0:00:01.934,0:00:05.268 Here are some images of clusters of galaxies. 0:00:05.268,0:00:07.101 They're exactly what they sound like. 0:00:07.101,0:00:09.654 They are these huge collections of galaxies, 0:00:09.654,0:00:11.909 bound together by their mutual gravity. 0:00:11.909,0:00:14.571 So most of the points that you see on the screen 0:00:14.571,0:00:17.100 are not individual stars, 0:00:17.100,0:00:19.936 but collections of stars, or galaxies. 0:00:19.936,0:00:22.267 Now, by showing you some of these images, 0:00:22.267,0:00:24.014 I hope that you will quickly see that 0:00:24.014,0:00:26.694 galaxy clusters are these beautiful objects, 0:00:26.694,0:00:28.187 but more than that, 0:00:28.187,0:00:30.850 I think galaxy clusters are mysterious, 0:00:30.850,0:00:32.403 they are surprising, 0:00:32.403,0:00:33.820 and they're useful. 0:00:33.820,0:00:37.351 Useful as the universe's most massive laboratories. 0:00:37.351,0:00:40.987 And as laboratories, to describe galaxy clusters 0:00:40.987,0:00:42.852 is to describe the experiments 0:00:42.852,0:00:44.405 that you can do with them. 0:00:44.405,0:00:46.685 And I think there are four major types, 0:00:46.685,0:00:49.269 and the first type that I want to describe 0:00:49.269,0:00:51.488 is probing the very big. 0:00:51.488,0:00:53.183 So, how big? 0:00:53.183,0:00:57.434 Well, here is an image of a particular galaxy cluster. 0:00:57.434,0:01:00.403 It is so massive that the light passing through it 0:01:00.403,0:01:03.018 is being bent, it's being distorted 0:01:03.018,0:01:05.527 by the extreme gravity of this cluster. 0:01:05.527,0:01:07.331 And, in fact, if you look very carefully 0:01:07.331,0:01:10.031 you'll be able to see rings around this cluster. 0:01:10.031,0:01:11.580 Now, to give you a number, 0:01:11.580,0:01:13.363 this particular galaxy cluster 0:01:13.363,0:01:17.571 has a mass of over one million billion suns. 0:01:17.571,0:01:20.884 It's just mind-boggling how [br]massive these systems can get. 0:01:20.884,0:01:22.385 But more than their mass, 0:01:22.385,0:01:24.359 they have this additional feature. 0:01:24.359,0:01:26.703 They are essentially isolated systems, 0:01:26.703,0:01:28.925 so if we like, we can think of them 0:01:28.925,0:01:32.195 as a scaled-down version of the entire universe. 0:01:32.195,0:01:34.197 And many of the questions that we might have 0:01:34.197,0:01:36.165 about the universe at large scales, 0:01:36.165,0:01:38.080 such as, how does gravity work? 0:01:38.080,0:01:41.112 might be answered by studying these systems. 0:01:41.112,0:01:42.445 So that was very big. 0:01:42.445,0:01:44.330 The second things is very hot. 0:01:44.330,0:01:47.113 Okay, if I take an image of a galaxy cluster, 0:01:47.113,0:01:50.030 and I subtract away all of the starlight, 0:01:50.030,0:01:52.825 what I'm left with is this big, blue blob. 0:01:52.825,0:01:54.310 This is in false color. 0:01:54.310,0:01:56.659 It's actually X-ray light that we're seeing. 0:01:56.659,0:01:59.320 And the question is, if it's not galaxies, 0:01:59.320,0:02:01.519 what is emitting this light? 0:02:01.519,0:02:03.419 The answer is hot gas, 0:02:03.419,0:02:05.242 million-degree gas -- 0:02:05.242,0:02:06.899 in fact, it's plasma. 0:02:06.899,0:02:08.558 And the reason why it's so hot 0:02:08.558,0:02:10.538 goes back to the previous slide. 0:02:10.538,0:02:13.278 The extreme gravity of these systems 0:02:13.278,0:02:16.320 is accelerating particles of gas to great speeds, 0:02:16.320,0:02:18.902 and great speeds means great temperatures. 0:02:18.902,0:02:20.621 So this is the main idea, 0:02:20.621,0:02:22.987 but science is a rough draft. 0:02:22.987,0:02:25.538 There are many basic properties about this plasma 0:02:25.538,0:02:27.236 that still confuse us, 0:02:27.236,0:02:28.569 still puzzle us, 0:02:28.569,0:02:30.655 and still push our understanding 0:02:30.655,0:02:32.694 of the physics of the very hot. 0:02:32.694,0:02:36.121 Third thing: probing the very small. 0:02:36.121,0:02:38.967 Now, to explain this, I need to tell you 0:02:38.967,0:02:40.987 a very disturbing fact. 0:02:40.987,0:02:43.777 Most of the universe's matter 0:02:43.777,0:02:45.588 is not made up of atoms. 0:02:45.588,0:02:47.538 You were lied to. 0:02:47.538,0:02:50.665 Most of it is made up of something [br]very, very mysterious, 0:02:50.665,0:02:52.787 which we call dark matter. 0:02:52.787,0:02:56.454 Dark matter is something that [br]doesn't like to interact very much, 0:02:56.454,0:02:58.122 except through gravity, 0:02:58.122,0:02:59.734 and of course we would like to learn more about it. 0:02:59.734,0:03:01.277 If you're a particle physicist, 0:03:01.277,0:03:03.786 you want to know what happens [br]when we smash things together. 0:03:03.786,0:03:05.776 And dark matter is no exception. 0:03:05.776,0:03:07.436 Well, how do we do this? 0:03:07.436,0:03:08.927 To answer that question, 0:03:08.927,0:03:10.320 I'm going to have to ask another one, 0:03:10.320,0:03:13.486 which is, what happens when galaxy clusters collide? 0:03:13.486,0:03:15.796 Here is an image. 0:03:15.796,0:03:18.987 Since galaxy clusters are representative 0:03:18.987,0:03:22.111 slices of the universe, scaled-down versions. 0:03:22.111,0:03:24.234 They are mostly made up of dark matter, 0:03:24.234,0:03:26.787 and that's what you see in this bluish purple. 0:03:26.787,0:03:28.404 The red represents the hot gas, 0:03:28.404,0:03:30.371 and, of course, you can see many galaxies. 0:03:30.371,0:03:32.736 What's happened is a particle accelerator 0:03:32.736,0:03:34.627 at a huge, huge scale. 0:03:34.627,0:03:36.404 And this is very important, 0:03:36.404,0:03:38.287 because what it means is that very, very small 0:03:38.287,0:03:41.371 effects that might be difficult to detect in the lab, 0:03:41.371,0:03:43.550 might be compounded and compounded 0:03:43.550,0:03:47.359 into something that we could [br]possibly observe in nature. 0:03:47.359,0:03:48.871 So, it's very funny. 0:03:48.871,0:03:50.819 The reason why galaxy clusters 0:03:50.819,0:03:52.485 can teach us about dark matter, 0:03:52.485,0:03:54.235 the reason why galaxy clusters 0:03:54.235,0:03:56.788 can teach us about the physics of the very small, 0:03:56.788,0:03:59.819 is precisely because they are so very big. 0:03:59.819,0:04:03.904 Fourth thing: the physics of the very strange. 0:04:03.904,0:04:06.954 Certainly what I've said so far is crazy. 0:04:06.954,0:04:09.237 Okay, if there's anything stranger 0:04:09.237,0:04:11.654 I think it has to be dark energy. 0:04:11.654,0:04:13.371 If I throw a ball into the air, 0:04:13.371,0:04:15.169 I expect it to go up. 0:04:15.169,0:04:17.293 What I don't expect is that it go up 0:04:17.293,0:04:19.459 at an ever-increasing rate. 0:04:19.459,0:04:22.204 Similarly, cosmologists understand why 0:04:22.204,0:04:23.786 the universe is expanding. 0:04:23.786,0:04:25.954 They don't understand why it's expanding 0:04:25.954,0:04:27.892 at an ever-increasing rate. 0:04:27.892,0:04:29.570 They give the cause of this 0:04:29.570,0:04:31.162 accelerated expansion a name, 0:04:31.162,0:04:33.046 and they call it dark energy. 0:04:33.046,0:04:35.734 And, again, we want to learn more about it. 0:04:35.734,0:04:38.121 So, one particular question that we have is, 0:04:38.121,0:04:40.816 how does dark energy affect the universe 0:04:40.816,0:04:42.434 at the largest scales? 0:04:42.434,0:04:43.893 Depending on how strong it is, 0:04:43.893,0:04:47.154 maybe structure forms faster or slower. 0:04:47.154,0:04:49.703 Well, the problem with the large-scale structure 0:04:49.703,0:04:52.041 of the universe is that it's horribly complicated. 0:04:52.041,0:04:54.370 Here is a computer simulation. 0:04:54.370,0:04:56.498 And we need a way to simplify it. 0:04:56.498,0:05:00.069 Well, I like to think about this using an analogy. 0:05:00.069,0:05:02.787 If I want to understand the sinking of the Titanic, 0:05:02.787,0:05:04.453 the most important thing to do 0:05:04.453,0:05:06.234 is not to model the little positions 0:05:06.234,0:05:09.155 of every single little piece of the boat that broke off. 0:05:09.155,0:05:10.820 The most important thing to do is 0:05:10.820,0:05:12.821 to track the two biggest parts. 0:05:12.821,0:05:16.653 Similarly, I can learn a lot about the universe 0:05:16.653,0:05:18.120 at the largest scales 0:05:18.120,0:05:19.871 by tracking its biggest pieces 0:05:19.871,0:05:23.862 and those biggest pieces are clusters of galaxies. 0:05:23.862,0:05:26.818 So, as I come to a close, 0:05:26.818,0:05:28.820 you might feel slightly cheated. 0:05:28.820,0:05:31.070 I mean, I began by talking about 0:05:31.070,0:05:33.153 how galaxy clusters are useful, 0:05:33.153,0:05:34.653 and I've given some reasons, 0:05:34.653,0:05:37.253 but what is their use really? 0:05:37.253,0:05:39.370 Well, to answer this, 0:05:39.370,0:05:41.986 I want to give you a quote by Henry Ford 0:05:41.986,0:05:43.472 when he was asked about cars. 0:05:43.472,0:05:44.831 He had this to say: 0:05:44.831,0:05:47.120 "If I had asked people what they wanted, 0:05:47.120,0:05:49.903 they would have said faster horses." 0:05:49.903,0:05:52.404 Today, we as a society are faced 0:05:52.404,0:05:54.622 with many, many difficult problems. 0:05:54.622,0:05:58.453 And the solutions to these [br]problems are not obvious. 0:05:58.453,0:06:00.620 They are not faster horses. 0:06:00.620,0:06:02.862 They will require an enormous amount of 0:06:02.862,0:06:04.568 scientific ingenuity. 0:06:04.568,0:06:06.154 So, yes, we need to focus, 0:06:06.154,0:06:07.904 yes, we need to concentrate, 0:06:07.904,0:06:09.870 but we also need to remember that 0:06:09.870,0:06:12.705 innovation, ingenuity, inspiration -- 0:06:12.705,0:06:14.037 these things come 0:06:14.037,0:06:15.725 when we broaden our field of vision 0:06:15.725,0:06:17.065 when we step back 0:06:17.065,0:06:18.414 when we zoom out. 0:06:18.414,0:06:20.237 And I can't think of a better way to do this than 0:06:20.237,0:06:23.992 by studying the universe around us. Thanks. 0:06:23.992,0:06:26.941 (Applause)