1 00:00:01,502 --> 00:00:04,937 There's a mystery at the very heart of the universe. 2 00:00:05,039 --> 00:00:08,340 We don't know how old the cosmos is. 3 00:00:08,409 --> 00:00:10,387 Understanding the age of the universe 4 00:00:10,411 --> 00:00:14,091 is fundamental to understanding the universe at all. 5 00:00:14,115 --> 00:00:16,815 It's at the heart of everything. 6 00:00:16,950 --> 00:00:19,196 It's more than just celebrating a birthday. 7 00:00:19,220 --> 00:00:20,931 We want to know how much mass is in it, 8 00:00:20,955 --> 00:00:23,155 how much energy is in it, how it behaves. 9 00:00:23,224 --> 00:00:25,657 We have to have this number nailed down. 10 00:00:25,759 --> 00:00:27,104 The age of the universe 11 00:00:27,128 --> 00:00:30,907 enables us to not only understand where we came from, 12 00:00:30,931 --> 00:00:33,699 but potentially, the fate of the universe, 13 00:00:33,801 --> 00:00:37,280 what will happen millions and billions of years from now. 14 00:00:37,304 --> 00:00:40,539 But our quest to discover the age of the universe is 15 00:00:40,608 --> 00:00:42,085 starting a war. 16 00:00:42,109 --> 00:00:44,287 Usually Nature just whispers to us. 17 00:00:44,311 --> 00:00:47,190 Now Nature is screaming in our ear 18 00:00:47,214 --> 00:00:50,616 that we're doing something wrong, and that's exciting. 19 00:00:55,076 --> 00:00:57,122 DvX3M www.opensubtitles.org 20 00:01:02,229 --> 00:01:04,830 We think the universe started with a bang. 21 00:01:06,200 --> 00:01:09,435 Everything that has ever existed is squashed up 22 00:01:09,536 --> 00:01:12,037 in this space smaller than a pinhead, 23 00:01:12,106 --> 00:01:16,286 and all of a sudden, space just starts expanding 24 00:01:16,310 --> 00:01:17,743 everywhere at once. 25 00:01:17,812 --> 00:01:20,512 The idea that the universe grew from 26 00:01:20,614 --> 00:01:24,716 a ball smaller than a pinhead is hard to understand, 27 00:01:24,819 --> 00:01:26,852 but figuring out when it happened 28 00:01:26,954 --> 00:01:29,721 sounds like it should be more straightforward. 29 00:01:29,823 --> 00:01:31,802 It seems like a simple question right? 30 00:01:31,826 --> 00:01:33,036 But it turns out, getting 31 00:01:33,060 --> 00:01:35,305 the age of the universe is pretty tricky. 32 00:01:35,329 --> 00:01:38,608 Scientists have just a single fact 33 00:01:38,632 --> 00:01:40,332 as their starting point... 34 00:01:40,401 --> 00:01:43,380 The universe is expanding. 35 00:01:43,404 --> 00:01:45,582 When people realized the universe was expanding, 36 00:01:45,606 --> 00:01:46,649 they thought they finally had 37 00:01:46,673 --> 00:01:48,718 a way to estimate the age of the universe. 38 00:01:48,742 --> 00:01:51,944 Take the universe now and run it backwards in time. 39 00:01:52,012 --> 00:01:54,212 Things get closer and closer until they come 40 00:01:54,248 --> 00:01:55,347 to a single point. 41 00:01:55,449 --> 00:01:59,196 That time to that point is the age of the universe. 42 00:01:59,220 --> 00:02:02,666 The expansion rate is so important, 43 00:02:02,690 --> 00:02:05,157 it's been given its own name... 44 00:02:05,226 --> 00:02:06,959 The Hubble constant. 45 00:02:07,061 --> 00:02:10,963 The Hubble constant is the present day 46 00:02:11,031 --> 00:02:12,509 expansion rate of the universe. 47 00:02:12,533 --> 00:02:15,134 It is a key ingredient to understanding 48 00:02:15,202 --> 00:02:20,706 the entire expansion history of our universe and its age. 49 00:02:20,808 --> 00:02:22,888 Scientists discovered a strange 50 00:02:22,943 --> 00:02:25,711 radio signal permeating the cosmos. 51 00:02:25,813 --> 00:02:30,716 It's the remnants of ancient light from the early universe. 52 00:02:30,818 --> 00:02:35,165 We call it the cosmic microwave background, or CMB 53 00:02:35,189 --> 00:02:36,622 for short. 54 00:02:36,724 --> 00:02:39,324 The cosmic microwave background radiation is 55 00:02:39,393 --> 00:02:42,405 simply the afterglow of our Big Bang, 56 00:02:42,429 --> 00:02:47,032 the way the universe looked when it was 400,000 years old. 57 00:02:48,302 --> 00:02:52,282 The European Space Agency launched the Planck satellite. 58 00:02:52,306 --> 00:02:54,339 Using sensitive radio receivers, 59 00:02:54,441 --> 00:02:57,109 the orbiter studied the sky in every direction, 60 00:02:57,244 --> 00:03:01,091 measuring tiny changes in the temperature and polarization 61 00:03:01,115 --> 00:03:02,592 of the radiation signal. 62 00:03:02,616 --> 00:03:06,251 The CMB has all these variations 63 00:03:06,320 --> 00:03:09,221 in temperature, and they're not randomly generated. 64 00:03:09,323 --> 00:03:11,868 They are there because of physical processes 65 00:03:11,892 --> 00:03:13,503 that occurred when the universe 66 00:03:13,527 --> 00:03:16,161 was in its primordial fireball phase. 67 00:03:16,197 --> 00:03:19,264 The red blobs are where matter was hottest, 68 00:03:19,366 --> 00:03:22,201 and the blue areas are where matter was cooler. 69 00:03:22,336 --> 00:03:24,681 The smallest red blobs are where 70 00:03:24,705 --> 00:03:27,940 hot material was packed tightly together. 71 00:03:28,041 --> 00:03:29,786 That's where material in the universe would have 72 00:03:29,810 --> 00:03:32,211 been denser, and that's where galaxies would 73 00:03:32,346 --> 00:03:34,213 preferentially form. 74 00:03:34,348 --> 00:03:39,318 It's so cool to get to look at those blueprints and study them 75 00:03:39,420 --> 00:03:41,720 and see how that baby universe 76 00:03:41,822 --> 00:03:45,524 later grew up into the universe we see around us today. 77 00:03:45,626 --> 00:03:47,304 Although it doesn't look like much, 78 00:03:47,328 --> 00:03:50,429 hidden within this picture is almost everything 79 00:03:50,531 --> 00:03:53,065 we can know about the universe. 80 00:03:53,133 --> 00:03:55,178 In a complex process using 81 00:03:55,202 --> 00:03:57,035 different mathematical models, 82 00:03:57,104 --> 00:04:00,739 cosmologists figured out how the ancient cosmos 83 00:04:00,808 --> 00:04:02,241 captured in the CMB 84 00:04:02,342 --> 00:04:05,344 became the universe we see today. 85 00:04:05,445 --> 00:04:07,891 They worked out how the universe got from 86 00:04:07,915 --> 00:04:12,451 small to big and how fast that expansion happened. 87 00:04:13,487 --> 00:04:15,899 The data from the cosmic microwave background 88 00:04:15,923 --> 00:04:19,603 is absolutely the gold standard for cosmology. 89 00:04:19,627 --> 00:04:23,629 It's beautifully clean, we can understand it really well, 90 00:04:23,731 --> 00:04:25,575 and we have a lot of confidence 91 00:04:25,599 --> 00:04:29,334 that what we learn from it is pretty robust. 92 00:04:29,436 --> 00:04:31,381 By running the expansion backwards, 93 00:04:31,405 --> 00:04:33,038 we get an age... 94 00:04:34,508 --> 00:04:38,410 13.82 billion years. 95 00:04:38,545 --> 00:04:39,945 Job finished! 96 00:04:41,615 --> 00:04:44,216 But it's not quite a slam dunk. 97 00:04:44,318 --> 00:04:46,652 The figure must be verified. 98 00:04:46,720 --> 00:04:48,398 We don't make a single measurement 99 00:04:48,422 --> 00:04:49,621 using a single technique. 100 00:04:49,690 --> 00:04:53,525 We make multiple measurements via multiple techniques. 101 00:04:53,627 --> 00:04:55,605 Another group of scientists use 102 00:04:55,629 --> 00:04:56,962 a totally different method 103 00:04:57,097 --> 00:04:59,264 to calculate the age of the cosmos, 104 00:04:59,333 --> 00:05:01,733 measuring objects that we can see 105 00:05:01,802 --> 00:05:05,804 in our universe to determine how far away they are and how 106 00:05:05,906 --> 00:05:09,786 fast they're moving away from us as the universe expands. 107 00:05:09,810 --> 00:05:13,045 The most direct and most accurate measurements 108 00:05:13,113 --> 00:05:15,747 are using what is known as parallax. 109 00:05:15,816 --> 00:05:20,352 Parallax is the apparent shift in an object relative 110 00:05:20,454 --> 00:05:22,666 to the background when it's viewed 111 00:05:22,690 --> 00:05:24,056 from two different locations. 112 00:05:24,157 --> 00:05:28,126 So if I look at my thumb with one eye, and then I close it 113 00:05:28,228 --> 00:05:30,962 and look at the other eye, it looks like my thumb moves. 114 00:05:32,633 --> 00:05:34,878 If I move my thumb closer to my face, 115 00:05:34,902 --> 00:05:38,337 then the distance it moves back and forth changes. 116 00:05:38,438 --> 00:05:40,884 It appears to move back and forth more. 117 00:05:40,908 --> 00:05:43,887 That parallax difference as we move the thumb closer 118 00:05:43,911 --> 00:05:45,288 and farther from the face 119 00:05:45,312 --> 00:05:47,946 is the way we measure distances to distant objects. 120 00:05:48,048 --> 00:05:51,149 Using parallax, we can measure 121 00:05:51,251 --> 00:05:54,798 the distance to bright stars called cepheids 122 00:05:54,822 --> 00:05:56,254 in the Milky Way. 123 00:05:56,356 --> 00:05:59,569 Cepheids are stars that burn 100,000 124 00:05:59,593 --> 00:06:01,226 times brighter than our sun, 125 00:06:01,328 --> 00:06:04,162 so they're extremely bright, and they pulsate, meaning they 126 00:06:04,231 --> 00:06:08,178 get brighter and dimmer over a regular time period. 127 00:06:08,202 --> 00:06:10,013 Cepheids that pulsate at 128 00:06:10,037 --> 00:06:13,138 the same rate have the same brightness. 129 00:06:13,207 --> 00:06:15,941 They're known as a standard candle. 130 00:06:16,043 --> 00:06:19,244 A standard candle is something that is a standard, meaning 131 00:06:19,313 --> 00:06:21,847 we know how intrinsically bright it is. 132 00:06:21,982 --> 00:06:23,826 So all we have to do is measure 133 00:06:23,850 --> 00:06:26,996 the brightness that we appear to perceive on Earth, 134 00:06:27,020 --> 00:06:28,698 and then you solve for the distance. 135 00:06:28,722 --> 00:06:32,001 So imagine that you're on the street. 136 00:06:32,025 --> 00:06:33,903 By looking down the street, 137 00:06:33,927 --> 00:06:36,728 you'll see that the street lights get dimmer and dimmer 138 00:06:36,797 --> 00:06:38,208 the farther away they are, 139 00:06:38,232 --> 00:06:40,276 but that's not their intrinsic brightness. 140 00:06:40,300 --> 00:06:42,420 Their intrinsic brightness is the same. 141 00:06:42,536 --> 00:06:46,405 So by seeing how faint the farthest away ones are, 142 00:06:46,540 --> 00:06:50,409 you can understand how far away they are from you. 143 00:06:50,544 --> 00:06:53,390 We can use standard candles to measure 144 00:06:53,414 --> 00:06:55,892 the distance to stars farther away. 145 00:06:55,916 --> 00:06:59,596 But there's a big problem... Throughout the universe, 146 00:06:59,620 --> 00:07:03,221 there's a competition between the expansion pushing things 147 00:07:03,356 --> 00:07:08,204 apart and gravity pulling things together. 148 00:07:08,228 --> 00:07:09,228 In the Milky Way, 149 00:07:09,262 --> 00:07:11,863 there's so much matter that gravity wins. 150 00:07:11,965 --> 00:07:14,933 Even looking at galaxies in our neighborhood, 151 00:07:15,002 --> 00:07:17,335 the expansion is tiny, 152 00:07:17,404 --> 00:07:21,540 but at cosmic scales of very different galaxies, 153 00:07:21,608 --> 00:07:25,388 matter is more spread out, and expansion wins, 154 00:07:25,412 --> 00:07:29,214 so we can only measure expansion over massive distances. 155 00:07:30,217 --> 00:07:32,495 The way we start to measure distances to things that 156 00:07:32,519 --> 00:07:34,252 are farther and farther away 157 00:07:34,321 --> 00:07:37,055 is to use something we call the distance ladder. 158 00:07:37,124 --> 00:07:40,859 Each category of object that we observe 159 00:07:40,994 --> 00:07:44,129 is on a separate rung of this ladder. 160 00:07:44,231 --> 00:07:49,034 Measuring the distance to one will then inform us how far 161 00:07:49,135 --> 00:07:52,103 away the second rung is and then the third rung. 162 00:07:52,206 --> 00:07:56,786 So each rung depends on the previous rung, and from 163 00:07:56,810 --> 00:08:00,946 stacking these together, we can start to measure things very, 164 00:08:01,048 --> 00:08:02,714 very far away from us. 165 00:08:03,851 --> 00:08:06,496 Using parallax to measure cepheid stars in 166 00:08:06,520 --> 00:08:08,653 the Milky Way gives us a benchmark. 167 00:08:08,755 --> 00:08:12,524 We can then use their standard brightness 168 00:08:12,626 --> 00:08:14,706 to measure cepheids in other galaxies. 169 00:08:14,761 --> 00:08:18,730 The next rung is a brighter standard candle called 170 00:08:18,799 --> 00:08:20,932 Type 1A supernovas. 171 00:08:21,034 --> 00:08:23,935 They can be seen in galaxies farther away. 172 00:08:24,037 --> 00:08:27,305 Finally, we can measure light from distant elliptical 173 00:08:27,407 --> 00:08:31,343 galaxies, and by looking at how red the light is, 174 00:08:31,444 --> 00:08:35,547 we can work out how fast they're moving away from us. 175 00:08:35,616 --> 00:08:39,217 So those three things give us the nearby universe, 176 00:08:39,319 --> 00:08:42,499 the somewhat far away universe, and the very distant universe, 177 00:08:42,523 --> 00:08:43,655 rung by rung. 178 00:08:46,527 --> 00:08:49,060 March 2021. 179 00:08:49,162 --> 00:08:52,397 Scientists measure the light from 63 180 00:08:52,533 --> 00:08:54,277 giant elliptical galaxies, 181 00:08:54,301 --> 00:08:57,502 the farthest rung of the distance ladder. 182 00:08:57,604 --> 00:09:00,350 They hope to get the most accurate measurement of 183 00:09:00,374 --> 00:09:04,509 the Hubble constant to date and a precise age 184 00:09:04,578 --> 00:09:05,810 for the universe. 185 00:09:07,247 --> 00:09:09,948 Their calculations make the universe 186 00:09:10,050 --> 00:09:12,929 13.3 billion years old, 187 00:09:12,953 --> 00:09:15,353 not too far away from the figure of 188 00:09:15,455 --> 00:09:17,756 13.82 billion years 189 00:09:17,857 --> 00:09:20,058 given by the cosmic microwave background, 190 00:09:20,126 --> 00:09:22,906 a difference of around 6%. 191 00:09:22,930 --> 00:09:26,776 That sounds trivial, but that equates to hundreds of millions 192 00:09:26,800 --> 00:09:29,434 of years of cosmic history that either happened 193 00:09:29,503 --> 00:09:31,336 or didn't happen. 194 00:09:31,405 --> 00:09:34,083 50 years ago, when we weren't quite as good 195 00:09:34,107 --> 00:09:35,785 at measuring everything about the universe, 196 00:09:35,809 --> 00:09:36,919 we would have been thrilled to 197 00:09:36,943 --> 00:09:39,022 have our numbers agreeing to this level. 198 00:09:39,046 --> 00:09:41,413 But nowadays, having a difference like this, 199 00:09:41,515 --> 00:09:43,448 it's unacceptable. 200 00:09:43,517 --> 00:09:46,551 Clearly, the two techniques do not agree. 201 00:09:46,620 --> 00:09:49,621 Cosmologists split into two camps. 202 00:09:49,723 --> 00:09:52,223 We had hoped that these two methods were like building 203 00:09:52,326 --> 00:09:56,061 a bridge from either side and then meeting in the middle. 204 00:09:56,162 --> 00:09:57,529 But they're not. 205 00:09:57,631 --> 00:09:59,409 Now we know that something is going on 206 00:09:59,433 --> 00:10:01,132 we don't understand. 207 00:10:01,234 --> 00:10:02,579 Even though these measurements 208 00:10:02,603 --> 00:10:03,868 are roughly the same, 209 00:10:03,937 --> 00:10:06,883 it's really dangerous to just accept them and assume that 210 00:10:06,907 --> 00:10:09,207 everything's fine, because in science, 211 00:10:09,309 --> 00:10:13,144 usually, the initial really big discoveries start off 212 00:10:13,246 --> 00:10:15,347 as small differences, but then you pull 213 00:10:15,415 --> 00:10:18,950 on that thread, and something wonderful emerges. 214 00:10:19,019 --> 00:10:20,979 So does a simple question, 215 00:10:21,021 --> 00:10:24,956 how old is the universe, unravel everything? 216 00:10:34,601 --> 00:10:37,280 The universe is expanding outwards. 217 00:10:37,304 --> 00:10:40,984 The rate it's growing is called the Hubble constant, 218 00:10:41,008 --> 00:10:44,809 and it's the key to working out the age of the universe. 219 00:10:44,911 --> 00:10:47,212 So the Hubble constant might just seem 220 00:10:47,314 --> 00:10:51,149 like some academic number that doesn't mean anything, 221 00:10:51,251 --> 00:10:55,654 but that number contains information about 222 00:10:55,722 --> 00:10:57,055 the composition, 223 00:10:57,157 --> 00:11:00,425 the evolution, and the fate of the universe. 224 00:11:01,461 --> 00:11:02,905 It's an important number, 225 00:11:02,929 --> 00:11:04,474 but there's a problem. 226 00:11:04,498 --> 00:11:08,333 Our best measurement methods don't match. 227 00:11:08,402 --> 00:11:10,480 It's incredibly frustrating to not 228 00:11:10,504 --> 00:11:12,482 know how old the universe is. 229 00:11:12,506 --> 00:11:14,484 It's even more frustrating to know that 230 00:11:14,508 --> 00:11:17,409 there's two experiments, which are excellent experiments 231 00:11:17,511 --> 00:11:19,110 that we firmly believe in, 232 00:11:19,212 --> 00:11:20,857 that completely disagree with each other. 233 00:11:20,881 --> 00:11:24,649 My hair fell out a long time ago over this kind of stuff. 234 00:11:24,751 --> 00:11:27,419 This has been the number-one question 235 00:11:27,521 --> 00:11:29,254 for over half a decade. 236 00:11:30,323 --> 00:11:33,758 There must be something wrong with one of the methods. 237 00:11:33,860 --> 00:11:36,205 There's a definite sense in the community 238 00:11:36,229 --> 00:11:39,130 that whichever camp you happen to fall into, 239 00:11:39,232 --> 00:11:41,800 the problems lie on the other side of the fence. 240 00:11:41,935 --> 00:11:43,913 So if you're mainly working with the cosmic 241 00:11:43,937 --> 00:11:46,304 microwave background, you probably think 242 00:11:46,406 --> 00:11:49,207 something is up with the distance ladder. 243 00:11:49,342 --> 00:11:51,743 If there's a problem with the distance ladder, 244 00:11:51,845 --> 00:11:53,812 there's a prime suspect. 245 00:11:53,880 --> 00:11:57,048 The ladder relies on stars that have a predictable 246 00:11:57,117 --> 00:11:59,896 brightness called standard candles. 247 00:11:59,920 --> 00:12:04,556 But there's evidence that these stars are not always 248 00:12:04,657 --> 00:12:05,957 the same brightness. 249 00:12:06,059 --> 00:12:10,261 So if you expect an object to have a particular brightness, 250 00:12:10,297 --> 00:12:12,141 and it has a different brightness, 251 00:12:12,165 --> 00:12:15,878 then whatever conclusion you draw that relies on 252 00:12:15,902 --> 00:12:17,213 the brightness of that object is 253 00:12:17,237 --> 00:12:18,881 gonna be off somewhat. 254 00:12:18,905 --> 00:12:21,639 Think of the stars like streetlights. 255 00:12:21,741 --> 00:12:24,943 If one light is broken and dimmer than the others, 256 00:12:25,045 --> 00:12:27,645 you might think it's farther away. 257 00:12:27,747 --> 00:12:30,315 The concern with the distance ladder is that 258 00:12:30,416 --> 00:12:33,696 if any of the single rungs is not perfect, 259 00:12:33,720 --> 00:12:36,054 then the entire ladder might be out of whack 260 00:12:36,156 --> 00:12:38,076 by the time you get to the top. 261 00:12:38,158 --> 00:12:40,358 What we need is a fresh approach 262 00:12:40,460 --> 00:12:42,660 to measuring the age of the universe. 263 00:12:42,729 --> 00:12:45,630 We're hoping we could bring in a tie breaker, 264 00:12:45,732 --> 00:12:47,832 a referee, a brand new method 265 00:12:47,934 --> 00:12:50,279 that didn't care about any of this or any 266 00:12:50,303 --> 00:12:54,506 of that, and tell us what is the Hubble constant. 267 00:12:54,608 --> 00:12:57,308 We may have just found one. 268 00:12:57,410 --> 00:13:01,212 This observatory doesn't have a telescope. 269 00:13:01,314 --> 00:13:03,893 It's hunting for an invisible wave, 270 00:13:03,917 --> 00:13:07,864 a disturbance in spacetime itself, caused 271 00:13:07,888 --> 00:13:12,201 by massive objects accelerating or colliding. 272 00:13:12,225 --> 00:13:14,459 It's known as LIGO. 273 00:13:14,528 --> 00:13:17,006 LIGO stands for the Laser 274 00:13:17,030 --> 00:13:20,131 Interferometer Gravitational Wave Observatory, 275 00:13:20,233 --> 00:13:23,835 and it is a ground-based gravitational wave detector. 276 00:13:23,970 --> 00:13:27,539 A perfectly stabilized beam of laser light bounces 277 00:13:27,607 --> 00:13:30,942 in a five-mile-long, L-shaped tunnel. 278 00:13:31,044 --> 00:13:34,445 As a gravitational wave passes through the detector, 279 00:13:34,514 --> 00:13:36,259 space stretches, 280 00:13:36,283 --> 00:13:39,717 forcing the light to travel a tiny bit farther. 281 00:13:39,819 --> 00:13:43,955 You're bouncing a laser over an incredible distance 282 00:13:44,057 --> 00:13:47,659 and trying to measure as spacetime itself 283 00:13:47,727 --> 00:13:49,661 gets stretched and deformed 284 00:13:49,729 --> 00:13:52,764 whether that lazar had to travel a tiny bit further 285 00:13:52,799 --> 00:13:53,876 or a tiny bit shorter, 286 00:13:53,900 --> 00:13:57,335 and a tiny bit here is the width of a single atom 287 00:13:57,437 --> 00:13:59,704 over miles and miles of distance. 288 00:14:00,907 --> 00:14:04,409 LIGO has already detected colliding black holes, 289 00:14:06,046 --> 00:14:09,692 but it's also received a signal from something 290 00:14:09,716 --> 00:14:11,594 less massive. 291 00:14:11,618 --> 00:14:14,597 Neutron stars are the densest thing in 292 00:14:14,621 --> 00:14:16,754 the universe other than black holes. 293 00:14:16,823 --> 00:14:19,858 They're the last stopping point before you would collapse 294 00:14:19,960 --> 00:14:22,104 all the way to form a black hole. 295 00:14:22,128 --> 00:14:24,829 They're the size of Washington, D.C., 296 00:14:24,931 --> 00:14:28,233 but they can have the mass of two suns. 297 00:14:28,301 --> 00:14:32,937 A collision between neutron stars is incredibly powerful. 298 00:14:33,039 --> 00:14:35,840 It's one of the most energetic events in the universe, and it 299 00:14:35,942 --> 00:14:38,788 distorts the fabric of spacetime very strongly, 300 00:14:38,812 --> 00:14:41,346 because their gravity is so strong. 301 00:14:41,381 --> 00:14:43,092 But unlike black hole mergers, 302 00:14:43,116 --> 00:14:46,517 neutron star collisions can also send out light. 303 00:14:48,321 --> 00:14:52,156 In 2017, LIGO sent out an alert... more than 304 00:14:52,225 --> 00:14:57,061 70 telescopes on Earth and in space swung into action. 305 00:14:57,130 --> 00:15:00,331 This binary neutron star merger was the first time 306 00:15:00,433 --> 00:15:02,533 we had witnessed gravitational waves 307 00:15:02,636 --> 00:15:04,796 and light waves coming from the same event. 308 00:15:07,340 --> 00:15:09,607 It was groundbreaking. 309 00:15:09,709 --> 00:15:13,689 This event is ideal for Hubble constant hunters. 310 00:15:13,713 --> 00:15:15,713 The light tells us how fast 311 00:15:15,815 --> 00:15:18,561 the colliding stars are moving away from us. 312 00:15:18,585 --> 00:15:22,565 Gravitational waves give us the distance. 313 00:15:22,589 --> 00:15:26,224 If we know how far away it is and how fast it's moving, 314 00:15:26,359 --> 00:15:28,304 that's the Hubble constant. 315 00:15:28,328 --> 00:15:32,408 Having neutron star mergers added to your arsenal 316 00:15:32,432 --> 00:15:34,132 of ways of measuring 317 00:15:34,233 --> 00:15:36,779 the universe's expansion is great, because it's 318 00:15:36,803 --> 00:15:38,303 completely independent. 319 00:15:38,405 --> 00:15:40,805 It uses physics that's not related to either 320 00:15:40,907 --> 00:15:44,042 of the two competing methods we have so far. 321 00:15:44,110 --> 00:15:45,830 Sounds perfect. 322 00:15:45,912 --> 00:15:47,712 The result? 323 00:15:47,847 --> 00:15:49,425 So this brand-new measurement that 324 00:15:49,449 --> 00:15:51,529 were hoping would be a tie breaker... 325 00:15:53,520 --> 00:15:57,055 ended up coming right in between these two extremes. 326 00:15:58,124 --> 00:16:00,825 Thanks for the help. 327 00:16:00,927 --> 00:16:04,228 But it might not be as bad as it sounds. 328 00:16:04,331 --> 00:16:07,176 The number of neutron star collisions where 329 00:16:07,200 --> 00:16:11,502 we have detected gravitational waves and light... one. 330 00:16:12,639 --> 00:16:15,039 We shouldn't be at all disheartened by the fact 331 00:16:15,108 --> 00:16:17,386 that this hasn't actually decided 332 00:16:17,410 --> 00:16:20,144 the problem, because there's a huge margin for error 333 00:16:20,246 --> 00:16:22,213 when you have just one object. 334 00:16:22,315 --> 00:16:24,816 We would like something like 100 events 335 00:16:24,951 --> 00:16:27,352 like this neutron star merger. 336 00:16:27,453 --> 00:16:29,954 That might seem like a huge improvement we need, 337 00:16:30,023 --> 00:16:31,400 but actually, it's very feasible that 338 00:16:31,424 --> 00:16:33,803 in the next decade, we'll get there. 339 00:16:33,827 --> 00:16:37,261 Gravitational waves may give us a precise age of 340 00:16:37,363 --> 00:16:39,564 the universe, but there is a chance 341 00:16:39,632 --> 00:16:42,934 they'll tell us the problem isn't with our measurements, 342 00:16:43,036 --> 00:16:45,403 but with our understanding of the cosmos. 343 00:16:45,505 --> 00:16:48,239 If we keep getting different answers for the Hubble constant, 344 00:16:48,308 --> 00:16:50,641 especially depending on the method we use, 345 00:16:50,710 --> 00:16:52,488 that's a big clue that we don't 346 00:16:52,512 --> 00:16:54,812 understand something fundamental about 347 00:16:54,914 --> 00:16:56,214 the universe's evolution, 348 00:16:56,315 --> 00:16:58,275 its makeup, something important. 349 00:16:59,619 --> 00:17:02,798 Our search for the age of the universe just might 350 00:17:02,822 --> 00:17:06,424 destroy our model of how we think the cosmos works, 351 00:17:07,427 --> 00:17:10,628 plunging physics into chaos. 352 00:17:20,540 --> 00:17:23,107 We don't know the age of the universe. 353 00:17:23,209 --> 00:17:26,689 We had hoped that the results from our experiments would be 354 00:17:26,713 --> 00:17:31,093 like building a bridge, starting at opposite ends 355 00:17:31,117 --> 00:17:33,051 and meeting in the middle. 356 00:17:33,152 --> 00:17:34,852 As time goes on, 357 00:17:34,954 --> 00:17:38,100 as the evidence accumulates, 358 00:17:38,124 --> 00:17:40,903 these two sides of the bridge are not gonna meet. 359 00:17:40,927 --> 00:17:44,106 Something has to give. 360 00:17:44,130 --> 00:17:46,308 Some believe the problem lies in the way 361 00:17:46,332 --> 00:17:49,879 we've interpreted the picture of the early universe, 362 00:17:49,903 --> 00:17:54,338 the pattern hidden in the cosmic microwave background. 363 00:17:54,407 --> 00:17:56,685 We're really confident in the data that we have from 364 00:17:56,709 --> 00:17:59,210 the CMB, but it's actually an indirect 365 00:17:59,312 --> 00:18:01,290 measurement of the universe's age. 366 00:18:01,314 --> 00:18:04,315 It depends on our model of the universe being right. 367 00:18:04,417 --> 00:18:08,953 It could be, it could very well be that our fundamental 368 00:18:09,055 --> 00:18:11,901 cosmological model that we've used 369 00:18:11,925 --> 00:18:15,805 to successfully describe the universe is coming up short, 370 00:18:15,829 --> 00:18:17,840 that there's something wrong in there, 371 00:18:17,864 --> 00:18:20,531 that that engine is broken. 372 00:18:20,633 --> 00:18:24,936 That engine is the standard cosmological model. 373 00:18:25,071 --> 00:18:27,483 Based on our knowledge of particle physics 374 00:18:27,507 --> 00:18:29,185 and general relativity, 375 00:18:29,209 --> 00:18:31,387 it's like an instruction manual 376 00:18:31,411 --> 00:18:33,311 for how the universe works. 377 00:18:33,413 --> 00:18:36,948 Rewriting it is a radical suggestion. 378 00:18:37,049 --> 00:18:40,096 For the most part, it matches what we see, 379 00:18:40,120 --> 00:18:43,321 but it does struggle with one thing. 380 00:18:43,389 --> 00:18:46,824 As the universe expands away from the Big Bang, 381 00:18:46,959 --> 00:18:51,129 the intuitive thing you would expect is for gravity to start 382 00:18:51,230 --> 00:18:52,708 pulling it back together again. 383 00:18:52,732 --> 00:18:55,377 So over time, gravity would just reverse that 384 00:18:55,401 --> 00:18:59,982 and pull everything back in, back to a single point. 385 00:19:00,006 --> 00:19:03,508 But what we see in the data is completely opposite. 386 00:19:03,643 --> 00:19:06,188 What we see is that the universe is not only 387 00:19:06,212 --> 00:19:07,812 continuing to expand, 388 00:19:07,914 --> 00:19:10,893 but it's speeding up faster and faster all the time. 389 00:19:10,917 --> 00:19:13,117 To explain this weird phenomenon, 390 00:19:13,252 --> 00:19:15,253 the cosmological model relies on 391 00:19:15,354 --> 00:19:18,656 the existence of a strange, unknown force... 392 00:19:18,725 --> 00:19:20,458 Dark energy. 393 00:19:20,560 --> 00:19:22,827 Dark energy is the most perplexing 394 00:19:22,929 --> 00:19:25,530 and mysterious thing I've encountered in my research. 395 00:19:25,665 --> 00:19:28,177 Dark energy is a term that we slap 396 00:19:28,201 --> 00:19:30,779 on this idea that the universal expansion 397 00:19:30,803 --> 00:19:33,082 is accelerating. 398 00:19:33,106 --> 00:19:35,239 That's about all we know about it. 399 00:19:35,308 --> 00:19:36,718 We don't know what's causing it. 400 00:19:36,742 --> 00:19:37,786 We don't know how it behaves. 401 00:19:37,810 --> 00:19:38,988 We don't know what it was like in 402 00:19:39,012 --> 00:19:40,689 the past or what it's like in the future. 403 00:19:40,713 --> 00:19:43,814 So we just call it dark energy. 404 00:19:43,950 --> 00:19:47,852 It's invisible... It fills the whole universe 405 00:19:47,954 --> 00:19:50,955 and pushes galaxies apart. 406 00:19:51,024 --> 00:19:53,057 In some sense, it's like a spring, 407 00:19:53,159 --> 00:19:56,305 a contracted spring, and you let it go, and it wants to push 408 00:19:56,329 --> 00:19:58,307 everything away. 409 00:19:58,331 --> 00:19:59,808 And things get stranger. 410 00:19:59,832 --> 00:20:04,802 Dark energy doesn't dilute as the universe expands. 411 00:20:04,904 --> 00:20:07,038 As empty space gets created 412 00:20:07,139 --> 00:20:10,886 or expands, the dark energy associated with that 413 00:20:10,910 --> 00:20:12,009 stays the same. 414 00:20:12,045 --> 00:20:14,590 It basically populates all this empty space. 415 00:20:14,614 --> 00:20:16,392 Imagine I'm draining a bucket of water, 416 00:20:16,416 --> 00:20:19,595 and water just magically appears out of nowhere. 417 00:20:19,619 --> 00:20:21,752 That's like how dark energy behaves 418 00:20:21,821 --> 00:20:23,899 as the universe is expanding. 419 00:20:23,923 --> 00:20:25,768 Dark energy plays an important 420 00:20:25,792 --> 00:20:28,404 role in the standard cosmological model. 421 00:20:28,428 --> 00:20:31,362 If our understanding of it is wrong, 422 00:20:31,464 --> 00:20:33,008 then so too is the model, 423 00:20:33,032 --> 00:20:37,379 which means the age of the universe we get from the CMB 424 00:20:37,403 --> 00:20:39,437 is wrong, too. 425 00:20:39,505 --> 00:20:41,717 Since nobody has a clue what dark energy is, 426 00:20:41,741 --> 00:20:43,018 there are a lot of different theories. 427 00:20:43,042 --> 00:20:46,844 But the biggest question of all is simply, is it constant? 428 00:20:47,947 --> 00:20:51,315 Our standard assumption about dark energy is that 429 00:20:51,417 --> 00:20:53,251 it's pushing apart the universe 430 00:20:53,352 --> 00:20:55,430 with the same strength throughout the history 431 00:20:55,454 --> 00:20:56,654 of the universe. 432 00:20:57,724 --> 00:21:02,204 Now physicists are wondering if that idea is wrong. 433 00:21:02,228 --> 00:21:06,675 Maybe, in the early universe, dark energy acted differently. 434 00:21:06,699 --> 00:21:10,334 Hey, you know that whole dark energy thing that's messing 435 00:21:10,436 --> 00:21:11,780 with the universe today? 436 00:21:11,804 --> 00:21:15,740 Maybe it messed with the universe back then. 437 00:21:15,808 --> 00:21:19,510 It could be that dark energy really has affected 438 00:21:19,645 --> 00:21:22,113 the rate of expansion a lot more than we thought. 439 00:21:22,215 --> 00:21:25,494 This is gonna throw a big monkey wrench into our idea of 440 00:21:25,518 --> 00:21:26,862 how old the universe is 441 00:21:26,886 --> 00:21:28,998 and what it was like at different eras. 442 00:21:29,022 --> 00:21:34,203 The theory is called new early dark energy. 443 00:21:34,227 --> 00:21:38,007 So the idea behind new early dark energy is that 444 00:21:38,031 --> 00:21:41,577 dark energy was present during the very early periods 445 00:21:41,601 --> 00:21:44,168 of the universe, but in a very different state. 446 00:21:44,203 --> 00:21:48,205 Just like you can think of water being present in 447 00:21:48,341 --> 00:21:50,286 two states, 448 00:21:50,310 --> 00:21:54,445 it can be liquid water if the environment is quite hot, 449 00:21:54,514 --> 00:21:56,847 or it can be frozen water 450 00:21:56,949 --> 00:21:59,094 if the environment is colder. 451 00:21:59,118 --> 00:22:01,218 We call that a phase change. 452 00:22:01,321 --> 00:22:03,198 Maybe in the early universe, 453 00:22:03,222 --> 00:22:05,701 dark energy underwent a phase change, as well. 454 00:22:05,725 --> 00:22:09,204 It was different before then and acts differently now. 455 00:22:09,228 --> 00:22:13,164 According to the theory, this more energetic state of 456 00:22:13,265 --> 00:22:15,533 early dark energy pushed apart 457 00:22:15,601 --> 00:22:18,681 the early universe much faster than we thought. 458 00:22:18,705 --> 00:22:22,707 So that speeds things up in the opening moments of 459 00:22:22,809 --> 00:22:23,986 our universe, 460 00:22:24,010 --> 00:22:27,589 which starts to actually bring things back into agreement 461 00:22:27,613 --> 00:22:29,792 when you look at interpreting both 462 00:22:29,816 --> 00:22:32,249 the cosmic microwave background and the distance 463 00:22:32,318 --> 00:22:33,996 ladder measurements. 464 00:22:34,020 --> 00:22:36,554 One of the things that we see in the universe 465 00:22:36,622 --> 00:22:39,056 is that things change with time, density changes, 466 00:22:39,158 --> 00:22:40,602 matter changes, energy changes. 467 00:22:40,626 --> 00:22:42,426 Why not dark energy? 468 00:22:42,528 --> 00:22:45,363 Adding new early dark energy 469 00:22:45,464 --> 00:22:48,332 to the early universe changes the standard model. 470 00:22:48,401 --> 00:22:52,737 The CMB gives a higher figure for the expansion of 471 00:22:52,805 --> 00:22:54,939 the universe, and finally, 472 00:22:55,040 --> 00:22:58,042 an age that matches the one given by 473 00:22:58,144 --> 00:23:00,611 the distance ladder method. 474 00:23:00,713 --> 00:23:04,315 If you think about that bridge analogy, where the two parts 475 00:23:04,417 --> 00:23:07,596 just don't meet, the early dark energy adjusts 476 00:23:07,620 --> 00:23:11,400 the angle of the early universe part of the bridge, 477 00:23:11,424 --> 00:23:14,903 and it just gets them to actually meet in the middle. 478 00:23:14,927 --> 00:23:19,130 It's still controversial, but new dark energy may be 479 00:23:19,232 --> 00:23:22,032 detected in detailed measurements of 480 00:23:22,134 --> 00:23:24,279 the cosmic microwave background. 481 00:23:24,303 --> 00:23:26,704 I mean, in one sense, like, 482 00:23:26,839 --> 00:23:28,584 do we really need to overcomplicate 483 00:23:28,608 --> 00:23:30,386 the universe here? But you know what? 484 00:23:30,410 --> 00:23:34,089 The universe is under no obligation to be simple. 485 00:23:34,113 --> 00:23:38,060 But there's one thing physicists can agree on. 486 00:23:38,084 --> 00:23:41,118 Dark energy truly is a can of worms we've just 487 00:23:41,253 --> 00:23:43,954 opened, and there may be some big changes coming up. 488 00:23:44,056 --> 00:23:46,257 There is a more radical possibility. 489 00:23:46,325 --> 00:23:51,407 Maybe we need to ditch dark energy altogether and question 490 00:23:51,431 --> 00:23:56,200 one of the most famous theories of all, general relativity. 491 00:23:56,335 --> 00:23:57,601 Is it possible? 492 00:23:57,703 --> 00:24:00,638 Did Einstein make a colossal mistake? 493 00:24:09,749 --> 00:24:12,149 In trying to work out the age of the universe, 494 00:24:12,218 --> 00:24:15,553 physicists have started a revolution, 495 00:24:15,654 --> 00:24:18,722 a revolution that could overturn everything we thought 496 00:24:18,824 --> 00:24:21,503 we knew about how the universe works, 497 00:24:21,527 --> 00:24:24,361 including the bedrock of modern physics, 498 00:24:24,463 --> 00:24:26,964 Einstein's theory of gravity, 499 00:24:27,033 --> 00:24:28,799 general relativity. 500 00:24:28,901 --> 00:24:31,402 Underlying everything, 501 00:24:31,537 --> 00:24:34,538 all of cosmology, is general relativity, 502 00:24:34,607 --> 00:24:38,987 but maybe we need a completely new understanding 503 00:24:39,011 --> 00:24:40,845 of gravity. 504 00:24:40,947 --> 00:24:43,948 Gravity is a strange force. 505 00:24:44,083 --> 00:24:46,317 It's always attractive. 506 00:24:46,419 --> 00:24:49,453 The Earth pulling on us gives us our weight. 507 00:24:49,555 --> 00:24:53,702 The force of gravity acts over huge distances. 508 00:24:53,726 --> 00:24:57,862 The sun tugs on objects throughout the solar system. 509 00:24:57,964 --> 00:25:00,865 The Milky Way pulls on other galaxies. 510 00:25:00,933 --> 00:25:04,635 On the one hand, gravity is incredibly familiar 511 00:25:04,737 --> 00:25:07,037 to us, you know, the apple falling from the tree 512 00:25:07,139 --> 00:25:08,639 and all of that stuff, 513 00:25:08,741 --> 00:25:11,342 and we also know that gravity behaves in a very 514 00:25:11,443 --> 00:25:13,811 predictable way throughout our solar system 515 00:25:13,913 --> 00:25:16,592 from all the spacecraft and things we've sent out. 516 00:25:16,616 --> 00:25:20,551 But when it comes to how it behaves on incredibly tiny 517 00:25:20,653 --> 00:25:23,120 scales and also on incredibly 518 00:25:23,222 --> 00:25:25,956 large scales, covering the whole universe, 519 00:25:26,025 --> 00:25:29,059 it's possible that we just don't yet have the right 520 00:25:29,128 --> 00:25:31,629 picture of what's going on. 521 00:25:31,731 --> 00:25:34,064 Einstein's model of gravity has remained 522 00:25:34,133 --> 00:25:36,700 largely the same for 100 years. 523 00:25:36,802 --> 00:25:39,737 So much of modern physics is really standing on 524 00:25:39,838 --> 00:25:41,305 Einstein's shoulders, 525 00:25:41,374 --> 00:25:43,407 but at the same time, 526 00:25:43,509 --> 00:25:46,588 we can't ever take anything for granted. 527 00:25:46,612 --> 00:25:50,392 Claudia de Rham works on a theory called massive gravity. 528 00:25:50,416 --> 00:25:52,616 It's based on a key part of 529 00:25:52,718 --> 00:25:56,598 Einstein's theory that says gravity doesn't have mass. 530 00:25:56,622 --> 00:26:00,157 Once you understand that general relativity is the theory 531 00:26:00,259 --> 00:26:01,625 of a massless particle, 532 00:26:01,727 --> 00:26:03,305 the immediate response should be, 533 00:26:03,329 --> 00:26:05,674 well, what if it was massive? 534 00:26:05,698 --> 00:26:08,065 The theoretical particle that carries 535 00:26:08,133 --> 00:26:10,879 gravity is called the graviton. 536 00:26:10,903 --> 00:26:13,737 If gravitons don't have any weight, 537 00:26:13,773 --> 00:26:16,740 then there's nothing to slow them down as they speed 538 00:26:16,842 --> 00:26:18,108 through the universe. 539 00:26:18,177 --> 00:26:21,189 They can act over infinite distances, 540 00:26:21,213 --> 00:26:23,547 just like photons of light. 541 00:26:23,649 --> 00:26:28,118 So one galaxy on this side of the universe can actually pull 542 00:26:28,220 --> 00:26:31,221 on a galaxy that's right on the other side of the universe. 543 00:26:31,324 --> 00:26:35,059 But if gravity has weight, things change. 544 00:26:35,127 --> 00:26:36,371 In some sense, 545 00:26:36,395 --> 00:26:39,330 if we attach a little backpack to our graviton particle, 546 00:26:39,432 --> 00:26:45,803 its effect is to slowly slow it down just enough so as to 547 00:26:45,938 --> 00:26:49,139 make its effect on very large distances 548 00:26:49,241 --> 00:26:53,744 being a tiny little bit weaker, and that's our way to 549 00:26:53,813 --> 00:26:55,346 switch off the effect of 550 00:26:55,414 --> 00:26:59,995 gravity on huge cosmological distances. 551 00:27:00,019 --> 00:27:04,054 If gravity is a little bit weaker, a galaxy on this side 552 00:27:04,123 --> 00:27:05,956 of the universe can't pull on 553 00:27:06,058 --> 00:27:08,303 one on the other side of the cosmos. 554 00:27:08,327 --> 00:27:12,229 It has a huge effect on the expansion of the universe. 555 00:27:13,833 --> 00:27:17,601 If the force of gravity actually just switches off at 556 00:27:17,703 --> 00:27:19,136 large distances, 557 00:27:19,205 --> 00:27:22,684 then you no longer have to counter the fact that 558 00:27:22,708 --> 00:27:23,873 everything is pulling 559 00:27:23,909 --> 00:27:26,221 everything else together, because it isn't anymore. 560 00:27:26,245 --> 00:27:29,213 So that would quite naturally explain why 561 00:27:29,315 --> 00:27:32,349 the expansion of our universe would be speeding up. 562 00:27:32,418 --> 00:27:38,122 This acceleration is what we see in the universe today. 563 00:27:38,257 --> 00:27:42,526 Currently, we use dark energy to explain it. 564 00:27:42,661 --> 00:27:45,162 So, if the graviton has mass, 565 00:27:45,231 --> 00:27:48,832 that means that we can get out of the universe 566 00:27:48,934 --> 00:27:51,802 what we see without the need for dark energy. 567 00:27:51,904 --> 00:27:54,383 What if actually what we were observing 568 00:27:54,407 --> 00:27:58,387 is simply the first sign of gravity 569 00:27:58,411 --> 00:28:01,089 switching off at very large distances. 570 00:28:01,113 --> 00:28:04,393 Maybe we're just observing the first effect of 571 00:28:04,417 --> 00:28:06,395 the graviton having a mass. 572 00:28:06,419 --> 00:28:09,197 Without dark energy to deal with, 573 00:28:09,221 --> 00:28:12,456 the universe is a lot easier to explain. 574 00:28:12,524 --> 00:28:15,559 Maybe we don't need these complicated physics. 575 00:28:15,628 --> 00:28:19,107 Maybe it's just all the normal ingredients of the universe, 576 00:28:19,131 --> 00:28:22,477 but operating under a different set of rules. 577 00:28:22,501 --> 00:28:26,982 Claudia hopes her theory will soon be put to the test. 578 00:28:27,006 --> 00:28:30,786 Around 2037, 579 00:28:30,810 --> 00:28:33,744 we'll have a new gravitational wave detector, 580 00:28:33,813 --> 00:28:37,915 the Laser Interferometer Space Antenna, or LISA. 581 00:28:39,218 --> 00:28:43,220 It'll be bigger than LIGO and will orbit the Earth. 582 00:28:43,322 --> 00:28:45,233 When LISA get out there in space, 583 00:28:45,257 --> 00:28:47,558 we'll even have a bigger handle on 584 00:28:47,659 --> 00:28:51,006 gravitational waves evolving throughout the whole universe, 585 00:28:51,030 --> 00:28:54,276 and so it will allow us to go very deep in our understanding 586 00:28:54,300 --> 00:28:55,677 of gravity. 587 00:28:55,701 --> 00:29:00,182 LISA is a system of three satellites arranged in a giant 588 00:29:00,206 --> 00:29:05,309 triangular formation, 1.5 million miles apart. 589 00:29:06,445 --> 00:29:08,285 It should pick up very low 590 00:29:08,347 --> 00:29:13,395 frequency gravitational waves from more ancient events, 591 00:29:13,419 --> 00:29:17,955 perhaps even shockwaves from the birth of the universe. 592 00:29:18,090 --> 00:29:21,002 If the graviton has mass, 593 00:29:21,026 --> 00:29:24,406 then the waves will arrive more slowly than predicted, 594 00:29:24,430 --> 00:29:28,432 but until we receive those signals, all bets are off. 595 00:29:28,501 --> 00:29:32,035 It's a big deal to propose a difference in gravity, 596 00:29:32,104 --> 00:29:35,139 but then again, we don't know. 597 00:29:35,207 --> 00:29:36,384 I'm making no bets. 598 00:29:36,408 --> 00:29:40,310 The universe has proven itself to be so deceptive. 599 00:29:40,412 --> 00:29:43,147 So I'm gonna wait until it tells me what it is. 600 00:29:45,518 --> 00:29:47,751 The question of the age of the universe 601 00:29:47,853 --> 00:29:50,154 opens Pandora's box, 602 00:29:50,255 --> 00:29:52,756 and the expansion rate of the universe 603 00:29:52,825 --> 00:29:55,270 holds another secret, 604 00:29:55,294 --> 00:29:59,863 our ultimate fate... How the universe will end. 605 00:30:11,210 --> 00:30:13,510 We know exactly how the Earth will end. 606 00:30:15,714 --> 00:30:18,715 In around 5.4 billion years, 607 00:30:18,818 --> 00:30:21,096 the sun will turn into a red giant, 608 00:30:21,120 --> 00:30:24,454 expanding to 1,000 times its current size. 609 00:30:26,325 --> 00:30:28,205 The Earth will be destroyed. 610 00:30:30,029 --> 00:30:32,262 Humans, if we still exist, 611 00:30:32,298 --> 00:30:34,932 will have long deserted our home planet. 612 00:30:37,203 --> 00:30:39,403 But how will the universe end? 613 00:30:40,940 --> 00:30:43,385 The age of the universe enables us to 614 00:30:43,409 --> 00:30:46,109 not only understand where we came from, 615 00:30:46,212 --> 00:30:48,924 but potentially, the fate of the universe. 616 00:30:48,948 --> 00:30:52,216 What will happen millions and billions of years from now? 617 00:30:53,219 --> 00:30:55,352 If scientists confirm the value 618 00:30:55,421 --> 00:30:56,720 of the Hubble constant, 619 00:30:56,822 --> 00:31:00,001 the elusive figure that tells us just how fast the universe 620 00:31:00,025 --> 00:31:02,359 is expanding, it will tell us the age of 621 00:31:02,461 --> 00:31:06,563 the universe, and it will help us predict its end. 622 00:31:06,632 --> 00:31:08,577 Measuring the Hubble constant is 623 00:31:08,601 --> 00:31:10,934 measuring the expansion rate today, 624 00:31:11,036 --> 00:31:14,605 right now, it's like checking your speedometer at one moment. 625 00:31:14,740 --> 00:31:16,685 But just because it's your speed now, 626 00:31:16,709 --> 00:31:19,087 it doesn't mean it was the same speed when you left 627 00:31:19,111 --> 00:31:21,890 your home or the same speed when you'll be on the freeway. 628 00:31:21,914 --> 00:31:25,215 How the expansion changes over time 629 00:31:25,317 --> 00:31:27,517 will control the fate of the cosmos. 630 00:31:27,620 --> 00:31:29,397 So depending on the Hubble constant, 631 00:31:29,421 --> 00:31:31,722 the universe could continue to expand. 632 00:31:31,824 --> 00:31:35,303 It could accelerate its expansion rate, 633 00:31:35,327 --> 00:31:37,728 or it could be decelerating. 634 00:31:38,831 --> 00:31:41,832 At the moment, galaxies are racing apart. 635 00:31:43,135 --> 00:31:45,836 A continually expanding universe will 636 00:31:45,938 --> 00:31:48,984 cool down as it spreads out. 637 00:31:49,008 --> 00:31:53,310 Another name for this eternal expansion is the Big Freeze, 638 00:31:53,445 --> 00:31:56,113 because as everything gets spread out, 639 00:31:56,248 --> 00:31:59,616 the density is lower, and there's no more opportunities 640 00:31:59,718 --> 00:32:01,296 for temperature differences. 641 00:32:01,320 --> 00:32:04,621 Everything just gets colder and colder and colder and colder, 642 00:32:04,723 --> 00:32:09,604 slowly, eternally approaching absolute zero. 643 00:32:09,628 --> 00:32:11,962 The more matter is spread out, 644 00:32:12,031 --> 00:32:14,765 the less chance there is for star formation. 645 00:32:14,800 --> 00:32:18,480 And so the universe's continued expansion means our 646 00:32:18,504 --> 00:32:23,585 night sky and every night sky in the universe will inevitably 647 00:32:23,609 --> 00:32:26,388 continue to get darker and darker and darker as things 648 00:32:26,412 --> 00:32:29,546 move further away and as stars die off. 649 00:32:29,615 --> 00:32:33,750 Eventually, all the stars will go out, 650 00:32:33,852 --> 00:32:36,398 and there'll just be the leftovers, 651 00:32:36,422 --> 00:32:40,101 which we call the degenerates, black holes, 652 00:32:40,125 --> 00:32:42,359 white dwarfs, rogue planets. 653 00:32:42,428 --> 00:32:46,007 It's gonna be a very, very sad place. 654 00:32:46,031 --> 00:32:48,276 The last refuge of any matter at all 655 00:32:48,300 --> 00:32:50,278 will be black holes. 656 00:32:50,302 --> 00:32:52,782 You've got a big black hole in the middle of each galaxy, 657 00:32:52,838 --> 00:32:56,685 over trillions of years, everything in galaxies fall in, 658 00:32:56,709 --> 00:33:00,188 so finally, you're left with big black holes over vast 659 00:33:00,212 --> 00:33:01,945 distances, separated 660 00:33:02,047 --> 00:33:04,492 almost universes away. 661 00:33:04,516 --> 00:33:07,951 So getting towards the Big Freeze, 662 00:33:08,053 --> 00:33:10,954 black holes themselves start to evaporate. 663 00:33:11,056 --> 00:33:14,458 There won't even be black holes at the end of this 664 00:33:14,559 --> 00:33:16,271 accelerating universe. 665 00:33:16,295 --> 00:33:18,729 All that's left is very, 666 00:33:18,864 --> 00:33:22,277 very low energy photons and a little bit of matter dispersed 667 00:33:22,301 --> 00:33:25,135 throughout the universe, and there's nothing left. 668 00:33:25,237 --> 00:33:26,414 That's it. 669 00:33:26,438 --> 00:33:29,084 We call that the heat death of the universe. 670 00:33:29,108 --> 00:33:31,786 There's no longer any place that has more energy 671 00:33:31,810 --> 00:33:36,146 or more heat... it's all just thin, barely there photons. 672 00:33:36,215 --> 00:33:38,515 It's fascinating scientifically, 673 00:33:38,617 --> 00:33:40,417 but from a human standpoint, 674 00:33:40,519 --> 00:33:42,797 not a lot of fun to think about. 675 00:33:42,821 --> 00:33:44,366 But if the Hubble constant, 676 00:33:44,390 --> 00:33:48,403 the expansion rate of the universe, keeps increasing, 677 00:33:48,427 --> 00:33:52,529 then the end of the universe could be a lot scarier 678 00:33:52,631 --> 00:33:54,631 and come a lot sooner. 679 00:33:57,036 --> 00:33:59,703 One possibility is that the expansion 680 00:33:59,838 --> 00:34:01,683 of the universe will accelerate 681 00:34:01,707 --> 00:34:03,985 and continue to accelerate forever, 682 00:34:04,009 --> 00:34:05,709 faster and faster and faster. 683 00:34:05,844 --> 00:34:08,945 And if that happens, we face a scenario that we call 684 00:34:09,047 --> 00:34:11,248 the Big Rip, where actually the whole 685 00:34:11,316 --> 00:34:14,217 of space essentially just gets ripped to shreds. 686 00:34:15,821 --> 00:34:18,121 So the solar system is gonna get ripped apart, 687 00:34:18,256 --> 00:34:21,558 then the sun and the planets themselves will start to get 688 00:34:21,593 --> 00:34:22,925 ripped apart. 689 00:34:22,961 --> 00:34:26,163 And finally, it works its way down to atoms, and atoms get 690 00:34:26,264 --> 00:34:28,376 ripped apart, and we're starting to see effects on 691 00:34:28,400 --> 00:34:29,800 space and time. 692 00:34:31,036 --> 00:34:33,236 Space is ripped apart. 693 00:34:33,338 --> 00:34:34,671 Time comes to a stop. 694 00:34:37,042 --> 00:34:41,411 So in this scenario, time and space have no meaning. 695 00:34:41,513 --> 00:34:44,392 If everything is infinitely far apart, 696 00:34:44,416 --> 00:34:48,496 then space doesn't really exist. 697 00:34:48,520 --> 00:34:51,521 It's sort of beyond our comprehension. 698 00:34:51,623 --> 00:34:54,224 Working out the expansion rate will 699 00:34:54,293 --> 00:34:57,405 tell us which scenario we face, 700 00:34:57,429 --> 00:35:01,331 but for now, the lifespan of the universe is unknown. 701 00:35:02,534 --> 00:35:07,482 Maybe we need to investigate the other end of the timeline. 702 00:35:07,506 --> 00:35:09,584 But how can we get a fix on 703 00:35:09,608 --> 00:35:13,043 the age of the universe without understanding 704 00:35:13,112 --> 00:35:14,544 its origin? 705 00:35:14,613 --> 00:35:17,292 As you go back in time towards the Big Bang, 706 00:35:17,316 --> 00:35:20,717 our knowledge of physics really goes out the window. 707 00:35:21,954 --> 00:35:24,621 Temperatures off the scale, pressure off the scale... 708 00:35:24,723 --> 00:35:27,958 The way everything behaved is just so different that 709 00:35:28,059 --> 00:35:30,327 the rules we have now do not apply. 710 00:35:31,396 --> 00:35:33,396 The biggest problem of all... 711 00:35:33,432 --> 00:35:37,100 What came just before the Big Bang? 712 00:35:37,202 --> 00:35:40,937 Einstein's general relativity predicts that all the matter 713 00:35:41,039 --> 00:35:42,739 and energy in the universe 714 00:35:42,841 --> 00:35:45,887 was concentrated down to a single point, 715 00:35:45,911 --> 00:35:47,155 the Singularity. 716 00:35:47,179 --> 00:35:50,147 The Singularity is like the part 717 00:35:50,215 --> 00:35:53,717 of those old maps that says, "Here be dragons." 718 00:35:53,819 --> 00:35:57,398 Singularities are a problem. We don't like them. 719 00:35:57,422 --> 00:36:00,201 This is where basically you have a finite amount of matter 720 00:36:00,225 --> 00:36:02,103 in the universe, but it's squeezed down 721 00:36:02,127 --> 00:36:05,228 into zero volume, so it would be infinitely dense. 722 00:36:06,431 --> 00:36:09,299 Infinite densities don't actually happen in nature. 723 00:36:09,434 --> 00:36:12,514 This is a sign that our math is breaking down. 724 00:36:12,538 --> 00:36:13,781 This is a sign that we need 725 00:36:13,805 --> 00:36:17,507 to replace that with a new understanding. 726 00:36:18,810 --> 00:36:21,411 Many now believe Einstein was wrong. 727 00:36:21,513 --> 00:36:25,048 There was no Singularity begging the question, 728 00:36:25,150 --> 00:36:28,552 could the age of the universe be infinite? 729 00:36:36,428 --> 00:36:38,588 Scientists investigating the age of 730 00:36:38,630 --> 00:36:42,132 the universe are struggling to understand its origins. 731 00:36:43,202 --> 00:36:46,803 Could that be because there was no beginning? 732 00:36:46,905 --> 00:36:50,106 Could the universe be infinite? 733 00:36:50,209 --> 00:36:52,187 Because we think we live and we die, 734 00:36:52,211 --> 00:36:54,055 we project that onto the universe. 735 00:36:54,079 --> 00:36:56,213 But that may not be the case. 736 00:36:56,348 --> 00:36:58,760 The idea of an infinite universe is 737 00:36:58,784 --> 00:37:01,296 no more strange than the idea of a singularity. 738 00:37:01,320 --> 00:37:03,954 And in fact, throughout most of history, 739 00:37:04,055 --> 00:37:06,100 astronomers thought that the universe was 740 00:37:06,124 --> 00:37:08,525 probably infinite. 741 00:37:08,627 --> 00:37:10,605 The foundation of our mathematical 742 00:37:10,629 --> 00:37:12,106 understanding of the universe, 743 00:37:12,130 --> 00:37:15,632 Einstein's general relativity, has a problem. 744 00:37:15,701 --> 00:37:18,935 It doesn't translate to the world of the very tiny, 745 00:37:19,037 --> 00:37:23,384 which is why its laws break down close to the Big Bang. 746 00:37:23,408 --> 00:37:26,988 General relativity does a great job at describing things 747 00:37:27,012 --> 00:37:28,445 on scales that you and I are 748 00:37:28,513 --> 00:37:31,559 familiar with and things like how planets move 749 00:37:31,583 --> 00:37:34,050 and how galaxies evolve, all the big stuff. 750 00:37:34,119 --> 00:37:37,621 Quantum mechanics, on the other hand, describes the world of 751 00:37:37,723 --> 00:37:40,724 the very small, the world of the atoms. 752 00:37:40,826 --> 00:37:43,226 The problem is that these two theories 753 00:37:43,328 --> 00:37:45,462 don't fit well together at all. 754 00:37:46,531 --> 00:37:50,033 A new theory known as loop quantum gravity, 755 00:37:50,135 --> 00:37:54,082 brings quantum theory and relativity together, 756 00:37:54,106 --> 00:37:56,940 and it makes a stunning prediction. 757 00:37:57,042 --> 00:38:01,645 So one possibility is that the end of the universe 758 00:38:01,746 --> 00:38:04,848 could kind of match onto the beginning of a new universe 759 00:38:04,950 --> 00:38:07,250 and create a cycle of universes, 760 00:38:07,352 --> 00:38:08,997 one after the other. 761 00:38:09,021 --> 00:38:10,954 Nicknamed the Big Bounce, 762 00:38:11,056 --> 00:38:12,800 it predicts a universe that 763 00:38:12,824 --> 00:38:16,804 stops expanding and switches into reverse. 764 00:38:16,828 --> 00:38:19,963 And the idea here is that the universe can expand for 765 00:38:20,065 --> 00:38:22,065 a time, stop expanding, 766 00:38:22,133 --> 00:38:23,578 and then begin to contract again. 767 00:38:23,602 --> 00:38:26,036 And some have suggested that perhaps 768 00:38:26,137 --> 00:38:28,783 there's a cycle of expanding and compressing. 769 00:38:28,807 --> 00:38:30,985 It bounces back over again. 770 00:38:31,009 --> 00:38:33,943 One of the appeals of the bouncing model is 771 00:38:34,045 --> 00:38:36,846 that it allows us to get beyond the Singularity. 772 00:38:36,948 --> 00:38:39,316 A bit like recycling on Earth. 773 00:38:39,418 --> 00:38:43,753 All the components get crushed down and then reused, 774 00:38:43,822 --> 00:38:47,524 giving the cosmos no beginning and no end. 775 00:38:47,659 --> 00:38:49,826 If the universe is cyclic, 776 00:38:49,928 --> 00:38:51,906 does the age even have a meaning? 777 00:38:51,930 --> 00:38:54,864 Age is a construct of humanity, 778 00:38:54,966 --> 00:38:56,433 because we need to count time. 779 00:38:56,501 --> 00:38:58,079 But if the universe is infinite, 780 00:38:58,103 --> 00:39:01,037 maybe it doesn't matter in the big scheme of things. 781 00:39:01,139 --> 00:39:03,540 A contracting and expanding universe 782 00:39:03,641 --> 00:39:06,509 messes with the concept of age. 783 00:39:06,611 --> 00:39:07,944 But the very idea of 784 00:39:08,046 --> 00:39:12,093 an expanding universe provides another cosmic curveball. 785 00:39:12,117 --> 00:39:14,451 It might not be alone... It might 786 00:39:14,519 --> 00:39:18,900 be just one ageless universe among many. 787 00:39:18,924 --> 00:39:23,304 It's an idea embedded in the math of the Big Bang. 788 00:39:23,328 --> 00:39:26,329 The most popular theory we have in astrophysics, 789 00:39:26,431 --> 00:39:29,977 what put the bang into our Big Bang, is inflation. 790 00:39:30,001 --> 00:39:32,080 This idea that there was a kind of dark energy 791 00:39:32,104 --> 00:39:34,682 on steroids that made our universe double over and over 792 00:39:34,706 --> 00:39:36,784 not every seven billion years, 793 00:39:36,808 --> 00:39:39,687 but every split second, 794 00:39:39,711 --> 00:39:43,446 creating out of almost nothing, a big bang. 795 00:39:43,548 --> 00:39:47,595 When the universe was just a hundredth of a billionth 796 00:39:47,619 --> 00:39:51,054 of a trillionth of a trillionth of a second old, 797 00:39:51,156 --> 00:39:54,657 it underwent a period of rapid expansion called inflation. 798 00:39:54,726 --> 00:39:59,062 It doubled in size at least 90 times, going from 799 00:39:59,164 --> 00:40:02,276 the size of a subatomic particle to that of 800 00:40:02,300 --> 00:40:03,633 a golf ball. 801 00:40:03,735 --> 00:40:06,581 The problem with this inflation is that 802 00:40:06,605 --> 00:40:07,737 it doesn't really stop. 803 00:40:07,839 --> 00:40:10,740 It just makes this ever bigger space and says that, 804 00:40:10,809 --> 00:40:13,443 yeah, well, okay, there was one region of space where 805 00:40:13,512 --> 00:40:16,591 this crazy doubling stopped and galaxies formed, 806 00:40:16,615 --> 00:40:17,913 and that's us. 807 00:40:17,949 --> 00:40:20,517 But there's this vast realm out there where inflation is 808 00:40:20,619 --> 00:40:22,051 still happening. 809 00:40:22,153 --> 00:40:24,721 In the spots where inflation stops, 810 00:40:24,823 --> 00:40:27,257 parallel universes form. 811 00:40:27,325 --> 00:40:30,204 This eternal inflation means that 812 00:40:30,228 --> 00:40:32,907 new universes are popping into existence all the time, 813 00:40:32,931 --> 00:40:35,251 but they're completely separated one from the other. 814 00:40:35,333 --> 00:40:39,580 Many of my colleagues hate parallel universes. 815 00:40:39,604 --> 00:40:42,183 They just don't like the idea that our universe is 816 00:40:42,207 --> 00:40:45,041 so big and most of it is off limits for us. 817 00:40:45,110 --> 00:40:48,890 If you are willing to be a bit more humble and accept 818 00:40:48,914 --> 00:40:51,347 that the reality might be much, much bigger 819 00:40:51,416 --> 00:40:53,149 than we will ever see, 820 00:40:53,251 --> 00:40:56,853 then parallel universes feel pretty natural. 821 00:40:58,523 --> 00:41:00,301 It's really interesting how everything 822 00:41:00,325 --> 00:41:01,735 in the universe is tied together. 823 00:41:01,759 --> 00:41:05,006 We can start with a simple question like how old is 824 00:41:05,030 --> 00:41:08,631 the universe, and here we are, questioning virtually 825 00:41:08,733 --> 00:41:10,099 everything about the universe. 826 00:41:11,102 --> 00:41:12,180 Cosmology's 827 00:41:12,204 --> 00:41:15,383 century-long search for the age of the universe 828 00:41:15,407 --> 00:41:18,842 forces us to question our cosmological model, 829 00:41:18,944 --> 00:41:24,047 the nature of gravity, and even time itself. 830 00:41:24,115 --> 00:41:26,549 The age of the universe does bring up sort of 831 00:41:26,618 --> 00:41:30,720 profound philosophical questions about how 832 00:41:30,822 --> 00:41:32,655 a universe can even start, 833 00:41:32,724 --> 00:41:35,558 how can you create something from nothing? 834 00:41:38,063 --> 00:41:39,707 The vast majority of whatever 835 00:41:39,731 --> 00:41:43,678 the universe is, is eternally hidden to us. 836 00:41:43,702 --> 00:41:46,681 So we answered the questions how big, how old, 837 00:41:46,705 --> 00:41:50,340 and those very answers show us that we don't 838 00:41:50,408 --> 00:41:52,942 even know if we've asked the right questions to begin with.