0:00:00.000,0:00:02.000 The universe 0:00:02.000,0:00:04.000 is really big. 0:00:04.000,0:00:07.000 We live in a galaxy, the Milky Way Galaxy. 0:00:07.000,0:00:10.000 There are about a hundred billion stars in the Milky Way Galaxy. 0:00:10.000,0:00:12.000 And if you take a camera 0:00:12.000,0:00:14.000 and you point it at a random part of the sky, 0:00:14.000,0:00:16.000 and you just keep the shutter open, 0:00:16.000,0:00:19.000 as long as your camera is attached to the Hubble Space Telescope, 0:00:19.000,0:00:21.000 it will see something like this. 0:00:21.000,0:00:24.000 Every one of these little blobs 0:00:24.000,0:00:26.000 is a galaxy roughly the size of our Milky Way -- 0:00:26.000,0:00:29.000 a hundred billion stars in each of those blobs. 0:00:29.000,0:00:32.000 There are approximately a hundred billion galaxies 0:00:32.000,0:00:34.000 in the observable universe. 0:00:34.000,0:00:36.000 100 billion is the only number you need to know. 0:00:36.000,0:00:39.000 The age of the universe, between now and the Big Bang, 0:00:39.000,0:00:41.000 is a hundred billion in dog years. 0:00:41.000,0:00:43.000 (Laughter) 0:00:43.000,0:00:46.000 Which tells you something about our place in the universe. 0:00:46.000,0:00:48.000 One thing you can do with a picture like this is simply admire it. 0:00:48.000,0:00:50.000 It's extremely beautiful. 0:00:50.000,0:00:53.000 I've often wondered, what is the evolutionary pressure 0:00:53.000,0:00:56.000 that made our ancestors in the Veldt adapt and evolve 0:00:56.000,0:00:58.000 to really enjoy pictures of galaxies 0:00:58.000,0:01:00.000 when they didn't have any. 0:01:00.000,0:01:02.000 But we would also like to understand it. 0:01:02.000,0:01:06.000 As a cosmologist, I want to ask, why is the universe like this? 0:01:06.000,0:01:09.000 One big clue we have is that the universe is changing with time. 0:01:09.000,0:01:12.000 If you looked at one of these galaxies and measured its velocity, 0:01:12.000,0:01:14.000 it would be moving away from you. 0:01:14.000,0:01:16.000 And if you look at a galaxy even farther away, 0:01:16.000,0:01:18.000 it would be moving away faster. 0:01:18.000,0:01:20.000 So we say the universe is expanding. 0:01:20.000,0:01:22.000 What that means, of course, is that, in the past, 0:01:22.000,0:01:24.000 things were closer together. 0:01:24.000,0:01:26.000 In the past, the universe was more dense, 0:01:26.000,0:01:28.000 and it was also hotter. 0:01:28.000,0:01:30.000 If you squeeze things together, the temperature goes up. 0:01:30.000,0:01:32.000 That kind of makes sense to us. 0:01:32.000,0:01:34.000 The thing that doesn't make sense to us as much 0:01:34.000,0:01:37.000 is that the universe, at early times, near the Big Bang, 0:01:37.000,0:01:39.000 was also very, very smooth. 0:01:39.000,0:01:41.000 You might think that that's not a surprise. 0:01:41.000,0:01:43.000 The air in this room is very smooth. 0:01:43.000,0:01:46.000 You might say, "Well, maybe things just smoothed themselves out." 0:01:46.000,0:01:49.000 But the conditions near the Big Bang are very, very different 0:01:49.000,0:01:51.000 than the conditions of the air in this room. 0:01:51.000,0:01:53.000 In particular, things were a lot denser. 0:01:53.000,0:01:55.000 The gravitational pull of things 0:01:55.000,0:01:57.000 was a lot stronger near the Big Bang. 0:01:57.000,0:01:59.000 What you have to think about 0:01:59.000,0:02:01.000 is we have a universe with a hundred billion galaxies, 0:02:01.000,0:02:03.000 a hundred billion stars each. 0:02:03.000,0:02:06.000 At early times, those hundred billion galaxies 0:02:06.000,0:02:09.000 were squeezed into a region about this big -- 0:02:09.000,0:02:11.000 literally -- at early times. 0:02:11.000,0:02:13.000 And you have to imagine doing that squeezing 0:02:13.000,0:02:15.000 without any imperfections, 0:02:15.000,0:02:17.000 without any little spots 0:02:17.000,0:02:19.000 where there were a few more atoms than somewhere else. 0:02:19.000,0:02:22.000 Because if there had been, they would have collapsed under the gravitational pull 0:02:22.000,0:02:24.000 into a huge black hole. 0:02:24.000,0:02:27.000 Keeping the universe very, very smooth at early times 0:02:27.000,0:02:29.000 is not easy; it's a delicate arrangement. 0:02:29.000,0:02:31.000 It's a clue 0:02:31.000,0:02:33.000 that the early universe is not chosen randomly. 0:02:33.000,0:02:35.000 There is something that made it that way. 0:02:35.000,0:02:37.000 We would like to know what. 0:02:37.000,0:02:40.000 So part of our understanding of this was given to us by Ludwig Boltzmann, 0:02:40.000,0:02:43.000 an Austrian physicist in the 19th century. 0:02:43.000,0:02:46.000 And Boltzmann's contribution was that he helped us understand entropy. 0:02:46.000,0:02:48.000 You've heard of entropy. 0:02:48.000,0:02:51.000 It's the randomness, the disorder, the chaoticness of some systems. 0:02:51.000,0:02:53.000 Boltzmann gave us a formula -- 0:02:53.000,0:02:55.000 engraved on his tombstone now -- 0:02:55.000,0:02:57.000 that really quantifies what entropy is. 0:02:57.000,0:02:59.000 And it's basically just saying 0:02:59.000,0:03:01.000 that entropy is the number of ways 0:03:01.000,0:03:04.000 we can rearrange the constituents of a system so that you don't notice, 0:03:04.000,0:03:06.000 so that macroscopically it looks the same. 0:03:06.000,0:03:08.000 If you have the air in this room, 0:03:08.000,0:03:11.000 you don't notice each individual atom. 0:03:11.000,0:03:13.000 A low entropy configuration 0:03:13.000,0:03:15.000 is one in which there's only a few arrangements that look that way. 0:03:15.000,0:03:17.000 A high entropy arrangement 0:03:17.000,0:03:19.000 is one that there are many arrangements that look that way. 0:03:19.000,0:03:21.000 This is a crucially important insight 0:03:21.000,0:03:23.000 because it helps us explain 0:03:23.000,0:03:25.000 the second law of thermodynamics -- 0:03:25.000,0:03:28.000 the law that says that entropy increases in the universe, 0:03:28.000,0:03:30.000 or in some isolated bit of the universe. 0:03:30.000,0:03:32.000 The reason why entropy increases 0:03:32.000,0:03:35.000 is simply because there are many more ways 0:03:35.000,0:03:37.000 to be high entropy than to be low entropy. 0:03:37.000,0:03:39.000 That's a wonderful insight, 0:03:39.000,0:03:41.000 but it leaves something out. 0:03:41.000,0:03:43.000 This insight that entropy increases, by the way, 0:03:43.000,0:03:46.000 is what's behind what we call the arrow of time, 0:03:46.000,0:03:48.000 the difference between the past and the future. 0:03:48.000,0:03:50.000 Every difference that there is 0:03:50.000,0:03:52.000 between the past and the future 0:03:52.000,0:03:54.000 is because entropy is increasing -- 0:03:54.000,0:03:57.000 the fact that you can remember the past, but not the future. 0:03:57.000,0:04:00.000 The fact that you are born, and then you live, and then you die, 0:04:00.000,0:04:02.000 always in that order, 0:04:02.000,0:04:04.000 that's because entropy is increasing. 0:04:04.000,0:04:06.000 Boltzmann explained that if you start with low entropy, 0:04:06.000,0:04:08.000 it's very natural for it to increase 0:04:08.000,0:04:11.000 because there's more ways to be high entropy. 0:04:11.000,0:04:13.000 What he didn't explain 0:04:13.000,0:04:16.000 was why the entropy was ever low in the first place. 0:04:16.000,0:04:18.000 The fact that the entropy of the universe was low 0:04:18.000,0:04:20.000 was a reflection of the fact 0:04:20.000,0:04:22.000 that the early universe was very, very smooth. 0:04:22.000,0:04:24.000 We'd like to understand that. 0:04:24.000,0:04:26.000 That's our job as cosmologists. 0:04:26.000,0:04:28.000 Unfortunately, it's actually not a problem 0:04:28.000,0:04:30.000 that we've been giving enough attention to. 0:04:30.000,0:04:32.000 It's not one of the first things people would say, 0:04:32.000,0:04:34.000 if you asked a modern cosmologist, 0:04:34.000,0:04:36.000 "What are the problems we're trying to address?" 0:04:36.000,0:04:38.000 One of the people who did understand that this was a problem 0:04:38.000,0:04:40.000 was Richard Feynman. 0:04:40.000,0:04:42.000 50 years ago, he gave a series of a bunch of different lectures. 0:04:42.000,0:04:44.000 He gave the popular lectures 0:04:44.000,0:04:46.000 that became "The Character of Physical Law." 0:04:46.000,0:04:48.000 He gave lectures to Caltech undergrads 0:04:48.000,0:04:50.000 that became "The Feynman Lectures on Physics." 0:04:50.000,0:04:52.000 He gave lectures to Caltech graduate students 0:04:52.000,0:04:54.000 that became "The Feynman Lectures on Gravitation." 0:04:54.000,0:04:57.000 In every one of these books, every one of these sets of lectures, 0:04:57.000,0:04:59.000 he emphasized this puzzle: 0:04:59.000,0:05:02.000 Why did the early universe have such a small entropy? 0:05:02.000,0:05:04.000 So he says -- I'm not going to do the accent -- 0:05:04.000,0:05:07.000 he says, "For some reason, the universe, at one time, 0:05:07.000,0:05:10.000 had a very low entropy for its energy content, 0:05:10.000,0:05:12.000 and since then the entropy has increased. 0:05:12.000,0:05:15.000 The arrow of time cannot be completely understood 0:05:15.000,0:05:18.000 until the mystery of the beginnings of the history of the universe 0:05:18.000,0:05:20.000 are reduced still further 0:05:20.000,0:05:22.000 from speculation to understanding." 0:05:22.000,0:05:24.000 So that's our job. 0:05:24.000,0:05:26.000 We want to know -- this is 50 years ago, "Surely," you're thinking, 0:05:26.000,0:05:28.000 "we've figured it out by now." 0:05:28.000,0:05:30.000 It's not true that we've figured it out by now. 0:05:30.000,0:05:32.000 The reason the problem has gotten worse, 0:05:32.000,0:05:34.000 rather than better, 0:05:34.000,0:05:36.000 is because in 1998 0:05:36.000,0:05:39.000 we learned something crucial about the universe that we didn't know before. 0:05:39.000,0:05:41.000 We learned that it's accelerating. 0:05:41.000,0:05:43.000 The universe is not only expanding. 0:05:43.000,0:05:45.000 If you look at the galaxy, it's moving away. 0:05:45.000,0:05:47.000 If you come back a billion years later and look at it again, 0:05:47.000,0:05:50.000 it will be moving away faster. 0:05:50.000,0:05:53.000 Individual galaxies are speeding away from us faster and faster 0:05:53.000,0:05:55.000 so we say the universe is accelerating. 0:05:55.000,0:05:57.000 Unlike the low entropy of the early universe, 0:05:57.000,0:05:59.000 even though we don't know the answer for this, 0:05:59.000,0:06:01.000 we at least have a good theory that can explain it, 0:06:01.000,0:06:03.000 if that theory is right, 0:06:03.000,0:06:05.000 and that's the theory of dark energy. 0:06:05.000,0:06:08.000 It's just the idea that empty space itself has energy. 0:06:08.000,0:06:11.000 In every little cubic centimeter of space, 0:06:11.000,0:06:13.000 whether or not there's stuff, 0:06:13.000,0:06:15.000 whether or not there's particles, matter, radiation or whatever, 0:06:15.000,0:06:18.000 there's still energy, even in the space itself. 0:06:18.000,0:06:20.000 And this energy, according to Einstein, 0:06:20.000,0:06:23.000 exerts a push on the universe. 0:06:23.000,0:06:25.000 It is a perpetual impulse 0:06:25.000,0:06:27.000 that pushes galaxies apart from each other. 0:06:27.000,0:06:30.000 Because dark energy, unlike matter or radiation, 0:06:30.000,0:06:33.000 does not dilute away as the universe expands. 0:06:33.000,0:06:35.000 The amount of energy in each cubic centimeter 0:06:35.000,0:06:37.000 remains the same, 0:06:37.000,0:06:39.000 even as the universe gets bigger and bigger. 0:06:39.000,0:06:42.000 This has crucial implications 0:06:42.000,0:06:45.000 for what the universe is going to do in the future. 0:06:45.000,0:06:47.000 For one thing, the universe will expand forever. 0:06:47.000,0:06:49.000 Back when I was your age, 0:06:49.000,0:06:51.000 we didn't know what the universe was going to do. 0:06:51.000,0:06:54.000 Some people thought that the universe would recollapse in the future. 0:06:54.000,0:06:56.000 Einstein was fond of this idea. 0:06:56.000,0:06:59.000 But if there's dark energy, and the dark energy does not go away, 0:06:59.000,0:07:02.000 the universe is just going to keep expanding forever and ever and ever. 0:07:02.000,0:07:04.000 14 billion years in the past, 0:07:04.000,0:07:06.000 100 billion dog years, 0:07:06.000,0:07:09.000 but an infinite number of years into the future. 0:07:09.000,0:07:12.000 Meanwhile, for all intents and purposes, 0:07:12.000,0:07:14.000 space looks finite to us. 0:07:14.000,0:07:16.000 Space may be finite or infinite, 0:07:16.000,0:07:18.000 but because the universe is accelerating, 0:07:18.000,0:07:20.000 there are parts of it we cannot see 0:07:20.000,0:07:22.000 and never will see. 0:07:22.000,0:07:24.000 There's a finite region of space that we have access to, 0:07:24.000,0:07:26.000 surrounded by a horizon. 0:07:26.000,0:07:28.000 So even though time goes on forever, 0:07:28.000,0:07:30.000 space is limited to us. 0:07:30.000,0:07:33.000 Finally, empty space has a temperature. 0:07:33.000,0:07:35.000 In the 1970s, Stephen Hawking told us 0:07:35.000,0:07:37.000 that a black hole, even though you think it's black, 0:07:37.000,0:07:39.000 it actually emits radiation 0:07:39.000,0:07:41.000 when you take into account quantum mechanics. 0:07:41.000,0:07:44.000 The curvature of space-time around the black hole 0:07:44.000,0:07:47.000 brings to life the quantum mechanical fluctuation, 0:07:47.000,0:07:49.000 and the black hole radiates. 0:07:49.000,0:07:52.000 A precisely similar calculation by Hawking and Gary Gibbons 0:07:52.000,0:07:55.000 showed that if you have dark energy in empty space, 0:07:55.000,0:07:58.000 then the whole universe radiates. 0:07:58.000,0:08:00.000 The energy of empty space 0:08:00.000,0:08:02.000 brings to life quantum fluctuations. 0:08:02.000,0:08:04.000 And so even though the universe will last forever, 0:08:04.000,0:08:07.000 and ordinary matter and radiation will dilute away, 0:08:07.000,0:08:09.000 there will always be some radiation, 0:08:09.000,0:08:11.000 some thermal fluctuations, 0:08:11.000,0:08:13.000 even in empty space. 0:08:13.000,0:08:15.000 So what this means 0:08:15.000,0:08:17.000 is that the universe is like a box of gas 0:08:17.000,0:08:19.000 that lasts forever. 0:08:19.000,0:08:21.000 Well what is the implication of that? 0:08:21.000,0:08:24.000 That implication was studied by Boltzmann back in the 19th century. 0:08:24.000,0:08:27.000 He said, well, entropy increases 0:08:27.000,0:08:29.000 because there are many, many more ways 0:08:29.000,0:08:32.000 for the universe to be high entropy, rather than low entropy. 0:08:32.000,0:08:35.000 But that's a probabilistic statement. 0:08:35.000,0:08:37.000 It will probably increase, 0:08:37.000,0:08:39.000 and the probability is enormously huge. 0:08:39.000,0:08:41.000 It's not something you have to worry about -- 0:08:41.000,0:08:45.000 the air in this room all gathering over one part of the room and suffocating us. 0:08:45.000,0:08:47.000 It's very, very unlikely. 0:08:47.000,0:08:49.000 Except if they locked the doors 0:08:49.000,0:08:51.000 and kept us here literally forever, 0:08:51.000,0:08:53.000 that would happen. 0:08:53.000,0:08:55.000 Everything that is allowed, 0:08:55.000,0:08:58.000 every configuration that is allowed to be obtained by the molecules in this room, 0:08:58.000,0:09:00.000 would eventually be obtained. 0:09:00.000,0:09:03.000 So Boltzmann says, look, you could start with a universe 0:09:03.000,0:09:05.000 that was in thermal equilibrium. 0:09:05.000,0:09:08.000 He didn't know about the Big Bang. He didn't know about the expansion of the universe. 0:09:08.000,0:09:11.000 He thought that space and time were explained by Isaac Newton -- 0:09:11.000,0:09:13.000 they were absolute; they just stuck there forever. 0:09:13.000,0:09:15.000 So his idea of a natural universe 0:09:15.000,0:09:18.000 was one in which the air molecules were just spread out evenly everywhere -- 0:09:18.000,0:09:20.000 the everything molecules. 0:09:20.000,0:09:23.000 But if you're Boltzmann, you know that if you wait long enough, 0:09:23.000,0:09:26.000 the random fluctuations of those molecules 0:09:26.000,0:09:28.000 will occasionally bring them 0:09:28.000,0:09:30.000 into lower entropy configurations. 0:09:30.000,0:09:32.000 And then, of course, in the natural course of things, 0:09:32.000,0:09:34.000 they will expand back. 0:09:34.000,0:09:36.000 So it's not that entropy must always increase -- 0:09:36.000,0:09:39.000 you can get fluctuations into lower entropy, 0:09:39.000,0:09:41.000 more organized situations. 0:09:41.000,0:09:43.000 Well if that's true, 0:09:43.000,0:09:45.000 Boltzmann then goes onto invent 0:09:45.000,0:09:47.000 two very modern-sounding ideas -- 0:09:47.000,0:09:50.000 the multiverse and the anthropic principle. 0:09:50.000,0:09:52.000 He says, the problem with thermal equilibrium 0:09:52.000,0:09:54.000 is that we can't live there. 0:09:54.000,0:09:57.000 Remember, life itself depends on the arrow of time. 0:09:57.000,0:09:59.000 We would not be able to process information, 0:09:59.000,0:10:01.000 metabolize, walk and talk, 0:10:01.000,0:10:03.000 if we lived in thermal equilibrium. 0:10:03.000,0:10:05.000 So if you imagine a very, very big universe, 0:10:05.000,0:10:07.000 an infinitely big universe, 0:10:07.000,0:10:09.000 with randomly bumping into each other particles, 0:10:09.000,0:10:12.000 there will occasionally be small fluctuations in the lower entropy states, 0:10:12.000,0:10:14.000 and then they relax back. 0:10:14.000,0:10:16.000 But there will also be large fluctuations. 0:10:16.000,0:10:18.000 Occasionally, you will make a planet 0:10:18.000,0:10:20.000 or a star or a galaxy 0:10:20.000,0:10:22.000 or a hundred billion galaxies. 0:10:22.000,0:10:24.000 So Boltzmann says, 0:10:24.000,0:10:27.000 we will only live in the part of the multiverse, 0:10:27.000,0:10:30.000 in the part of this infinitely big set of fluctuating particles, 0:10:30.000,0:10:32.000 where life is possible. 0:10:32.000,0:10:34.000 That's the region where entropy is low. 0:10:34.000,0:10:37.000 Maybe our universe is just one of those things 0:10:37.000,0:10:39.000 that happens from time to time. 0:10:39.000,0:10:41.000 Now your homework assignment 0:10:41.000,0:10:43.000 is to really think about this, to contemplate what it means. 0:10:43.000,0:10:45.000 Carl Sagan once famously said 0:10:45.000,0:10:47.000 that "in order to make an apple pie, 0:10:47.000,0:10:50.000 you must first invent the universe." 0:10:50.000,0:10:52.000 But he was not right. 0:10:52.000,0:10:55.000 In Boltzmann's scenario, if you want to make an apple pie, 0:10:55.000,0:10:58.000 you just wait for the random motion of atoms 0:10:58.000,0:11:00.000 to make you an apple pie. 0:11:00.000,0:11:02.000 That will happen much more frequently 0:11:02.000,0:11:04.000 than the random motions of atoms 0:11:04.000,0:11:06.000 making you an apple orchard 0:11:06.000,0:11:08.000 and some sugar and an oven, 0:11:08.000,0:11:10.000 and then making you an apple pie. 0:11:10.000,0:11:13.000 So this scenario makes predictions. 0:11:13.000,0:11:15.000 And the predictions are 0:11:15.000,0:11:18.000 that the fluctuations that make us are minimal. 0:11:18.000,0:11:21.000 Even if you imagine that this room we are in now 0:11:21.000,0:11:23.000 exists and is real and here we are, 0:11:23.000,0:11:25.000 and we have, not only our memories, 0:11:25.000,0:11:27.000 but our impression that outside there's something 0:11:27.000,0:11:31.000 called Caltech and the United States and the Milky Way Galaxy, 0:11:31.000,0:11:34.000 it's much easier for all those impressions to randomly fluctuate into your brain 0:11:34.000,0:11:36.000 than for them actually to randomly fluctuate 0:11:36.000,0:11:39.000 into Caltech, the United States and the galaxy. 0:11:39.000,0:11:41.000 The good news is that, 0:11:41.000,0:11:44.000 therefore, this scenario does not work; it is not right. 0:11:44.000,0:11:47.000 This scenario predicts that we should be a minimal fluctuation. 0:11:47.000,0:11:49.000 Even if you left our galaxy out, 0:11:49.000,0:11:51.000 you would not get a hundred billion other galaxies. 0:11:51.000,0:11:53.000 And Feynman also understood this. 0:11:53.000,0:11:57.000 Feynman says, "From the hypothesis that the world is a fluctuation, 0:11:57.000,0:11:59.000 all the predictions are that 0:11:59.000,0:12:01.000 if we look at a part of the world we've never seen before, 0:12:01.000,0:12:03.000 we will find it mixed up, and not like the piece we've just looked at -- 0:12:03.000,0:12:05.000 high entropy. 0:12:05.000,0:12:07.000 If our order were due to a fluctuation, 0:12:07.000,0:12:09.000 we would not expect order anywhere but where we have just noticed it. 0:12:09.000,0:12:13.000 We therefore conclude the universe is not a fluctuation." 0:12:13.000,0:12:16.000 So that's good. The question is then what is the right answer? 0:12:16.000,0:12:18.000 If the universe is not a fluctuation, 0:12:18.000,0:12:21.000 why did the early universe have a low entropy? 0:12:21.000,0:12:24.000 And I would love to tell you the answer, but I'm running out of time. 0:12:24.000,0:12:26.000 (Laughter) 0:12:26.000,0:12:28.000 Here is the universe that we tell you about, 0:12:28.000,0:12:30.000 versus the universe that really exists. 0:12:30.000,0:12:32.000 I just showed you this picture. 0:12:32.000,0:12:34.000 The universe is expanding for the last 10 billion years or so. 0:12:34.000,0:12:36.000 It's cooling off. 0:12:36.000,0:12:38.000 But we now know enough about the future of the universe 0:12:38.000,0:12:40.000 to say a lot more. 0:12:40.000,0:12:42.000 If the dark energy remains around, 0:12:42.000,0:12:45.000 the stars around us will use up their nuclear fuel, they will stop burning. 0:12:45.000,0:12:47.000 They will fall into black holes. 0:12:47.000,0:12:49.000 We will live in a universe 0:12:49.000,0:12:51.000 with nothing in it but black holes. 0:12:51.000,0:12:55.000 That universe will last 10 to the 100 years -- 0:12:55.000,0:12:57.000 a lot longer than our little universe has lived. 0:12:57.000,0:12:59.000 The future is much longer than the past. 0:12:59.000,0:13:01.000 But even black holes don't last forever. 0:13:01.000,0:13:03.000 They will evaporate, 0:13:03.000,0:13:05.000 and we will be left with nothing but empty space. 0:13:05.000,0:13:09.000 That empty space lasts essentially forever. 0:13:09.000,0:13:12.000 However, you notice, since empty space gives off radiation, 0:13:12.000,0:13:14.000 there's actually thermal fluctuations, 0:13:14.000,0:13:16.000 and it cycles around 0:13:16.000,0:13:18.000 all the different possible combinations 0:13:18.000,0:13:21.000 of the degrees of freedom that exist in empty space. 0:13:21.000,0:13:23.000 So even though the universe lasts forever, 0:13:23.000,0:13:25.000 there's only a finite number of things 0:13:25.000,0:13:27.000 that can possibly happen in the universe. 0:13:27.000,0:13:29.000 They all happen over a period of time 0:13:29.000,0:13:32.000 equal to 10 to the 10 to the 120 years. 0:13:32.000,0:13:34.000 So here's two questions for you. 0:13:34.000,0:13:37.000 Number one: If the universe lasts for 10 to the 10 to the 120 years, 0:13:37.000,0:13:39.000 why are we born 0:13:39.000,0:13:42.000 in the first 14 billion years of it, 0:13:42.000,0:13:45.000 in the warm, comfortable afterglow of the Big Bang? 0:13:45.000,0:13:47.000 Why aren't we in empty space? 0:13:47.000,0:13:49.000 You might say, "Well there's nothing there to be living," 0:13:49.000,0:13:51.000 but that's not right. 0:13:51.000,0:13:53.000 You could be a random fluctuation out of the nothingness. 0:13:53.000,0:13:55.000 Why aren't you? 0:13:55.000,0:13:58.000 More homework assignment for you. 0:13:58.000,0:14:00.000 So like I said, I don't actually know the answer. 0:14:00.000,0:14:02.000 I'm going to give you my favorite scenario. 0:14:02.000,0:14:05.000 Either it's just like that. There is no explanation. 0:14:05.000,0:14:07.000 This is a brute fact about the universe 0:14:07.000,0:14:10.000 that you should learn to accept and stop asking questions. 0:14:11.000,0:14:13.000 Or maybe the Big Bang 0:14:13.000,0:14:15.000 is not the beginning of the universe. 0:14:15.000,0:14:18.000 An egg, an unbroken egg, is a low entropy configuration, 0:14:18.000,0:14:20.000 and yet, when we open our refrigerator, 0:14:20.000,0:14:22.000 we do not go, "Hah, how surprising to find 0:14:22.000,0:14:24.000 this low entropy configuration in our refrigerator." 0:14:24.000,0:14:27.000 That's because an egg is not a closed system; 0:14:27.000,0:14:29.000 it comes out of a chicken. 0:14:29.000,0:14:33.000 Maybe the universe comes out of a universal chicken. 0:14:33.000,0:14:35.000 Maybe there is something that naturally, 0:14:35.000,0:14:38.000 through the growth of the laws of physics, 0:14:38.000,0:14:40.000 gives rise to universe like ours 0:14:40.000,0:14:42.000 in low entropy configurations. 0:14:42.000,0:14:44.000 If that's true, it would happen more than once; 0:14:44.000,0:14:47.000 we would be part of a much bigger multiverse. 0:14:47.000,0:14:49.000 That's my favorite scenario. 0:14:49.000,0:14:52.000 So the organizers asked me to end with a bold speculation. 0:14:52.000,0:14:54.000 My bold speculation 0:14:54.000,0:14:57.000 is that I will be absolutely vindicated by history. 0:14:57.000,0:14:59.000 And 50 years from now, 0:14:59.000,0:15:02.000 all of my current wild ideas will be accepted as truths 0:15:02.000,0:15:05.000 by the scientific and external communities. 0:15:05.000,0:15:07.000 We will all believe that our little universe 0:15:07.000,0:15:10.000 is just a small part of a much larger multiverse. 0:15:10.000,0:15:13.000 And even better, we will understand what happened at the Big Bang 0:15:13.000,0:15:15.000 in terms of a theory 0:15:15.000,0:15:17.000 that we will be able to compare to observations. 0:15:17.000,0:15:19.000 This is a prediction. I might be wrong. 0:15:19.000,0:15:21.000 But we've been thinking as a human race 0:15:21.000,0:15:23.000 about what the universe was like, 0:15:23.000,0:15:26.000 why it came to be in the way it did for many, many years. 0:15:26.000,0:15:29.000 It's exciting to think we may finally know the answer someday. 0:15:29.000,0:15:31.000 Thank you. 0:15:31.000,0:15:33.000 (Applause)