1 00:00:03,169 --> 00:00:07,272 ROWE: First there was light, visible light. 2 00:00:07,374 --> 00:00:12,811 Then, we viewed the universe in radio waves and X-rays. 3 00:00:12,912 --> 00:00:15,481 Ever since there's been astronomy, 4 00:00:15,582 --> 00:00:19,118 we've been looking at different kinds of light 5 00:00:19,219 --> 00:00:22,721 and opening up the universe a little bit more of the time. 6 00:00:24,391 --> 00:00:28,394 But then in 2015, like, the roof came off. 7 00:00:29,696 --> 00:00:31,530 THALLER: Something happened that changed everything, 8 00:00:31,631 --> 00:00:35,200 the ability to see waves in space and time itself. 9 00:00:35,301 --> 00:00:37,669 ROWE: Gravitational waves. 10 00:00:37,771 --> 00:00:41,273 They help us roll back the clock to the dawn of time, 11 00:00:41,374 --> 00:00:44,877 discover epic cosmic collisions, 12 00:00:44,978 --> 00:00:48,447 on make Earth-shaking discoveries. 13 00:00:48,548 --> 00:00:51,316 Gravitational waves are the biggest game changer 14 00:00:51,418 --> 00:00:53,285 since the invention of the telescope. 15 00:00:53,386 --> 00:00:57,156 We have a completely new universe to view now. 16 00:00:57,257 --> 00:01:00,159 ROWE: A new exploration of space is just beginning. 17 00:01:00,260 --> 00:01:02,628 [electricity buzzing] 18 00:01:04,564 --> 00:01:06,198 [explosion blasts] 19 00:01:12,772 --> 00:01:16,675 Long ago, 17 billion light-years away, 20 00:01:16,776 --> 00:01:19,111 a cataclysmic showdown plays out. 21 00:01:20,680 --> 00:01:22,915 Two black holes locked together 22 00:01:23,016 --> 00:01:25,117 in a deadly cosmic dance. 23 00:01:25,218 --> 00:01:30,389 Black holes are unimaginably dense objects with gravity 24 00:01:30,490 --> 00:01:33,192 so intense that if you get too close to them, 25 00:01:33,293 --> 00:01:34,893 you're gone. 26 00:01:39,732 --> 00:01:42,401 ROWE: Their immense gravitational pull causes 27 00:01:42,502 --> 00:01:45,237 them to spiral towards each other. 28 00:01:45,338 --> 00:01:48,740 OLUSEYI: When black holes collide, they don't just run 29 00:01:48,842 --> 00:01:49,608 into each other. 30 00:01:49,709 --> 00:01:51,910 They're in orbit about each other. 31 00:01:52,011 --> 00:01:55,647 So what we're talking about is an inspiralling orbit 32 00:01:55,748 --> 00:01:58,750 that goes faster and faster and faster and faster. 33 00:01:58,852 --> 00:02:02,221 ROWE: Until they finally collide in a fatal embrace. 34 00:02:05,358 --> 00:02:07,926 [explosion blasts] 35 00:02:08,027 --> 00:02:10,095 But astronomers don't see a thing. 36 00:02:11,631 --> 00:02:13,999 The problem with observing colliding black holes is all 37 00:02:14,100 --> 00:02:16,735 about the name, black holes, they give off no light. 38 00:02:16,836 --> 00:02:19,071 How can astronomers see something that 39 00:02:19,172 --> 00:02:20,939 no telescope can detect? 40 00:02:22,842 --> 00:02:24,076 ROWE: Across the universe, 41 00:02:24,177 --> 00:02:27,379 extraordinary events take place. 42 00:02:27,480 --> 00:02:33,352 But we sometimes miss them, because we rely on light. 43 00:02:33,453 --> 00:02:37,389 Now, astronomers have a new toolkit that's 44 00:02:37,490 --> 00:02:41,226 revealing the cosmos in a totally different way... 45 00:02:43,396 --> 00:02:46,732 ...using the very fabric of our universe 46 00:02:46,833 --> 00:02:49,935 we call spacetime. 47 00:02:50,036 --> 00:02:53,772 Everything with mass, like stars, 48 00:02:53,873 --> 00:02:57,743 planets, and black holes, all curve this fabric. 49 00:02:59,846 --> 00:03:01,613 The more massive the object, 50 00:03:01,714 --> 00:03:04,416 the bigger the distortion of spacetime. 51 00:03:04,517 --> 00:03:06,852 The classical analogy is 52 00:03:06,953 --> 00:03:08,954 this stretched rubber sheet, right? 53 00:03:09,055 --> 00:03:12,824 And, like, a mass, like, the sun is, like, a ball on this sheet, 54 00:03:12,926 --> 00:03:14,893 and it distorts and warps the sheet 55 00:03:14,994 --> 00:03:16,161 into this valley, right? 56 00:03:16,262 --> 00:03:19,398 And if you roll a marble across it like the marble is 57 00:03:19,499 --> 00:03:21,133 a planet, the marble will be 58 00:03:21,234 --> 00:03:23,902 pulled into orbit around the ball because 59 00:03:24,003 --> 00:03:25,804 of the curvature of the sheet. 60 00:03:28,875 --> 00:03:31,276 ROWE: But that's only half the picture. 61 00:03:31,377 --> 00:03:34,947 If an object has mass and is accelerating through 62 00:03:35,048 --> 00:03:37,416 spacetime, it creates ripples 63 00:03:37,517 --> 00:03:39,218 in that fabric of spacetime, 64 00:03:39,319 --> 00:03:41,553 and we call these gravitational waves. 65 00:03:41,654 --> 00:03:46,191 ROWE: Gravitational waves give us vital clues 66 00:03:46,292 --> 00:03:48,860 about distant objects that we can't see. 67 00:03:50,263 --> 00:03:53,865 The more massive the object that produces them and the faster 68 00:03:53,967 --> 00:03:57,536 it's moving, the bigger the ripples. 69 00:03:57,637 --> 00:04:01,673 These ripples pass through planets, stars, and galaxies 70 00:04:01,741 --> 00:04:03,008 with ease. 71 00:04:04,711 --> 00:04:07,913 When a gravitational wave passes through an object like 72 00:04:08,014 --> 00:04:10,215 a star or a planet or a person, 73 00:04:10,316 --> 00:04:12,217 it stretches and compresses them, 74 00:04:12,318 --> 00:04:13,619 like with this tennis ball. 75 00:04:15,021 --> 00:04:17,756 Now, if you're close to a powerful source of 76 00:04:17,857 --> 00:04:18,724 gravitational waves, 77 00:04:18,825 --> 00:04:21,159 like merging supermassive black holes, 78 00:04:21,261 --> 00:04:23,829 those waves are incredibly strong, and they're capable of 79 00:04:23,930 --> 00:04:26,098 actually destroying a planet. 80 00:04:26,165 --> 00:04:29,368 ROWE: But like the ripples on a pond, 81 00:04:29,469 --> 00:04:32,771 their strength and size diminishes over distance. 82 00:04:34,073 --> 00:04:36,708 The farther away you are, the weaker they get. 83 00:04:36,809 --> 00:04:39,511 And when they're hundreds of millions of light-years away, 84 00:04:39,612 --> 00:04:42,648 they're actually smaller than the size of an atom. 85 00:04:42,749 --> 00:04:45,517 ROWE: So, to listen for gravitational waves, 86 00:04:45,618 --> 00:04:48,720 scientists built the most sensitive measuring device on 87 00:04:48,821 --> 00:04:49,888 the planet. 88 00:04:54,294 --> 00:04:56,795 This is LIGO, 89 00:04:56,896 --> 00:05:01,466 the Laser Interferometer Gravitational-Wave Observatory, 90 00:05:03,236 --> 00:05:07,339 two enormous detectors located almost 2,000 miles 91 00:05:07,440 --> 00:05:10,642 apart in Louisiana and Washington state. 92 00:05:12,412 --> 00:05:16,648 Each sensor has L-shaped arms, measuring 2.5 miles. 93 00:05:19,118 --> 00:05:21,086 Inside the LIGO detectors, 94 00:05:21,187 --> 00:05:25,757 inside these concrete tunnels, there is a laser system. 95 00:05:25,858 --> 00:05:29,761 It's called an interferometer, so light comes in from 96 00:05:29,862 --> 00:05:34,800 a laser beam and is split into two paths. 97 00:05:36,536 --> 00:05:39,805 ROWE: Normally, the lengths of the two beams are the same. 98 00:05:41,140 --> 00:05:43,342 That changes when gravitational waves 99 00:05:43,443 --> 00:05:44,476 hit the beams. 100 00:05:46,479 --> 00:05:49,181 When a gravitational wave passes through, 101 00:05:49,282 --> 00:05:53,752 it changes the distance that light travels along these arms, 102 00:05:53,853 --> 00:05:58,190 so one arm effectively gets longer, and the other one 103 00:05:58,291 --> 00:05:59,524 gets shorter. 104 00:05:59,625 --> 00:06:03,795 The length of those two beams varies just ever so slightly, 105 00:06:03,896 --> 00:06:07,733 and the very sensitive apparatus in LIGO is able to pick that up. 106 00:06:08,835 --> 00:06:11,436 ROWE: With this ultra-sensitive laser system, 107 00:06:11,537 --> 00:06:16,575 LIGO picks up distortions in spacetime, narrower than 108 00:06:16,676 --> 00:06:20,112 one millionth of the diameter of an atom. 109 00:06:20,213 --> 00:06:21,346 Just that feat, 110 00:06:21,447 --> 00:06:23,482 just the fact that we were able to build 111 00:06:23,583 --> 00:06:26,351 a detector to detect gravitational waves 112 00:06:26,452 --> 00:06:28,954 is just mind-boggling. 113 00:06:29,021 --> 00:06:30,722 All of a sudden now, we were listening 114 00:06:30,823 --> 00:06:33,024 to the faintest whispers of the universe. 115 00:06:34,927 --> 00:06:39,431 ROWE: In 2015, LIGO picked up a whisper that had 116 00:06:39,532 --> 00:06:42,300 been traveling towards Earth for over a billion years. 117 00:06:44,470 --> 00:06:48,573 Its source? Two colliding stellar black holes. 118 00:06:48,674 --> 00:06:52,277 Watching two black holes spiral in and merge -- 119 00:06:52,378 --> 00:06:54,546 That's not something we can do using optical 120 00:06:54,647 --> 00:06:57,482 telescopes or X-ray telescopes or anything like that. 121 00:06:57,583 --> 00:07:01,486 But with LIGO, we could actually detect that event. 122 00:07:01,587 --> 00:07:03,588 [explosion blasts] 123 00:07:11,564 --> 00:07:13,198 ROWE: Now, scientists can paint 124 00:07:13,299 --> 00:07:15,667 accurate pictures of invisible objects. 125 00:07:18,504 --> 00:07:20,772 You can tell you're looking at black holes. 126 00:07:20,873 --> 00:07:23,475 You can get their masses, you can get their distance. 127 00:07:23,576 --> 00:07:27,245 There's a phenomenal amount of information in that wave. 128 00:07:29,315 --> 00:07:30,649 ROWE: The colliding black holes are 129 00:07:30,750 --> 00:07:33,251 the most massive LIGO has ever detected. 130 00:07:34,320 --> 00:07:37,322 One is 66 times the mass of our sun, 131 00:07:38,524 --> 00:07:41,526 the other, 85 times the mass of our sun. 132 00:07:43,129 --> 00:07:45,530 PLAIT: As two black holes are spiraling in, 133 00:07:45,631 --> 00:07:47,132 they are moving faster and faster 134 00:07:47,233 --> 00:07:49,000 as they get closer and closer. 135 00:07:49,101 --> 00:07:50,869 That means that the gravitational waves 136 00:07:50,970 --> 00:07:54,072 they're emitting have a higher and higher frequency. 137 00:07:54,173 --> 00:07:57,309 So as time goes on, the pitch gets higher. 138 00:07:57,410 --> 00:07:59,511 So it goes... [increasing pitch] ooop! 139 00:07:59,612 --> 00:08:00,745 [increasing pitch] Ooop! 140 00:08:00,847 --> 00:08:01,947 [increasing pitch] Zhhhrp! 141 00:08:02,048 --> 00:08:06,017 [repeating noise with increasing pitch] 142 00:08:08,254 --> 00:08:11,556 ROWE: When they finally merge, they create a giant. 143 00:08:16,996 --> 00:08:19,498 PONTZEN: By analyzing that data, 144 00:08:19,599 --> 00:08:23,168 it's possible to establish that the new black hole from 145 00:08:23,269 --> 00:08:26,905 the merger of these two original black holes weighs as much as 146 00:08:27,006 --> 00:08:31,109 something like 140 times the mass of our sun. 147 00:08:32,912 --> 00:08:34,613 It's really difficult to overstate 148 00:08:34,714 --> 00:08:36,615 the importance of gravitational wave detection. 149 00:08:36,716 --> 00:08:39,150 It's like adding on an entirely new sense -- 150 00:08:39,252 --> 00:08:41,686 All of a sudden, there's a brand-new way 151 00:08:41,787 --> 00:08:43,288 to explore the rest of the universe. 152 00:08:46,993 --> 00:08:49,027 ROWE: Invisible cosmic collisions are just 153 00:08:49,128 --> 00:08:51,263 the beginning of what gravitational wave 154 00:08:51,364 --> 00:08:52,931 astronomy can reveal to us. 155 00:08:54,300 --> 00:08:58,370 Now, scientists are using gravitational waves 156 00:08:58,471 --> 00:09:00,939 to revisit other long-standing mysteries, 157 00:09:01,040 --> 00:09:05,677 like what causes the brightest explosions 158 00:09:05,778 --> 00:09:08,146 in the cosmos? 159 00:09:08,247 --> 00:09:10,115 This is not an everyday car crash. 160 00:09:10,216 --> 00:09:12,717 This is the most dramatic event that 161 00:09:12,818 --> 00:09:16,388 you're ever gonna see in our universe. 162 00:09:27,366 --> 00:09:29,000 ROWE: Across the universe, 163 00:09:29,101 --> 00:09:32,304 strange bursts of light puzzle astronomers. 164 00:09:34,073 --> 00:09:36,241 For just a fraction of a second, 165 00:09:36,375 --> 00:09:38,944 they shine more than a trillion times brighter 166 00:09:39,045 --> 00:09:43,782 than the sun -- Then, they vanish. 167 00:09:43,883 --> 00:09:46,484 These brief flashes of light are known 168 00:09:46,586 --> 00:09:50,455 as gamma-ray bursts or GRBs for short, 169 00:09:50,556 --> 00:09:52,857 and they're such a mystery, because they are 170 00:09:52,959 --> 00:09:57,696 insanely energetic, and we don't know what causes them. 171 00:09:57,797 --> 00:09:59,397 [explosion blasts] 172 00:09:59,498 --> 00:10:02,500 ROWE: For decades, these short gamma-ray bursts 173 00:10:02,602 --> 00:10:04,736 have been an enigma. 174 00:10:04,837 --> 00:10:08,640 No explanation was off limits, no matter how wild. 175 00:10:09,942 --> 00:10:11,443 Is it a supernova? 176 00:10:11,544 --> 00:10:13,979 Is it on alien civilization saying hello? 177 00:10:14,080 --> 00:10:15,647 You know, we just don't know. 178 00:10:17,717 --> 00:10:21,086 ROWE: In August 2017, the Fermi Gamma-ray Telescope 179 00:10:21,187 --> 00:10:24,756 detected another short gamma-ray burst, 180 00:10:26,492 --> 00:10:29,160 but this one was different, 181 00:10:29,261 --> 00:10:33,231 So a gamma-ray burst went off 130 million light-years away, 182 00:10:33,332 --> 00:10:36,101 and it actually produced a ripple in space and time 183 00:10:36,202 --> 00:10:37,335 that LIGO could detect. 184 00:10:38,404 --> 00:10:40,271 ROWE: Gravitational waves could help 185 00:10:40,373 --> 00:10:42,574 finally reveal what causes 186 00:10:42,675 --> 00:10:45,343 one of the brightest explosions in the universe. 187 00:10:48,314 --> 00:10:51,449 LIGOS data suggests the culprit could be two 188 00:10:51,550 --> 00:10:54,519 massive objects spiraling towards each other 189 00:10:54,620 --> 00:10:56,721 and colliding. 190 00:10:56,822 --> 00:10:59,224 But based on the gravitational wave data, 191 00:10:59,325 --> 00:11:04,095 these two objects were too small to be black holes. 192 00:11:04,196 --> 00:11:05,797 They had to be something else. 193 00:11:07,833 --> 00:11:10,368 ROWE: Not black holes, but the ultra dense 194 00:11:10,469 --> 00:11:14,072 cores of collapsed stars called neutron stars. 195 00:11:15,941 --> 00:11:18,777 ESQUIVE: A neutron star is what's left over 196 00:11:18,844 --> 00:11:21,913 after a massive star collapses in on itself. 197 00:11:22,014 --> 00:11:25,750 It's very, very dense, because it took all, essentially, 198 00:11:25,851 --> 00:11:30,755 the mass of the core and contracted it into a really, 199 00:11:30,856 --> 00:11:32,757 really small radius. 200 00:11:36,562 --> 00:11:39,831 ROWE: As the dense neutron stars spiral ever closer, 201 00:11:39,932 --> 00:11:44,602 the gravitational wave signal gets stronger and stronger, 202 00:11:44,704 --> 00:11:47,472 until they collide, releasing 203 00:11:47,573 --> 00:11:50,442 an epic burst of gravitational waves. 204 00:11:52,511 --> 00:11:54,279 PONTZEN: Because they're not black holes, 205 00:11:54,380 --> 00:11:57,482 light can get out. 206 00:11:57,583 --> 00:12:00,819 And if you smash two things together at these kind of 207 00:12:00,920 --> 00:12:02,520 absolutely massive speeds, 208 00:12:02,621 --> 00:12:04,756 there's a huge amount of energy involved. 209 00:12:04,857 --> 00:12:08,059 ROWE: Energy we detected both as 210 00:12:08,160 --> 00:12:13,198 invisible gravitational waves and visible light. 211 00:12:14,734 --> 00:12:16,935 Could this light be a mysterious 212 00:12:17,036 --> 00:12:19,471 and ultra-powerful gamma-ray burst? 213 00:12:21,340 --> 00:12:24,642 How could these colliding dead stars be associated 214 00:12:24,744 --> 00:12:25,944 with gamma-ray bursts, 215 00:12:26,045 --> 00:12:28,313 which are in fact, the most energetic explosions we see in 216 00:12:28,414 --> 00:12:29,481 the entire universe? 217 00:12:31,083 --> 00:12:33,818 ROWE: Neutron stars have powerful magnetic fields 218 00:12:33,919 --> 00:12:36,354 that trap particles of gas and dust. 219 00:12:38,257 --> 00:12:39,491 During a collision, 220 00:12:39,592 --> 00:12:41,893 the swirling magnetic fields twist up, 221 00:12:41,994 --> 00:12:44,796 building up more and more energy. 222 00:12:46,832 --> 00:12:50,068 You have lots of little particles of matter that are 223 00:12:50,169 --> 00:12:53,238 trying to keep up with these rapidly spinning magnetic 224 00:12:53,339 --> 00:12:57,609 fields -- that starts swooshing them round until they reach 225 00:12:57,710 --> 00:12:59,978 pretty much the speed of light, and eventually, 226 00:13:00,079 --> 00:13:03,915 they're kind of shot out of the remnant in a tight beam. 227 00:13:05,785 --> 00:13:09,087 ROWE: The beam is a gamma-ray burst, 228 00:13:09,188 --> 00:13:11,489 but they're not always easy to detect. 229 00:13:13,392 --> 00:13:16,227 If the jet coming out is pointed right at you, 230 00:13:16,328 --> 00:13:19,430 then you see this extremely high energy event, 231 00:13:19,532 --> 00:13:21,132 the gamma-ray burst. 232 00:13:22,434 --> 00:13:24,836 ROWE: If it's not pointed at us, 233 00:13:24,937 --> 00:13:26,437 we might miss it. 234 00:13:26,539 --> 00:13:28,706 Fortunately, the gravitational waves 235 00:13:28,808 --> 00:13:30,575 show us where to look. 236 00:13:36,248 --> 00:13:37,882 Following the gamma-ray burst, 237 00:13:37,950 --> 00:13:43,054 we spotted a strange red cloud, evidence of a heavy 238 00:13:43,155 --> 00:13:46,057 element factory. 239 00:13:46,158 --> 00:13:47,592 SUTTER: After the initial collision, 240 00:13:47,693 --> 00:13:50,962 there is a shell of debris moving outwards, 241 00:13:51,063 --> 00:13:56,067 but then, high-energy neutrons come slamming into this 242 00:13:56,168 --> 00:13:59,637 material and start to build heavier elements, 243 00:13:59,738 --> 00:14:02,473 one after another. 244 00:14:02,608 --> 00:14:07,712 We can see the gold, we can see the potassium, 245 00:14:07,813 --> 00:14:11,616 we can see the plutonium being created 246 00:14:11,717 --> 00:14:13,885 before our very eyes. 247 00:14:16,989 --> 00:14:18,790 ROWE: The neutron star collision 248 00:14:18,891 --> 00:14:22,093 produced huge quantities of heavy elements, 249 00:14:22,194 --> 00:14:25,597 blasting out enough gold and platinum to weigh more than 10 250 00:14:25,698 --> 00:14:27,832 times the mass of the Earth, 251 00:14:27,933 --> 00:14:30,301 solving a long-standing mystery. 252 00:14:30,402 --> 00:14:31,803 We knew that 253 00:14:31,904 --> 00:14:34,639 supernova explosions did create some 254 00:14:34,740 --> 00:14:35,874 of the heavier elements. 255 00:14:35,975 --> 00:14:39,344 But from everything we've observed about supernova, 256 00:14:39,478 --> 00:14:43,815 they don't happen often enough to really populate a galaxy 257 00:14:43,916 --> 00:14:46,751 with all of the heavier elements that we observed. 258 00:14:46,852 --> 00:14:49,921 This was the missing piece. 259 00:14:50,022 --> 00:14:52,924 The gold on your wedding ring, 260 00:14:53,025 --> 00:14:55,159 the gold in your jewelry, 261 00:14:55,261 --> 00:14:57,395 was formed and forged from 262 00:14:57,496 --> 00:15:00,932 a titanic collision before the Earth even existed. 263 00:15:03,869 --> 00:15:06,137 ROWE: The combination of gravitational waves 264 00:15:06,238 --> 00:15:07,238 and telescopes 265 00:15:08,974 --> 00:15:12,010 proves that neutron star collisions create 266 00:15:12,111 --> 00:15:14,412 precious metals 267 00:15:14,513 --> 00:15:17,382 and cause super-bright gamma-ray bursts. 268 00:15:18,884 --> 00:15:23,421 When you can measure a gravitational wave signal 269 00:15:23,522 --> 00:15:25,857 and a light signal like a gamma-ray burst, 270 00:15:25,958 --> 00:15:29,027 you get a whole new way to solve complicated, 271 00:15:29,128 --> 00:15:31,696 intertwined physical processes. 272 00:15:33,866 --> 00:15:36,534 TREMBLAY: It's like you're watching a symphony on mute, 273 00:15:36,635 --> 00:15:38,903 and then you hit that button, and the sound comes on, 274 00:15:39,004 --> 00:15:41,372 and it's just a completely different picture. 275 00:15:45,144 --> 00:15:49,881 ROWE: The sounds of the cosmos don't just reveal collisions. 276 00:15:53,953 --> 00:15:57,422 It turns out, we can use gravitational waves to help us 277 00:15:57,523 --> 00:16:00,591 understand some of the biggest mysteries of the cosmos. 278 00:16:13,572 --> 00:16:15,707 ROWE: Gravitational waves are a new way 279 00:16:15,808 --> 00:16:19,577 to listen to the universe, revealing unseen, 280 00:16:19,678 --> 00:16:24,115 epic cosmic events and adding vital details to our picture 281 00:16:24,216 --> 00:16:25,416 of the cosmos. 282 00:16:28,187 --> 00:16:31,756 Every new way we figure out to probe the universe is 283 00:16:31,857 --> 00:16:34,892 a good thing, and detecting gravitational waves, 284 00:16:34,994 --> 00:16:37,395 it's a new dimension to being able to study the universe. 285 00:16:37,496 --> 00:16:39,664 It's like -- it's like having a new sense. 286 00:16:42,501 --> 00:16:45,269 ROWE: This new sense could be just what astronomers need 287 00:16:45,371 --> 00:16:48,906 to answer some of the biggest questions in physics, 288 00:16:49,008 --> 00:16:51,676 like, "What is the speed of gravity?" 289 00:16:53,178 --> 00:16:56,114 And, "Does it travel at the universe's speed limit?" 290 00:16:57,916 --> 00:17:00,284 OLUSEYI: One of the things we learn early in science is that 291 00:17:00,386 --> 00:17:02,854 the universe has an absolute speed limit, 292 00:17:02,955 --> 00:17:05,423 which is the speed of light in a vacuum, 293 00:17:05,524 --> 00:17:08,693 which is 186,000 miles per second. 294 00:17:11,563 --> 00:17:13,464 ROWE: Light from the sun takes eight minutes 295 00:17:13,565 --> 00:17:15,833 and 20 seconds to reach Earth. 296 00:17:15,934 --> 00:17:19,370 So, if the sun disappeared, 297 00:17:19,471 --> 00:17:21,439 we wouldn't miss its light immediately. 298 00:17:23,142 --> 00:17:25,943 But how quickly would we notice its missing gravity? 299 00:17:27,546 --> 00:17:30,681 OLUSEYI: The first thing that we'd notice is nothing. 300 00:17:30,783 --> 00:17:35,720 Things would seem very normal, but then they wouldn't. 301 00:17:35,821 --> 00:17:40,558 There would be nothing curving space where Earth is located, 302 00:17:40,659 --> 00:17:43,795 and so Earth would take off in a straight line, 303 00:17:43,896 --> 00:17:46,464 moving at the same speed at which it orbits the sun. 304 00:17:46,565 --> 00:17:52,236 And things will get cold and lonely really, really fast. 305 00:17:54,006 --> 00:17:55,406 ROWE: According to Albert Einstein, 306 00:17:55,507 --> 00:17:59,143 our skies would go dark, and the earth would be flung into 307 00:17:59,244 --> 00:18:02,080 deep space at exactly the same time. 308 00:18:03,782 --> 00:18:07,085 It's a foundation of his famous Theory of Relativity, 309 00:18:07,186 --> 00:18:11,022 still the most complete theory of how our universe works. 310 00:18:12,491 --> 00:18:14,358 PONTZEN: Einstein's theory of relativity has been 311 00:18:14,460 --> 00:18:15,927 a fantastic theory. 312 00:18:16,028 --> 00:18:19,664 It explains so many things for us, including gravity. 313 00:18:19,765 --> 00:18:21,966 But when we look out at the universe, 314 00:18:22,067 --> 00:18:25,069 there are many mysteries, there are things that are quite hard 315 00:18:25,170 --> 00:18:26,137 to explain. 316 00:18:28,474 --> 00:18:30,541 ROWE: At the top of the list -- 317 00:18:30,642 --> 00:18:33,010 The mystery of our expanding universe. 318 00:18:35,848 --> 00:18:39,083 There is something pushing outward that is 319 00:18:39,184 --> 00:18:43,554 making that expansion rate ever and ever faster. 320 00:18:43,655 --> 00:18:46,924 ROWE: Astronomers call this something dark energy. 321 00:18:49,328 --> 00:18:53,831 It accounts for 70% of the total energy in the universe. 322 00:18:57,102 --> 00:19:00,004 Einstein's models of the universe need dark energy 323 00:19:00,105 --> 00:19:02,907 to work, but we have no idea 324 00:19:03,008 --> 00:19:04,709 what it is. 325 00:19:06,478 --> 00:19:09,847 Dark energy is not something we actually understand. 326 00:19:09,948 --> 00:19:12,150 It's kind of a placeholder term 327 00:19:12,251 --> 00:19:14,218 for something we don't understand. 328 00:19:14,319 --> 00:19:17,889 And so people naturally are looking for better theories, 329 00:19:17,990 --> 00:19:20,291 theories that are a bit like Einstein's theory 330 00:19:20,392 --> 00:19:22,860 but just go that bit further and explain 331 00:19:22,961 --> 00:19:26,531 some of these things that we don't currently understand. 332 00:19:26,632 --> 00:19:30,201 ROWE: One way to excise dark energy 333 00:19:30,302 --> 00:19:31,969 is with a new theory of gravity, 334 00:19:33,438 --> 00:19:35,806 one where the speed of gravitational waves 335 00:19:35,908 --> 00:19:39,143 is different from the speed of light. 336 00:19:39,244 --> 00:19:40,511 TREMBLAY: There are some so-called 337 00:19:40,612 --> 00:19:42,280 non-Einsteinian theories for 338 00:19:42,381 --> 00:19:45,383 the structure of spacetime itself that don't actually 339 00:19:45,484 --> 00:19:46,851 require dark energy. 340 00:19:46,952 --> 00:19:49,654 For example, if gravity doesn't propagate through 341 00:19:49,755 --> 00:19:52,757 spacetime at the same speed that light does, 342 00:19:52,858 --> 00:19:55,326 you could find models that don't actually require 343 00:19:55,427 --> 00:19:58,930 dark energy -- it could be a clean, simple, albeit very, 344 00:19:59,031 --> 00:20:02,133 very profound solution to this underlying problem. 345 00:20:04,203 --> 00:20:06,204 ROWE: In order to overthrow Einstein 346 00:20:06,305 --> 00:20:07,672 and eliminate dark energy, 347 00:20:07,773 --> 00:20:11,442 the speeds of light and gravity must be different. 348 00:20:12,911 --> 00:20:14,745 We know the speed of light. 349 00:20:14,846 --> 00:20:18,349 So how do we test the speed of gravity? 350 00:20:20,118 --> 00:20:21,953 In order to test the speed of gravity, 351 00:20:22,054 --> 00:20:24,088 you need to have a system that emits both 352 00:20:24,189 --> 00:20:26,123 gravitational waves and light. 353 00:20:28,227 --> 00:20:30,895 ROWE: The colliding neutron stars detected by LIGO 354 00:20:30,996 --> 00:20:33,731 in 2017 are part of the solution. 355 00:20:35,467 --> 00:20:38,436 The collision released a flash of light, 356 00:20:38,537 --> 00:20:40,805 along with a burst of gravitational waves. 357 00:20:45,577 --> 00:20:49,280 But the universe threw a curveball. 358 00:20:49,381 --> 00:20:52,450 The light signal arrived 1.7 seconds 359 00:20:52,551 --> 00:20:55,786 after the gravitational wave signal. 360 00:20:55,887 --> 00:20:57,722 Does that mean gravitational waves 361 00:20:57,823 --> 00:20:59,790 travel slightly faster than light? 362 00:21:02,361 --> 00:21:05,463 Albert Einstein predicted that gravitational waves 363 00:21:05,564 --> 00:21:07,398 would move at the speed of light. 364 00:21:07,499 --> 00:21:10,167 So what if Albert Einstein was wrong? 365 00:21:10,269 --> 00:21:11,902 I know, sounds crazy, right? 366 00:21:12,004 --> 00:21:15,006 That's like almost as crazy as me being wrong, right? 367 00:21:15,107 --> 00:21:18,876 But if Einstein was wrong, that's one thing. 368 00:21:18,977 --> 00:21:21,946 But a bigger problem is that we'd have to rethink 369 00:21:22,047 --> 00:21:23,247 our physics. 370 00:21:25,517 --> 00:21:26,584 ROWE: Before we do that, 371 00:21:26,685 --> 00:21:29,153 let's take a closer look at the neutron star 372 00:21:29,254 --> 00:21:30,488 collision site. 373 00:21:30,589 --> 00:21:31,989 [explosion blasts] 374 00:21:32,090 --> 00:21:34,925 It's surrounded by a shroud of gas and dust. 375 00:21:36,495 --> 00:21:39,363 Light is made of particles called photons, 376 00:21:39,464 --> 00:21:42,667 which scatter when they hit obstacles. 377 00:21:42,768 --> 00:21:46,203 But gravitational waves pass through anything. 378 00:21:47,472 --> 00:21:50,241 OLUSEYI: They pass right through everything like it's not there. 379 00:21:50,342 --> 00:21:51,909 Light, on the other hand, 380 00:21:52,010 --> 00:21:55,246 was slowed down by interactions with that matter. 381 00:21:55,347 --> 00:21:57,481 It didn't just escape immediately 382 00:21:57,582 --> 00:21:59,884 like the gravitational wave signal did. 383 00:22:01,653 --> 00:22:03,854 ROWE: The debris gave the gravitational waves 384 00:22:03,955 --> 00:22:07,258 a head start by slowing the light. 385 00:22:07,359 --> 00:22:10,127 So gravitational waves and light do, 386 00:22:10,228 --> 00:22:13,230 in fact, travel at the same speed. 387 00:22:13,298 --> 00:22:14,665 Einstein was right. 388 00:22:16,601 --> 00:22:19,403 PONTZEN: This one event ruled out the other theories of 389 00:22:19,504 --> 00:22:21,939 gravity that are competing with Einstein's theory, 390 00:22:22,040 --> 00:22:25,042 things that people have been working on all their life 391 00:22:25,143 --> 00:22:27,244 and overnight, it's gone. 392 00:22:29,114 --> 00:22:30,815 ROWE: Thanks to gravitational waves, 393 00:22:30,916 --> 00:22:34,618 dark energy remains our best explanation for why 394 00:22:34,720 --> 00:22:37,321 the universe's expansion is accelerating. 395 00:22:38,824 --> 00:22:41,592 Maybe dark energy isn't what we think it is, and maybe 396 00:22:41,693 --> 00:22:43,194 tomorrow, or maybe next year, 397 00:22:43,295 --> 00:22:44,862 or maybe next decade or next century, 398 00:22:44,963 --> 00:22:49,133 we will discover that. - Gravitational waves are a huge 399 00:22:49,234 --> 00:22:52,603 step forward in our effort to understand the universe, 400 00:22:52,704 --> 00:22:54,238 and I mean everything. 401 00:22:54,373 --> 00:22:57,475 Space, time, matter, dark energy. 402 00:22:57,576 --> 00:23:00,978 We have a completely new universe to view now. 403 00:23:03,749 --> 00:23:05,483 ROWE: Now astronomers want to use 404 00:23:05,584 --> 00:23:08,552 gravitational waves to answer another mystery. 405 00:23:10,155 --> 00:23:14,358 What happens when supermassive black holes collide? 406 00:23:26,004 --> 00:23:30,708 ROWE: We first detected gravitational waves in 2015. 407 00:23:30,809 --> 00:23:33,444 Since then, they've revealed colliding 408 00:23:33,545 --> 00:23:35,913 black holes across the universe. 409 00:23:38,116 --> 00:23:40,184 Prior to LIGO going online, 410 00:23:40,285 --> 00:23:43,821 we never witnessed black hole collisions directly, 411 00:23:43,922 --> 00:23:47,358 but now that we can witness them with our observatories, 412 00:23:47,459 --> 00:23:50,294 we're finding them pretty regularly. 413 00:23:50,395 --> 00:23:53,264 We're seeing gravitational waves come 414 00:23:53,365 --> 00:23:55,433 across the LIGO experiment 415 00:23:55,534 --> 00:23:57,368 left and right. 416 00:23:57,469 --> 00:24:00,204 ROWE: But LIGO has only been listening for gravitational 417 00:24:00,305 --> 00:24:02,006 waves from black holes 418 00:24:02,107 --> 00:24:04,475 on the smaller end of the cosmic scale. 419 00:24:06,044 --> 00:24:09,847 When we look at the cosmic zoo of black holes out there, 420 00:24:09,948 --> 00:24:14,285 we find small ones weighing, you know, 10, maybe 30 times as 421 00:24:14,386 --> 00:24:18,022 much as the sun, and then large all the way up to extra-large 422 00:24:18,123 --> 00:24:19,056 going from, like, a million 423 00:24:19,157 --> 00:24:20,858 to a billion times as much as the sun. 424 00:24:22,327 --> 00:24:24,628 ROWE: These supermassive black holes 425 00:24:24,729 --> 00:24:27,531 lurk at the hearts of galaxies. 426 00:24:27,632 --> 00:24:31,035 When Galaxies merge, supermassive black holes 427 00:24:31,136 --> 00:24:33,304 should merge, too. 428 00:24:37,108 --> 00:24:39,543 But even though we see galaxies colliding 429 00:24:39,644 --> 00:24:40,644 across the universe, 430 00:24:40,745 --> 00:24:44,982 we've never seen two supermassive black holes 431 00:24:45,083 --> 00:24:47,918 collide, because they have too 432 00:24:48,019 --> 00:24:51,322 much orbital energy to get close enough to merge. 433 00:24:55,994 --> 00:25:00,064 OLUSEYI: That orbital energy has to go somewhere, 434 00:25:00,165 --> 00:25:03,100 and what supermassive black holes do is they throw out 435 00:25:03,201 --> 00:25:06,203 stars that are around the core of the galaxy. 436 00:25:06,304 --> 00:25:09,640 But when they get sufficiently close, there are just no more 437 00:25:09,741 --> 00:25:11,175 stars to throw out, 438 00:25:11,276 --> 00:25:14,645 and so the idea is, they can't merge. 439 00:25:14,746 --> 00:25:15,713 So there's a problem. 440 00:25:15,814 --> 00:25:19,049 How is it that they managed to bridge that gap 441 00:25:19,150 --> 00:25:21,218 and finally spiral in? 442 00:25:21,319 --> 00:25:23,354 The only way to understand if supermassive 443 00:25:23,455 --> 00:25:25,523 black holes merge is by looking 444 00:25:25,624 --> 00:25:28,626 at their gravitational wave signal. 445 00:25:28,727 --> 00:25:30,794 ROWE: Two supermassive black holes 446 00:25:30,896 --> 00:25:34,498 merging should release a burst of gravitational waves 447 00:25:34,566 --> 00:25:36,400 millions of times more powerful 448 00:25:36,501 --> 00:25:38,802 than a stellar mass black hole merger. 449 00:25:41,606 --> 00:25:43,941 But LIGO won't hear a thing. 450 00:25:45,176 --> 00:25:47,311 The problem with using LIGO to detect the merger 451 00:25:47,412 --> 00:25:50,614 of supermassive black holes is actually a scale of time. 452 00:25:54,119 --> 00:25:57,321 One wave, as these things move around each other very slowly, 453 00:25:57,422 --> 00:26:02,259 would take over 10 years to go by, just one wave. 454 00:26:02,360 --> 00:26:04,495 MINGARELLI: In order to detect a gravitational wave with 455 00:26:04,596 --> 00:26:05,796 periods of decades, 456 00:26:05,897 --> 00:26:09,733 you also need an experiment that can be extremely stable 457 00:26:09,834 --> 00:26:11,068 over that amount of time. 458 00:26:12,971 --> 00:26:15,439 ROWE: Vibrations from earthquakes, 459 00:26:15,540 --> 00:26:17,408 weather, or even nearby traffic 460 00:26:17,509 --> 00:26:21,545 prevent LIGO from listening for a decade, just to hear one wave. 461 00:26:23,848 --> 00:26:26,717 But there may be another way to 462 00:26:26,818 --> 00:26:30,521 detect gravitational waves from supermassive black holes, 463 00:26:30,622 --> 00:26:35,492 using a strange type of dead star called a pulsar. 464 00:26:35,594 --> 00:26:38,996 A pulsar is a kind of 465 00:26:39,097 --> 00:26:42,166 neutron star that is rapidly spinning 466 00:26:42,267 --> 00:26:45,603 and has a beam of radiation that makes 467 00:26:45,737 --> 00:26:48,372 wide circles across the sky. 468 00:26:48,473 --> 00:26:52,710 And when that flash of circle washes over the planet Earth, 469 00:26:52,811 --> 00:26:55,145 we get a little beep, a little beep. 470 00:26:55,246 --> 00:26:58,248 We get pulses of radiation, hence pulsar. 471 00:27:00,418 --> 00:27:03,253 ROWE: Pulsars are the best timekeepers in the universe, 472 00:27:04,589 --> 00:27:09,159 but passing gravitational waves make them miss a beat. 473 00:27:09,260 --> 00:27:11,996 THALLER: What if we noticed that the frequency of a pulsar was 474 00:27:12,097 --> 00:27:14,298 shifting very, very slowly, 475 00:27:14,399 --> 00:27:17,434 year to year to year, over 10 years or more, 476 00:27:17,535 --> 00:27:21,572 just slightly getting a little bit longer as space itself was 477 00:27:21,673 --> 00:27:23,340 changing between us and the pulsar? 478 00:27:25,944 --> 00:27:28,379 ROWE: By monitoring dozens of pulsars, 479 00:27:28,480 --> 00:27:31,181 Chiara Mingarelli and a team of astronomers 480 00:27:31,282 --> 00:27:36,453 have created a galaxy-sized gravitational wave detector. 481 00:27:36,554 --> 00:27:42,326 It's called a pulsar timing array. 482 00:27:42,427 --> 00:27:45,863 You can really look for deviations in those arrival 483 00:27:45,964 --> 00:27:47,331 times over decades, 484 00:27:47,432 --> 00:27:50,901 almost like a tsunami warning system to show you when 485 00:27:51,002 --> 00:27:53,837 a gravitational wave is passing by. 486 00:27:56,341 --> 00:27:58,509 ROWE: After 12 years, the team detected 487 00:27:58,610 --> 00:28:01,445 the same change in a number of pulsars. 488 00:28:03,348 --> 00:28:04,615 MINGARELLI: These pulsars are all 489 00:28:04,716 --> 00:28:06,784 thousands of light-years apart. 490 00:28:06,851 --> 00:28:07,818 If you think about it, 491 00:28:07,919 --> 00:28:11,255 it's difficult to make a signal that's the same 492 00:28:11,356 --> 00:28:12,956 in all of these pulsars. 493 00:28:13,058 --> 00:28:16,326 This has to be this common signal from something like 494 00:28:16,428 --> 00:28:18,696 a gravitational wave event. 495 00:28:21,733 --> 00:28:24,468 ROWE: The signal the team detected wasn't created 496 00:28:24,569 --> 00:28:27,938 by just two supermassive black holes colliding. 497 00:28:29,441 --> 00:28:33,811 It's evidence of gravitational waves from hundreds of pairs of 498 00:28:33,912 --> 00:28:35,846 supermassive black holes, 499 00:28:35,947 --> 00:28:38,682 all in different stages of merging. 500 00:28:41,052 --> 00:28:44,688 Because it takes so long for one of these individual 501 00:28:44,789 --> 00:28:48,492 binary systems to merge, there could be thousands, 502 00:28:48,560 --> 00:28:52,196 if not millions, of these signals all being emitted at 503 00:28:52,297 --> 00:28:54,231 the same time, all of them. 504 00:28:54,332 --> 00:28:56,600 They all create this gravitational wave background 505 00:28:56,701 --> 00:28:59,303 that we're just starting to see the first signs of now. 506 00:29:02,474 --> 00:29:05,008 ROWE: Astronomers predict this gravitational 507 00:29:05,076 --> 00:29:07,711 wave background fills our universe. 508 00:29:09,581 --> 00:29:12,216 If the signal the team detected is confirmed, 509 00:29:12,317 --> 00:29:16,286 it's proof that supermassive black holes do merge. 510 00:29:18,089 --> 00:29:21,825 The next step is to observe that as it happens. 511 00:29:23,261 --> 00:29:25,996 MINGARELLI: It would be a dream to see two supermassive 512 00:29:26,097 --> 00:29:27,297 black holes merging, 513 00:29:27,398 --> 00:29:30,134 emitting gravitational waves, and also being able to point 514 00:29:30,235 --> 00:29:33,470 a telescope at them and to see the physics of how they merge. 515 00:29:34,939 --> 00:29:37,608 ROWE: Gravitational waves reveal the hidden workings 516 00:29:37,709 --> 00:29:39,843 of the cosmos. 517 00:29:39,944 --> 00:29:43,213 They reach the farthest corners of our universe. 518 00:29:44,549 --> 00:29:47,251 Now, astronomers are using gravitational 519 00:29:47,352 --> 00:29:49,753 waves to look back in time. 520 00:29:51,055 --> 00:29:53,423 They'll let us see all the way back 521 00:29:53,525 --> 00:29:55,359 to the earliest moments of our Big Bang. 522 00:29:56,528 --> 00:29:57,961 [explosion blasts] 523 00:30:15,980 --> 00:30:18,315 ROWE: 13.8 billion years ago, 524 00:30:20,285 --> 00:30:22,452 the universe sparks into life. 525 00:30:24,255 --> 00:30:28,258 The tiny speck of energy expands and cools. 526 00:30:28,359 --> 00:30:33,664 The infant cosmos is a fog of tiny particles of matter. 527 00:30:33,765 --> 00:30:38,969 Over time, the particles form atoms of hydrogen and helium. 528 00:30:40,438 --> 00:30:44,074 The fog clears, and the first light races across 529 00:30:44,175 --> 00:30:45,642 the universe. 530 00:30:45,743 --> 00:30:49,646 We call that light the cosmic microwave background. 531 00:30:50,849 --> 00:30:52,649 The cosmic microwave background is simply 532 00:30:52,750 --> 00:30:54,551 the most distant light we can see. 533 00:30:54,652 --> 00:30:57,621 So, looking at it give us baby pictures of our universe 534 00:30:57,722 --> 00:31:00,424 the way it looked 400,000 years after a big bang. 535 00:31:02,193 --> 00:31:04,695 ROWE: What happened before these baby pictures 536 00:31:04,796 --> 00:31:06,997 remains a mystery. 537 00:31:08,199 --> 00:31:11,001 The leading theory is that in the very first second 538 00:31:11,102 --> 00:31:12,836 of the Big Bang, 539 00:31:12,937 --> 00:31:15,572 our infant universe had a growth spurt. 540 00:31:18,376 --> 00:31:21,211 Scientists call this idea inflation. 541 00:31:22,380 --> 00:31:26,316 In a billionth of a billionth of a billionth of a second, 542 00:31:26,417 --> 00:31:30,287 our universe grew a billion, billion, billion, 543 00:31:30,388 --> 00:31:33,790 billion, billion, billion times bigger. 544 00:31:33,892 --> 00:31:37,494 That is the mother of all growth spurts -- it laid 545 00:31:37,595 --> 00:31:41,565 the foundations for the entire cosmos that we know today. 546 00:31:49,040 --> 00:31:51,141 ROWE: Inflation is just a theory, 547 00:31:51,242 --> 00:31:54,845 but there may be a way to prove it happened. 548 00:31:54,946 --> 00:31:57,481 Scientists think that during that brief moment of 549 00:31:57,615 --> 00:31:58,849 cosmic expansion, 550 00:31:58,950 --> 00:32:02,786 inflation stretched tiny fluctuations of gravity. 551 00:32:02,887 --> 00:32:06,823 That is such a violent process that it actually causes ripples 552 00:32:06,925 --> 00:32:08,191 and distortions in the very 553 00:32:08,293 --> 00:32:10,694 shape and fabric of space itself, 554 00:32:10,795 --> 00:32:13,096 which we can see today as gravitational waves. 555 00:32:14,599 --> 00:32:17,434 ROWE: Scientists call these theoretical ripples through 556 00:32:17,535 --> 00:32:21,004 the early universe primordial gravitational waves. 557 00:32:22,874 --> 00:32:25,108 SUTTER: When they were first released, 558 00:32:25,209 --> 00:32:27,678 these were deafening. 559 00:32:27,779 --> 00:32:30,747 But in the billions of years since, our universe has grown 560 00:32:30,848 --> 00:32:32,282 bigger and colder, 561 00:32:32,383 --> 00:32:35,252 and these gravitational waves have diluted 562 00:32:35,353 --> 00:32:38,655 so that they barely even exist today. 563 00:32:38,756 --> 00:32:43,260 ROWE: Scientists searched for signs of these very weak, 564 00:32:43,361 --> 00:32:46,029 primordial gravitational waves in the cosmic 565 00:32:46,130 --> 00:32:48,365 microwave background. 566 00:32:48,466 --> 00:32:50,934 And in 2014, 567 00:32:51,035 --> 00:32:54,604 a teen, using their purpose-built microwave array 568 00:32:54,706 --> 00:32:56,907 in Antarctica called BICEP, 569 00:32:57,008 --> 00:32:59,643 found a strange swirling pattern. 570 00:32:59,744 --> 00:33:03,947 SUTTER: When they saw those swirls, they saw those patterns, 571 00:33:04,048 --> 00:33:05,082 they thought they had seen 572 00:33:05,183 --> 00:33:08,185 the signature of primordial gravitational waves. 573 00:33:08,286 --> 00:33:11,955 Now this is really the conclusive 574 00:33:12,056 --> 00:33:15,058 evidence that inflation had to have happened. 575 00:33:15,159 --> 00:33:18,528 ROWE: The results were exciting, 576 00:33:18,629 --> 00:33:21,498 but there was a glitch. 577 00:33:21,599 --> 00:33:23,567 This amazement lasted 578 00:33:23,634 --> 00:33:28,572 for a few months until cracks started appearing in this, 579 00:33:28,673 --> 00:33:30,173 and gradually, it all collapsed. 580 00:33:33,144 --> 00:33:34,778 ROWE: The signal, thought to be proof of 581 00:33:34,879 --> 00:33:39,082 primordial gravitational waves and the theory of inflation, 582 00:33:39,183 --> 00:33:41,985 turned out to be a case of mistaken identity. 583 00:33:45,656 --> 00:33:47,691 SUTTER: As this light from the ancient universe, 584 00:33:47,792 --> 00:33:50,494 from the cosmic microwave background, travels 585 00:33:50,595 --> 00:33:53,196 through the universe, it had to travel through dust 586 00:33:53,297 --> 00:33:58,468 before reaching our detectors, and the dust itself can affect 587 00:33:58,569 --> 00:34:01,271 the light and mimic what 588 00:34:01,372 --> 00:34:03,707 the primordial gravitational waves can do. 589 00:34:05,510 --> 00:34:07,878 ROWE: The primordial gravitational wave signal 590 00:34:07,979 --> 00:34:11,214 turned out to be mainly clouds of dust 591 00:34:11,315 --> 00:34:12,783 floating through space. 592 00:34:15,386 --> 00:34:17,754 That's how BICEP bit the dust. 593 00:34:19,590 --> 00:34:21,391 ROWE: BICEP failed to detect 594 00:34:21,492 --> 00:34:24,594 primordial gravitational waves. 595 00:34:24,695 --> 00:34:27,097 Can LIGO do any better? 596 00:34:27,198 --> 00:34:30,967 OLUSEYI: Unfortunately, LIGO can't help us 597 00:34:31,069 --> 00:34:33,770 in observing primordial gravitational waves. 598 00:34:33,871 --> 00:34:35,939 It can't even observe supermassive black holes 599 00:34:36,040 --> 00:34:37,240 at the centers of galaxies. 600 00:34:37,341 --> 00:34:39,509 It is designed to observe in a particular 601 00:34:39,610 --> 00:34:41,278 frequency range. 602 00:34:41,379 --> 00:34:44,181 SUTTER: Primordial gravitational waves 603 00:34:44,282 --> 00:34:46,349 are at such a low frequency in 604 00:34:46,451 --> 00:34:48,251 such a low amplitude 605 00:34:48,352 --> 00:34:52,989 that there is no hope of LIGO being able to detect them. 606 00:34:55,560 --> 00:34:58,228 ROWE: But scientists hope that an ambitious project 607 00:34:58,329 --> 00:35:00,197 called LISA will. 608 00:35:01,732 --> 00:35:05,268 Not on Earth, but from 30 million miles above. 609 00:35:07,205 --> 00:35:10,474 LISA is like LIGO, 610 00:35:10,575 --> 00:35:12,476 but bigger and in space. 611 00:35:16,314 --> 00:35:17,914 ROWE: LISA, 612 00:35:18,015 --> 00:35:22,052 or the Laser Interferometer Space Antenna, will be a system 613 00:35:22,153 --> 00:35:26,890 of three satellites arranged in a giant triangular formation, 614 00:35:26,991 --> 00:35:29,893 1.5 million miles apart. 615 00:35:29,994 --> 00:35:32,262 PLAIT: If a gravitational wave passes through them 616 00:35:32,363 --> 00:35:33,563 and changes that distance, 617 00:35:33,664 --> 00:35:36,066 they can detect that -- because the satellites are so much 618 00:35:36,167 --> 00:35:39,436 farther apart, a very low frequency wave 619 00:35:39,537 --> 00:35:40,937 can make a detectable change. 620 00:35:41,038 --> 00:35:44,875 LIGO wouldn't be able to see that, but LISA could. 621 00:35:44,976 --> 00:35:47,277 ROWE: As well as listening for low frequency 622 00:35:47,378 --> 00:35:50,514 gravitational wave sources, like supermassive black 623 00:35:50,615 --> 00:35:51,948 hole mergers, 624 00:35:52,049 --> 00:35:55,318 LISA will listen for primordial gravitational 625 00:35:55,419 --> 00:35:58,355 waves from the dawn of time. 626 00:35:58,456 --> 00:35:59,523 If it detects them, 627 00:35:59,624 --> 00:36:03,760 we will know that the infant universe inflated. 628 00:36:04,929 --> 00:36:08,131 Inflation has explained almost everything 629 00:36:08,232 --> 00:36:10,767 we measure in modern cosmology. 630 00:36:10,868 --> 00:36:12,636 It's an incredibly successful theory. 631 00:36:12,737 --> 00:36:15,305 The icing on the cake would be if we could 632 00:36:15,406 --> 00:36:17,974 also discover these gravitational waves 633 00:36:18,075 --> 00:36:21,411 that it's supposed to have created. 634 00:36:22,880 --> 00:36:25,982 ROWE: From the Big Bang to the most massive black holes, 635 00:36:27,084 --> 00:36:30,487 the universe talks to us using gravitational waves. 636 00:36:32,757 --> 00:36:37,494 Just like with telescopes, we're using gravitational waves to 637 00:36:37,595 --> 00:36:39,629 look at different types of objects -- 638 00:36:39,730 --> 00:36:44,300 Neutron star mergers and black hole mergers -- and learn more 639 00:36:44,368 --> 00:36:45,969 about the universe around us. 640 00:36:48,372 --> 00:36:49,639 ROWE: They could even reveal 641 00:36:49,740 --> 00:36:53,043 the most elusive substance in the universe -- 642 00:36:53,144 --> 00:36:55,045 Dark matter. 643 00:36:55,146 --> 00:36:57,881 SUTTER: If anything's gonna help us understand 644 00:36:57,982 --> 00:36:59,216 the nature of dark matter, 645 00:36:59,317 --> 00:37:01,651 it might just be gravitational waves. 646 00:37:11,195 --> 00:37:12,562 ROWE: Across the universe, 647 00:37:12,663 --> 00:37:15,832 an invisible substance holds galaxies together. 648 00:37:15,900 --> 00:37:20,070 Without it, they would fly apart. 649 00:37:20,171 --> 00:37:23,974 THALLER: The Milky Way should've dispersed long ago, 650 00:37:24,075 --> 00:37:26,142 and the Magellanic clouds right in front of us are 651 00:37:26,244 --> 00:37:27,577 exactly the same. 652 00:37:27,678 --> 00:37:29,479 These things should be just shedding stars 653 00:37:29,580 --> 00:37:32,215 left and right as they fly off this rotating galaxy. 654 00:37:32,316 --> 00:37:34,751 Instead, they're not. They're holding together. 655 00:37:34,852 --> 00:37:37,320 There are motions in the stars that we just cannot 656 00:37:37,421 --> 00:37:39,723 account for unless there's something holding 657 00:37:39,824 --> 00:37:40,790 the whole thing together. 658 00:37:42,560 --> 00:37:45,695 ROWE: We call this mysterious substance dark matter. 659 00:37:45,796 --> 00:37:50,267 It doesn't interact with light, so we can't see it. 660 00:37:50,368 --> 00:37:52,335 But we cannot ignore it. 661 00:37:54,305 --> 00:37:57,741 From the motions of stars inside of galaxies to 662 00:37:57,842 --> 00:38:00,610 the motions of galaxies inside of clusters 663 00:38:00,711 --> 00:38:04,481 to the very structure of the universe itself, 664 00:38:04,582 --> 00:38:08,718 we see evidence for dark matter everywhere we look. 665 00:38:11,389 --> 00:38:13,690 ROWE: We think dark matter makes up 666 00:38:13,791 --> 00:38:17,761 85% of the matter in the universe. 667 00:38:17,862 --> 00:38:22,065 But because we can't see dark matter with telescopes, 668 00:38:22,166 --> 00:38:23,867 we know very little about it. 669 00:38:25,636 --> 00:38:27,904 ESQUIVE: While we know that it's there, 670 00:38:28,005 --> 00:38:31,541 we haven't actually answered the question of what it is 671 00:38:31,642 --> 00:38:35,979 or how it interacts or why it's there or how it's created. 672 00:38:36,080 --> 00:38:37,847 So you have to be really creative 673 00:38:37,948 --> 00:38:39,249 if you want to go after this stuff 674 00:38:39,350 --> 00:38:42,185 and really understand what's it made out of? 675 00:38:45,556 --> 00:38:48,258 ROWE: One creative theory suggests that black holes 676 00:38:48,359 --> 00:38:50,560 make up dark matter, 677 00:38:50,661 --> 00:38:54,564 not the regular stellar mass black holes that LIGO detects, 678 00:38:56,000 --> 00:38:58,401 or the supermassive black holes that 679 00:38:58,502 --> 00:39:00,704 lurk at the center of galaxies 680 00:39:00,805 --> 00:39:05,442 but tiny, primordial black holes born during the period of 681 00:39:05,576 --> 00:39:09,412 rapid expansion in the first moments of the Big Bang. 682 00:39:10,881 --> 00:39:13,350 Primordial black holes could be 683 00:39:13,451 --> 00:39:16,920 potential explanations for what we call dark matter. 684 00:39:17,021 --> 00:39:19,222 And if there's enough of them, they can hold an entire 685 00:39:19,323 --> 00:39:21,057 galaxy together. 686 00:39:21,158 --> 00:39:24,961 ROWE: We don't know if primordial black holes exist, 687 00:39:25,062 --> 00:39:30,033 but gravitational waves could change that. 688 00:39:30,167 --> 00:39:32,535 PONTZEN: When you form a primordial black hole, 689 00:39:32,636 --> 00:39:35,205 you send out a burst of gravitational waves 690 00:39:35,306 --> 00:39:38,174 that, in principle, carries on traveling through the universe, 691 00:39:38,275 --> 00:39:39,809 and you might be able to detect it 692 00:39:39,910 --> 00:39:41,177 still today. 693 00:39:41,278 --> 00:39:43,880 TREMBLAY: The problem is that these things would have emitted 694 00:39:43,981 --> 00:39:46,149 gravitational waves at a frequency that is not 695 00:39:46,250 --> 00:39:47,617 detectable by LIGO. 696 00:39:47,718 --> 00:39:50,987 And so it's very hard to discern whether or not they 697 00:39:51,088 --> 00:39:54,357 are plentiful enough to actually serve as a compelling 698 00:39:54,458 --> 00:39:55,458 dark matter candidate. 699 00:39:58,028 --> 00:40:00,430 ROWE: If primordial black holes do exist, 700 00:40:00,531 --> 00:40:03,633 they still might not explain all the dark matter in 701 00:40:03,734 --> 00:40:05,335 the universe. 702 00:40:05,436 --> 00:40:07,737 They might be working with another type 703 00:40:07,838 --> 00:40:10,407 of dark matter to hold galaxies together. 704 00:40:11,809 --> 00:40:14,878 The upcoming LISA mission may fill in the blanks. 705 00:40:16,514 --> 00:40:18,748 SUTTER: What we call dark matter could be simple. 706 00:40:18,849 --> 00:40:22,452 It could just be made of one thing that absolutely floods 707 00:40:22,553 --> 00:40:24,687 the universe, or it can be made of 708 00:40:24,789 --> 00:40:28,291 many different things that all work together to combine 709 00:40:28,392 --> 00:40:29,893 to make this effect. 710 00:40:29,994 --> 00:40:32,095 Is dark matter all primordial black holes? 711 00:40:32,196 --> 00:40:34,898 Is it something else that we haven't thought of yet? 712 00:40:34,999 --> 00:40:38,268 Gravitational waves could provide those answers. 713 00:40:40,471 --> 00:40:43,706 ROWE: The detection of tiny gravitational waves generated 714 00:40:43,808 --> 00:40:45,542 by primordial black holes 715 00:40:45,643 --> 00:40:49,879 will be a huge advance in our understanding of dark matter. 716 00:40:49,980 --> 00:40:51,581 OLUSEYI: With gravitational wave astronomy, 717 00:40:51,682 --> 00:40:54,918 we're seeing things that we have never seen before. 718 00:40:55,019 --> 00:40:56,519 So who knows 719 00:40:56,620 --> 00:40:59,088 what we're gonna see as we continue to look out into space? 720 00:41:01,826 --> 00:41:04,861 MINGARELLI: We've been able to see dozens of black holes 721 00:41:04,962 --> 00:41:07,597 merge, two neutron stars merging, 722 00:41:07,698 --> 00:41:10,633 and discovered from that merger that neutron stars 723 00:41:10,734 --> 00:41:12,735 can make platinum and gold. 724 00:41:15,806 --> 00:41:18,374 From thinking that we would never be able to see 725 00:41:18,476 --> 00:41:22,712 gravitational waves to seeing gravitational wave signals 726 00:41:22,813 --> 00:41:25,815 happen on the regular -- It's just crazy. 727 00:41:27,518 --> 00:41:30,720 ROWE: Already, we've heard epic explosions. 728 00:41:32,289 --> 00:41:35,825 We've identified the brightest lights in the cosmos, 729 00:41:37,328 --> 00:41:40,497 and we have solved some of the biggest mysteries 730 00:41:40,598 --> 00:41:42,565 in astronomy. 731 00:41:42,666 --> 00:41:46,069 But that is just the beginning. 732 00:41:46,170 --> 00:41:49,172 THALLER: Right now is a golden age in astronomy. 733 00:41:49,273 --> 00:41:51,574 Think of the time that you're living in -- the first detection 734 00:41:51,675 --> 00:41:54,811 of gravitational waves by LIGO was only a couple of years ago. 735 00:41:54,912 --> 00:41:56,679 You were here of the birth of 736 00:41:56,780 --> 00:41:58,681 this entirely new view of the universe.