1 00:00:01,507 --> 00:00:04,942 ROWE: There's a mystery at the very heart of the universe. 2 00:00:05,044 --> 00:00:08,345 We don't know how old the cosmos is. 3 00:00:08,414 --> 00:00:10,347 OLUSEYI: Understanding the age of the universe 4 00:00:10,416 --> 00:00:14,118 is fundamental to understanding the universe at all. 5 00:00:14,120 --> 00:00:16,820 It's at the heart of everything. 6 00:00:16,955 --> 00:00:19,223 ROWE: It's more than just celebrating a birthday. 7 00:00:19,225 --> 00:00:20,858 PLAIT: We want to know how much mass is in it, 8 00:00:20,960 --> 00:00:23,160 how much energy is in it, how it behaves. 9 00:00:23,229 --> 00:00:25,662 We have to have this number nailed down. 10 00:00:25,764 --> 00:00:27,064 MINGARELLI: The age of the universe 11 00:00:27,133 --> 00:00:30,934 enables us to not only understand where we came from, 12 00:00:30,936 --> 00:00:33,704 but potentially, the fate of the universe, 13 00:00:33,806 --> 00:00:37,307 what will happen millions and billions of years from now. 14 00:00:37,309 --> 00:00:40,544 ROWE: But our quest to discover the age of the universe is 15 00:00:40,613 --> 00:00:42,112 starting a war. 16 00:00:42,114 --> 00:00:44,248 SUTTER: Usually Nature just whispers to us. 17 00:00:44,316 --> 00:00:47,217 Now Nature is screaming in our ear 18 00:00:47,219 --> 00:00:50,621 that we're doing something wrong, and that's exciting. 19 00:01:02,234 --> 00:01:04,835 ROWE: We think the universe started with a bang. 20 00:01:06,205 --> 00:01:09,440 Everything that has ever existed is squashed up 21 00:01:09,541 --> 00:01:12,042 in this space smaller than a pinhead, 22 00:01:12,111 --> 00:01:16,313 and all of a sudden, space just starts expanding 23 00:01:16,315 --> 00:01:17,748 everywhere at once. 24 00:01:17,817 --> 00:01:20,517 ROWE: The idea that the universe grew from 25 00:01:20,619 --> 00:01:24,721 a ball smaller than a pinhead is hard to understand, 26 00:01:24,824 --> 00:01:26,857 but figuring out when it happened 27 00:01:26,959 --> 00:01:29,726 sounds like it should be more straightforward. 28 00:01:29,828 --> 00:01:31,829 It seems like a simple question right? 29 00:01:31,831 --> 00:01:32,963 But it turns out, getting 30 00:01:33,065 --> 00:01:35,332 the age of the universe is pretty tricky. 31 00:01:35,334 --> 00:01:38,635 ROWE: Scientists have just a single fact 32 00:01:38,637 --> 00:01:40,337 as their starting point -- 33 00:01:40,406 --> 00:01:43,407 the universe is expanding. 34 00:01:43,409 --> 00:01:45,609 When people realized the universe was expanding, 35 00:01:45,611 --> 00:01:46,676 they thought they finally had 36 00:01:46,678 --> 00:01:48,645 a way to estimate the age of the universe. 37 00:01:48,747 --> 00:01:51,949 Take the universe now and run it backwards in time. 38 00:01:52,017 --> 00:01:54,118 Things get closer and closer until they come 39 00:01:54,253 --> 00:01:55,352 to a single point. 40 00:01:55,454 --> 00:01:59,223 That time to that point is the age of the universe. 41 00:01:59,225 --> 00:02:02,693 ROWE: The expansion rate is so important, 42 00:02:02,695 --> 00:02:05,162 it's been given its own name -- 43 00:02:05,231 --> 00:02:06,964 the Hubble constant. 44 00:02:07,066 --> 00:02:10,968 SUTTER: The Hubble constant is the present day 45 00:02:11,036 --> 00:02:12,469 expansion rate of the universe. 46 00:02:12,538 --> 00:02:15,139 It is a key ingredient to understanding 47 00:02:15,207 --> 00:02:20,711 the entire expansion history of our universe and its age. 48 00:02:20,813 --> 00:02:22,813 ROWE: Scientists discovered a strange 49 00:02:22,948 --> 00:02:25,716 radio signal permeating the cosmos. 50 00:02:25,818 --> 00:02:30,721 It's the remnants of ancient light from the early universe. 51 00:02:30,823 --> 00:02:35,192 We call it the cosmic microwave background, or CMB 52 00:02:35,194 --> 00:02:36,627 for short. 53 00:02:36,729 --> 00:02:39,329 The cosmic microwave background radiation is 54 00:02:39,398 --> 00:02:42,432 simply the afterglow of our Big Bang, 55 00:02:42,434 --> 00:02:47,037 the way the universe looked when it was 400,000 years old. 56 00:02:48,307 --> 00:02:52,309 ROWE: The European Space Agency launched the Planck satellite. 57 00:02:52,311 --> 00:02:54,344 Using sensitive radio receivers, 58 00:02:54,446 --> 00:02:57,114 the orbiter studied the sky in every direction, 59 00:02:57,249 --> 00:03:01,118 measuring tiny changes in the temperature and polarization 60 00:03:01,120 --> 00:03:02,619 of the radiation signal. 61 00:03:02,621 --> 00:03:06,256 The CMB has all these variations 62 00:03:06,325 --> 00:03:09,226 in temperature, and they're not randomly generated. 63 00:03:09,328 --> 00:03:11,895 They are there because of physical processes 64 00:03:11,897 --> 00:03:13,463 that occurred when the universe 65 00:03:13,532 --> 00:03:16,166 was in its primordial fireball phase. 66 00:03:16,202 --> 00:03:19,269 ROWE: The red blobs are where matter was hottest, 67 00:03:19,371 --> 00:03:22,206 and the blue areas are where matter was cooler. 68 00:03:22,341 --> 00:03:24,708 The smallest red blobs are where 69 00:03:24,710 --> 00:03:27,945 hot material was packed tightly together. 70 00:03:28,046 --> 00:03:29,813 That's where material in the universe would have 71 00:03:29,815 --> 00:03:32,216 been denser, and that's where galaxies would 72 00:03:32,351 --> 00:03:34,218 preferentially form. 73 00:03:34,353 --> 00:03:39,323 It's so cool to get to look at those blueprints and study them 74 00:03:39,425 --> 00:03:41,725 and see how that baby universe 75 00:03:41,827 --> 00:03:45,529 later grew up into the universe we see around us today. 76 00:03:45,631 --> 00:03:47,331 ROWE: Although it doesn't look like much, 77 00:03:47,333 --> 00:03:50,434 hidden within this picture is almost everything 78 00:03:50,536 --> 00:03:53,070 we can know about the universe. 79 00:03:53,138 --> 00:03:55,205 In a complex process using 80 00:03:55,207 --> 00:03:57,040 different mathematical models, 81 00:03:57,109 --> 00:04:00,744 cosmologists figured out how the ancient cosmos 82 00:04:00,813 --> 00:04:02,246 captured in the CMB 83 00:04:02,347 --> 00:04:05,349 became the universe we see today. 84 00:04:05,450 --> 00:04:07,918 They worked out how the universe got from 85 00:04:07,920 --> 00:04:12,456 small to big and how fast that expansion happened. 86 00:04:13,492 --> 00:04:15,926 PONTZEN: The data from the cosmic microwave background 87 00:04:15,928 --> 00:04:19,630 is absolutely the gold standard for cosmology. 88 00:04:19,632 --> 00:04:23,634 It's beautifully clean, we can understand it really well, 89 00:04:23,736 --> 00:04:25,602 and we have a lot of confidence 90 00:04:25,604 --> 00:04:29,339 that what we learn from it is pretty robust. 91 00:04:29,441 --> 00:04:31,308 ROWE: By running the expansion backwards, 92 00:04:31,410 --> 00:04:33,043 we get an age... 93 00:04:34,513 --> 00:04:38,415 13.82 billion years. 94 00:04:38,550 --> 00:04:39,950 Job finished! 95 00:04:41,620 --> 00:04:44,221 But it's not quite a slam dunk. 96 00:04:44,323 --> 00:04:46,657 The figure must be verified. 97 00:04:46,725 --> 00:04:48,358 We don't make a single measurement 98 00:04:48,427 --> 00:04:49,626 using a single technique. 99 00:04:49,695 --> 00:04:53,530 We make multiple measurements via multiple techniques. 100 00:04:53,632 --> 00:04:55,565 ROWE: Another group of scientists use 101 00:04:55,634 --> 00:04:56,967 a totally different method 102 00:04:57,102 --> 00:04:59,269 to calculate the age of the cosmos, 103 00:04:59,338 --> 00:05:01,738 measuring objects that we can see 104 00:05:01,807 --> 00:05:05,809 in our universe to determine how far away they are and how 105 00:05:05,911 --> 00:05:09,813 fast they're moving away from us as the universe expands. 106 00:05:09,815 --> 00:05:13,050 The most direct and most accurate measurements 107 00:05:13,118 --> 00:05:15,752 are using what is known as parallax. 108 00:05:15,821 --> 00:05:20,357 Parallax is the apparent shift in an object relative 109 00:05:20,459 --> 00:05:22,693 to the background when it's viewed 110 00:05:22,695 --> 00:05:24,061 from two different locations. 111 00:05:24,162 --> 00:05:28,131 So if I look at my thumb with one eye, and then I close it 112 00:05:28,233 --> 00:05:30,967 and look at the other eye, it looks like my thumb moves. 113 00:05:32,638 --> 00:05:34,905 If I move my thumb closer to my face, 114 00:05:34,907 --> 00:05:38,342 then the distance it moves back and forth changes. 115 00:05:38,443 --> 00:05:40,911 It appears to move back and forth more. 116 00:05:40,913 --> 00:05:43,914 That parallax difference as we move the thumb closer 117 00:05:43,916 --> 00:05:45,315 and farther from the face 118 00:05:45,317 --> 00:05:47,951 is the way we measure distances to distant objects. 119 00:05:48,053 --> 00:05:51,154 ROWE: Using parallax, we can measure 120 00:05:51,256 --> 00:05:54,825 the distance to bright stars called cepheids 121 00:05:54,827 --> 00:05:56,259 in the Milky Way. 122 00:05:56,361 --> 00:05:59,596 NANCE: Cepheids are stars that burn 100,000 123 00:05:59,598 --> 00:06:01,231 times brighter than our sun, 124 00:06:01,333 --> 00:06:04,167 so they're extremely bright, and they pulsate, meaning they 125 00:06:04,236 --> 00:06:08,205 get brighter and dimmer over a regular time period. 126 00:06:08,207 --> 00:06:09,940 ROWE: Cepheids that pulsate at 127 00:06:10,042 --> 00:06:13,143 the same rate have the same brightness. 128 00:06:13,212 --> 00:06:15,946 They're known as a standard candle. 129 00:06:16,048 --> 00:06:19,249 A standard candle is something that is a standard, meaning 130 00:06:19,318 --> 00:06:21,852 we know how intrinsically bright it is. 131 00:06:21,987 --> 00:06:23,720 So all we have to do is measure 132 00:06:23,855 --> 00:06:27,023 the brightness that we appear to perceive on Earth, 133 00:06:27,025 --> 00:06:28,658 and then you solve for the distance. 134 00:06:28,727 --> 00:06:32,028 So imagine that you're on the street. 135 00:06:32,030 --> 00:06:33,930 By looking down the street, 136 00:06:33,932 --> 00:06:36,733 you'll see that the street lights get dimmer and dimmer 137 00:06:36,802 --> 00:06:38,235 the farther away they are, 138 00:06:38,237 --> 00:06:40,237 but that's not their intrinsic brightness. 139 00:06:40,305 --> 00:06:42,406 Their intrinsic brightness is the same. 140 00:06:42,541 --> 00:06:46,410 So by seeing how faint the farthest away ones are, 141 00:06:46,545 --> 00:06:50,414 you can understand how far away they are from you. 142 00:06:50,549 --> 00:06:53,417 ROWE: We can use standard candles to measure 143 00:06:53,419 --> 00:06:55,919 the distance to stars farther away. 144 00:06:55,921 --> 00:06:59,623 But there's a big problem -- throughout the universe, 145 00:06:59,625 --> 00:07:03,226 there's a competition between the expansion pushing things 146 00:07:03,361 --> 00:07:08,231 apart and gravity pulling things together. 147 00:07:08,233 --> 00:07:09,199 In the Milky Way, 148 00:07:09,267 --> 00:07:11,868 there's so much matter that gravity wins. 149 00:07:11,970 --> 00:07:14,938 Even looking at galaxies in our neighborhood, 150 00:07:15,007 --> 00:07:17,340 the expansion is tiny, 151 00:07:17,409 --> 00:07:21,545 but at cosmic scales of very different galaxies, 152 00:07:21,613 --> 00:07:25,415 matter is more spread out, and expansion wins, 153 00:07:25,417 --> 00:07:29,219 so we can only measure expansion over massive distances. 154 00:07:30,222 --> 00:07:32,422 The way we start to measure distances to things that 155 00:07:32,524 --> 00:07:34,257 are farther and farther away 156 00:07:34,326 --> 00:07:37,060 is to use something we call the distance ladder. 157 00:07:37,129 --> 00:07:40,864 NANCE: Each category of object that we observe 158 00:07:40,999 --> 00:07:44,134 is on a separate rung of this ladder. 159 00:07:44,236 --> 00:07:49,039 Measuring the distance to one will then inform us how far 160 00:07:49,140 --> 00:07:52,108 away the second rung is and then the third rung. 161 00:07:52,211 --> 00:07:56,813 So each rung depends on the previous rung, and from 162 00:07:56,815 --> 00:08:00,951 stacking these together, we can start to measure things very, 163 00:08:01,053 --> 00:08:02,719 very far away from us. 164 00:08:03,856 --> 00:08:06,523 ROWE: Using parallax to measure cepheid stars in 165 00:08:06,525 --> 00:08:08,658 the Milky Way gives us a benchmark. 166 00:08:08,760 --> 00:08:12,529 We can then use their standard brightness 167 00:08:12,631 --> 00:08:14,664 to measure cepheids in other galaxies. 168 00:08:14,766 --> 00:08:18,735 The next rung is a brighter standard candle called 169 00:08:18,804 --> 00:08:20,937 Type 1A supernovas. 170 00:08:21,039 --> 00:08:23,940 They can be seen in galaxies farther away. 171 00:08:24,042 --> 00:08:27,310 Finally, we can measure light from distant elliptical 172 00:08:27,412 --> 00:08:31,348 galaxies, and by looking at how red the light is, 173 00:08:31,449 --> 00:08:35,552 we can work out how fast they're moving away from us. 174 00:08:35,621 --> 00:08:39,222 So those three things give us the nearby universe, 175 00:08:39,324 --> 00:08:42,526 the somewhat far away universe, and the very distant universe, 176 00:08:42,528 --> 00:08:43,660 rung by rung. 177 00:08:46,532 --> 00:08:49,065 ROWE: March 2021. 178 00:08:49,167 --> 00:08:52,402 Scientists measure the light from 63 179 00:08:52,538 --> 00:08:54,304 giant elliptical galaxies, 180 00:08:54,306 --> 00:08:57,507 the farthest rung of the distance ladder. 181 00:08:57,609 --> 00:09:00,377 They hope to get the most accurate measurement of 182 00:09:00,379 --> 00:09:04,514 the Hubble constant to date and a precise age 183 00:09:04,583 --> 00:09:05,815 for the universe. 184 00:09:07,252 --> 00:09:09,953 Their calculations make the universe 185 00:09:10,055 --> 00:09:12,956 13.3 billion years old, 186 00:09:12,958 --> 00:09:15,358 not too far away from the figure of 187 00:09:15,460 --> 00:09:17,761 13.82 billion years 188 00:09:17,862 --> 00:09:20,063 given by the cosmic microwave background, 189 00:09:20,131 --> 00:09:22,933 a difference of around 6%. 190 00:09:22,935 --> 00:09:26,803 That sounds trivial, but that equates to hundreds of millions 191 00:09:26,805 --> 00:09:29,439 of years of cosmic history that either happened 192 00:09:29,508 --> 00:09:31,341 or didn't happen. 193 00:09:31,410 --> 00:09:34,110 PLAIT: 50 years ago, when we weren't quite as good 194 00:09:34,112 --> 00:09:35,812 at measuring everything about the universe, 195 00:09:35,814 --> 00:09:36,846 we would have been thrilled to 196 00:09:36,948 --> 00:09:38,949 have our numbers agreeing to this level. 197 00:09:39,051 --> 00:09:41,418 But nowadays, having a difference like this, 198 00:09:41,520 --> 00:09:43,453 it's unacceptable. 199 00:09:43,522 --> 00:09:46,556 ROWE: Clearly, the two techniques do not agree. 200 00:09:46,625 --> 00:09:49,626 Cosmologists split into two camps. 201 00:09:49,728 --> 00:09:52,228 We had hoped that these two methods were like building 202 00:09:52,331 --> 00:09:56,066 a bridge from either side and then meeting in the middle. 203 00:09:56,167 --> 00:09:57,534 But they're not. 204 00:09:57,636 --> 00:09:59,336 Now we know that something is going on 205 00:09:59,438 --> 00:10:01,137 we don't understand. 206 00:10:01,239 --> 00:10:02,505 BULLOCK: Even though these measurements 207 00:10:02,608 --> 00:10:03,873 are roughly the same, 208 00:10:03,942 --> 00:10:06,910 it's really dangerous to just accept them and assume that 209 00:10:06,912 --> 00:10:09,212 everything's fine, because in science, 210 00:10:09,314 --> 00:10:13,149 usually, the initial really big discoveries start off 211 00:10:13,251 --> 00:10:15,352 as small differences, but then you pull 212 00:10:15,420 --> 00:10:18,955 on that thread, and something wonderful emerges. 213 00:10:19,024 --> 00:10:20,957 ROWE: So does a simple question, 214 00:10:21,026 --> 00:10:24,961 how old is the universe, unravel everything? 215 00:10:34,606 --> 00:10:37,307 ROWE: The universe is expanding outwards. 216 00:10:37,309 --> 00:10:41,011 The rate it's growing is called the Hubble constant, 217 00:10:41,013 --> 00:10:44,814 and it's the key to working out the age of the universe. 218 00:10:44,916 --> 00:10:47,217 So the Hubble constant might just seem 219 00:10:47,319 --> 00:10:51,154 like some academic number that doesn't mean anything, 220 00:10:51,256 --> 00:10:55,659 but that number contains information about 221 00:10:55,727 --> 00:10:57,060 the composition, 222 00:10:57,162 --> 00:11:00,430 the evolution, and the fate of the universe. 223 00:11:01,466 --> 00:11:02,832 ROWE: It's an important number, 224 00:11:02,934 --> 00:11:04,501 but there's a problem. 225 00:11:04,503 --> 00:11:08,338 Our best measurement methods don't match. 226 00:11:08,407 --> 00:11:10,507 It's incredibly frustrating to not 227 00:11:10,509 --> 00:11:12,509 know how old the universe is. 228 00:11:12,511 --> 00:11:14,444 It's even more frustrating to know that 229 00:11:14,513 --> 00:11:17,414 there's two experiments, which are excellent experiments 230 00:11:17,516 --> 00:11:19,115 that we firmly believe in, 231 00:11:19,217 --> 00:11:20,884 that completely disagree with each other. 232 00:11:20,886 --> 00:11:24,654 My hair fell out a long time ago over this kind of stuff. 233 00:11:24,756 --> 00:11:27,424 This has been the number-one question 234 00:11:27,526 --> 00:11:29,259 for over half a decade. 235 00:11:30,328 --> 00:11:33,763 ROWE: There must be something wrong with one of the methods. 236 00:11:33,865 --> 00:11:36,232 PONTZEN: There's a definite sense in the community 237 00:11:36,234 --> 00:11:39,135 that whichever camp you happen to fall into, 238 00:11:39,237 --> 00:11:41,805 the problems lie on the other side of the fence. 239 00:11:41,940 --> 00:11:43,840 So if you're mainly working with the cosmic 240 00:11:43,942 --> 00:11:46,309 microwave background, you probably think 241 00:11:46,411 --> 00:11:49,212 something is up with the distance ladder. 242 00:11:49,347 --> 00:11:51,748 ROWE: If there's a problem with the distance ladder, 243 00:11:51,850 --> 00:11:53,817 there's a prime suspect. 244 00:11:53,885 --> 00:11:57,053 The ladder relies on stars that have a predictable 245 00:11:57,122 --> 00:11:59,923 brightness called standard candles. 246 00:11:59,925 --> 00:12:04,561 But there's evidence that these stars are not always 247 00:12:04,662 --> 00:12:05,962 the same brightness. 248 00:12:06,064 --> 00:12:10,266 So if you expect an object to have a particular brightness, 249 00:12:10,302 --> 00:12:12,068 and it has a different brightness, 250 00:12:12,170 --> 00:12:15,905 then whatever conclusion you draw that relies on 251 00:12:15,907 --> 00:12:17,107 the brightness of that object is 252 00:12:17,242 --> 00:12:18,908 gonna be off somewhat. 253 00:12:18,910 --> 00:12:21,644 Think of the stars like streetlights. 254 00:12:21,746 --> 00:12:24,948 If one light is broken and dimmer than the others, 255 00:12:25,050 --> 00:12:27,650 you might think it's farther away. 256 00:12:27,752 --> 00:12:30,320 PONTZEN: The concern with the distance ladder is that 257 00:12:30,421 --> 00:12:33,723 if any of the single rungs is not perfect, 258 00:12:33,725 --> 00:12:36,059 then the entire ladder might be out of whack 259 00:12:36,161 --> 00:12:38,061 by the time you get to the top. 260 00:12:38,163 --> 00:12:40,363 ROWE: What we need is a fresh approach 261 00:12:40,465 --> 00:12:42,665 to measuring the age of the universe. 262 00:12:42,734 --> 00:12:45,635 SUTTER: We're hoping we could bring in a tie breaker, 263 00:12:45,737 --> 00:12:47,837 a referee, a brand new method 264 00:12:47,939 --> 00:12:50,306 that didn't care about any of this or any 265 00:12:50,308 --> 00:12:54,511 of that, and tell us what is the Hubble constant. 266 00:12:54,613 --> 00:12:57,313 ROWE: We may have just found one. 267 00:12:57,415 --> 00:13:01,217 This observatory doesn't have a telescope. 268 00:13:01,319 --> 00:13:03,920 It's hunting for an invisible wave, 269 00:13:03,922 --> 00:13:07,891 a disturbance in spacetime itself, caused 270 00:13:07,893 --> 00:13:12,228 by massive objects accelerating or colliding. 271 00:13:12,230 --> 00:13:14,464 It's known as LIGO. 272 00:13:14,533 --> 00:13:17,033 MINGARELLI: LIGO stands for the Laser 273 00:13:17,035 --> 00:13:20,136 Interferometer Gravitational Wave Observatory, 274 00:13:20,238 --> 00:13:23,840 and it is a ground-based gravitational wave detector. 275 00:13:23,975 --> 00:13:27,544 ROWE: A perfectly stabilized beam of laser light bounces 276 00:13:27,612 --> 00:13:30,947 in a five-mile-long, L-shaped tunnel. 277 00:13:31,049 --> 00:13:34,450 As a gravitational wave passes through the detector, 278 00:13:34,519 --> 00:13:36,286 space stretches, 279 00:13:36,288 --> 00:13:39,722 forcing the light to travel a tiny bit farther. 280 00:13:39,824 --> 00:13:43,960 You're bouncing a laser over an incredible distance 281 00:13:44,062 --> 00:13:47,664 and trying to measure as spacetime itself 282 00:13:47,732 --> 00:13:49,666 gets stretched and deformed 283 00:13:49,734 --> 00:13:52,769 whether that lazar had to travel a tiny bit further 284 00:13:52,804 --> 00:13:53,903 or a tiny bit shorter, 285 00:13:53,905 --> 00:13:57,340 and a tiny bit here is the width of a single atom 286 00:13:57,442 --> 00:13:59,709 over miles and miles of distance. 287 00:14:00,912 --> 00:14:04,414 ROWE: LIGO has already detected colliding black holes, 288 00:14:06,051 --> 00:14:09,719 but it's also received a signal from something 289 00:14:09,721 --> 00:14:11,621 less massive. 290 00:14:11,623 --> 00:14:14,624 Neutron stars are the densest thing in 291 00:14:14,626 --> 00:14:16,759 the universe other than black holes. 292 00:14:16,828 --> 00:14:19,863 They're the last stopping point before you would collapse 293 00:14:19,965 --> 00:14:22,131 all the way to form a black hole. 294 00:14:22,133 --> 00:14:24,834 ROWE: They're the size of Washington, D.C., 295 00:14:24,936 --> 00:14:28,238 but they can have the mass of two suns. 296 00:14:28,306 --> 00:14:32,942 A collision between neutron stars is incredibly powerful. 297 00:14:33,044 --> 00:14:35,845 It's one of the most energetic events in the universe, and it 298 00:14:35,947 --> 00:14:38,815 distorts the fabric of spacetime very strongly, 299 00:14:38,817 --> 00:14:41,351 because their gravity is so strong. 300 00:14:41,386 --> 00:14:43,119 ROWE: But unlike black hole mergers, 301 00:14:43,121 --> 00:14:46,522 neutron star collisions can also send out light. 302 00:14:48,326 --> 00:14:52,161 In 2017, LIGO sent out an alert -- more than 303 00:14:52,230 --> 00:14:57,066 70 telescopes on Earth and in space swung into action. 304 00:14:57,135 --> 00:15:00,336 This binary neutron star merger was the first time 305 00:15:00,438 --> 00:15:02,538 we had witnessed gravitational waves 306 00:15:02,641 --> 00:15:04,507 and light waves coming from the same event. 307 00:15:07,345 --> 00:15:09,612 It was groundbreaking. 308 00:15:09,714 --> 00:15:13,716 ROWE: This event is ideal for Hubble constant hunters. 309 00:15:13,718 --> 00:15:15,718 The light tells us how fast 310 00:15:15,820 --> 00:15:18,588 the colliding stars are moving away from us. 311 00:15:18,590 --> 00:15:22,592 Gravitational waves give us the distance. 312 00:15:22,594 --> 00:15:26,229 If we know how far away it is and how fast it's moving, 313 00:15:26,364 --> 00:15:28,331 that's the Hubble constant. 314 00:15:28,333 --> 00:15:32,435 PONTZEN: Having neutron star mergers added to your arsenal 315 00:15:32,437 --> 00:15:34,137 of ways of measuring 316 00:15:34,238 --> 00:15:36,806 the universe's expansion is great, because it's 317 00:15:36,808 --> 00:15:38,308 completely independent. 318 00:15:38,410 --> 00:15:40,810 It uses physics that's not related to either 319 00:15:40,912 --> 00:15:44,047 of the two competing methods we have so far. 320 00:15:44,115 --> 00:15:45,815 ROWE: Sounds perfect. 321 00:15:45,917 --> 00:15:47,717 The result? 322 00:15:47,852 --> 00:15:49,352 So this brand-new measurement that 323 00:15:49,454 --> 00:15:51,154 were hoping would be a tie breaker... 324 00:15:53,525 --> 00:15:57,060 ended up coming right in between these two extremes. 325 00:15:58,129 --> 00:16:00,830 Thanks for the help. 326 00:16:00,932 --> 00:16:04,233 ROWE: But it might not be as bad as it sounds. 327 00:16:04,336 --> 00:16:07,203 The number of neutron star collisions where 328 00:16:07,205 --> 00:16:11,507 we have detected gravitational waves and light...one. 329 00:16:12,644 --> 00:16:15,044 We shouldn't be at all disheartened by the fact 330 00:16:15,113 --> 00:16:17,413 that this hasn't actually decided 331 00:16:17,415 --> 00:16:20,149 the problem, because there's a huge margin for error 332 00:16:20,251 --> 00:16:22,218 when you have just one object. 333 00:16:22,320 --> 00:16:24,821 We would like something like 100 events 334 00:16:24,956 --> 00:16:27,357 like this neutron star merger. 335 00:16:27,458 --> 00:16:29,959 That might seem like a huge improvement we need, 336 00:16:30,028 --> 00:16:31,427 but actually, it's very feasible that 337 00:16:31,429 --> 00:16:33,830 in the next decade, we'll get there. 338 00:16:33,832 --> 00:16:37,266 ROWE: Gravitational waves may give us a precise age of 339 00:16:37,368 --> 00:16:39,569 the universe, but there is a chance 340 00:16:39,637 --> 00:16:42,939 they'll tell us the problem isn't with our measurements, 341 00:16:43,041 --> 00:16:45,408 but with our understanding of the cosmos. 342 00:16:45,510 --> 00:16:48,244 If we keep getting different answers for the Hubble constant, 343 00:16:48,313 --> 00:16:50,646 especially depending on the method we use, 344 00:16:50,715 --> 00:16:52,515 that's a big clue that we don't 345 00:16:52,517 --> 00:16:54,817 understand something fundamental about 346 00:16:54,919 --> 00:16:56,219 the universe's evolution, 347 00:16:56,320 --> 00:16:58,154 its makeup, something important. 348 00:16:59,624 --> 00:17:02,825 ROWE: Our search for the age of the universe just might 349 00:17:02,827 --> 00:17:06,429 destroy our model of how we think the cosmos works, 350 00:17:07,432 --> 00:17:10,633 plunging physics into chaos. 351 00:17:20,545 --> 00:17:23,112 ROWE: We don't know the age of the universe. 352 00:17:23,214 --> 00:17:26,716 We had hoped that the results from our experiments would be 353 00:17:26,718 --> 00:17:31,120 like building a bridge, starting at opposite ends 354 00:17:31,122 --> 00:17:33,056 and meeting in the middle. 355 00:17:33,157 --> 00:17:34,857 As time goes on, 356 00:17:34,959 --> 00:17:38,127 as the evidence accumulates, 357 00:17:38,129 --> 00:17:40,930 these two sides of the bridge are not gonna meet. 358 00:17:40,932 --> 00:17:44,133 Something has to give. 359 00:17:44,135 --> 00:17:46,235 ROWE: Some believe the problem lies in the way 360 00:17:46,337 --> 00:17:49,906 we've interpreted the picture of the early universe, 361 00:17:49,908 --> 00:17:54,343 the pattern hidden in the cosmic microwave background. 362 00:17:54,412 --> 00:17:56,612 We're really confident in the data that we have from 363 00:17:56,714 --> 00:17:59,215 the CMB, but it's actually an indirect 364 00:17:59,317 --> 00:18:01,250 measurement of the universe's age. 365 00:18:01,319 --> 00:18:04,320 It depends on our model of the universe being right. 366 00:18:04,422 --> 00:18:08,958 It could be, it could very well be that our fundamental 367 00:18:09,060 --> 00:18:11,928 cosmological model that we've used 368 00:18:11,930 --> 00:18:15,832 to successfully describe the universe is coming up short, 369 00:18:15,834 --> 00:18:17,733 that there's something wrong in there, 370 00:18:17,869 --> 00:18:20,536 that that engine is broken. 371 00:18:20,638 --> 00:18:24,941 ROWE: That engine is the standard cosmological model. 372 00:18:25,076 --> 00:18:27,510 Based on our knowledge of particle physics 373 00:18:27,512 --> 00:18:29,212 and general relativity, 374 00:18:29,214 --> 00:18:31,414 it's like an instruction manual 375 00:18:31,416 --> 00:18:33,316 for how the universe works. 376 00:18:33,418 --> 00:18:36,953 Rewriting it is a radical suggestion. 377 00:18:37,054 --> 00:18:40,123 For the most part, it matches what we see, 378 00:18:40,125 --> 00:18:43,326 but it does struggle with one thing. 379 00:18:43,394 --> 00:18:46,829 As the universe expands away from the Big Bang, 380 00:18:46,964 --> 00:18:51,134 the intuitive thing you would expect is for gravity to start 381 00:18:51,235 --> 00:18:52,735 pulling it back together again. 382 00:18:52,737 --> 00:18:55,404 So over time, gravity would just reverse that 383 00:18:55,406 --> 00:19:00,009 and pull everything back in, back to a single point. 384 00:19:00,011 --> 00:19:03,513 But what we see in the data is completely opposite. 385 00:19:03,648 --> 00:19:06,215 What we see is that the universe is not only 386 00:19:06,217 --> 00:19:07,817 continuing to expand, 387 00:19:07,919 --> 00:19:10,920 but it's speeding up faster and faster all the time. 388 00:19:10,922 --> 00:19:13,122 To explain this weird phenomenon, 389 00:19:13,257 --> 00:19:15,258 the cosmological model relies on 390 00:19:15,359 --> 00:19:18,661 the existence of a strange, unknown force -- 391 00:19:18,730 --> 00:19:20,463 dark energy. 392 00:19:20,565 --> 00:19:22,832 Dark energy is the most perplexing 393 00:19:22,934 --> 00:19:25,535 and mysterious thing I've encountered in my research. 394 00:19:25,670 --> 00:19:28,204 PLAIT: Dark energy is a term that we slap 395 00:19:28,206 --> 00:19:30,806 on this idea that the universal expansion 396 00:19:30,808 --> 00:19:33,109 is accelerating. 397 00:19:33,111 --> 00:19:35,244 That's about all we know about it. 398 00:19:35,313 --> 00:19:36,646 We don't know what's causing it. 399 00:19:36,747 --> 00:19:37,813 We don't know how it behaves. 400 00:19:37,815 --> 00:19:39,015 We don't know what it was like in 401 00:19:39,017 --> 00:19:40,650 the past or what it's like in the future. 402 00:19:40,718 --> 00:19:43,819 So we just call it dark energy. 403 00:19:43,955 --> 00:19:47,857 ROWE: It's invisible -- it fills the whole universe 404 00:19:47,959 --> 00:19:50,960 and pushes galaxies apart. 405 00:19:51,029 --> 00:19:53,062 In some sense, it's like a spring, 406 00:19:53,164 --> 00:19:56,332 a contracted spring, and you let it go, and it wants to push 407 00:19:56,334 --> 00:19:58,334 everything away. 408 00:19:58,336 --> 00:19:59,735 ROWE: And things get stranger. 409 00:19:59,837 --> 00:20:04,807 Dark energy doesn't dilute as the universe expands. 410 00:20:04,909 --> 00:20:07,043 HOPKINS: As empty space gets created 411 00:20:07,144 --> 00:20:10,913 or expands, the dark energy associated with that 412 00:20:10,915 --> 00:20:12,014 stays the same. 413 00:20:12,050 --> 00:20:14,617 It basically populates all this empty space. 414 00:20:14,619 --> 00:20:16,352 Imagine I'm draining a bucket of water, 415 00:20:16,421 --> 00:20:19,622 and water just magically appears out of nowhere. 416 00:20:19,624 --> 00:20:21,757 That's like how dark energy behaves 417 00:20:21,826 --> 00:20:23,926 as the universe is expanding. 418 00:20:23,928 --> 00:20:25,795 ROWE: Dark energy plays an important 419 00:20:25,797 --> 00:20:28,431 role in the standard cosmological model. 420 00:20:28,433 --> 00:20:31,367 If our understanding of it is wrong, 421 00:20:31,469 --> 00:20:33,035 then so too is the model, 422 00:20:33,037 --> 00:20:37,406 which means the age of the universe we get from the CMB 423 00:20:37,408 --> 00:20:39,442 is wrong, too. 424 00:20:39,510 --> 00:20:41,644 TEGMARK: Since nobody has a clue what dark energy is, 425 00:20:41,746 --> 00:20:42,945 there are a lot of different theories. 426 00:20:43,047 --> 00:20:46,849 But the biggest question of all is simply, is it constant? 427 00:20:47,952 --> 00:20:51,320 PONTZEN: Our standard assumption about dark energy is that 428 00:20:51,422 --> 00:20:53,256 it's pushing apart the universe 429 00:20:53,357 --> 00:20:55,358 with the same strength throughout the history 430 00:20:55,459 --> 00:20:56,659 of the universe. 431 00:20:57,729 --> 00:21:02,231 ROWE: Now physicists are wondering if that idea is wrong. 432 00:21:02,233 --> 00:21:06,702 Maybe, in the early universe, dark energy acted differently. 433 00:21:06,704 --> 00:21:10,339 Hey, you know that whole dark energy thing that's messing 434 00:21:10,441 --> 00:21:11,807 with the universe today? 435 00:21:11,809 --> 00:21:15,745 Maybe it messed with the universe back then. 436 00:21:15,813 --> 00:21:19,515 THALLER: It could be that dark energy really has affected 437 00:21:19,650 --> 00:21:22,118 the rate of expansion a lot more than we thought. 438 00:21:22,220 --> 00:21:25,521 This is gonna throw a big monkey wrench into our idea of 439 00:21:25,523 --> 00:21:26,889 how old the universe is 440 00:21:26,891 --> 00:21:29,025 and what it was like at different eras. 441 00:21:29,027 --> 00:21:34,230 ROWE: The theory is called new early dark energy. 442 00:21:34,232 --> 00:21:38,034 DE RHAM: So the idea behind new early dark energy is that 443 00:21:38,036 --> 00:21:41,604 dark energy was present during the very early periods 444 00:21:41,606 --> 00:21:44,173 of the universe, but in a very different state. 445 00:21:44,208 --> 00:21:48,210 Just like you can think of water being present in 446 00:21:48,346 --> 00:21:50,313 two states, 447 00:21:50,315 --> 00:21:54,450 it can be liquid water if the environment is quite hot, 448 00:21:54,519 --> 00:21:56,852 or it can be frozen water 449 00:21:56,954 --> 00:21:59,121 if the environment is colder. 450 00:21:59,123 --> 00:22:01,223 PLAIT: We call that a phase change. 451 00:22:01,326 --> 00:22:03,225 Maybe in the early universe, 452 00:22:03,227 --> 00:22:05,728 dark energy underwent a phase change, as well. 453 00:22:05,730 --> 00:22:09,231 It was different before then and acts differently now. 454 00:22:09,233 --> 00:22:13,169 ROWE: According to the theory, this more energetic state of 455 00:22:13,270 --> 00:22:15,538 early dark energy pushed apart 456 00:22:15,606 --> 00:22:18,708 the early universe much faster than we thought. 457 00:22:18,710 --> 00:22:22,712 PONTZEN: So that speeds things up in the opening moments of 458 00:22:22,814 --> 00:22:24,013 our universe, 459 00:22:24,015 --> 00:22:27,616 which starts to actually bring things back into agreement 460 00:22:27,618 --> 00:22:29,819 when you look at interpreting both 461 00:22:29,821 --> 00:22:32,254 the cosmic microwave background and the distance 462 00:22:32,323 --> 00:22:34,023 ladder measurements. 463 00:22:34,025 --> 00:22:36,559 PLAIT: One of the things that we see in the universe 464 00:22:36,627 --> 00:22:39,061 is that things change with time, density changes, 465 00:22:39,163 --> 00:22:40,629 matter changes, energy changes. 466 00:22:40,631 --> 00:22:42,431 Why not dark energy? 467 00:22:42,533 --> 00:22:45,368 ROWE: Adding new early dark energy 468 00:22:45,469 --> 00:22:48,337 to the early universe changes the standard model. 469 00:22:48,406 --> 00:22:52,742 The CMB gives a higher figure for the expansion of 470 00:22:52,810 --> 00:22:54,944 the universe, and finally, 471 00:22:55,045 --> 00:22:58,047 an age that matches the one given by 472 00:22:58,149 --> 00:23:00,616 the distance ladder method. 473 00:23:00,718 --> 00:23:04,320 If you think about that bridge analogy, where the two parts 474 00:23:04,422 --> 00:23:07,623 just don't meet, the early dark energy adjusts 475 00:23:07,625 --> 00:23:11,427 the angle of the early universe part of the bridge, 476 00:23:11,429 --> 00:23:14,930 and it just gets them to actually meet in the middle. 477 00:23:14,932 --> 00:23:19,135 ROWE: It's still controversial, but new dark energy may be 478 00:23:19,237 --> 00:23:22,037 detected in detailed measurements of 479 00:23:22,139 --> 00:23:24,306 the cosmic microwave background. 480 00:23:24,308 --> 00:23:26,709 SUTTER: I mean, in one sense, like, 481 00:23:26,844 --> 00:23:28,511 do we really need to overcomplicate 482 00:23:28,613 --> 00:23:30,413 the universe here? But you know what? 483 00:23:30,415 --> 00:23:34,116 The universe is under no obligation to be simple. 484 00:23:34,118 --> 00:23:38,087 ROWE: But there's one thing physicists can agree on. 485 00:23:38,089 --> 00:23:41,123 Dark energy truly is a can of worms we've just 486 00:23:41,258 --> 00:23:43,959 opened, and there may be some big changes coming up. 487 00:23:44,061 --> 00:23:46,262 ROWE: There is a more radical possibility. 488 00:23:46,330 --> 00:23:51,434 Maybe we need to ditch dark energy altogether and question 489 00:23:51,436 --> 00:23:56,205 one of the most famous theories of all, general relativity. 490 00:23:56,340 --> 00:23:57,606 Is it possible? 491 00:23:57,708 --> 00:24:00,643 Did Einstein make a colossal mistake? 492 00:24:09,754 --> 00:24:12,154 ROWE: In trying to work out the age of the universe, 493 00:24:12,223 --> 00:24:15,558 physicists have started a revolution, 494 00:24:15,659 --> 00:24:18,727 a revolution that could overturn everything we thought 495 00:24:18,829 --> 00:24:21,530 we knew about how the universe works, 496 00:24:21,532 --> 00:24:24,366 including the bedrock of modern physics, 497 00:24:24,468 --> 00:24:26,969 Einstein's theory of gravity, 498 00:24:27,038 --> 00:24:28,804 general relativity. 499 00:24:28,906 --> 00:24:31,407 Underlying everything, 500 00:24:31,542 --> 00:24:34,543 all of cosmology, is general relativity, 501 00:24:34,612 --> 00:24:39,014 but maybe we need a completely new understanding 502 00:24:39,016 --> 00:24:40,850 of gravity. 503 00:24:40,952 --> 00:24:43,953 ROWE: Gravity is a strange force. 504 00:24:44,088 --> 00:24:46,322 It's always attractive. 505 00:24:46,424 --> 00:24:49,458 The Earth pulling on us gives us our weight. 506 00:24:49,560 --> 00:24:53,729 The force of gravity acts over huge distances. 507 00:24:53,731 --> 00:24:57,867 The sun tugs on objects throughout the solar system. 508 00:24:57,969 --> 00:25:00,870 The Milky Way pulls on other galaxies. 509 00:25:00,938 --> 00:25:04,640 PONTZEN: On the one hand, gravity is incredibly familiar 510 00:25:04,742 --> 00:25:07,042 to us, you know, the apple falling from the tree 511 00:25:07,144 --> 00:25:08,644 and all of that stuff, 512 00:25:08,746 --> 00:25:11,347 and we also know that gravity behaves in a very 513 00:25:11,448 --> 00:25:13,816 predictable way throughout our solar system 514 00:25:13,918 --> 00:25:16,619 from all the spacecraft and things we've sent out. 515 00:25:16,621 --> 00:25:20,556 But when it comes to how it behaves on incredibly tiny 516 00:25:20,658 --> 00:25:23,125 scales and also on incredibly 517 00:25:23,227 --> 00:25:25,961 large scales, covering the whole universe, 518 00:25:26,030 --> 00:25:29,064 it's possible that we just don't yet have the right 519 00:25:29,133 --> 00:25:31,634 picture of what's going on. 520 00:25:31,736 --> 00:25:34,069 ROWE: Einstein's model of gravity has remained 521 00:25:34,138 --> 00:25:36,705 largely the same for 100 years. 522 00:25:36,807 --> 00:25:39,742 So much of modern physics is really standing on 523 00:25:39,843 --> 00:25:41,310 Einstein's shoulders, 524 00:25:41,379 --> 00:25:43,412 but at the same time, 525 00:25:43,514 --> 00:25:46,615 we can't ever take anything for granted. 526 00:25:46,617 --> 00:25:50,419 ROWE: Claudia de Rham works on a theory called massive gravity. 527 00:25:50,421 --> 00:25:52,621 It's based on a key part of 528 00:25:52,723 --> 00:25:56,625 Einstein's theory that says gravity doesn't have mass. 529 00:25:56,627 --> 00:26:00,162 Once you understand that general relativity is the theory 530 00:26:00,264 --> 00:26:01,630 of a massless particle, 531 00:26:01,732 --> 00:26:03,332 the immediate response should be, 532 00:26:03,334 --> 00:26:05,701 well, what if it was massive? 533 00:26:05,703 --> 00:26:08,070 ROWE: The theoretical particle that carries 534 00:26:08,138 --> 00:26:10,906 gravity is called the graviton. 535 00:26:10,908 --> 00:26:13,742 If gravitons don't have any weight, 536 00:26:13,778 --> 00:26:16,745 then there's nothing to slow them down as they speed 537 00:26:16,847 --> 00:26:18,113 through the universe. 538 00:26:18,182 --> 00:26:21,216 They can act over infinite distances, 539 00:26:21,218 --> 00:26:23,552 just like photons of light. 540 00:26:23,654 --> 00:26:28,123 So one galaxy on this side of the universe can actually pull 541 00:26:28,225 --> 00:26:31,226 on a galaxy that's right on the other side of the universe. 542 00:26:31,329 --> 00:26:35,064 ROWE: But if gravity has weight, things change. 543 00:26:35,132 --> 00:26:36,398 DE RHAM: In some sense, 544 00:26:36,400 --> 00:26:39,335 if we attach a little backpack to our graviton particle, 545 00:26:39,437 --> 00:26:45,808 its effect is to slowly slow it down just enough so as to 546 00:26:45,943 --> 00:26:49,144 make its effect on very large distances 547 00:26:49,246 --> 00:26:53,749 being a tiny little bit weaker, and that's our way to 548 00:26:53,818 --> 00:26:55,351 switch off the effect of 549 00:26:55,419 --> 00:27:00,022 gravity on huge cosmological distances. 550 00:27:00,024 --> 00:27:04,059 ROWE: If gravity is a little bit weaker, a galaxy on this side 551 00:27:04,128 --> 00:27:05,961 of the universe can't pull on 552 00:27:06,063 --> 00:27:08,330 one on the other side of the cosmos. 553 00:27:08,332 --> 00:27:12,234 It has a huge effect on the expansion of the universe. 554 00:27:13,838 --> 00:27:17,606 PONTZEN: If the force of gravity actually just switches off at 555 00:27:17,708 --> 00:27:19,141 large distances, 556 00:27:19,210 --> 00:27:22,711 then you no longer have to counter the fact that 557 00:27:22,713 --> 00:27:23,878 everything is pulling 558 00:27:23,914 --> 00:27:26,115 everything else together, because it isn't anymore. 559 00:27:26,250 --> 00:27:29,218 So that would quite naturally explain why 560 00:27:29,320 --> 00:27:32,354 the expansion of our universe would be speeding up. 561 00:27:32,423 --> 00:27:38,127 ROWE: This acceleration is what we see in the universe today. 562 00:27:38,262 --> 00:27:42,531 Currently, we use dark energy to explain it. 563 00:27:42,666 --> 00:27:45,167 OLUSEYI: So, if the graviton has mass, 564 00:27:45,236 --> 00:27:48,837 that means that we can get out of the universe 565 00:27:48,939 --> 00:27:51,807 what we see without the need for dark energy. 566 00:27:51,909 --> 00:27:54,410 DE RHAM: What if actually what we were observing 567 00:27:54,412 --> 00:27:58,414 is simply the first sign of gravity 568 00:27:58,416 --> 00:28:01,116 switching off at very large distances. 569 00:28:01,118 --> 00:28:04,420 Maybe we're just observing the first effect of 570 00:28:04,422 --> 00:28:06,422 the graviton having a mass. 571 00:28:06,424 --> 00:28:09,224 ROWE: Without dark energy to deal with, 572 00:28:09,226 --> 00:28:12,461 the universe is a lot easier to explain. 573 00:28:12,529 --> 00:28:15,564 SUTTER: Maybe we don't need these complicated physics. 574 00:28:15,633 --> 00:28:19,134 Maybe it's just all the normal ingredients of the universe, 575 00:28:19,136 --> 00:28:22,504 but operating under a different set of rules. 576 00:28:22,506 --> 00:28:27,009 ROWE: Claudia hopes her theory will soon be put to the test. 577 00:28:27,011 --> 00:28:30,813 Around 2037, 578 00:28:30,815 --> 00:28:33,749 we'll have a new gravitational wave detector, 579 00:28:33,818 --> 00:28:37,920 the Laser Interferometer Space Antenna, or LISA. 580 00:28:39,223 --> 00:28:43,225 It'll be bigger than LIGO and will orbit the Earth. 581 00:28:43,327 --> 00:28:45,127 When LISA get out there in space, 582 00:28:45,262 --> 00:28:47,563 we'll even have a bigger handle on 583 00:28:47,664 --> 00:28:51,033 gravitational waves evolving throughout the whole universe, 584 00:28:51,035 --> 00:28:54,303 and so it will allow us to go very deep in our understanding 585 00:28:54,305 --> 00:28:55,704 of gravity. 586 00:28:55,706 --> 00:29:00,209 ROWE: LISA is a system of three satellites arranged in a giant 587 00:29:00,211 --> 00:29:05,314 triangular formation, 1.5 million miles apart. 588 00:29:06,450 --> 00:29:08,217 It should pick up very low 589 00:29:08,352 --> 00:29:13,422 frequency gravitational waves from more ancient events, 590 00:29:13,424 --> 00:29:17,960 perhaps even shockwaves from the birth of the universe. 591 00:29:18,095 --> 00:29:21,029 If the graviton has mass, 592 00:29:21,031 --> 00:29:24,433 then the waves will arrive more slowly than predicted, 593 00:29:24,435 --> 00:29:28,437 but until we receive those signals, all bets are off. 594 00:29:28,506 --> 00:29:32,040 It's a big deal to propose a difference in gravity, 595 00:29:32,109 --> 00:29:35,144 but then again, we don't know. 596 00:29:35,212 --> 00:29:36,411 OLUSEYI: I'm making no bets. 597 00:29:36,413 --> 00:29:40,315 The universe has proven itself to be so deceptive. 598 00:29:40,417 --> 00:29:43,152 So I'm gonna wait until it tells me what it is. 599 00:29:45,523 --> 00:29:47,756 ROWE: The question of the age of the universe 600 00:29:47,858 --> 00:29:50,159 opens Pandora's box, 601 00:29:50,260 --> 00:29:52,761 and the expansion rate of the universe 602 00:29:52,830 --> 00:29:55,297 holds another secret, 603 00:29:55,299 --> 00:29:59,868 our ultimate fate -- how the universe will end. 604 00:30:11,215 --> 00:30:13,515 ROWE: We know exactly how the Earth will end. 605 00:30:15,719 --> 00:30:18,720 In around 5.4 billion years, 606 00:30:18,823 --> 00:30:21,123 the sun will turn into a red giant, 607 00:30:21,125 --> 00:30:24,459 expanding to 1,000 times its current size. 608 00:30:26,330 --> 00:30:28,063 The Earth will be destroyed. 609 00:30:30,034 --> 00:30:32,267 Humans, if we still exist, 610 00:30:32,303 --> 00:30:34,937 will have long deserted our home planet. 611 00:30:37,208 --> 00:30:39,408 But how will the universe end? 612 00:30:40,945 --> 00:30:43,412 MINGARELLI: The age of the universe enables us to 613 00:30:43,414 --> 00:30:46,114 not only understand where we came from, 614 00:30:46,217 --> 00:30:48,951 but potentially, the fate of the universe. 615 00:30:48,953 --> 00:30:52,221 What will happen millions and billions of years from now? 616 00:30:53,224 --> 00:30:55,357 ROWE: If scientists confirm the value 617 00:30:55,426 --> 00:30:56,725 of the Hubble constant, 618 00:30:56,827 --> 00:31:00,028 the elusive figure that tells us just how fast the universe 619 00:31:00,030 --> 00:31:02,364 is expanding, it will tell us the age of 620 00:31:02,466 --> 00:31:06,568 the universe, and it will help us predict its end. 621 00:31:06,637 --> 00:31:08,604 SUTTER: Measuring the Hubble constant is 622 00:31:08,606 --> 00:31:10,939 measuring the expansion rate today, 623 00:31:11,041 --> 00:31:14,610 right now, it's like checking your speedometer at one moment. 624 00:31:14,745 --> 00:31:16,712 But just because it's your speed now, 625 00:31:16,714 --> 00:31:19,114 it doesn't mean it was the same speed when you left 626 00:31:19,116 --> 00:31:21,917 your home or the same speed when you'll be on the freeway. 627 00:31:21,919 --> 00:31:25,220 ROWE: How the expansion changes over time 628 00:31:25,322 --> 00:31:27,522 will control the fate of the cosmos. 629 00:31:27,625 --> 00:31:29,358 MINGARELLI: So depending on the Hubble constant, 630 00:31:29,426 --> 00:31:31,727 the universe could continue to expand. 631 00:31:31,829 --> 00:31:35,330 It could accelerate its expansion rate, 632 00:31:35,332 --> 00:31:37,733 or it could be decelerating. 633 00:31:38,836 --> 00:31:41,837 ROWE: At the moment, galaxies are racing apart. 634 00:31:43,140 --> 00:31:45,841 A continually expanding universe will 635 00:31:45,943 --> 00:31:49,011 cool down as it spreads out. 636 00:31:49,013 --> 00:31:53,315 Another name for this eternal expansion is the Big Freeze, 637 00:31:53,450 --> 00:31:56,118 because as everything gets spread out, 638 00:31:56,253 --> 00:31:59,621 the density is lower, and there's no more opportunities 639 00:31:59,723 --> 00:32:01,323 for temperature differences. 640 00:32:01,325 --> 00:32:04,626 Everything just gets colder and colder and colder and colder, 641 00:32:04,728 --> 00:32:09,631 slowly, eternally approaching absolute zero. 642 00:32:09,633 --> 00:32:11,967 ROWE: The more matter is spread out, 643 00:32:12,036 --> 00:32:14,770 the less chance there is for star formation. 644 00:32:14,805 --> 00:32:18,507 And so the universe's continued expansion means our 645 00:32:18,509 --> 00:32:23,612 night sky and every night sky in the universe will inevitably 646 00:32:23,614 --> 00:32:26,415 continue to get darker and darker and darker as things 647 00:32:26,417 --> 00:32:29,551 move further away and as stars die off. 648 00:32:29,620 --> 00:32:33,755 SUTTER: Eventually, all the stars will go out, 649 00:32:33,857 --> 00:32:36,425 and there'll just be the leftovers, 650 00:32:36,427 --> 00:32:40,128 which we call the degenerates, black holes, 651 00:32:40,130 --> 00:32:42,364 white dwarfs, rogue planets. 652 00:32:42,433 --> 00:32:46,034 It's gonna be a very, very sad place. 653 00:32:46,036 --> 00:32:48,303 ROWE: The last refuge of any matter at all 654 00:32:48,305 --> 00:32:50,305 will be black holes. 655 00:32:50,307 --> 00:32:52,741 You've got a big black hole in the middle of each galaxy, 656 00:32:52,843 --> 00:32:56,712 over trillions of years, everything in galaxies fall in, 657 00:32:56,714 --> 00:33:00,215 so finally, you're left with big black holes over vast 658 00:33:00,217 --> 00:33:01,950 distances, separated 659 00:33:02,052 --> 00:33:04,519 almost universes away. 660 00:33:04,521 --> 00:33:07,956 MINGARELLI: So getting towards the Big Freeze, 661 00:33:08,058 --> 00:33:10,959 black holes themselves start to evaporate. 662 00:33:11,061 --> 00:33:14,463 There won't even be black holes at the end of this 663 00:33:14,564 --> 00:33:16,298 accelerating universe. 664 00:33:16,300 --> 00:33:18,734 PLAIT: All that's left is very, 665 00:33:18,869 --> 00:33:22,304 very low energy photons and a little bit of matter dispersed 666 00:33:22,306 --> 00:33:25,140 throughout the universe, and there's nothing left. 667 00:33:25,242 --> 00:33:26,441 That's it. 668 00:33:26,443 --> 00:33:29,111 THALLER: We call that the heat death of the universe. 669 00:33:29,113 --> 00:33:31,813 There's no longer any place that has more energy 670 00:33:31,815 --> 00:33:36,151 or more heat -- it's all just thin, barely there photons. 671 00:33:36,220 --> 00:33:38,520 It's fascinating scientifically, 672 00:33:38,622 --> 00:33:40,422 but from a human standpoint, 673 00:33:40,524 --> 00:33:42,824 not a lot of fun to think about. 674 00:33:42,826 --> 00:33:44,393 ROWE: But if the Hubble constant, 675 00:33:44,395 --> 00:33:48,430 the expansion rate of the universe, keeps increasing, 676 00:33:48,432 --> 00:33:52,534 then the end of the universe could be a lot scarier 677 00:33:52,636 --> 00:33:54,636 and come a lot sooner. 678 00:33:57,041 --> 00:33:59,708 One possibility is that the expansion 679 00:33:59,843 --> 00:34:01,610 of the universe will accelerate 680 00:34:01,712 --> 00:34:04,012 and continue to accelerate forever, 681 00:34:04,014 --> 00:34:05,714 faster and faster and faster. 682 00:34:05,849 --> 00:34:08,950 And if that happens, we face a scenario that we call 683 00:34:09,052 --> 00:34:11,253 the Big Rip, where actually the whole 684 00:34:11,321 --> 00:34:14,222 of space essentially just gets ripped to shreds. 685 00:34:15,826 --> 00:34:18,126 So the solar system is gonna get ripped apart, 686 00:34:18,261 --> 00:34:21,563 then the sun and the planets themselves will start to get 687 00:34:21,598 --> 00:34:22,930 ripped apart. 688 00:34:22,966 --> 00:34:26,168 And finally, it works its way down to atoms, and atoms get 689 00:34:26,269 --> 00:34:28,370 ripped apart, and we're starting to see effects on 690 00:34:28,405 --> 00:34:29,805 space and time. 691 00:34:31,041 --> 00:34:33,241 Space is ripped apart. 692 00:34:33,343 --> 00:34:34,676 Time comes to a stop. 693 00:34:37,047 --> 00:34:41,416 NANCE: So in this scenario, time and space have no meaning. 694 00:34:41,518 --> 00:34:44,419 If everything is infinitely far apart, 695 00:34:44,421 --> 00:34:48,523 then space doesn't really exist. 696 00:34:48,525 --> 00:34:51,526 It's sort of beyond our comprehension. 697 00:34:51,628 --> 00:34:54,229 ROWE: Working out the expansion rate will 698 00:34:54,298 --> 00:34:57,432 tell us which scenario we face, 699 00:34:57,434 --> 00:35:01,336 but for now, the lifespan of the universe is unknown. 700 00:35:02,539 --> 00:35:07,509 Maybe we need to investigate the other end of the timeline. 701 00:35:07,511 --> 00:35:09,611 But how can we get a fix on 702 00:35:09,613 --> 00:35:13,048 the age of the universe without understanding 703 00:35:13,117 --> 00:35:14,549 its origin? 704 00:35:14,618 --> 00:35:17,319 BULLOCK: As you go back in time towards the Big Bang, 705 00:35:17,321 --> 00:35:20,722 our knowledge of physics really goes out the window. 706 00:35:21,959 --> 00:35:24,626 PLAIT: Temperatures off the scale, pressure off the scale -- 707 00:35:24,728 --> 00:35:27,963 the way everything behaved is just so different that 708 00:35:28,064 --> 00:35:30,332 the rules we have now do not apply. 709 00:35:31,401 --> 00:35:33,368 ROWE: The biggest problem of all -- 710 00:35:33,437 --> 00:35:37,105 what came just before the Big Bang? 711 00:35:37,207 --> 00:35:40,942 Einstein's general relativity predicts that all the matter 712 00:35:41,044 --> 00:35:42,744 and energy in the universe 713 00:35:42,846 --> 00:35:45,914 was concentrated down to a single point, 714 00:35:45,916 --> 00:35:47,182 the Singularity. 715 00:35:47,184 --> 00:35:50,152 The Singularity is like the part 716 00:35:50,220 --> 00:35:53,722 of those old maps that says, "Here be dragons." 717 00:35:53,824 --> 00:35:57,425 Singularities are a problem. We don't like them. 718 00:35:57,427 --> 00:36:00,228 This is where basically you have a finite amount of matter 719 00:36:00,230 --> 00:36:02,130 in the universe, but it's squeezed down 720 00:36:02,132 --> 00:36:05,233 into zero volume, so it would be infinitely dense. 721 00:36:06,436 --> 00:36:09,304 SUTTER: Infinite densities don't actually happen in nature. 722 00:36:09,439 --> 00:36:12,541 This is a sign that our math is breaking down. 723 00:36:12,543 --> 00:36:13,808 This is a sign that we need 724 00:36:13,810 --> 00:36:17,512 to replace that with a new understanding. 725 00:36:18,815 --> 00:36:21,416 ROWE: Many now believe Einstein was wrong. 726 00:36:21,518 --> 00:36:25,053 There was no Singularity begging the question, 727 00:36:25,155 --> 00:36:28,557 could the age of the universe be infinite? 728 00:36:36,433 --> 00:36:38,567 ROWE: Scientists investigating the age of 729 00:36:38,635 --> 00:36:42,137 the universe are struggling to understand its origins. 730 00:36:43,207 --> 00:36:46,808 Could that be because there was no beginning? 731 00:36:46,910 --> 00:36:50,111 Could the universe be infinite? 732 00:36:50,214 --> 00:36:52,147 Because we think we live and we die, 733 00:36:52,216 --> 00:36:54,049 we project that onto the universe. 734 00:36:54,084 --> 00:36:56,218 But that may not be the case. 735 00:36:56,353 --> 00:36:58,787 STRAUGHN: The idea of an infinite universe is 736 00:36:58,789 --> 00:37:01,323 no more strange than the idea of a singularity. 737 00:37:01,325 --> 00:37:03,959 And in fact, throughout most of history, 738 00:37:04,060 --> 00:37:06,061 astronomers thought that the universe was 739 00:37:06,129 --> 00:37:08,530 probably infinite. 740 00:37:08,632 --> 00:37:10,532 ROWE: The foundation of our mathematical 741 00:37:10,634 --> 00:37:12,067 understanding of the universe, 742 00:37:12,135 --> 00:37:15,637 Einstein's general relativity, has a problem. 743 00:37:15,706 --> 00:37:18,940 It doesn't translate to the world of the very tiny, 744 00:37:19,042 --> 00:37:23,411 which is why its laws break down close to the Big Bang. 745 00:37:23,413 --> 00:37:27,015 General relativity does a great job at describing things 746 00:37:27,017 --> 00:37:28,450 on scales that you and I are 747 00:37:28,518 --> 00:37:31,586 familiar with and things like how planets move 748 00:37:31,588 --> 00:37:34,055 and how galaxies evolve, all the big stuff. 749 00:37:34,124 --> 00:37:37,626 Quantum mechanics, on the other hand, describes the world of 750 00:37:37,728 --> 00:37:40,729 the very small, the world of the atoms. 751 00:37:40,831 --> 00:37:43,231 The problem is that these two theories 752 00:37:43,333 --> 00:37:45,467 don't fit well together at all. 753 00:37:46,536 --> 00:37:50,038 ROWE: A new theory known as loop quantum gravity, 754 00:37:50,140 --> 00:37:54,109 brings quantum theory and relativity together, 755 00:37:54,111 --> 00:37:56,945 and it makes a stunning prediction. 756 00:37:57,047 --> 00:38:01,650 PONTZEN: So one possibility is that the end of the universe 757 00:38:01,751 --> 00:38:04,853 could kind of match onto the beginning of a new universe 758 00:38:04,955 --> 00:38:07,255 and create a cycle of universes, 759 00:38:07,357 --> 00:38:09,024 one after the other. 760 00:38:09,026 --> 00:38:10,959 ROWE: Nicknamed the Big Bounce, 761 00:38:11,061 --> 00:38:12,827 it predicts a universe that 762 00:38:12,829 --> 00:38:16,831 stops expanding and switches into reverse. 763 00:38:16,833 --> 00:38:19,968 OLUSEYI: And the idea here is that the universe can expand for 764 00:38:20,070 --> 00:38:22,070 a time, stop expanding, 765 00:38:22,138 --> 00:38:23,605 and then begin to contract again. 766 00:38:23,607 --> 00:38:26,041 And some have suggested that perhaps 767 00:38:26,142 --> 00:38:28,810 there's a cycle of expanding and compressing. 768 00:38:28,812 --> 00:38:31,012 It bounces back over again. 769 00:38:31,014 --> 00:38:33,948 One of the appeals of the bouncing model is 770 00:38:34,050 --> 00:38:36,851 that it allows us to get beyond the Singularity. 771 00:38:36,953 --> 00:38:39,321 ROWE: A bit like recycling on Earth. 772 00:38:39,423 --> 00:38:43,758 All the components get crushed down and then reused, 773 00:38:43,827 --> 00:38:47,529 giving the cosmos no beginning and no end. 774 00:38:47,664 --> 00:38:49,831 If the universe is cyclic, 775 00:38:49,933 --> 00:38:51,833 does the age even have a meaning? 776 00:38:51,935 --> 00:38:54,869 STRAUGHN: Age is a construct of humanity, 777 00:38:54,971 --> 00:38:56,438 because we need to count time. 778 00:38:56,506 --> 00:38:58,106 But if the universe is infinite, 779 00:38:58,108 --> 00:39:01,042 maybe it doesn't matter in the big scheme of things. 780 00:39:01,144 --> 00:39:03,545 ROWE: A contracting and expanding universe 781 00:39:03,646 --> 00:39:06,514 messes with the concept of age. 782 00:39:06,616 --> 00:39:07,949 But the very idea of 783 00:39:08,051 --> 00:39:12,120 an expanding universe provides another cosmic curveball. 784 00:39:12,122 --> 00:39:14,456 It might not be alone -- it might 785 00:39:14,524 --> 00:39:18,927 be just one ageless universe among many. 786 00:39:18,929 --> 00:39:23,331 It's an idea embedded in the math of the Big Bang. 787 00:39:23,333 --> 00:39:26,334 The most popular theory we have in astrophysics, 788 00:39:26,436 --> 00:39:30,004 what put the bang into our Big Bang, is inflation. 789 00:39:30,006 --> 00:39:32,107 This idea that there was a kind of dark energy 790 00:39:32,109 --> 00:39:34,709 on steroids that made our universe double over and over 791 00:39:34,711 --> 00:39:36,811 not every seven billion years, 792 00:39:36,813 --> 00:39:39,714 but every split second, 793 00:39:39,716 --> 00:39:43,451 creating out of almost nothing, a big bang. 794 00:39:43,553 --> 00:39:47,622 When the universe was just a hundredth of a billionth 795 00:39:47,624 --> 00:39:51,059 of a trillionth of a trillionth of a second old, 796 00:39:51,161 --> 00:39:54,662 it underwent a period of rapid expansion called inflation. 797 00:39:54,731 --> 00:39:59,067 It doubled in size at least 90 times, going from 798 00:39:59,169 --> 00:40:02,303 the size of a subatomic particle to that of 799 00:40:02,305 --> 00:40:03,638 a golf ball. 800 00:40:03,740 --> 00:40:06,608 TEGMARK: The problem with this inflation is that 801 00:40:06,610 --> 00:40:07,742 it doesn't really stop. 802 00:40:07,844 --> 00:40:10,745 It just makes this ever bigger space and says that, 803 00:40:10,814 --> 00:40:13,448 yeah, well, okay, there was one region of space where 804 00:40:13,517 --> 00:40:16,618 this crazy doubling stopped and galaxies formed, 805 00:40:16,620 --> 00:40:17,918 and that's us. 806 00:40:17,954 --> 00:40:20,522 But there's this vast realm out there where inflation is 807 00:40:20,624 --> 00:40:22,056 still happening. 808 00:40:22,158 --> 00:40:24,726 ROWE: In the spots where inflation stops, 809 00:40:24,828 --> 00:40:27,262 parallel universes form. 810 00:40:27,330 --> 00:40:30,231 This eternal inflation means that 811 00:40:30,233 --> 00:40:32,934 new universes are popping into existence all the time, 812 00:40:32,936 --> 00:40:35,236 but they're completely separated one from the other. 813 00:40:35,338 --> 00:40:39,607 TEGMARK: Many of my colleagues hate parallel universes. 814 00:40:39,609 --> 00:40:42,210 They just don't like the idea that our universe is 815 00:40:42,212 --> 00:40:45,046 so big and most of it is off limits for us. 816 00:40:45,115 --> 00:40:48,917 If you are willing to be a bit more humble and accept 817 00:40:48,919 --> 00:40:51,352 that the reality might be much, much bigger 818 00:40:51,421 --> 00:40:53,154 than we will ever see, 819 00:40:53,256 --> 00:40:56,858 then parallel universes feel pretty natural. 820 00:40:58,528 --> 00:41:00,228 OLUSEYI: It's really interesting how everything 821 00:41:00,330 --> 00:41:01,663 in the universe is tied together. 822 00:41:01,764 --> 00:41:05,033 We can start with a simple question like how old is 823 00:41:05,035 --> 00:41:08,636 the universe, and here we are, questioning virtually 824 00:41:08,738 --> 00:41:10,104 everything about the universe. 825 00:41:11,107 --> 00:41:12,207 ROWE: Cosmology's 826 00:41:12,209 --> 00:41:15,410 century-long search for the age of the universe 827 00:41:15,412 --> 00:41:18,847 forces us to question our cosmological model, 828 00:41:18,949 --> 00:41:24,052 the nature of gravity, and even time itself. 829 00:41:24,120 --> 00:41:26,554 The age of the universe does bring up sort of 830 00:41:26,623 --> 00:41:30,725 profound philosophical questions about how 831 00:41:30,827 --> 00:41:32,660 a universe can even start, 832 00:41:32,729 --> 00:41:35,563 how can you create something from nothing? 833 00:41:38,068 --> 00:41:39,734 THALLER: The vast majority of whatever 834 00:41:39,736 --> 00:41:43,705 the universe is, is eternally hidden to us. 835 00:41:43,707 --> 00:41:46,708 So we answered the questions how big, how old, 836 00:41:46,710 --> 00:41:50,345 and those very answers show us that we don't 837 00:41:50,413 --> 00:41:52,947 even know if we've asked the right questions to begin with.