0:00:02.934,0:00:06.268 Here are some images of clusters of galaxies. 0:00:06.268,0:00:08.101 They're exactly what they sound like. 0:00:08.101,0:00:10.654 They are these huge collections of galaxies, 0:00:10.654,0:00:12.909 bound together by their mutual gravity. 0:00:12.909,0:00:15.571 So most of the points that you see on the screen 0:00:15.571,0:00:18.100 are not individual stars, 0:00:18.100,0:00:20.936 but collections of stars, or galaxies. 0:00:20.936,0:00:23.267 Now, by showing you some of these images, 0:00:23.267,0:00:25.014 I hope that you will quickly see that 0:00:25.014,0:00:27.694 galaxy clusters are these beautiful objects, 0:00:27.694,0:00:29.187 but more than that, 0:00:29.187,0:00:31.850 I think galaxy clusters are mysterious, 0:00:31.850,0:00:33.403 they are surprising, 0:00:33.403,0:00:34.820 and they're useful. 0:00:34.820,0:00:38.351 Useful as the universe's most massive laboratories. 0:00:38.351,0:00:41.987 And as laboratories, to describe galaxy clusters 0:00:41.987,0:00:43.852 is to describe the experiments 0:00:43.852,0:00:45.405 that you can do with them. 0:00:45.405,0:00:47.685 And I think there are four major types, 0:00:47.685,0:00:50.269 and the first type that I want to describe 0:00:50.269,0:00:52.488 is probing the very big. 0:00:52.488,0:00:54.183 So, how big? 0:00:54.183,0:00:58.434 Well, here is an image of a particular galaxy cluster. 0:00:58.434,0:01:01.403 It is so massive that the light passing through it 0:01:01.403,0:01:04.018 is being bent, it's being distorted 0:01:04.018,0:01:06.527 by the extreme gravity of this cluster. 0:01:06.527,0:01:08.331 And, in fact, if you look very carefully 0:01:08.331,0:01:11.031 you'll be able to see rings around this cluster. 0:01:11.031,0:01:12.580 Now, to give you a number, 0:01:12.580,0:01:14.363 this particular galaxy cluster 0:01:14.363,0:01:18.571 has a mass of over one million billion suns. 0:01:18.571,0:01:21.884 It's just mind-boggling how [br]massive these systems can get. 0:01:21.884,0:01:23.385 But more than their mass, 0:01:23.385,0:01:25.359 they have this additional feature. 0:01:25.359,0:01:27.703 They are essentially isolated systems, 0:01:27.703,0:01:29.925 so if we like, we can think of them 0:01:29.925,0:01:33.195 as a scaled-down version of the entire universe. 0:01:33.195,0:01:35.197 And many of the questions that we might have 0:01:35.197,0:01:37.165 about the universe at large scales, 0:01:37.165,0:01:39.080 such as, how does gravity work? 0:01:39.080,0:01:42.112 might be answered by studying these systems. 0:01:42.112,0:01:43.445 So that was very big. 0:01:43.445,0:01:45.330 The second things is very hot. 0:01:45.330,0:01:48.113 Okay, if I take an image of a galaxy cluster, 0:01:48.113,0:01:51.030 and I subtract away all of the starlight, 0:01:51.030,0:01:53.825 what I'm left with is this big, blue blob. 0:01:53.825,0:01:55.310 This is in false color. 0:01:55.310,0:01:57.659 It's actually X-ray light that we're seeing. 0:01:57.659,0:02:00.320 And the question is, if it's not galaxies, 0:02:00.320,0:02:02.519 what is emitting this light? 0:02:02.519,0:02:04.419 The answer is hot gas, 0:02:04.419,0:02:06.242 million-degree gas -- 0:02:06.242,0:02:07.899 in fact, it's plasma. 0:02:07.899,0:02:09.558 And the reason why it's so hot 0:02:09.558,0:02:11.538 goes back to the previous slide. 0:02:11.538,0:02:14.278 The extreme gravity of these systems 0:02:14.278,0:02:17.320 is accelerating particles of gas to great speeds, 0:02:17.320,0:02:19.902 and great speeds means great temperatures. 0:02:19.902,0:02:21.621 So this is the main idea, 0:02:21.621,0:02:23.987 but science is a rough draft. 0:02:23.987,0:02:26.538 There are many basic properties about this plasma 0:02:26.538,0:02:28.236 that still confuse us, 0:02:28.236,0:02:29.569 still puzzle us, 0:02:29.569,0:02:31.655 and still push our understanding 0:02:31.655,0:02:33.694 of the physics of the very hot. 0:02:33.694,0:02:37.121 Third thing: probing the very small. 0:02:37.121,0:02:39.967 Now, to explain this, I need to tell you 0:02:39.967,0:02:41.987 a very disturbing fact. 0:02:41.987,0:02:44.777 Most of the universe's matter 0:02:44.777,0:02:46.588 is not made up of atoms. 0:02:46.588,0:02:48.538 You were lied to. 0:02:48.538,0:02:51.665 Most of it is made up of something [br]very, very mysterious, 0:02:51.665,0:02:53.787 which we call dark matter. 0:02:53.787,0:02:57.454 Dark matter is something that [br]doesn't like to interact very much, 0:02:57.454,0:02:59.122 except through gravity, 0:02:59.122,0:03:00.734 and of course we would like to learn more about it. 0:03:00.734,0:03:02.277 If you're a particle physicist, 0:03:02.277,0:03:04.786 you want to know what happens [br]when we smash things together. 0:03:04.786,0:03:06.776 And dark matter is no exception. 0:03:06.776,0:03:08.436 Well, how do we do this? 0:03:08.436,0:03:09.927 To answer that question, 0:03:09.927,0:03:11.320 I'm going to have to ask another one, 0:03:11.320,0:03:14.486 which is, what happens when galaxy clusters collide? 0:03:14.486,0:03:16.796 Here is an image. 0:03:16.796,0:03:19.987 Since galaxy clusters are representative 0:03:19.987,0:03:23.111 slices of the universe, scaled-down versions. 0:03:23.111,0:03:25.234 They are mostly made up of dark matter, 0:03:25.234,0:03:27.787 and that's what you see in this bluish purple. 0:03:27.787,0:03:29.404 The red represents the hot gas, 0:03:29.404,0:03:31.371 and, of course, you can see many galaxies. 0:03:31.371,0:03:33.736 What's happened is a particle accelerator 0:03:33.736,0:03:35.627 at a huge, huge scale. 0:03:35.627,0:03:37.404 And this is very important, 0:03:37.404,0:03:39.287 because what it means is that very, very small 0:03:39.287,0:03:42.371 effects that might be difficult to detect in the lab, 0:03:42.371,0:03:44.550 might be compounded and compounded 0:03:44.550,0:03:48.359 into something that we could [br]possibly observe in nature. 0:03:48.359,0:03:49.871 So, it's very funny. 0:03:49.871,0:03:51.819 The reason why galaxy clusters 0:03:51.819,0:03:53.485 can teach us about dark matter, 0:03:53.485,0:03:55.235 the reason why galaxy clusters 0:03:55.235,0:03:57.788 can teach us about the physics of the very small, 0:03:57.788,0:04:00.819 is precisely because they are so very big. 0:04:00.819,0:04:04.904 Fourth thing: the physics of the very strange. 0:04:04.904,0:04:07.954 Certainly what I've said so far is crazy. 0:04:07.954,0:04:10.237 Okay, if there's anything stranger 0:04:10.237,0:04:12.654 I think it has to be dark energy. 0:04:12.654,0:04:14.371 If I throw a ball into the air, 0:04:14.371,0:04:16.168 I expect it to go up. 0:04:16.168,0:04:18.293 What I don't expect is that it go up 0:04:18.293,0:04:20.459 at an ever-increasing rate. 0:04:20.459,0:04:23.204 Similarly, cosmologists understand why 0:04:23.204,0:04:24.786 the universe is expanding. 0:04:24.786,0:04:26.954 They don't understand why it's expanding 0:04:26.954,0:04:28.892 at an ever-increasing rate. 0:04:28.892,0:04:30.570 They give the cause of this 0:04:30.570,0:04:32.162 accelerated expansion a name, 0:04:32.162,0:04:34.046 and they call it dark energy. 0:04:34.046,0:04:36.734 And, again, we want to learn more about it. 0:04:36.734,0:04:39.121 So, one particular question that we have is, 0:04:39.121,0:04:41.816 how does dark energy affect the universe 0:04:41.816,0:04:43.434 at the largest scales? 0:04:43.434,0:04:44.893 Depending on how strong it is, 0:04:44.893,0:04:48.154 maybe structure forms faster or slower. 0:04:48.154,0:04:50.703 Well, the problem with the large-scale structure 0:04:50.703,0:04:53.041 of the universe is that it's horribly complicated. 0:04:53.041,0:04:55.370 Here is a computer simulation. 0:04:55.370,0:04:57.498 And we need a way to simplify it. 0:04:57.498,0:05:01.069 Well, I like to think about this using an analogy. 0:05:01.069,0:05:03.787 If I want to understand the sinking of the Titanic, 0:05:03.787,0:05:05.453 the most important thing to do 0:05:05.453,0:05:07.234 is not to model the little positions 0:05:07.234,0:05:10.155 of every single little piece of the boat that broke off. 0:05:10.155,0:05:11.820 The most important thing to do is 0:05:11.820,0:05:13.821 to track the two biggest parts. 0:05:13.821,0:05:17.653 Similarly, I can learn a lot about the universe 0:05:17.653,0:05:19.120 at the largest scales 0:05:19.120,0:05:20.871 by tracking its biggest pieces 0:05:20.871,0:05:24.862 and those biggest pieces are clusters of galaxies. 0:05:24.862,0:05:27.818 So, as I come to a close, 0:05:27.818,0:05:29.820 you might feel slightly cheated. 0:05:29.820,0:05:32.070 I mean, I began by talking about 0:05:32.070,0:05:34.153 how galaxy clusters are useful, 0:05:34.153,0:05:35.653 and I've given some reasons, 0:05:35.653,0:05:38.253 but what is their use really? 0:05:38.253,0:05:40.370 Well, to answer this, 0:05:40.370,0:05:42.986 I want to give you a quote by Henry Ford 0:05:42.986,0:05:44.472 when he was asked about cars. 0:05:44.472,0:05:45.831 He had this to say: 0:05:45.831,0:05:48.120 "If I had asked people what they wanted, 0:05:48.120,0:05:50.903 they would have said faster horses." 0:05:50.903,0:05:53.404 Today, we as a society are faced 0:05:53.404,0:05:55.622 with many, many difficult problems. 0:05:55.622,0:05:59.453 And the solutions to these [br]problems are not obvious. 0:05:59.453,0:06:01.620 They are not faster horses. 0:06:01.620,0:06:03.862 They will require an enormous amount of 0:06:03.862,0:06:05.568 scientific ingenuity. 0:06:05.568,0:06:07.154 So, yes, we need to focus, 0:06:07.154,0:06:08.904 yes, we need to concentrate, 0:06:08.904,0:06:10.870 but we also need to remember that 0:06:10.870,0:06:13.705 innovation, ingenuity, inspiration -- 0:06:13.705,0:06:15.037 these things come 0:06:15.037,0:06:16.725 when we broaden our field of vision 0:06:16.725,0:06:18.065 when we step back 0:06:18.065,0:06:19.414 when we zoom out. 0:06:19.414,0:06:21.237 And I can't think of a better way to do this than 0:06:21.237,0:06:24.992 by studying the universe around us. Thanks. 0:06:24.992,0:06:27.941 (Applause)