,050 And so the challenge then, in understanding how galaxies form, why a galaxy like our Milky Way has the stellar mass it does. 381 00:49:01,320 --> 00:49:06,180 Why? It's mostly a disk with a little bit of a spherical thing. 382 00:49:06,480 --> 00:49:13,230 Spherical bulge at the centre lies in understanding the interplay between all of these different processes. 383 00:49:13,650 --> 00:49:23,610 And so a lot of effort is going into identifying what physical processes are the most important blowing gas out of galaxies. 384 00:49:23,850 --> 00:49:32,040 How does it get back into galaxies? And then trying to incorporate that into more and more realistic numerical simulations. 385 00:49:32,430 --> 00:49:40,409 And so I want to show you a few examples of work done by our group where we try to incorporate this physics into, 386 00:49:40,410 --> 00:49:52,050 at least in physically reasonable but still simplified ways into understanding not how dark matter behaves when galaxies form, 387 00:49:52,350 --> 00:49:56,430 but into understanding how gas and stars behave when galaxies. 388 00:49:57,720 --> 00:50:06,840 And so this is a numerical simulation about a region about the size of the stars in our own galaxy. 389 00:50:07,200 --> 00:50:11,730 The simulation is showing you the gas in a galaxy. 390 00:50:12,210 --> 00:50:16,250 The movie is only a few hundred million years long. 391 00:50:16,260 --> 00:50:20,370 That's actually pretty short for the scale of the universe as a whole. 392 00:50:20,670 --> 00:50:30,240 The white regions are dense regions where gravity causes gas and galaxies to collapse and form stars. 393 00:50:30,600 --> 00:50:43,140 And then you see little explosions going off, which are the processes of stellar death, stellar radiation, stellar explosions pushing gas around. 394 00:50:43,440 --> 00:50:51,840 And in fact, in the kind of side projection, you can see stuff being flung up out of the galaxy. 395 00:50:52,410 --> 00:51:02,640 And so this is attempting to realise now in a computer simulation these processes that actually dictate how gas turns into stars, 396 00:51:02,910 --> 00:51:11,760 how gas gets blown out of galaxies. Since those are the critical processes that govern the type of galaxy that will result. 397 00:51:12,990 --> 00:51:18,580 And again, this movie is about 30,000 light years across. 398 00:51:18,600 --> 00:51:25,920 That's about the size of the stars that make up our galaxy and now show you a similar computer simulation. 399 00:51:26,220 --> 00:51:29,610 But over a region that's about a million light years across. 400 00:51:29,970 --> 00:51:36,600 That's the region that makes up our the entire dark matter distribution that makes up our galaxy. 401 00:51:37,260 --> 00:51:42,120 So this computer simulation starts when the universe was much younger, goes to the present day. 402 00:51:42,510 --> 00:51:47,180 It's about a million light years across. This shows the density of gas. 403 00:51:47,190 --> 00:51:51,059 So dark regions, lots of gas. This shows the temperature. 404 00:51:51,060 --> 00:52:06,720 The red is hot regions. And you can see literally like explosions driven by gas getting blown out of galaxies, heating up the surroundings. 405 00:52:07,530 --> 00:52:13,860 That process actually shuts off star formation for a while in the galaxy. 406 00:52:14,100 --> 00:52:23,250 Things settle down a little bit. New gas falls in fuelling a new generation of star formation, which blows out gas again. 407 00:52:23,460 --> 00:52:33,360 Right. And this is again, this is kind of an illustration of this cycle of gas flowing into galaxies and getting blown out. 408 00:52:33,810 --> 00:52:42,600 And really only in the last few years have somewhat first principles, 409 00:52:42,600 --> 00:52:56,910 calculations incorporating this physics been successfully able to even explain why galaxies roughly have the stellar masses that they do. 410 00:52:57,270 --> 00:53:08,910 And so this is just an illustration of that. So this shows the stellar mass of a galaxy as a function of how much dark matter there is. 411 00:53:09,360 --> 00:53:17,790 The dotted line would be a simple story in which stars just track dark matter. 412 00:53:18,810 --> 00:53:23,430 About 10% of the mass of dark matter ends up in stars. 413 00:53:24,030 --> 00:53:29,520 Reality observations are indicated by the solid and dashed lines. 414 00:53:29,850 --> 00:53:36,930 So in small galaxies, the amount of stars is actually very small compared to the amount of dark matter, 415 00:53:37,200 --> 00:53:40,500 because most of the stuff has actually been blown out. 416 00:53:40,890 --> 00:53:49,140 And so really, only in the last few years have computer simulations reached the point where we 417 00:53:49,140 --> 00:53:55,320 can kind of successfully produce galaxies of roughly the right stellar masses. 418 00:53:55,650 --> 00:54:04,440 And that's shown by the different symbols. Here are different computer simulations by a couple of different groups trying to incorporate this physics. 419 00:54:04,920 --> 00:54:08,760 These ideas had been around for decades, 420 00:54:09,060 --> 00:54:17,870 but really incorporating them in the first principles way into our understanding of how galaxies form is a relatively, relatively recent. 421 00:54:17,910 --> 00:54:29,160 And so some of the current frontiers then are trying to understand what makes some galaxies disc like rather than elliptical or spherical. 422 00:54:29,160 --> 00:54:33,160 That remains a rather thorny theoretical problem. 423 00:54:33,180 --> 00:54:38,890 There's a lot of good ideas for what determines that distinction, but which of those ideas is right? 424 00:54:39,390 --> 00:54:42,630 We still don't really know. Okay. 425 00:54:42,750 --> 00:54:52,770 So I'll end there. I hope I've given you a little bit of a feel for how Gravity and the action star 426 00:54:52,770 --> 00:55:00,209 formation in black hole growth has caused the universe to evolve from its very simple, 427 00:55:00,210 --> 00:55:06,720 smooth initial conditions that we see in the Big Bang, in the cosmic microwave background, 428 00:55:07,020 --> 00:55:11,490 to the much more rich inhomogeneous structure that we see in the present day universe. 429 00:55:11,520 --> 00:55:12,720 So thanks a lot.