WEBVTT 00:00:06.494 --> 00:00:07.890 Looking up at the night sky, 00:00:07.890 --> 00:00:10.910 we are amazed by how it seems to go on forever. 00:00:10.910 --> 00:00:12.395 But what will the sky look like 00:00:12.395 --> 00:00:13.865 billions of years from now? 00:00:13.865 --> 00:00:15.372 A particular type of scientist, 00:00:15.372 --> 00:00:16.616 called a cosmologist, 00:00:16.616 --> 00:00:19.531 spends her time thinking about that very question. 00:00:19.531 --> 00:00:21.924 The end of the universe is intimately linked 00:00:21.924 --> 00:00:23.868 to what the universe contains. 00:00:23.868 --> 00:00:25.170 Over 100 years ago, 00:00:25.170 --> 00:00:27.879 Einstein developed the Theory of General Relativity, 00:00:27.879 --> 00:00:29.923 formed of equations that help us 00:00:29.923 --> 00:00:31.258 understand the relationship 00:00:31.258 --> 00:00:33.462 between what a universe is made of 00:00:33.462 --> 00:00:34.511 and its shape. 00:00:34.511 --> 00:00:36.124 It turns out that the universe 00:00:36.124 --> 00:00:38.178 could be curved like a ball or sphere. 00:00:38.178 --> 00:00:40.676 We call this positively curved or closed. 00:00:40.676 --> 00:00:42.228 Or it could be shaped like a saddle. 00:00:42.228 --> 00:00:44.488 We call this negatively curved or open. 00:00:44.488 --> 00:00:46.033 Or it could be flat. 00:00:46.033 --> 00:00:47.155 And that shape determines 00:00:47.155 --> 00:00:49.537 how the universe will live and die. 00:00:49.537 --> 00:00:52.632 We now know that the universe is very close to flat. 00:00:52.632 --> 00:00:54.338 However, the components of the universe 00:00:54.338 --> 00:00:56.454 can still affect its eventual fate. 00:00:56.454 --> 00:00:58.033 We can predict how the universe 00:00:58.033 --> 00:00:59.611 will change with time 00:00:59.611 --> 00:01:01.793 if we measure the amounts or energy densities 00:01:01.793 --> 00:01:04.588 of the various components in the universe today. 00:01:04.588 --> 00:01:06.727 So, what is the universe made of? 00:01:06.727 --> 00:01:09.393 The universe contains all the things that we can see, 00:01:09.393 --> 00:01:11.564 like stars, gas, and planets. 00:01:11.564 --> 00:01:14.733 We call these things ordinary or baryonic matter. 00:01:14.733 --> 00:01:16.593 Even though we see them all around us, 00:01:16.593 --> 00:01:18.702 the total energy density of these components 00:01:18.702 --> 00:01:20.440 is actually very small, 00:01:20.440 --> 00:01:23.530 around 5% of the total energy of the universe. 00:01:23.530 --> 00:01:26.675 So, now let's talk about what the other 95% is. 00:01:26.675 --> 00:01:29.081 Just under 27% of the rest 00:01:29.081 --> 00:01:30.913 of the energy density of the universe 00:01:30.913 --> 00:01:33.825 is made up of what we call dark matter. 00:01:33.825 --> 00:01:36.928 Dark matter is only very weakly interacting with light, 00:01:36.928 --> 00:01:39.208 which means it doesn't shine or reflect light 00:01:39.208 --> 00:01:41.191 in the way that stars and planets do, 00:01:41.191 --> 00:01:42.288 but, in every other way, 00:01:42.288 --> 00:01:44.259 it behaves like ordinary matter -- 00:01:44.259 --> 00:01:46.128 it attracts things gravitationally. 00:01:46.128 --> 00:01:48.761 In fact, the only way we can detect this dark matter 00:01:48.761 --> 00:01:51.052 is through this gravitational interaction, 00:01:51.052 --> 00:01:52.343 how things orbit around it 00:01:52.343 --> 00:01:53.595 and how it bends light 00:01:53.595 --> 00:01:56.008 as it curves the space around it. 00:01:56.008 --> 00:01:58.343 We have yet to discover a dark matter particle, 00:01:58.343 --> 00:02:00.815 but scientists all over the world are searching 00:02:00.815 --> 00:02:02.950 for this elusive particle or particles 00:02:02.950 --> 00:02:05.843 and the effects of dark matter on the universe. 00:02:05.843 --> 00:02:08.256 But this still doesn't add up to 100%. 00:02:08.256 --> 00:02:09.784 The remaining 68% 00:02:09.784 --> 00:02:11.680 of the energy density of the universe 00:02:11.680 --> 00:02:13.759 is made up of dark energy, 00:02:13.759 --> 00:02:16.457 which is even more mysterious than dark matter. 00:02:16.457 --> 00:02:18.422 This dark energy doesn't behave 00:02:18.422 --> 00:02:20.590 like any other substance we know at all 00:02:20.590 --> 00:02:23.013 and acts more like anti-gravity force. 00:02:23.013 --> 00:02:25.420 We say that it has a gravitational pressure, 00:02:25.420 --> 00:02:28.330 which ordinary matter and dark matter do not. 00:02:28.330 --> 00:02:30.213 Instead of pulling the universe together, 00:02:30.213 --> 00:02:32.004 as we would expect gravity to do, 00:02:32.004 --> 00:02:34.287 the universe appears to be expanding apart 00:02:34.287 --> 00:02:36.110 at an ever-increasing rate. 00:02:36.110 --> 00:02:37.938 The leading idea for dark energy 00:02:37.938 --> 00:02:40.148 is that it is a cosmological constant. 00:02:40.148 --> 00:02:42.245 That means it has the strange property 00:02:42.245 --> 00:02:45.434 that it expands as the volume of space increases 00:02:45.434 --> 00:02:47.606 to keep its energy density constant. 00:02:47.606 --> 00:02:49.441 So, as the universe expands 00:02:49.441 --> 00:02:50.772 as it is doing right now, 00:02:50.772 --> 00:02:52.742 there will be more and more dark energy. 00:02:52.742 --> 00:02:54.574 Dark matter and baryonic matter, 00:02:54.574 --> 00:02:55.277 on the other hand, 00:02:55.277 --> 00:02:56.622 don't expand with the universe 00:02:56.622 --> 00:02:58.409 and become more diluted. 00:02:58.409 --> 00:02:59.335 Because of this property 00:02:59.335 --> 00:03:00.694 of the cosmological constant, 00:03:00.694 --> 00:03:03.451 the future universe will be more and more dominated 00:03:03.451 --> 00:03:04.592 by dark energy, 00:03:04.592 --> 00:03:06.330 becoming colder and colder 00:03:06.330 --> 00:03:08.715 and expanding faster and faster. 00:03:08.715 --> 00:03:10.736 Eventually, the universe will run out of gas 00:03:10.736 --> 00:03:11.842 to form stars, 00:03:11.842 --> 00:03:13.941 and the stars themselves will run out of fuel 00:03:13.941 --> 00:03:15.192 and burn out, 00:03:15.192 --> 00:03:18.062 leaving the universe with only black holes in it. 00:03:18.062 --> 00:03:19.255 Given enough time, 00:03:19.255 --> 00:03:21.610 even these black holes will evaporate, 00:03:21.610 --> 00:03:24.523 leaving a universe that is completely cold and empty. 00:03:24.523 --> 00:03:28.127 That is what we call the heat death of the universe. 00:03:28.127 --> 00:03:29.569 While it might sound depressing 00:03:29.569 --> 00:03:30.694 living in a universe 00:03:30.694 --> 00:03:32.735 that will end its lifetime cold 00:03:32.735 --> 00:03:34.237 and devoid of life, 00:03:34.237 --> 00:03:36.027 the end fate of our universe 00:03:36.027 --> 00:03:37.937 actually has a beautiful symmetry 00:03:37.937 --> 00:03:40.087 to its hot, fiery beginning. 00:03:40.087 --> 00:03:41.998 We call the accelerating end state 00:03:41.998 --> 00:03:44.111 of the universe a de Sitter phase, 00:03:44.111 --> 00:03:46.082 named after the Dutch mathematician 00:03:46.082 --> 00:03:47.927 Willem de Sitter. 00:03:47.927 --> 00:03:49.685 However, we also believe 00:03:49.685 --> 00:03:51.520 that the universe had another phase 00:03:51.520 --> 00:03:52.831 of de Sitter expansion 00:03:52.831 --> 00:03:54.607 in the earliest times of its life. 00:03:54.607 --> 00:03:57.228 We call this early period inflation, 00:03:57.228 --> 00:03:58.894 where, shortly after the Big Bang, 00:03:58.894 --> 00:04:01.219 the universe expanded extremely fast 00:04:01.219 --> 00:04:02.860 for a brief period. 00:04:02.860 --> 00:04:04.391 So, the universe will end 00:04:04.391 --> 00:04:06.858 in much the same state as it began, 00:04:06.858 --> 00:04:08.768 accelerating. 00:04:08.768 --> 00:04:10.778 We live at an extraordinary time 00:04:10.778 --> 00:04:12.350 in the life of the universe 00:04:12.350 --> 00:04:13.861 where we can start to understand 00:04:13.861 --> 00:04:15.153 the universe's journey 00:04:15.153 --> 00:04:16.437 and view a history 00:04:16.437 --> 00:04:18.520 that plays itself out on the sky 00:04:18.520 --> 00:04:20.622 for all of us to see.