0 00:00:00,010 --> 00:00:07,777 1 00:00:06,273 --> 00:00:11,076 Across the universe, an endless war rages, 2 00:00:11,078 --> 00:00:14,980 a bitter struggle between invisible forces. 3 00:00:14,982 --> 00:00:17,149 We've seen this cosmic battle go on 4 00:00:17,151 --> 00:00:19,785 for the past 14 billion years. 5 00:00:19,787 --> 00:00:23,288 Like two navies, fighting it out in the ocean of space time. 6 00:00:25,326 --> 00:00:27,259 Dark matter... 7 00:00:27,261 --> 00:00:29,428 And dark energy 8 00:00:29,430 --> 00:00:32,498 battling for control of the universe. 9 00:00:32,500 --> 00:00:35,000 They've shaped the entire history of the universe. 10 00:00:35,002 --> 00:00:37,636 They're not about to stop now. 11 00:00:37,638 --> 00:00:41,040 Shadowy forces dictating our past, 12 00:00:41,042 --> 00:00:43,942 our present, and our future. 13 00:00:43,944 --> 00:00:45,878 The dark universe is kind of a puppet master 14 00:00:45,880 --> 00:00:47,379 behind the scenes, 15 00:00:47,381 --> 00:00:50,149 guiding the universe that we can see. 16 00:00:50,151 --> 00:00:53,285 It's taken over the universe and, eventually, 17 00:00:53,287 --> 00:00:55,220 it may well destroy the universe. 18 00:01:06,634 --> 00:01:09,635 Captions paid for by discovery communications 19 00:01:17,011 --> 00:01:20,412 As we look out across the universe, 20 00:01:20,414 --> 00:01:23,515 we see nebulas, 21 00:01:23,517 --> 00:01:26,585 stars, 22 00:01:26,587 --> 00:01:28,287 and planets, 23 00:01:28,289 --> 00:01:32,558 all constructed from visible matter. 24 00:01:32,560 --> 00:01:37,763 But what we see is just a small part of the cosmos. 25 00:01:37,765 --> 00:01:40,099 The rest is invisible, 26 00:01:40,101 --> 00:01:43,001 unknown... 27 00:01:43,003 --> 00:01:46,572 The dark universe. 28 00:01:46,574 --> 00:01:50,876 Dark universe is a very common phrase to describe our universe, 29 00:01:50,878 --> 00:01:56,014 because it turns out most of our cosmos is dark. 30 00:01:56,016 --> 00:01:57,950 Dark not as in night. 31 00:01:57,952 --> 00:02:01,487 Dark as in, doesn't interact with light, 32 00:02:01,489 --> 00:02:04,656 and dark as in, we don't really understand it. 33 00:02:04,658 --> 00:02:06,992 All the objects in our daily experience... 34 00:02:06,994 --> 00:02:11,296 our bodies, the air, the chair that you're sitting in, 35 00:02:11,298 --> 00:02:13,298 the planets and stars, 36 00:02:13,300 --> 00:02:16,735 all of that only adds up to 5 percent of the universe. 37 00:02:19,273 --> 00:02:22,574 It's the other 95 percent that is the dark stuff, 38 00:02:22,576 --> 00:02:24,176 the dark universe. 39 00:02:26,847 --> 00:02:31,316 The dark universe is made of two forces... 40 00:02:31,318 --> 00:02:35,120 dark matter and dark energy. 41 00:02:36,423 --> 00:02:39,725 In a way, the dark matter and dark energy 42 00:02:39,727 --> 00:02:40,926 sort of oppose each other. 43 00:02:43,664 --> 00:02:47,799 Dark matter has positive gravity that pulls things together, 44 00:02:47,801 --> 00:02:49,902 and dark energy has repulsive gravity 45 00:02:49,904 --> 00:02:52,838 that pushes things apart. 46 00:02:52,840 --> 00:02:54,806 Freese: To encapsulate that in one sentence, 47 00:02:54,808 --> 00:02:58,944 dark matter is attractive, dark energy is repulsive. 48 00:02:58,946 --> 00:03:00,679 Since the dawn of time, 49 00:03:00,681 --> 00:03:04,983 two forces slugging it out for control of the universe. 50 00:03:04,985 --> 00:03:06,285 Dark matter and dark energy 51 00:03:06,287 --> 00:03:08,253 are locked in this epic struggle. 52 00:03:08,255 --> 00:03:11,056 The dark matter is trying to bring things together. 53 00:03:11,058 --> 00:03:14,459 The dark energy is trying to drive everything apart. 54 00:03:14,461 --> 00:03:16,628 So it's basically a battle. Who's going to win? 55 00:03:21,468 --> 00:03:23,235 It's a struggle that started 56 00:03:23,237 --> 00:03:26,505 13.8 billion years ago 57 00:03:26,507 --> 00:03:29,608 in the cosmic furnace of the big bang. 58 00:03:31,946 --> 00:03:36,582 The infant universe was a super hot ball of intense radiation, 59 00:03:36,584 --> 00:03:37,849 but suddenly... 60 00:03:37,851 --> 00:03:41,286 It started to transform. 61 00:03:41,288 --> 00:03:43,689 It cooled and expanded, 62 00:03:43,691 --> 00:03:47,426 triggering the birth of the regular universe 63 00:03:47,428 --> 00:03:50,762 and the dark universe. 64 00:03:50,764 --> 00:03:54,967 Scientists believe that both dark matter and dark energy 65 00:03:54,969 --> 00:03:58,637 formed in the first moments of the big bang. 66 00:03:58,639 --> 00:04:02,007 It would have formed probably just fractions of a second 67 00:04:02,009 --> 00:04:03,575 after the big bang, 68 00:04:03,577 --> 00:04:06,278 around the time that normal matter formed, 69 00:04:06,280 --> 00:04:07,813 and the processes that created 70 00:04:07,815 --> 00:04:09,681 the normal matter we know all about. 71 00:04:09,683 --> 00:04:13,318 Similar processes probably created the dark matter. 72 00:04:15,122 --> 00:04:16,855 During these first microseconds, 73 00:04:16,857 --> 00:04:22,027 the universe was a hot, dense ball of matter and energy. 74 00:04:22,029 --> 00:04:24,830 They're two sides of the same coin. 75 00:04:24,832 --> 00:04:27,199 Matter can convert into energy... 76 00:04:28,902 --> 00:04:32,604 And energy can convert directly into matter, 77 00:04:32,606 --> 00:04:36,808 visible particles of regular matter forming atoms, 78 00:04:36,810 --> 00:04:40,445 planets, stars, and us... 79 00:04:40,447 --> 00:04:42,648 and other particles. 80 00:04:42,650 --> 00:04:44,650 They are invisible. 81 00:04:44,652 --> 00:04:46,918 They are dark matter. 82 00:04:46,920 --> 00:04:50,389 One of the big mysteries that we as astronomers have to solve 83 00:04:50,391 --> 00:04:52,157 is what this dark matter is. 84 00:04:52,159 --> 00:04:54,826 We just don't know. 85 00:04:54,828 --> 00:04:58,297 The idea of a type of matter that you can't see 86 00:04:58,299 --> 00:05:00,699 and that acts differently than normal matter 87 00:05:00,701 --> 00:05:01,733 is sort of out there. 88 00:05:01,735 --> 00:05:04,670 It's sort of weird. 89 00:05:04,672 --> 00:05:08,573 But the dark universe does leave clues. 90 00:05:08,575 --> 00:05:10,142 It's like a crime scene. 91 00:05:10,144 --> 00:05:12,611 You know that a crime has been committed, 92 00:05:12,613 --> 00:05:15,280 even though you don't know the perpetrator yet. 93 00:05:15,282 --> 00:05:18,183 We see the hints. We see the signs. 94 00:05:18,185 --> 00:05:21,553 We see the signals that something funny 95 00:05:21,555 --> 00:05:23,221 is happening in our universe, 96 00:05:23,223 --> 00:05:26,892 even though we don't know exactly what's causing it. 97 00:05:26,894 --> 00:05:28,760 There are several contenders 98 00:05:28,762 --> 00:05:30,962 for what dark matter could be. 99 00:05:30,964 --> 00:05:34,132 It could be normal matter that we just don't see, 100 00:05:34,134 --> 00:05:36,234 or it could be some sort of exotic matter, 101 00:05:36,236 --> 00:05:39,204 a particle of some sort that we haven't detected yet. 102 00:05:39,206 --> 00:05:42,708 Sutter: It could be a species of particle, 103 00:05:42,710 --> 00:05:45,811 like an electron, like a proton, 104 00:05:45,813 --> 00:05:47,913 like a quark, 105 00:05:47,915 --> 00:05:51,950 but a special kind that doesn't interact with light. 106 00:05:51,952 --> 00:05:54,419 One of the favorite possible models of dark matter 107 00:05:54,421 --> 00:05:57,322 are wimps, weakly interacting massive particles. 108 00:05:58,892 --> 00:06:00,859 Wimps may not interact strongly 109 00:06:00,861 --> 00:06:03,462 with other matter or light, 110 00:06:03,464 --> 00:06:06,231 but they do exert a gravitational pull. 111 00:06:08,769 --> 00:06:11,937 So they're the best candidates for the particles of dark matter 112 00:06:11,939 --> 00:06:13,872 that formed in the big bang. 113 00:06:16,877 --> 00:06:20,846 The early universe was intensely hot and dense, 114 00:06:20,848 --> 00:06:24,182 full of new particles of both visible matter 115 00:06:24,184 --> 00:06:27,285 and invisible dark matter. 116 00:06:27,287 --> 00:06:30,956 But another force was present... 117 00:06:30,958 --> 00:06:34,025 dark energy. 118 00:06:34,027 --> 00:06:37,696 Dark energy has existed since the time of the big bang. 119 00:06:37,698 --> 00:06:39,564 Dark energy was always there. 120 00:06:39,566 --> 00:06:41,700 We don't know how much of it there was, 121 00:06:41,702 --> 00:06:44,302 but it's possible that the same amount of dark energy 122 00:06:44,304 --> 00:06:45,404 was always there. 123 00:06:47,908 --> 00:06:50,809 Our understanding of what that dark energy is 124 00:06:50,811 --> 00:06:53,245 is very limited. 125 00:06:53,247 --> 00:06:56,448 If you were to ask a roomful of 10 theoretical physicists 126 00:06:56,450 --> 00:06:57,816 on the nature of dark energy, 127 00:06:57,818 --> 00:07:00,952 you'd get about 12 different answers. 128 00:07:00,954 --> 00:07:05,457 We're not sure what dark energy is. 129 00:07:05,459 --> 00:07:08,727 Dark energy is just a fancy name for our ignorance. 130 00:07:08,729 --> 00:07:12,431 Dark energy is nothing more than a placeholder name 131 00:07:12,433 --> 00:07:15,066 for this enormous gap in our understanding 132 00:07:15,068 --> 00:07:16,501 of how the cosmos works. 133 00:07:16,503 --> 00:07:19,404 But we don't understand it at all. 134 00:07:19,406 --> 00:07:21,540 It's true. 135 00:07:23,377 --> 00:07:25,944 One potential answer to what dark energy 136 00:07:25,946 --> 00:07:31,583 actually is may be found in so-called empty space. 137 00:07:31,585 --> 00:07:34,886 Could this be the source of dark energy? 138 00:07:34,888 --> 00:07:38,290 We used to think of space as just boring emptiness. 139 00:07:38,292 --> 00:07:41,693 But now, I think it's healthier to think of space 140 00:07:41,695 --> 00:07:44,362 as a kind of substance. 141 00:07:44,364 --> 00:07:46,131 A substance that carries 142 00:07:46,133 --> 00:07:48,266 a strange type of energy. 143 00:07:48,268 --> 00:07:49,868 Every small region of space 144 00:07:49,870 --> 00:07:52,237 has a little bit of energy in it, 145 00:07:52,239 --> 00:07:55,106 just associated with the vacuum itself. 146 00:07:55,108 --> 00:07:59,478 Vacuum energy is the idea that vacuum isn't empty, 147 00:07:59,480 --> 00:08:01,079 that there is something there. 148 00:08:01,081 --> 00:08:04,549 There's an energy in it with a kind of antigravity. 149 00:08:06,954 --> 00:08:08,887 Perhaps this vacuum energy 150 00:08:08,889 --> 00:08:12,824 that pushes against gravity 151 00:08:12,826 --> 00:08:16,027 is the mysterious dark energy. 152 00:08:16,029 --> 00:08:19,731 We simply don't know. 153 00:08:19,733 --> 00:08:23,568 We're not sure that the dark energy is the vacuum energy. 154 00:08:23,570 --> 00:08:27,739 It could be a new type of energy that permeates all of space. 155 00:08:27,741 --> 00:08:31,109 This is what we're trying to measure now. 156 00:08:31,111 --> 00:08:33,378 Dark energy and dark matter, 157 00:08:33,380 --> 00:08:36,915 forged in the intense heat of the big bang, 158 00:08:36,917 --> 00:08:41,820 opposing forces, one attractive, one repulsive. 159 00:08:41,822 --> 00:08:44,890 But together, over 13.8 billions years, 160 00:08:44,892 --> 00:08:48,593 they will shape the history of the universe. 161 00:09:08,215 --> 00:09:10,515 The story of the universe is dominated 162 00:09:10,517 --> 00:09:15,320 by two powerful opposing forces... 163 00:09:15,322 --> 00:09:18,723 dark matter and dark energy. 164 00:09:18,725 --> 00:09:21,593 For 13.8 billions years, 165 00:09:21,595 --> 00:09:25,530 they've battled it out for control of the cosmos. 166 00:09:25,532 --> 00:09:27,732 Dark matter and dark energy are out there, 167 00:09:27,734 --> 00:09:30,135 and they've shaped the entire history of the universe. 168 00:09:30,137 --> 00:09:33,238 Our universe is actually the balance between dark forces. 169 00:09:33,240 --> 00:09:35,640 Dark matter is trying to draw everything together, 170 00:09:35,642 --> 00:09:39,945 and dark energy is trying to rip everything apart. 171 00:09:39,947 --> 00:09:44,082 After the big bang, the infant universe was small, 172 00:09:44,084 --> 00:09:47,419 intensely hot and intensely dense. 173 00:09:49,456 --> 00:09:53,291 Dark matter, the force bent on bringing things together, 174 00:09:53,293 --> 00:09:55,226 thrived. 175 00:09:55,228 --> 00:09:58,396 But in this compressed space, dark energy, 176 00:09:58,398 --> 00:10:03,935 the force trying to drive things apart, had no room to act. 177 00:10:03,937 --> 00:10:05,704 When things were closer together, 178 00:10:05,706 --> 00:10:08,840 the density of matter and radiation was bigger, 179 00:10:08,842 --> 00:10:12,510 so big that the dark energy didn't matter. 180 00:10:12,512 --> 00:10:17,482 The environment was also tough for normal matter. 181 00:10:17,484 --> 00:10:19,884 It was so hot, intense radiation 182 00:10:19,886 --> 00:10:25,490 prevented visible matter from bunching together to form atoms. 183 00:10:25,492 --> 00:10:27,659 If any normal matter tried to clump together 184 00:10:27,661 --> 00:10:29,461 through gravity or some other force, 185 00:10:29,463 --> 00:10:31,730 this energy would just basically blast it apart. 186 00:10:31,732 --> 00:10:33,365 In the very early universe, 187 00:10:33,367 --> 00:10:35,000 when our universe was a lot smaller 188 00:10:35,002 --> 00:10:37,102 and a lot hotter and a lot denser, 189 00:10:37,104 --> 00:10:41,206 matter, normal matter, tried to collect together, 190 00:10:41,208 --> 00:10:43,041 wanted to join the party. 191 00:10:43,043 --> 00:10:44,943 But it was prevented from doing so 192 00:10:44,945 --> 00:10:50,649 because there was also radiation that would throw it out. 193 00:10:50,651 --> 00:10:52,317 Unable to stick together, 194 00:10:52,319 --> 00:10:54,486 normal visible matter sped out 195 00:10:54,488 --> 00:10:58,356 across the infant cosmos in a blizzard of particles. 196 00:11:00,227 --> 00:11:02,327 But then dark matter, 197 00:11:02,329 --> 00:11:05,897 the force that brings things together, intervened. 198 00:11:07,601 --> 00:11:10,201 Dark matter doesn't talk to radiation, 199 00:11:10,203 --> 00:11:11,369 doesn't talk to light. 200 00:11:11,371 --> 00:11:12,971 Can do whatever it wants. 201 00:11:12,973 --> 00:11:16,341 It starts clumping together. 202 00:11:16,343 --> 00:11:19,044 Radiation pushes normal matter apart, 203 00:11:19,046 --> 00:11:22,781 stopping it from forming dense regions. 204 00:11:22,783 --> 00:11:27,052 But photons simply pass straight through the dark matter, 205 00:11:27,054 --> 00:11:32,724 allowing it to clump and fall into dense pockets or wells. 206 00:11:32,726 --> 00:11:36,895 The dark matter begins to clump together gravitationally, 207 00:11:36,897 --> 00:11:39,597 and this means that the matter is going to fall into 208 00:11:39,599 --> 00:11:42,834 those dark matter wells. 209 00:11:42,836 --> 00:11:45,737 Over time, more and more regular matter 210 00:11:45,739 --> 00:11:48,840 is pulled into the dark matter wells. 211 00:11:48,842 --> 00:11:50,542 The regions that have a little bit more stuff, 212 00:11:50,544 --> 00:11:52,010 gravity makes them bigger, 213 00:11:52,012 --> 00:11:54,846 and the regions that have less stuff, those expand more. 214 00:11:54,848 --> 00:11:58,616 So you have little pockets of slight extra matter, 215 00:11:58,618 --> 00:12:00,819 have more and more and more matter over time. 216 00:12:02,856 --> 00:12:05,156 Gravity-rich pockets of dark matter 217 00:12:05,158 --> 00:12:08,226 pull particles of regular matter together. 218 00:12:10,097 --> 00:12:13,431 Gradually, they form giant clouds of hydrogen 219 00:12:13,433 --> 00:12:16,801 and helium gas. 220 00:12:16,803 --> 00:12:21,473 Dark matter has laid the foundation stones of the cosmos. 221 00:12:21,475 --> 00:12:25,710 This force may be dark, but it's highly creative. 222 00:12:29,282 --> 00:12:34,352 And now, 180 million years after the big bang, 223 00:12:34,354 --> 00:12:35,854 everything is in place 224 00:12:35,856 --> 00:12:39,290 for the next dark matter construction milestone... 225 00:12:39,292 --> 00:12:42,761 the creation of stars. 226 00:12:42,763 --> 00:12:44,129 We know stars, 227 00:12:44,131 --> 00:12:46,331 in the very early universe at the edge of time, 228 00:12:46,333 --> 00:12:48,900 had to form from the collapse of gas clouds 229 00:12:48,902 --> 00:12:50,368 under their own gravity. 230 00:12:53,273 --> 00:12:54,939 But there's a problem. 231 00:12:54,941 --> 00:13:00,478 The clouds of hydrogen in the infant universe can't collapse, 232 00:13:00,480 --> 00:13:04,616 and no collapse means no stars. 233 00:13:04,618 --> 00:13:07,218 Bullock: The gas in the early universe has a lot of pressure, 234 00:13:07,220 --> 00:13:09,387 and this pressure keeps it from collapsing. 235 00:13:09,389 --> 00:13:10,655 The dark matter doesn't experience 236 00:13:10,657 --> 00:13:12,023 that kind of pressure. 237 00:13:12,025 --> 00:13:13,491 So the dark matter can clump up 238 00:13:13,493 --> 00:13:17,262 and make sites for structure formation. 239 00:13:17,264 --> 00:13:19,430 So dark matter comes to the rescue, 240 00:13:19,432 --> 00:13:23,067 creating regions of higher gravity, 241 00:13:23,069 --> 00:13:25,170 dragging in hydrogen gas, 242 00:13:25,172 --> 00:13:29,641 forcing the clouds to get denser and denser, 243 00:13:29,643 --> 00:13:34,445 creating the conditions for collapse and then creation. 244 00:13:40,854 --> 00:13:44,289 So it's only when the gravity of the dark matter 245 00:13:44,291 --> 00:13:46,691 overwhelms the pressure of gases 246 00:13:46,693 --> 00:13:50,862 that the gases can collapse and turn into stars. 247 00:13:50,864 --> 00:13:52,263 Bullock: As soon as the gas cools down, 248 00:13:52,265 --> 00:13:53,965 it can fall into those potential wells 249 00:13:53,967 --> 00:13:55,900 that the dark matter created, 250 00:13:55,902 --> 00:13:58,603 almost like little nurseries for stars, 251 00:13:58,605 --> 00:14:00,638 and they start forming in earnest. 252 00:14:02,642 --> 00:14:05,210 Dark matter provides a boost of gravity 253 00:14:05,212 --> 00:14:10,782 to kick-start hydrogen into constructing the first stars, 254 00:14:10,784 --> 00:14:14,652 stars that are the seeds of the first galaxies. 255 00:14:14,654 --> 00:14:17,355 So it's dark matter that would have coalesced 256 00:14:17,357 --> 00:14:20,692 in the early universe and grown from there, 257 00:14:20,694 --> 00:14:22,627 and then the luminous component of the universe, 258 00:14:22,629 --> 00:14:24,963 the things that we think of as being the universe itself, 259 00:14:24,965 --> 00:14:26,497 like stars and galaxies, 260 00:14:26,499 --> 00:14:28,900 would have just been along for the ride. 261 00:14:28,902 --> 00:14:34,239 Without the presence of dark matter to seed structures, 262 00:14:34,241 --> 00:14:39,043 there wasn't enough time in the early universe to form galaxies, 263 00:14:39,045 --> 00:14:42,914 which means you and me have to thank dark matter 264 00:14:42,916 --> 00:14:45,149 for our existence. 265 00:14:45,151 --> 00:14:47,252 And dark matter now begins 266 00:14:47,254 --> 00:14:51,890 a much more ambitious architectural project... 267 00:14:51,892 --> 00:14:56,194 to shape the entire universe itself, 268 00:14:56,196 --> 00:15:00,431 to build the biggest structure ever constructed... 269 00:15:00,433 --> 00:15:02,000 the cosmic web. 270 00:15:20,420 --> 00:15:23,254 The war between dark matter and dark energy 271 00:15:23,256 --> 00:15:25,990 has been raging since the birth of time. 272 00:15:29,062 --> 00:15:33,865 But in the early years, it's a one-sided contest. 273 00:15:33,867 --> 00:15:36,200 In the early universe, the only thing that really mattered 274 00:15:36,202 --> 00:15:39,771 was the dark matter and the normal matter. 275 00:15:39,773 --> 00:15:41,873 Dark matter, 276 00:15:41,875 --> 00:15:45,677 the force that brings things together, 277 00:15:45,679 --> 00:15:47,345 is in the driver's seat. 278 00:15:50,951 --> 00:15:55,019 Dark energy, the force that pulls things apart, 279 00:15:55,021 --> 00:15:57,021 is the underdog. 280 00:15:57,023 --> 00:15:59,590 We've seen this cosmic battle go on 281 00:15:59,592 --> 00:16:01,693 for the past 14 billion years. 282 00:16:01,695 --> 00:16:06,798 Fortunately for us, the dark energy got off to a slow start. 283 00:16:06,800 --> 00:16:10,201 Meanwhile, dark matter is busy at work, 284 00:16:10,203 --> 00:16:12,870 building the universe. 285 00:16:12,872 --> 00:16:17,742 Not only does it trigger the birth of the first stars, 286 00:16:17,744 --> 00:16:19,978 it embarks on an even more formidable 287 00:16:19,980 --> 00:16:22,947 construction project... 288 00:16:22,949 --> 00:16:26,451 the cosmic web. 289 00:16:26,453 --> 00:16:30,521 There's this large-scale structure of filaments 290 00:16:30,523 --> 00:16:32,757 that galaxies seem to form on, 291 00:16:32,759 --> 00:16:35,526 and that's what we call the cosmic web. 292 00:16:35,528 --> 00:16:38,429 And we can trace the formation of this cosmic web 293 00:16:38,431 --> 00:16:41,265 all the way back to the early universe. 294 00:16:41,267 --> 00:16:43,034 Now this is such a huge structure, 295 00:16:43,036 --> 00:16:44,736 we don't think there's time in the universe 296 00:16:44,738 --> 00:16:47,338 for matter's gravity alone to do this. 297 00:16:47,340 --> 00:16:49,507 There must have been an underlying scaffold 298 00:16:49,509 --> 00:16:51,242 of dark matter. 299 00:16:51,244 --> 00:16:53,578 The dark matter started forming into these filaments, 300 00:16:53,580 --> 00:16:55,580 and when the universe cooled enough, 301 00:16:55,582 --> 00:16:57,281 normal matter could start to stream 302 00:16:57,283 --> 00:17:00,385 into this gravitational attraction of the dark matter. 303 00:17:00,387 --> 00:17:01,986 That became the scaffolding 304 00:17:01,988 --> 00:17:05,857 on which this large-scale structure was built. 305 00:17:05,859 --> 00:17:08,793 The filaments of dark matter joined together, 306 00:17:08,795 --> 00:17:12,630 drawing in more and more hydrogen gas. 307 00:17:12,632 --> 00:17:16,801 Dense clouds of gas build up at the junctions of the filaments, 308 00:17:16,803 --> 00:17:19,871 the point where gravity is at its strongest. 309 00:17:22,909 --> 00:17:24,842 Slowly and surely, 310 00:17:24,844 --> 00:17:29,380 a familiar-looking structure starts to take shape. 311 00:17:29,382 --> 00:17:30,715 If you've ever gone outside, 312 00:17:30,717 --> 00:17:34,519 and you can see a spider web covered in dew, 313 00:17:34,521 --> 00:17:37,355 that's kind of like what happened with the universe. 314 00:17:37,357 --> 00:17:38,890 In this case, the spider web 315 00:17:38,892 --> 00:17:40,792 is the structure of the dark matter. 316 00:17:40,794 --> 00:17:43,327 It's all of these filaments, and the moisture in the air 317 00:17:43,329 --> 00:17:44,862 is what condenses around them, 318 00:17:44,864 --> 00:17:48,699 just like the normal matter fell into the dark matter web 319 00:17:48,701 --> 00:17:51,769 to form these gigantic structures in the universe. 320 00:17:51,771 --> 00:17:55,373 Dark matter, the universe's master builder, 321 00:17:55,375 --> 00:17:59,677 succeeds in stitching together a cosmic web. 322 00:17:59,679 --> 00:18:04,682 This will be the framework for the entire universe. 323 00:18:04,684 --> 00:18:07,618 And so it is dark matter that would choreograph 324 00:18:07,620 --> 00:18:10,121 and sculpt the shape of the universe itself. 325 00:18:10,123 --> 00:18:12,090 One of the amazing things about dark matter is, 326 00:18:12,092 --> 00:18:14,325 without it, we wouldn't be here. 327 00:18:14,327 --> 00:18:16,828 It's hard to imagine how you could have structure 328 00:18:16,830 --> 00:18:19,730 in the universe without dark matter. 329 00:18:19,732 --> 00:18:22,366 Galaxies, and then galactic clusters, 330 00:18:22,368 --> 00:18:25,169 form at the junctions of the filaments. 331 00:18:25,171 --> 00:18:30,675 Slowly, but surely, the universe begins to take shape. 332 00:18:30,677 --> 00:18:33,711 When we look at this structure over a cosmic scale, 333 00:18:33,713 --> 00:18:35,847 we see that it looks kind of like a sponge. 334 00:18:35,849 --> 00:18:39,383 You see voids with galaxies all over the edges of them. 335 00:18:39,385 --> 00:18:41,552 That is the structure that was formed 336 00:18:41,554 --> 00:18:45,523 by the dark matter in the early universe. 337 00:18:45,525 --> 00:18:47,658 Dark matter is the thing that enabled, 338 00:18:47,660 --> 00:18:49,660 that provided enough gravity 339 00:18:49,662 --> 00:18:53,331 for the initial seeds of structure formation to coalesce, 340 00:18:53,333 --> 00:18:55,500 for galaxies themselves to form. 341 00:18:57,303 --> 00:18:58,936 And, of course, without galaxies, 342 00:18:58,938 --> 00:19:01,005 there are no stars, and there's no planets, 343 00:19:01,007 --> 00:19:02,907 and there's no us. 344 00:19:02,909 --> 00:19:04,509 For 9 billion years, 345 00:19:04,511 --> 00:19:08,012 dark matter orchestrates the construction of the universe. 346 00:19:09,983 --> 00:19:12,984 In these, the first battles of the cosmos, 347 00:19:12,986 --> 00:19:17,021 this constructive force is the clear victor. 348 00:19:17,023 --> 00:19:19,590 For the time being, the dark matter has won. 349 00:19:19,592 --> 00:19:21,459 Galaxies continue to form. 350 00:19:21,461 --> 00:19:23,861 Clusters of galaxies are getting bigger over time. 351 00:19:27,066 --> 00:19:30,067 But dark matter's success in building up the universe 352 00:19:30,069 --> 00:19:33,404 sets in motion its potential downfall. 353 00:19:37,810 --> 00:19:41,679 As the cosmic web evolves into a more complex structure... 354 00:19:44,417 --> 00:19:48,953 Gaps form between the filaments, the cosmic voids. 355 00:19:50,723 --> 00:19:55,493 The cosmic voids formed because other, more dense regions 356 00:19:55,495 --> 00:20:00,364 of the universe gravitationally stole material away from them. 357 00:20:00,366 --> 00:20:02,400 So the dense parts of the universe 358 00:20:02,402 --> 00:20:07,038 accumulated more matter at the expense of the less dense parts, 359 00:20:07,040 --> 00:20:10,174 which then became voids. 360 00:20:10,176 --> 00:20:14,378 And lurking in these voids, dark energy. 361 00:20:14,380 --> 00:20:15,980 Since the dawn of time, 362 00:20:15,982 --> 00:20:19,717 it's been waiting for its opportunity. 363 00:20:19,719 --> 00:20:23,754 Now it's preparing an offensive that may help it 364 00:20:23,756 --> 00:20:26,123 conquer the universe. 365 00:20:26,125 --> 00:20:27,825 In the very earliest times, 366 00:20:27,827 --> 00:20:29,460 the dark matter dominated everything. 367 00:20:29,462 --> 00:20:32,797 It was the big brother pushing the little brother around. 368 00:20:32,799 --> 00:20:34,232 But in the long run, 369 00:20:34,234 --> 00:20:37,001 the dark energy is going to overpower dark matter, 370 00:20:37,003 --> 00:20:39,370 and so the relationship is entirely flipped. 371 00:20:41,007 --> 00:20:43,874 We used to think the fate of the cosmos itself 372 00:20:43,876 --> 00:20:45,276 depended on dark matter, 373 00:20:45,278 --> 00:20:47,878 and it turns out that's not the case at all. 374 00:20:47,880 --> 00:20:51,916 The fate of the universe depends entirely on dark energy. 375 00:20:51,918 --> 00:20:54,151 The long reign of dark matter 376 00:20:54,153 --> 00:20:56,153 may be coming to an end. 377 00:20:56,155 --> 00:20:58,889 Dark energy, the great destroyer, 378 00:20:58,891 --> 00:21:01,892 is hoping to take control. 379 00:21:01,894 --> 00:21:06,330 This destructive force has one overriding aim... 380 00:21:06,332 --> 00:21:09,533 to tear the universe apart. 381 00:21:25,084 --> 00:21:28,119 Our universe is at war, 382 00:21:28,121 --> 00:21:32,490 a relentless conflict between dark forces. 383 00:21:32,492 --> 00:21:38,429 For the first 9 billion years, dark energy is subjugated. 384 00:21:38,431 --> 00:21:42,700 Dark matter has the upper hand. 385 00:21:42,702 --> 00:21:45,036 Frenk: When the universe emerged from the big bang, 386 00:21:45,038 --> 00:21:47,638 the dark energy played no role. 387 00:21:47,640 --> 00:21:48,806 It was insignificant. 388 00:21:50,610 --> 00:21:53,144 But at some stage in the 14 billion years 389 00:21:53,146 --> 00:21:58,316 since the big bang, these roles became reversed. 390 00:21:58,318 --> 00:22:02,620 Dark energy came to be the more powerful force. 391 00:22:02,622 --> 00:22:05,756 The question was, when? 392 00:22:05,758 --> 00:22:09,627 The answer came at the end of the 20th century. 393 00:22:09,629 --> 00:22:12,830 So it was an amazing breakthrough, really important. 394 00:22:14,067 --> 00:22:15,733 In 1999, 395 00:22:15,735 --> 00:22:18,903 scientists measure the expansion of the universe. 396 00:22:20,406 --> 00:22:23,374 What they find shocks them. 397 00:22:23,376 --> 00:22:28,145 They expect the speed of expansion to be decreasing. 398 00:22:28,147 --> 00:22:31,349 In fact, it's actually increasing 399 00:22:31,351 --> 00:22:34,752 and getting faster all the time. 400 00:22:34,754 --> 00:22:38,155 The data indicate that for about the first 9 billion years, 401 00:22:38,157 --> 00:22:39,590 it was slowing down. 402 00:22:39,592 --> 00:22:41,992 But then, in the past 5 billion years, 403 00:22:41,994 --> 00:22:46,230 it started accelerating faster and faster. 404 00:22:46,232 --> 00:22:49,033 Alex filippenko was part of the team 405 00:22:49,035 --> 00:22:52,503 that made this explosive discovery. 406 00:22:52,505 --> 00:22:53,938 It befuddled us. 407 00:22:53,940 --> 00:22:56,974 This isn't how nature was supposed to be behaving, 408 00:22:56,976 --> 00:22:59,043 and, in fact, initially we thought 409 00:22:59,045 --> 00:23:00,444 that there was something wrong 410 00:23:00,446 --> 00:23:03,347 with either the observations or the measurements. 411 00:23:03,349 --> 00:23:05,816 I didn't believe it for the longest time. 412 00:23:05,818 --> 00:23:07,385 When the first data came out, I'm like, 413 00:23:07,387 --> 00:23:09,453 "nah, I don't believe this, no way." 414 00:23:09,455 --> 00:23:10,921 but it's in the data. 415 00:23:10,923 --> 00:23:12,723 It's there. You can't escape it. 416 00:23:12,725 --> 00:23:16,460 This is as shocking as if you held up a rock, let go of it, 417 00:23:16,462 --> 00:23:17,762 and it went up into the air. 418 00:23:20,800 --> 00:23:22,433 Five billion years ago, 419 00:23:22,435 --> 00:23:26,370 galaxies started moving apart faster than before. 420 00:23:26,372 --> 00:23:29,473 The question is, why? 421 00:23:29,475 --> 00:23:32,777 What could be causing that? Well, one thing is clear. 422 00:23:32,779 --> 00:23:35,513 It must be getting some extra energy from somewhere. 423 00:23:35,515 --> 00:23:37,848 There is one main contender 424 00:23:37,850 --> 00:23:41,519 for what may be supplying this extra energy, 425 00:23:41,521 --> 00:23:44,422 a force with repulsive gravity, 426 00:23:44,424 --> 00:23:47,825 a force that pushes things apart. 427 00:23:47,827 --> 00:23:50,327 This is what astronomers call dark energy. 428 00:23:50,329 --> 00:23:53,164 It's this mysterious repulsive force 429 00:23:53,166 --> 00:23:55,166 that we know exists in the universe, 430 00:23:55,168 --> 00:23:58,202 and we have no idea what it is. 431 00:23:58,204 --> 00:24:01,806 Physicists may not agree on what dark energy is, 432 00:24:01,808 --> 00:24:03,240 but there is a consensus 433 00:24:03,242 --> 00:24:08,312 on where this repulsive force has the most influence... 434 00:24:08,314 --> 00:24:13,184 in the regions between galaxies and galaxy clusters, 435 00:24:13,186 --> 00:24:15,186 the cosmic voids. 436 00:24:15,188 --> 00:24:19,690 They're actually filled to the brim with dark energy. 437 00:24:19,692 --> 00:24:21,392 The first time dark energy is really going to make 438 00:24:21,394 --> 00:24:24,161 its mark in the universe is going to be the time 439 00:24:24,163 --> 00:24:26,864 when the first cosmic voids begin to appear. 440 00:24:28,935 --> 00:24:33,237 We see dark energy's effects throughout the universe. 441 00:24:33,239 --> 00:24:36,407 But when we look into the cosmic voids, 442 00:24:36,409 --> 00:24:39,577 which are the most empty regions of our universe, 443 00:24:39,579 --> 00:24:42,413 this is where dark energy is strongest. 444 00:24:45,251 --> 00:24:47,518 Dark energy is the repulsive force 445 00:24:47,520 --> 00:24:49,286 pushing things apart. 446 00:24:49,288 --> 00:24:52,690 It prefers the voids where gravity is weak. 447 00:24:54,560 --> 00:24:56,994 Bullock: These are areas where there's a lot less dark matter, 448 00:24:56,996 --> 00:24:59,396 and because the overall density is low, 449 00:24:59,398 --> 00:25:01,398 that's where the dark energy starts to peek out 450 00:25:01,400 --> 00:25:04,201 and can really drive those voids to expand. 451 00:25:04,203 --> 00:25:07,438 So the expansion and acceleration of the universe 452 00:25:07,440 --> 00:25:10,174 are driven by the dark energy in those regions. 453 00:25:11,611 --> 00:25:14,879 Dark energy pushes thing apart, 454 00:25:14,881 --> 00:25:17,781 things that get in its way, 455 00:25:17,783 --> 00:25:21,585 things like the cosmic web. 456 00:25:21,587 --> 00:25:26,957 Dark matter and normal matter are also in its path 457 00:25:26,959 --> 00:25:30,594 and are bulldozed out across the cosmos. 458 00:25:30,596 --> 00:25:34,865 Slowly, but surely, the balance between dark energy 459 00:25:34,867 --> 00:25:37,368 and dark matter is changing. 460 00:25:39,906 --> 00:25:41,672 Imagine you have a giant swimming pool, 461 00:25:41,674 --> 00:25:44,241 and at the very bottom there's a puddle of water 462 00:25:44,243 --> 00:25:45,476 with a splash of whiskey. 463 00:25:45,478 --> 00:25:47,311 So you have sort of a strong whiskey drink 464 00:25:47,313 --> 00:25:49,446 down at the bottom of your pool. 465 00:25:49,448 --> 00:25:52,950 But now you start dumping water into your pool, no more whiskey, 466 00:25:52,952 --> 00:25:54,952 and it begins to get diluted and diluted, 467 00:25:54,954 --> 00:25:57,821 and eventually, you just have a swimming pool full of water, 468 00:25:57,823 --> 00:25:59,657 with one shot of whiskey mixed in. 469 00:25:59,659 --> 00:26:01,158 That's not a very strong drink. 470 00:26:01,160 --> 00:26:03,294 It's basically a water swimming pool. 471 00:26:03,296 --> 00:26:05,563 That's pretty much happening with the dark energy. 472 00:26:05,565 --> 00:26:07,197 At first, it's a one-to-one mixture 473 00:26:07,199 --> 00:26:08,899 of dark matter and dark energy. 474 00:26:08,901 --> 00:26:11,335 But in the long run, it's all dark energy 475 00:26:11,337 --> 00:26:13,370 and pretty much no dark matter left over. 476 00:26:15,474 --> 00:26:17,975 The forces of dark energy 477 00:26:17,977 --> 00:26:20,511 are on an unstoppable March, 478 00:26:20,513 --> 00:26:22,513 picking up more and more power 479 00:26:22,515 --> 00:26:26,183 from the vast scale of the cosmic voids. 480 00:26:26,185 --> 00:26:28,953 Dark energy is intrinsically very weak. 481 00:26:28,955 --> 00:26:30,854 There's very little dark energy 482 00:26:30,856 --> 00:26:34,825 and this repulsive effect in every cubic centimeter. 483 00:26:34,827 --> 00:26:36,594 But the universe is vast. 484 00:26:36,596 --> 00:26:38,829 Space is big. 485 00:26:38,831 --> 00:26:42,633 So cumulatively, all this small amount of stuff 486 00:26:42,635 --> 00:26:45,202 adds up to a very large amount, 487 00:26:45,204 --> 00:26:48,872 and over a scale encompassing the entire universe, 488 00:26:48,874 --> 00:26:51,442 the dark energy dominates. 489 00:26:53,312 --> 00:26:55,913 But it's been a very long process. 490 00:26:55,915 --> 00:26:57,615 After the big bang, 491 00:26:57,617 --> 00:27:01,318 dark matter dominates for the first 9 billion years. 492 00:27:03,556 --> 00:27:06,023 Then, 5 billion years ago, 493 00:27:06,025 --> 00:27:08,959 dark energy starts to get the upper hand. 494 00:27:08,961 --> 00:27:12,730 It causes the expansion of the universe to accelerate 495 00:27:12,732 --> 00:27:16,467 and the space in the voids to grow more rapidly. 496 00:27:16,469 --> 00:27:20,404 As the space expands, there's more and more dark energy 497 00:27:20,406 --> 00:27:22,172 because you have a bigger space. 498 00:27:22,174 --> 00:27:26,744 It sort of creates itself with the expansion of space. 499 00:27:26,746 --> 00:27:29,380 Dark energy has a sneaky way of taking over 500 00:27:29,382 --> 00:27:30,914 because it causes the space 501 00:27:30,916 --> 00:27:32,616 to stretch out and get twice as big. 502 00:27:32,618 --> 00:27:36,153 So now there's twice as much dark energy. 503 00:27:36,155 --> 00:27:39,623 Dark energy just can't stop pushing, 504 00:27:39,625 --> 00:27:42,393 causing the empty space of the voids 505 00:27:42,395 --> 00:27:44,194 to continuously expand. 506 00:27:47,133 --> 00:27:50,701 As the universe expands because of dark energy, 507 00:27:50,703 --> 00:27:53,170 more and more dark energy is being created. 508 00:27:53,172 --> 00:27:54,738 Dark energy is definitely gaining 509 00:27:54,740 --> 00:27:56,507 the upper hand on dark matter. 510 00:27:56,509 --> 00:28:01,545 It was always there, but it took over compared to other stuff. 511 00:28:03,082 --> 00:28:06,450 Eventually creating enough energy to supercharge 512 00:28:06,452 --> 00:28:08,552 the expansion of the universe. 513 00:28:10,923 --> 00:28:13,023 This acceleration continues. 514 00:28:13,025 --> 00:28:15,893 The universe is getting bigger and bigger, 515 00:28:15,895 --> 00:28:19,430 and it's all powered by the forces of repulsion, 516 00:28:19,432 --> 00:28:20,831 dark energy. 517 00:28:20,833 --> 00:28:25,836 And for the universe, that could be very bad news. 518 00:28:25,838 --> 00:28:30,174 If that's the case, dark energy may destroy the universe. 519 00:28:30,176 --> 00:28:31,642 It will get stronger and stronger 520 00:28:31,644 --> 00:28:34,878 until it literally rips apart the fabric of space-time. 521 00:28:49,762 --> 00:28:51,695 Dark matter and dark energy 522 00:28:51,697 --> 00:28:56,433 have been battling each other for 13.8 billion years. 523 00:28:56,435 --> 00:29:01,171 For the first 9 billion years, dark matter dominates. 524 00:29:01,173 --> 00:29:04,775 Dark matter exerts positive gravity, 525 00:29:04,777 --> 00:29:09,913 but pulling everything together leads to one inevitable outcome. 526 00:29:09,915 --> 00:29:14,118 If the universe was totally dominated by matter, 527 00:29:14,120 --> 00:29:16,787 eventually our expansion would slow down, 528 00:29:16,789 --> 00:29:20,023 glide to a stop, and then turn around 529 00:29:20,025 --> 00:29:24,027 and collapse into a small, dense state 530 00:29:24,029 --> 00:29:29,099 from where it came from, an event we call the big crunch. 531 00:29:29,101 --> 00:29:30,868 During the big crunch, 532 00:29:30,870 --> 00:29:34,605 gravity would play havoc with the cosmos. 533 00:29:34,607 --> 00:29:38,375 Galaxies would be dragged together. 534 00:29:38,377 --> 00:29:42,479 Stars and planets would smash into each other. 535 00:29:42,481 --> 00:29:46,216 The universe would collapse in a blazing inferno 536 00:29:46,218 --> 00:29:48,886 of superdense matter and energy. 537 00:29:52,925 --> 00:29:55,993 Fortunately, none of this will probably happen. 538 00:29:59,064 --> 00:30:03,233 Scientists have now dismissed the possibility of a big crunch. 539 00:30:06,338 --> 00:30:08,739 Sutter: We don't face that, because we have a universe 540 00:30:08,741 --> 00:30:11,108 filled with dark energy. 541 00:30:11,110 --> 00:30:14,144 Dark energy is causing the universe to do something else, 542 00:30:14,146 --> 00:30:16,914 something it would prefer not to do. 543 00:30:16,916 --> 00:30:20,751 It is accelerating the expansion of the universe. 544 00:30:20,753 --> 00:30:23,287 As dark energy gets stronger, 545 00:30:23,289 --> 00:30:26,590 it supercharges this expansion. 546 00:30:26,592 --> 00:30:32,396 The presence of dark energy is like a high-octane additive 547 00:30:32,398 --> 00:30:34,198 into a gas tank, 548 00:30:34,200 --> 00:30:37,835 where a car isn't just coasting along. 549 00:30:37,837 --> 00:30:40,137 It's boosting along, 550 00:30:40,139 --> 00:30:42,973 and that's what's happening with our universe. 551 00:30:42,975 --> 00:30:45,943 If that's correct, this supercharged expansion 552 00:30:45,945 --> 00:30:49,980 will continue for the next few billion years. 553 00:30:49,982 --> 00:30:54,418 The repulsive force of dark energy will become invincible, 554 00:30:54,420 --> 00:31:00,123 and such unbridled power will come with a high cost. 555 00:31:00,125 --> 00:31:03,627 I know of three ultimate doomsday weapons. 556 00:31:03,629 --> 00:31:07,397 One is galactus' ultimate nullifier. 557 00:31:07,399 --> 00:31:11,268 Another is the infinity stones in the hands of thanos. 558 00:31:11,270 --> 00:31:13,136 But those two are make-believe. 559 00:31:13,138 --> 00:31:15,272 The real one is dark energy. 560 00:31:15,274 --> 00:31:18,642 It's the real ultimate universe destroyer. 561 00:31:20,713 --> 00:31:23,747 The universe may pay the ultimate price 562 00:31:23,749 --> 00:31:28,352 for this ever-increasing expansion driven by dark energy. 563 00:31:28,354 --> 00:31:29,853 And if that continues forever, 564 00:31:29,855 --> 00:31:33,056 the future of the universe is very, very simple. 565 00:31:33,058 --> 00:31:35,492 It continues to expand, to accelerate. 566 00:31:35,494 --> 00:31:37,394 Everything moves apart from everything else, 567 00:31:37,396 --> 00:31:40,497 and the universe becomes empty. 568 00:31:40,499 --> 00:31:42,332 There'll be nothing left 569 00:31:42,334 --> 00:31:46,003 but cold, desolate, empty space forever. 570 00:31:47,673 --> 00:31:49,573 Dark energy will finally win 571 00:31:49,575 --> 00:31:52,242 the long war with dark matter. 572 00:31:52,244 --> 00:31:55,178 But the result won't be pretty. 573 00:31:55,180 --> 00:32:00,684 This future universe will be cold, dark, and empty. 574 00:32:00,686 --> 00:32:02,486 Eventually it's going to get so cold 575 00:32:02,488 --> 00:32:05,022 that really nothing can happen in it. 576 00:32:05,024 --> 00:32:06,690 It's the big chill. 577 00:32:06,692 --> 00:32:10,160 It gets colder and colder and darker and darker. 578 00:32:10,162 --> 00:32:13,530 Everything will fade out, and though it began with a bang, 579 00:32:13,532 --> 00:32:14,998 it's going to die in a whimper. 580 00:32:15,000 --> 00:32:19,036 The universe flatlines because of dark energy. 581 00:32:19,038 --> 00:32:22,539 The universe dies in a big chill. 582 00:32:22,541 --> 00:32:24,841 Galaxies are so far apart, 583 00:32:24,843 --> 00:32:29,112 they're distant islands in a sea of darkness. 584 00:32:29,114 --> 00:32:32,282 Gradually, the galaxies dies, too. 585 00:32:32,284 --> 00:32:36,954 Star birth stops, and the universe fades away. 586 00:32:38,991 --> 00:32:40,757 Or maybe not. 587 00:32:40,759 --> 00:32:45,429 There's another, far more violent scenario. 588 00:32:45,431 --> 00:32:50,767 Here, dark energy just goes from strength to strength. 589 00:32:50,769 --> 00:32:54,705 It could be that dark energy is so strong 590 00:32:54,707 --> 00:32:58,008 that it will multiply upon itself 591 00:32:58,010 --> 00:33:00,377 as the universe gets bigger. 592 00:33:00,379 --> 00:33:05,048 This is a process that we call phantom dark energy. 593 00:33:05,050 --> 00:33:09,753 Phantom energy is dark energy on steroids. 594 00:33:09,755 --> 00:33:13,623 It multiplies uncontrollably in the voids, 595 00:33:13,625 --> 00:33:16,026 tearing at the fabric of the universe 596 00:33:16,028 --> 00:33:20,597 in a process called the big rip. 597 00:33:20,599 --> 00:33:24,935 Dark energy is weird enough, but imagine the possibility 598 00:33:24,937 --> 00:33:28,405 that there is more and more of it as time goes on, 599 00:33:28,407 --> 00:33:32,509 and it's called phantom energy, and in that case, 600 00:33:32,511 --> 00:33:35,679 it would rip everything apart, even black holes. 601 00:33:39,885 --> 00:33:42,586 It will start to rip apart galaxies themselves... 602 00:33:45,657 --> 00:33:47,691 Rip apart solar systems... 603 00:33:51,563 --> 00:33:53,230 Rip apart people, 604 00:33:53,232 --> 00:33:56,666 rip apart atoms... 605 00:33:56,668 --> 00:33:57,868 Rip apart nuclei. 606 00:34:01,140 --> 00:34:03,774 Until finally, space itself is pulled apart. 607 00:34:05,044 --> 00:34:06,610 The universe, as we know it, 608 00:34:06,612 --> 00:34:08,378 will be destroyed, 609 00:34:08,380 --> 00:34:11,782 but the big rip may not be the end of everything. 610 00:34:15,821 --> 00:34:20,724 There will be no normal matter and no dark matter. 611 00:34:20,726 --> 00:34:22,826 And with nothing left to conquer, 612 00:34:22,828 --> 00:34:27,464 phantom energy may use its powers to become a creator, 613 00:34:27,466 --> 00:34:29,232 triggering a rebirth. 614 00:34:30,969 --> 00:34:35,405 Eventually, when you get to this ultimate stage of emptiness, 615 00:34:35,407 --> 00:34:36,873 because of the phantom energy, 616 00:34:36,875 --> 00:34:38,675 we're actually able to turn the universe around 617 00:34:38,677 --> 00:34:41,178 and get it to collapse again, 618 00:34:41,180 --> 00:34:44,114 and then go through a series of bounces. 619 00:34:44,116 --> 00:34:45,782 So we call it the phantom bounce. 620 00:34:47,352 --> 00:34:49,219 With this phantom bounce, 621 00:34:49,221 --> 00:34:51,721 universal life energy left in this dead universe 622 00:34:51,723 --> 00:34:54,357 starts to collapse. 623 00:34:54,359 --> 00:34:56,426 Freese: And eventually it becomes hotter and hotter 624 00:34:56,428 --> 00:34:59,463 and denser and denser, and then the fiery inferno 625 00:34:59,465 --> 00:35:02,933 eventually pushes you back out into another big bang, 626 00:35:02,935 --> 00:35:05,469 and this just keeps going on indefinitely. 627 00:35:07,139 --> 00:35:10,474 So the destructive and repulsive dark energy 628 00:35:10,476 --> 00:35:16,213 spawns a force that becomes the ultimate universe recycler. 629 00:35:16,215 --> 00:35:18,582 The end state of our universe 630 00:35:18,584 --> 00:35:20,851 would lead you back into another cycle, 631 00:35:20,853 --> 00:35:22,953 a whole new big bang from the beginning. 632 00:35:28,127 --> 00:35:31,962 In the end, dark energy may kill the cosmos, 633 00:35:31,964 --> 00:35:35,198 or it may create a new one. 634 00:35:35,200 --> 00:35:36,800 Dark energy is mysterious. 635 00:35:36,802 --> 00:35:38,335 Dark energy is unknown. 636 00:35:38,337 --> 00:35:41,705 Dark energy is going to do whatever it feels like. 637 00:35:41,707 --> 00:35:43,707 Maybe dark energy will go away. 638 00:35:43,709 --> 00:35:45,809 Maybe dark energy will decay 639 00:35:45,811 --> 00:35:48,912 and become a flood of new matter and radiation. 640 00:35:48,914 --> 00:35:51,781 Maybe dark matter will get stronger. 641 00:35:51,783 --> 00:35:52,916 We don't know. 642 00:35:56,255 --> 00:35:58,822 For now, we think dark energy 643 00:35:58,824 --> 00:36:02,159 will determine the fate of the universe. 644 00:36:02,161 --> 00:36:05,929 But all of our evidence is speculative. 645 00:36:05,931 --> 00:36:07,797 What if we have it all wrong? 646 00:36:07,799 --> 00:36:11,601 What if there is no dark universe at all? 647 00:36:29,821 --> 00:36:31,888 A large part of our understanding 648 00:36:31,890 --> 00:36:33,490 of the universe's past, 649 00:36:33,492 --> 00:36:37,227 present, and future is based on educated guesswork 650 00:36:37,229 --> 00:36:39,763 about two invisible forces... 651 00:36:42,067 --> 00:36:45,669 Dark matter and dark energy. 652 00:36:45,671 --> 00:36:49,072 But it's pure speculation. 653 00:36:49,074 --> 00:36:53,577 Perhaps dark matter and dark energy don't exist. 654 00:36:53,579 --> 00:36:56,279 There's not new stuff in the universe. 655 00:36:56,281 --> 00:36:57,514 Anything is possible. 656 00:36:57,516 --> 00:36:58,949 Dark energy, in particular, 657 00:36:58,951 --> 00:37:01,651 might not be real, so maybe there's something else 658 00:37:01,653 --> 00:37:03,353 that could be pushing the universe apart. 659 00:37:03,355 --> 00:37:05,021 So we could absolutely be wrong 660 00:37:05,023 --> 00:37:07,257 about dark energy and dark matter. 661 00:37:07,259 --> 00:37:08,959 Maybe they don't exist. 662 00:37:08,961 --> 00:37:11,962 Maybe tomorrow we'll discover that our understanding 663 00:37:11,964 --> 00:37:14,698 was wrong all along. 664 00:37:14,700 --> 00:37:16,733 That's an awful lot of maybes. 665 00:37:16,735 --> 00:37:19,035 Let's add one more. 666 00:37:19,037 --> 00:37:21,638 When it comes to finding answers, 667 00:37:21,640 --> 00:37:24,241 maybe we're looking in the wrong place. 668 00:37:26,345 --> 00:37:29,713 Filippenko: One possibility is that there are other universes 669 00:37:29,715 --> 00:37:34,484 out there pulling outward, so to speak, on our universe. 670 00:37:34,486 --> 00:37:35,885 That might be the answer. 671 00:37:35,887 --> 00:37:38,555 But most theoretical physicists and astrophysicists 672 00:37:38,557 --> 00:37:41,625 these days think that dark energy is real 673 00:37:41,627 --> 00:37:44,728 because that seems to be the simplest explanation 674 00:37:44,730 --> 00:37:47,564 for a wide variety of observations. 675 00:37:49,101 --> 00:37:52,736 No one really knows what dark energy is made of. 676 00:37:52,738 --> 00:37:56,273 Maybe the answer lies in the past. 677 00:37:56,275 --> 00:37:58,508 The best theory for dark energy we have right now 678 00:37:58,510 --> 00:38:00,810 is the simplest one and the oldest one, 679 00:38:00,812 --> 00:38:03,246 and that's the idea that it's a cosmological constant. 680 00:38:05,150 --> 00:38:07,384 Albert einstein came up with the idea 681 00:38:07,386 --> 00:38:12,289 of a cosmological constant in 1917. 682 00:38:12,291 --> 00:38:16,159 He suggested that space has its own energy, 683 00:38:16,161 --> 00:38:19,396 energy that can affect the way the universe expands. 684 00:38:21,867 --> 00:38:24,934 When edwin hubble proved the universe is expanding, 685 00:38:24,936 --> 00:38:27,937 einstein thought the cosmological constant 686 00:38:27,939 --> 00:38:30,073 was his biggest blunder. 687 00:38:30,075 --> 00:38:33,877 But observations that the expansion of the universe 688 00:38:33,879 --> 00:38:35,412 is accelerating 689 00:38:35,414 --> 00:38:38,948 reveal einstein was right all along. 690 00:38:38,950 --> 00:38:42,919 Filippenko: Well, here we are. We've reintroduced the idea. 691 00:38:42,921 --> 00:38:46,756 So einstein's biggest blunder may have actually 692 00:38:46,758 --> 00:38:49,292 conceptually been his greatest triumph. 693 00:38:52,831 --> 00:38:56,199 But to understand the true nature of the dark universe, 694 00:38:56,201 --> 00:39:00,537 we may need to re-evaluate what we think we know about gravity. 695 00:39:02,808 --> 00:39:05,542 When we're trying to understand dark energy and dark matter, 696 00:39:05,544 --> 00:39:09,379 there's a chance that just our fundamental theories of gravity 697 00:39:09,381 --> 00:39:13,783 are wrong, that general relativity isn't quite right. 698 00:39:13,785 --> 00:39:16,152 Einstein's theory of general relativity 699 00:39:16,154 --> 00:39:18,455 explains how gravity works, 700 00:39:18,457 --> 00:39:21,124 how stars orbit in galaxies, 701 00:39:21,126 --> 00:39:23,593 and planets orbit stars. 702 00:39:25,464 --> 00:39:28,398 Some scientists wonder if altering this theory 703 00:39:28,400 --> 00:39:32,669 will help us understand the dark universe. 704 00:39:32,671 --> 00:39:34,704 So you need not absolutely believe 705 00:39:34,706 --> 00:39:37,907 that there is something actually called dark matter. 706 00:39:37,909 --> 00:39:39,876 You need to only understand that there is something 707 00:39:39,878 --> 00:39:42,746 in the universe which behaves like dark matter. 708 00:39:42,748 --> 00:39:45,715 For example, you could effectively mimic the behavior 709 00:39:45,717 --> 00:39:49,986 of dark matter by modifying our current theory of gravity. 710 00:39:49,988 --> 00:39:51,921 But successfully modifying 711 00:39:51,923 --> 00:39:56,493 einstein's theories on gravity is a big challenge. 712 00:39:56,495 --> 00:39:58,795 Einstein's equations are very robust. 713 00:39:58,797 --> 00:40:02,399 You don't fluff around with einstein with impunity. 714 00:40:05,103 --> 00:40:07,370 For decades, theoretical physicists 715 00:40:07,372 --> 00:40:10,507 have toyed with einstein's equations, 716 00:40:10,509 --> 00:40:14,744 looking for ways to explain dark matter and dark energy, 717 00:40:14,746 --> 00:40:17,180 or make them go away. 718 00:40:17,182 --> 00:40:19,649 As yet, no one has managed. 719 00:40:19,651 --> 00:40:22,185 The dark universe persists. 720 00:40:23,722 --> 00:40:26,156 I think that the best description of the observations 721 00:40:26,158 --> 00:40:29,359 we have today is that dark matter exists. 722 00:40:29,361 --> 00:40:31,761 It's out there, as well as dark energy. 723 00:40:31,763 --> 00:40:33,730 I think dark energy exists. 724 00:40:33,732 --> 00:40:35,365 I think dark energy is real, 725 00:40:35,367 --> 00:40:38,435 but I must admit that sometimes, at 3 o'clock in the morning, 726 00:40:38,437 --> 00:40:41,771 I wake up screaming, worried that, in fact, 727 00:40:41,773 --> 00:40:44,741 we've settled on the wrong answer, 728 00:40:44,743 --> 00:40:46,810 and that in a couple hundred years, 729 00:40:46,812 --> 00:40:48,745 they're going to be laughing at us. 730 00:40:51,683 --> 00:40:54,884 Until then, our observations tell us the battle 731 00:40:54,886 --> 00:40:59,522 between dark matter and dark energy has shaped the universe. 732 00:41:02,928 --> 00:41:06,329 Dark matter dictated the past, 733 00:41:06,331 --> 00:41:09,632 built the galaxies, the stars, and the planets. 734 00:41:11,503 --> 00:41:14,604 Dark energy will determine its future, 735 00:41:14,606 --> 00:41:18,041 potentially tearing the universe apart. 736 00:41:18,043 --> 00:41:21,511 Just because we can't see dark matter and dark energy directly 737 00:41:21,513 --> 00:41:24,347 doesn't mean they have not had a profound effect 738 00:41:24,349 --> 00:41:26,683 on the evolution of the entire universe. 739 00:41:26,685 --> 00:41:30,019 The dark universe was there at the beginning of the universe, 740 00:41:30,021 --> 00:41:32,489 shaping it, and actually creating the conditions 741 00:41:32,491 --> 00:41:36,159 for us to be here, and it's taken over the universe. 742 00:41:36,161 --> 00:41:39,529 And, eventually, it may well destroy the universe. 743 00:41:41,166 --> 00:41:42,999 Our universe may be dominated 744 00:41:43,001 --> 00:41:47,470 by the long struggle between dark matter and dark energy. 745 00:41:47,472 --> 00:41:51,341 But all of this conflict has led to a creative outcome, 746 00:41:51,343 --> 00:41:55,678 an outcome for which we should all be grateful. 747 00:41:55,680 --> 00:41:59,849 The name dark matter suggests that it's something nefarious 748 00:41:59,851 --> 00:42:02,252 and somehow bad for us, but actually it's turned out 749 00:42:02,254 --> 00:42:04,821 that dark matter is very much our friend. 750 00:42:04,823 --> 00:42:07,390 Because if it weren't for the dark matter, 751 00:42:07,392 --> 00:42:08,691 we wouldn't be here. 752 00:42:09,895 --> 00:42:11,227 There's a wonderful irony 753 00:42:11,229 --> 00:42:13,096 to calling it the dark universe, 754 00:42:13,098 --> 00:42:14,998 because now we're actually beginning to shed light 755 00:42:15,000 --> 00:42:16,499 on how the universe began, 756 00:42:16,501 --> 00:42:18,935 how the largest structures in the universe evolved. 757 00:42:18,937 --> 00:42:20,803 We wouldn't be here without this dark universe. 758 00:42:20,805 --> 00:42:22,305 It's not dark at all. 759 00:42:22,307 --> 00:42:24,140 It's shedding light on our own reality.