1 00:00:01,595 --> 00:00:03,589 One day the universe will die. 2 00:00:04,144 --> 00:00:08,570 But why? And how? And will the universe be dead forever? 3 00:00:08,570 --> 00:00:10,652 And how do we know that? 4 00:00:10,652 --> 00:00:13,727 First of all, the universe is expanding. 5 00:00:13,727 --> 00:00:17,362 And not only that, the rate of its expansion is accelerating. 6 00:00:17,762 --> 00:00:19,668 The reason: dark energy. 7 00:00:20,293 --> 00:00:25,479 Dark energy is a strange phenomenon that scientists believe permeates the universe. 8 00:00:25,479 --> 00:00:29,624 Until 1998 we thought that the universe must work a bit like 9 00:00:29,624 --> 00:00:31,439 a ball that you throw into the sky. 10 00:00:31,829 --> 00:00:35,005 The ball moves up, but at some point it has to come down again. 11 00:00:35,515 --> 00:00:39,063 But the expansion of the universe is actually speeding up. 12 00:00:39,633 --> 00:00:42,781 That’s like throwing a ball up and watching it fly away 13 00:00:42,781 --> 00:00:44,769 faster and faster and faster. 14 00:00:45,269 --> 00:00:47,301 Where is this acceleration coming from? 15 00:00:48,071 --> 00:00:51,693 Well, we don’t know, but we call it “dark energy”. 16 00:00:52,603 --> 00:00:55,985 Einstein thought of it first and then decided it was stupid. 17 00:00:56,535 --> 00:01:00,138 Now, astrophysicists have decided it is plausible. 18 00:01:00,628 --> 00:01:04,716 Trouble is, this is all very theoretical, and we don’t actually know 19 00:01:04,716 --> 00:01:06,634 what the properties of dark energy are. 20 00:01:07,064 --> 00:01:10,845 But there are various theories and they lead us to three scenarios 21 00:01:10,845 --> 00:01:12,676 for the end of the universe. 22 00:01:19,856 --> 00:01:21,397 One: the Big Rip. 23 00:01:22,517 --> 00:01:25,316 Since its birth, the universe has been expanding. 24 00:01:25,646 --> 00:01:29,453 For unknown reasons new spaces created everywhere equally. 25 00:01:29,453 --> 00:01:33,230 The space between galaxies expands, so they move apart. 26 00:01:33,980 --> 00:01:38,391 The space inside galaxies also expands, but here, gravity is strong enough 27 00:01:38,391 --> 00:01:39,722 to keep them together. 28 00:01:40,392 --> 00:01:44,053 In the Big Rip scenario, the expansion accelerates up to a point where 29 00:01:44,053 --> 00:01:48,874 space expands so fast that gravity can’t compensate for this effect anymore. 30 00:01:49,084 --> 00:01:51,185 The result is a Big Rip. 31 00:01:51,725 --> 00:01:55,199 At first, only large structures like galaxies are torn apart, 32 00:01:55,199 --> 00:01:58,633 since space between the single objects expands very fast. 33 00:01:59,233 --> 00:02:03,159 Next, big bodies like black holes, stars, and planets die. 34 00:02:03,499 --> 00:02:06,130 Their gravity isn’t strong enough to keep them together, 35 00:02:06,130 --> 00:02:08,091 so they dissolve into their components. 36 00:02:08,731 --> 00:02:12,272 In the end, space would expand faster than the speed of light. 37 00:02:12,862 --> 00:02:16,220 Atoms would now be affected, and they would just disband. 38 00:02:16,790 --> 00:02:21,069 Once space is expanding faster than light, no particle in the universe can interact 39 00:02:21,069 --> 00:02:22,908 with any other particle anymore. 40 00:02:23,558 --> 00:02:28,205 The universe would dissolve into countless lonely particles that won’t be able to 41 00:02:28,205 --> 00:02:31,712 touch anything else in a strange, timeless universe. 42 00:02:33,002 --> 00:02:35,910 Hmm, and you thought you felt lonely! 43 00:02:37,213 --> 00:02:40,139 Two: Heat death or a Big Freeze. 44 00:02:41,029 --> 00:02:44,405 In a nutshell, the difference between the Big Rip and heat death is that 45 00:02:44,405 --> 00:02:48,089 in a heat death scenario matter stays intact and is converted 46 00:02:48,089 --> 00:02:51,915 over an incredibly long but finite period of time into radiation, 47 00:02:51,915 --> 00:02:53,951 while the universe expands forever. 48 00:02:54,401 --> 00:02:57,479 But how does this work? Let’s talk about entropy. 49 00:02:58,249 --> 00:03:01,382 Every system tends towards the state of highest entropy, 50 00:03:01,382 --> 00:03:04,105 like when we have a latte macchiato. 51 00:03:04,435 --> 00:03:07,625 Initially, it has different regions, but over time, they will 52 00:03:07,625 --> 00:03:10,421 cool down and disintegrate, until it’s uniform. 53 00:03:11,091 --> 00:03:13,457 And this also applies to the universe. 54 00:03:13,777 --> 00:03:16,366 So, while the universe gets bigger and bigger, 55 00:03:16,366 --> 00:03:19,075 matters slowly decays and spreads out. 56 00:03:19,655 --> 00:03:25,138 At some point, after lots of generations of stars, all the gas clouds necessary 57 00:03:25,138 --> 00:03:30,171 to form stars will be exhausted, so the universe will turn dark. 58 00:03:30,821 --> 00:03:32,451 The remaining suns will die; 59 00:03:32,891 --> 00:03:37,877 black holes will slowly degenerate and evaporate over trillions of years 60 00:03:37,877 --> 00:03:40,773 due to what’s known as Hawking radiation. 61 00:03:41,893 --> 00:03:47,330 When this process is complete, only a dilute gas of photons and light particles 62 00:03:47,330 --> 00:03:50,327 remains, until even this decays. 63 00:03:51,067 --> 00:03:54,371 All activity in the universe ceases at this point; 64 00:03:54,371 --> 00:03:59,275 entropy is at its maximum and the universe is dead forever. 65 00:04:00,255 --> 00:04:03,290 Unless… theoretically, it might be possible 66 00:04:03,290 --> 00:04:05,065 that after an incredibly long amount of time, 67 00:04:05,065 --> 00:04:10,270 there might be a spontaneous entropy decrease as a result of something called 68 00:04:10,270 --> 00:04:13,405 “quantum tunneling”, leading to a new Big Bang. 69 00:04:14,935 --> 00:04:17,750 Three: Big Crunch and Big Bounce. 70 00:04:18,400 --> 00:04:20,810 This is the most uplifting scenario. 71 00:04:21,030 --> 00:04:25,569 If there is less dark energy than we think or it decreases over time, gravity will be 72 00:04:25,569 --> 00:04:27,958 the dominating force in the universe one day. 73 00:04:28,238 --> 00:04:31,226 In a few trillion years, the rate of expansion of the universe 74 00:04:31,226 --> 00:04:32,534 will slow down and stop. 75 00:04:32,804 --> 00:04:34,842 After that, it reverses. 76 00:04:35,282 --> 00:04:38,635 Galaxies will race at each other, merging as the universe becomes 77 00:04:38,635 --> 00:04:39,868 smaller and smaller. 78 00:04:39,958 --> 00:04:43,660 Since a smaller universe also means a hotter universe, temperatures rise 79 00:04:43,660 --> 00:04:45,092 everywhere all at once. 80 00:04:45,752 --> 00:04:49,302 One hundred thousand years before the Big Crunch, background radiation would be 81 00:04:49,302 --> 00:04:52,119 hotter than the surfaces of the most stars, which means that 82 00:04:52,119 --> 00:04:54,036 they would be cooked from the outside. 83 00:04:54,646 --> 00:04:57,606 Minutes before the Big Crunch happens, atom cores are ripped apart, 84 00:04:57,606 --> 00:05:00,406 before supermassive black holes devour everything. 85 00:05:00,936 --> 00:05:05,106 Finally, all black holes would emerge into a supermassive mega-black hole 86 00:05:05,106 --> 00:05:07,909 that contains the entire mass of the universe, 87 00:05:07,909 --> 00:05:10,535 and in the last moment before the Big Crunch 88 00:05:10,535 --> 00:05:13,881 it would devour the universe, including itself. 89 00:05:14,671 --> 00:05:17,919 The Big Bounce theory states that this has happened a lot of times 90 00:05:17,919 --> 00:05:20,738 and that the universe goes through an infinite cycle of 91 00:05:20,738 --> 00:05:22,407 expansion and contraction. 92 00:05:22,797 --> 00:05:24,566 Well, wouldn’t that be nice? 93 00:05:25,336 --> 00:05:27,938 So what will actually happen to the universe in the end? 94 00:05:28,178 --> 00:05:33,021 At the moment, heat death seems the most likely, but we at Kurzgesagt hope that 95 00:05:33,021 --> 00:05:38,144 this “dead forever” stuff is wrong and the universe will start over and over again. 96 00:05:39,114 --> 00:05:42,348 We don't know for sure either way, so let’s just assume 97 00:05:42,348 --> 00:05:44,222 the most uplifting theory is true. 98 00:05:48,212 --> 00:05:50,688 By the way, we have a Twitter account.