1 00:00:04,104 --> 00:00:06,372 Narrator: Islands have edges. 2 00:00:08,075 --> 00:00:10,776 Planets have edges. 3 00:00:12,913 --> 00:00:16,982 Even galaxies have edges. 4 00:00:16,984 --> 00:00:20,386 But what about the universe? 5 00:00:20,388 --> 00:00:25,224 As explorers, as curious humans that we are, 6 00:00:25,226 --> 00:00:27,659 we're obsessed with boundaries and limits. 7 00:00:27,661 --> 00:00:28,961 And we want to know, 8 00:00:28,963 --> 00:00:33,899 "does the whole thing, the universe, have a limit?" 9 00:00:33,901 --> 00:00:38,871 does the universe have an edge? Well, the answer is yes and no. 10 00:00:38,873 --> 00:00:42,007 It depends on what you mean by edge. 11 00:00:42,009 --> 00:00:44,743 Narrator: The edge of what we can see? 12 00:00:44,745 --> 00:00:49,048 The edge of where we can go? 13 00:00:49,050 --> 00:00:52,051 Or the edge of reality itself? 14 00:00:53,988 --> 00:00:55,821 Thaller: Looking out to the edge of the universe 15 00:00:55,823 --> 00:00:58,157 is tremendously important to understand 16 00:00:58,159 --> 00:01:00,659 our place in the universe itself. 17 00:01:00,661 --> 00:01:02,127 Bullock: We're talking about our universe. 18 00:01:02,129 --> 00:01:04,530 We're talking about the thing that we exist within. 19 00:01:04,532 --> 00:01:06,165 The most fundamental thing there is, 20 00:01:06,167 --> 00:01:08,400 we're driven to understand it. 21 00:01:08,402 --> 00:01:12,438 There is always a desire to push the knowledge to the edge. 22 00:01:12,440 --> 00:01:15,741 Narrator: So, can we ever find 23 00:01:15,743 --> 00:01:17,843 the edge of the universe? 24 00:01:17,845 --> 00:01:20,512 -- Captions by vitac -- www.Vitac.Com 25 00:01:20,514 --> 00:01:23,582 captions paid for by discovery communications 26 00:01:28,322 --> 00:01:33,559 ♪ 27 00:01:33,561 --> 00:01:36,295 2016, the hubble space telescope 28 00:01:36,297 --> 00:01:39,098 turned toward a dark patch of sky 29 00:01:39,100 --> 00:01:42,034 in the constellation ursa major. 30 00:01:45,739 --> 00:01:51,443 It captured an image of an indistinct blob of light. 31 00:01:51,445 --> 00:01:56,682 The glow is from a distant galaxy called gn-z11... 32 00:01:56,684 --> 00:02:02,020 ♪ 33 00:02:02,022 --> 00:02:05,357 ...The most distant galaxy we've ever observed. 34 00:02:08,129 --> 00:02:12,664 But is this the edge of the universe? 35 00:02:12,666 --> 00:02:16,034 Thaller: The universe all around us is filled with galaxies, 36 00:02:16,036 --> 00:02:19,404 so it's kind of natural to say, "would there be a final galaxy? 37 00:02:19,406 --> 00:02:21,306 If you traveled far enough away, 38 00:02:21,308 --> 00:02:24,243 would you finally be at the very last galaxy in the universe 39 00:02:24,245 --> 00:02:25,811 looking out into empty space?" 40 00:02:28,115 --> 00:02:30,482 narrator: That's a difficult question to answer. 41 00:02:33,254 --> 00:02:36,889 Because there's a limit to how far we can see. 42 00:02:36,891 --> 00:02:40,792 It all comes down to the speed of light 43 00:02:40,794 --> 00:02:44,596 and the age of the universe. 44 00:02:44,598 --> 00:02:47,766 The key to understanding the edge of the universe 45 00:02:47,768 --> 00:02:51,403 is that light travels very, very fast, 46 00:02:51,405 --> 00:02:53,338 but not infinitely fast. 47 00:02:53,340 --> 00:02:55,641 It takes time for it to get from one place 48 00:02:55,643 --> 00:02:57,509 in the universe to the other. 49 00:02:57,511 --> 00:02:59,645 You open the curtains, light fills the room. 50 00:02:59,647 --> 00:03:01,747 It doesn't seem to travel at all. 51 00:03:01,749 --> 00:03:03,749 But over the vast distances of the universe, 52 00:03:03,751 --> 00:03:07,352 you actually notice this travel time. 53 00:03:07,354 --> 00:03:09,922 Even the sun 93 million miles away, 54 00:03:09,924 --> 00:03:12,624 the light takes eight minutes to get to us. 55 00:03:12,626 --> 00:03:15,060 When you look out at the stars, we start to think of distance 56 00:03:15,062 --> 00:03:18,197 in terms of light-years because it takes years 57 00:03:18,199 --> 00:03:20,699 for the light to get from those stars to us. 58 00:03:20,701 --> 00:03:23,001 Then when you look at galaxies, then you're talking about 59 00:03:23,003 --> 00:03:25,871 millions or billions of light-years. 60 00:03:28,075 --> 00:03:32,878 Narrator: When we look at the light from galaxy gn-z11, 61 00:03:32,880 --> 00:03:37,749 we're seeing light emitted 13.4 billion years ago. 62 00:03:42,556 --> 00:03:44,289 You can't really even find a galaxy 63 00:03:44,291 --> 00:03:45,924 too much farther away than that 64 00:03:45,926 --> 00:03:49,761 because the universe is only 13.8 billion years old, 65 00:03:49,763 --> 00:03:52,931 and it takes a certain amount of time for galaxies to even form. 66 00:03:52,933 --> 00:03:55,133 So we're not going to find too many more galaxies 67 00:03:55,135 --> 00:03:56,802 farther away than this. 68 00:03:56,804 --> 00:03:59,671 If things are far enough away, there is no way that light 69 00:03:59,673 --> 00:04:02,341 can get to us in the age of the universe. 70 00:04:02,343 --> 00:04:04,543 What this means is there's a hard limit 71 00:04:04,545 --> 00:04:07,312 to the edge of the universe that we can see, 72 00:04:07,314 --> 00:04:11,950 and this is set by the age of the universe. 73 00:04:11,952 --> 00:04:14,519 Narrator: Gn-z11 sparked into life 74 00:04:14,521 --> 00:04:17,422 early in the history of the universe, 75 00:04:17,424 --> 00:04:21,627 just 400 million years after the big bang. 76 00:04:21,629 --> 00:04:26,765 Before that, there were no stars to send out light. 77 00:04:26,767 --> 00:04:28,834 Thaller: If you look in any direction at all, 78 00:04:28,836 --> 00:04:31,303 you get all the way back to when there were no stars, 79 00:04:31,305 --> 00:04:34,906 no galaxies, nothing but very, very hot gas, 80 00:04:34,908 --> 00:04:39,311 and that sort of forms a shell around us. 81 00:04:39,313 --> 00:04:44,616 Narrator: That outer shell is the cosmic microwave background. 82 00:04:44,618 --> 00:04:48,587 It is the oldest light in the universe, 83 00:04:48,589 --> 00:04:51,623 the echo of the birth of the universe, 84 00:04:51,625 --> 00:04:54,293 the big bang. 85 00:04:54,295 --> 00:04:55,661 The edge of our universe, 86 00:04:55,663 --> 00:04:58,297 the very furthest thing that we can see, 87 00:04:58,299 --> 00:04:59,965 is one of the earliest relics 88 00:04:59,967 --> 00:05:01,933 of the formation of the universe itself. 89 00:05:01,935 --> 00:05:05,504 That is the cosmic microwave background. 90 00:05:05,506 --> 00:05:11,376 Narrator: We call this the edge of our observable universe. 91 00:05:11,378 --> 00:05:14,279 So we have an observable universe, but beyond that, 92 00:05:14,281 --> 00:05:15,480 even if there are things out there, 93 00:05:15,482 --> 00:05:17,349 there's no way we can see them 94 00:05:17,351 --> 00:05:20,585 because the light just could not have gotten to us by now. 95 00:05:25,192 --> 00:05:27,926 Narrator: As the name states, the observable universe 96 00:05:27,928 --> 00:05:32,364 is simply the part of the universe we can see. 97 00:05:32,366 --> 00:05:34,099 We can think of the observable universe 98 00:05:34,101 --> 00:05:35,634 sort of like a spotlight 99 00:05:35,636 --> 00:05:38,603 centered on wherever you're standing right now 100 00:05:38,605 --> 00:05:42,007 and you can see to the edge of your spotlight and not beyond. 101 00:05:44,478 --> 00:05:46,745 But if you move a little bit to the left, 102 00:05:46,747 --> 00:05:47,979 a little bit to the right, 103 00:05:47,981 --> 00:05:50,882 your observable universe actually moves with you. 104 00:05:54,722 --> 00:05:57,356 Narrator: For someone living in galaxy gn-z11, 105 00:05:57,358 --> 00:06:02,227 a totally different part of the universe would be observable. 106 00:06:02,229 --> 00:06:07,966 So that distant galaxy is at the edge of our observable universe, 107 00:06:07,968 --> 00:06:13,405 and we are at the edge of their observable universe. 108 00:06:13,407 --> 00:06:15,207 We have different spotlights. 109 00:06:15,209 --> 00:06:16,841 One of the wonderful things to think about 110 00:06:16,843 --> 00:06:19,378 is that there are other spheres around other galaxies, 111 00:06:19,380 --> 00:06:22,214 there are other aliens looking up into the sky tonight 112 00:06:22,216 --> 00:06:24,983 wondering what the true extent of the universe is. 113 00:06:27,221 --> 00:06:29,354 Narrator: The true extent of our universe 114 00:06:29,356 --> 00:06:32,657 doesn't end with galaxy gn-z11. 115 00:06:35,796 --> 00:06:39,030 But when astronomers use the hubble space telescope 116 00:06:39,032 --> 00:06:44,069 to accurately measure the distance to gn-z11, 117 00:06:44,071 --> 00:06:46,505 they find something shocking. 118 00:06:48,609 --> 00:06:53,044 It's 32 billion light-years away, 119 00:06:53,046 --> 00:06:55,914 three times further than thought possible. 120 00:06:57,684 --> 00:07:00,152 Plait: So if nothing can travel faster than light 121 00:07:00,154 --> 00:07:03,355 and we measure the distance to this galaxy, 122 00:07:03,357 --> 00:07:06,358 how can it be 32 billion light-years away? 123 00:07:08,462 --> 00:07:10,762 Narrator: There hasn't been enough time 124 00:07:10,764 --> 00:07:12,364 in the history of the universe 125 00:07:12,366 --> 00:07:15,567 for light from gn-z11 to reach us. 126 00:07:15,569 --> 00:07:19,604 There must be some mistake here. Right? 127 00:07:19,606 --> 00:07:21,072 Plait: At this point, your brain 128 00:07:21,074 --> 00:07:22,674 is probably thinking of leaping out 129 00:07:22,676 --> 00:07:24,776 of your skull and running around screaming. 130 00:07:24,778 --> 00:07:27,612 Trust me, I know. I'm an astronomer. 131 00:07:27,614 --> 00:07:29,247 I've been doing this my whole life, 132 00:07:29,249 --> 00:07:32,117 and this stuff twists my imagination up. 133 00:07:32,119 --> 00:07:34,686 It's really hard to grasp this. 134 00:07:36,623 --> 00:07:40,091 How do we see a galaxy that's 32 billion light-years away 135 00:07:40,093 --> 00:07:43,261 and only 13.4 billion years old? 136 00:07:46,300 --> 00:07:51,036 Narrator: Gn-z11 is further away than it should be 137 00:07:51,038 --> 00:07:55,373 because something strange is going on with our universe. 138 00:07:55,375 --> 00:07:57,242 It's expanding. 139 00:07:57,244 --> 00:07:59,845 And if the universe is expanding, 140 00:07:59,847 --> 00:08:03,381 then where does its edge lie 141 00:08:03,383 --> 00:08:06,184 and can we ever reach it? 142 00:08:15,629 --> 00:08:24,035 ♪ 143 00:08:24,037 --> 00:08:26,271 narrator: 13.8 billion years ago, 144 00:08:26,273 --> 00:08:29,174 a speck of energy burst into life. 145 00:08:32,679 --> 00:08:35,247 We call it the big bang -- 146 00:08:35,249 --> 00:08:39,251 space and time pushed out in all directions. 147 00:08:39,253 --> 00:08:42,687 Ever since, our universe has expanded. 148 00:08:44,925 --> 00:08:48,627 But the way it's expanding makes finding an edge 149 00:08:48,629 --> 00:08:51,930 a major challenge. 150 00:08:51,932 --> 00:08:53,632 The universe is expanding 151 00:08:53,634 --> 00:08:56,301 and expands according to a very simple law 152 00:08:56,303 --> 00:08:58,837 that the farther away a galaxy is from us, 153 00:08:58,839 --> 00:09:01,606 the faster it appears to be receding away from us. 154 00:09:03,977 --> 00:09:08,013 Narrator: The furthest galaxies are moving at very high speeds. 155 00:09:08,015 --> 00:09:12,817 The most distant galaxy we've ever spotted, gn-z11, 156 00:09:12,819 --> 00:09:17,856 seems to have moved 32 billion light-years away from us 157 00:09:17,858 --> 00:09:22,294 in just 13.4 billion years. 158 00:09:22,296 --> 00:09:26,031 That's faster than the speed of light. 159 00:09:26,033 --> 00:09:28,667 We can measure the speeds with which galaxies 160 00:09:28,669 --> 00:09:30,135 are moving away from us, 161 00:09:30,137 --> 00:09:32,504 and many, many galaxies are moving away from us 162 00:09:32,506 --> 00:09:35,440 at speeds faster than the speed of light. 163 00:09:35,442 --> 00:09:37,509 This sounds like it's breaking the law, right? 164 00:09:37,511 --> 00:09:39,778 There's this idea that you've all been told 165 00:09:39,780 --> 00:09:41,546 that relativity says nothing goes faster 166 00:09:41,548 --> 00:09:42,814 than the speed of light. 167 00:09:42,816 --> 00:09:46,017 Okay, you've been lied to. 168 00:09:46,019 --> 00:09:49,020 Plait: Space itself can do what it wants. 169 00:09:49,022 --> 00:09:52,624 It makes the rules, it can break the rules. 170 00:09:52,626 --> 00:09:55,594 That rule applies to matter, not to space itself. 171 00:09:55,596 --> 00:09:59,297 Space can expand at whatever rate it wants. 172 00:09:59,299 --> 00:10:02,200 Simple way to think of this expansion law is imagine 173 00:10:02,202 --> 00:10:04,669 standing on an infinite rubber sheet 174 00:10:04,671 --> 00:10:06,838 that stretches all the way out into the distance 175 00:10:06,840 --> 00:10:08,373 and you're standing on the same place. 176 00:10:08,375 --> 00:10:10,475 You can mark it with a little "x." 177 00:10:10,477 --> 00:10:14,446 now, all the sheet expands in every direction. 178 00:10:14,448 --> 00:10:16,848 So if it expands by a factor of two, 179 00:10:16,850 --> 00:10:19,751 another galaxy that was, say, one foot away from you 180 00:10:19,753 --> 00:10:22,954 is now two feet away from you as we stretch the sheet, 181 00:10:22,956 --> 00:10:25,590 but another galaxy was 10 feet away from you. 182 00:10:25,592 --> 00:10:27,192 Expand that by a factor of 2 183 00:10:27,194 --> 00:10:29,361 and now it's 20 feet away from you. 184 00:10:29,363 --> 00:10:33,231 So in the same amount of time, one galaxy moved one foot, 185 00:10:33,233 --> 00:10:36,034 where another galaxy moved 10 feet. 186 00:10:36,036 --> 00:10:37,869 So the more stuff there is, 187 00:10:37,871 --> 00:10:40,538 the more elastic between you and another galaxy, 188 00:10:40,540 --> 00:10:43,074 the more it seems to expand away from you. 189 00:10:45,545 --> 00:10:48,480 Narrator: Expansion means our observable universe 190 00:10:48,482 --> 00:10:52,951 stretches for a colossal 46 billion light-years 191 00:10:52,953 --> 00:10:57,255 in all directions, 92 billion light-years across... 192 00:10:59,760 --> 00:11:02,327 ...And getting bigger by the second. 193 00:11:06,566 --> 00:11:09,567 Bullock: This number is so incomprehensibly large 194 00:11:09,569 --> 00:11:12,971 that it's difficult to wrap your brain around. 195 00:11:12,973 --> 00:11:16,675 There are trillions of galaxies within this volume. 196 00:11:16,677 --> 00:11:18,143 It's staggering. 197 00:11:18,145 --> 00:11:22,180 It's so much larger than anything we're familiar with. 198 00:11:22,182 --> 00:11:23,748 Narrator: If we were to travel 199 00:11:23,750 --> 00:11:27,018 to the edge of the observable universe, 200 00:11:27,020 --> 00:11:31,589 we would enter even more unfamiliar territory. 201 00:11:31,591 --> 00:11:34,793 Imagine we're in an ultra-fast spaceship. 202 00:11:34,795 --> 00:11:37,929 We leave the solar system, then the milky way. 203 00:11:40,400 --> 00:11:43,668 As we travel deeper into intergalactic space, 204 00:11:43,670 --> 00:11:46,171 things start to get really weird. 205 00:11:49,676 --> 00:11:53,845 For every million light-years we go from the milky way, 206 00:11:53,847 --> 00:11:56,648 the galaxies move away from us 207 00:11:56,650 --> 00:12:00,518 at around 13 miles per second faster. 208 00:12:02,889 --> 00:12:05,890 We have to accelerate just to keep up. 209 00:12:05,892 --> 00:12:10,829 But the galaxies keep on moving, always beyond our reach. 210 00:12:10,831 --> 00:12:20,305 ♪ 211 00:12:20,307 --> 00:12:24,776 ♪ 212 00:12:24,778 --> 00:12:27,145 plait: Imagine you're a sprinter on a racetrack. 213 00:12:27,147 --> 00:12:28,646 If you're running towards the finish line, 214 00:12:28,648 --> 00:12:30,882 it may take you a few seconds to cross it. 215 00:12:30,884 --> 00:12:34,452 But now imagine that that finish line is moving away from you. 216 00:12:34,454 --> 00:12:36,821 If it's moving away from you at the same speed you're running, 217 00:12:36,823 --> 00:12:38,189 you'll never reach it. 218 00:12:38,191 --> 00:12:41,292 And if it's moving faster than the runner, 219 00:12:41,294 --> 00:12:43,528 then even faster runners won't reach it. 220 00:12:43,530 --> 00:12:47,732 And that's sort of what we're seeing here with the universe. 221 00:12:47,734 --> 00:12:49,334 Narrator: Beyond a certain distance, 222 00:12:49,336 --> 00:12:54,506 galaxies are racing away from us faster than the speed of light. 223 00:12:54,508 --> 00:12:58,109 It's a line called the cosmic event horizon. 224 00:13:00,347 --> 00:13:05,483 And 97% of galaxies we see in the observable universe 225 00:13:05,485 --> 00:13:08,853 are beyond this line and unreachable, 226 00:13:08,855 --> 00:13:11,556 including gn-z11. 227 00:13:13,126 --> 00:13:14,926 They're sort of teasing us to say, "look at me, 228 00:13:14,928 --> 00:13:16,494 what a nice piece of real estate." 229 00:13:16,496 --> 00:13:19,631 but we know even if we started going there now, 230 00:13:19,633 --> 00:13:21,266 we could never reach them. 231 00:13:24,004 --> 00:13:26,738 Narrator: Anything that has crossed the cosmic event horizon 232 00:13:26,740 --> 00:13:29,374 is out of our reach forever. 233 00:13:29,376 --> 00:13:31,075 But that's not the full picture 234 00:13:31,077 --> 00:13:34,546 because the expansion rate of the universe is changing. 235 00:13:36,483 --> 00:13:39,217 A little over 20 years ago, astronomers discovered 236 00:13:39,219 --> 00:13:42,320 that the current rate of the universe's expansion 237 00:13:42,322 --> 00:13:44,589 is accelerating, it's speeding up. 238 00:13:46,626 --> 00:13:51,396 Narrator: Astronomers suspect a mysterious force is at work -- 239 00:13:51,398 --> 00:13:54,432 dark energy. 240 00:13:54,434 --> 00:13:58,269 Dark energy is what we think is pushing the universe apart, 241 00:13:58,271 --> 00:14:01,406 causing this accelerating expansion. 242 00:14:01,408 --> 00:14:05,610 And the origin and true physical nature of dark energy 243 00:14:05,612 --> 00:14:07,245 is a big mystery. 244 00:14:09,683 --> 00:14:12,750 Narrator: Thanks to dark energy, more and more galaxies 245 00:14:12,752 --> 00:14:15,119 are crossing the cosmic event horizon 246 00:14:15,121 --> 00:14:17,655 and leaving the observable universe. 247 00:14:20,427 --> 00:14:22,327 These galaxies... 248 00:14:22,329 --> 00:14:24,796 Are lost to us forever. 249 00:14:27,467 --> 00:14:30,301 Plait: There are galaxies that we can see today 250 00:14:30,303 --> 00:14:33,805 that in a few million years, say, we won't be able to see 251 00:14:33,807 --> 00:14:35,673 because the edge of the observable universe 252 00:14:35,675 --> 00:14:39,077 has basically moved in closer than that galaxy. 253 00:14:39,079 --> 00:14:40,812 That's going to happen all the time. 254 00:14:40,814 --> 00:14:43,381 And in a trillion years or something like that, 255 00:14:43,383 --> 00:14:45,450 all these galaxies that we see in our sky 256 00:14:45,452 --> 00:14:46,985 will be completely invisible 257 00:14:46,987 --> 00:14:50,054 because they'll be beyond the edge of the universe. 258 00:14:53,126 --> 00:14:55,927 Thaller: So eventually, every last galaxy 259 00:14:55,929 --> 00:14:57,729 will be so far away from us 260 00:14:57,731 --> 00:15:01,332 that light cannot reach us through that expanding space. 261 00:15:01,334 --> 00:15:04,102 It's almost as if you're driving through a dark desert 262 00:15:04,104 --> 00:15:06,838 in your car and the very, very last town 263 00:15:06,840 --> 00:15:08,706 that ever exists has gone over the horizon 264 00:15:08,708 --> 00:15:10,975 and they'll never be any light again. 265 00:15:14,347 --> 00:15:16,814 We can see less and less of the universe 266 00:15:16,816 --> 00:15:18,983 as we go into the future. 267 00:15:18,985 --> 00:15:21,019 What a strange thought. 268 00:15:21,021 --> 00:15:25,189 So that means we should build all the telescopes we can now. 269 00:15:27,227 --> 00:15:29,894 Narrator: There's a limit to the universe we can see, 270 00:15:29,896 --> 00:15:33,731 even with the most advanced telescopes. 271 00:15:33,733 --> 00:15:36,234 But what lies beyond 272 00:15:36,236 --> 00:15:39,137 is one of the biggest mysteries in astronomy. 273 00:15:41,241 --> 00:15:44,509 The greater universe could be stranger 274 00:15:44,511 --> 00:15:46,611 than our wildest imagination. 275 00:15:48,982 --> 00:15:50,181 Plait: When you stand on the beach 276 00:15:50,183 --> 00:15:51,416 and you look at the horizon 277 00:15:51,418 --> 00:15:53,818 and you kind of think, "oh, what beautiful lands 278 00:15:53,820 --> 00:15:55,286 are there beyond the horizon? 279 00:15:55,288 --> 00:15:58,589 Things I've never imagined before." 280 00:15:58,591 --> 00:15:59,891 it's so natural. 281 00:15:59,893 --> 00:16:03,294 It's so human to ask, "what lies beyond that? 282 00:16:03,296 --> 00:16:05,830 What is the true extent of the universe?" 283 00:16:15,608 --> 00:16:21,145 ♪ 284 00:16:21,147 --> 00:16:23,147 narrator: The observable universe 285 00:16:23,149 --> 00:16:25,350 contains trillions of galaxies. 286 00:16:27,921 --> 00:16:31,122 It's about 92 billion light-years across... 287 00:16:33,360 --> 00:16:34,892 ...But astronomers believe 288 00:16:34,894 --> 00:16:37,762 this isn't the full extent of the universe. 289 00:16:41,001 --> 00:16:44,268 What we don't know is how much of the universe 290 00:16:44,270 --> 00:16:46,270 is our observable universe. 291 00:16:46,272 --> 00:16:49,307 It could be a tiny, microscopic speck of this 292 00:16:49,309 --> 00:16:51,776 much more vast universe. 293 00:16:51,778 --> 00:16:53,845 We just don't know. 294 00:16:53,847 --> 00:16:56,114 Carroll: We have no idea how much stuff there is 295 00:16:56,116 --> 00:16:57,582 outside the observable universe, 296 00:16:57,584 --> 00:17:01,419 but because by definition, it's outside the observable universe, 297 00:17:01,421 --> 00:17:04,222 we really don't know right now. 298 00:17:04,224 --> 00:17:06,290 Narrator: So what is out there? 299 00:17:06,292 --> 00:17:10,695 One theory says that space outside the observable universe 300 00:17:10,697 --> 00:17:16,134 is pretty much the same as our own cosmic neighborhood. 301 00:17:16,136 --> 00:17:18,636 It's just more universe. It's just like here. 302 00:17:18,638 --> 00:17:21,239 It's just far enough away that we can't see it. 303 00:17:21,241 --> 00:17:24,709 So it's not like there's bizarre places where time runs backwards 304 00:17:24,711 --> 00:17:27,078 or aliens have two heads, well, yeah, maybe. 305 00:17:29,549 --> 00:17:30,948 Narrator: But further out 306 00:17:30,950 --> 00:17:33,451 in the deepest parts of the greater universe, 307 00:17:33,453 --> 00:17:35,386 all bets are off. 308 00:17:37,657 --> 00:17:40,558 We expect that as you go sort of twice or three times 309 00:17:40,560 --> 00:17:42,060 beyond the observable universe, 310 00:17:42,062 --> 00:17:44,929 it's probably very similar to the universe we inhabit. 311 00:17:44,931 --> 00:17:48,433 But if you go a thousand times or a million times farther, 312 00:17:48,435 --> 00:17:50,134 who knows what you might see? 313 00:17:52,172 --> 00:17:55,206 It might be very, very different if we go far enough away. 314 00:17:57,844 --> 00:17:59,777 Narrator: Strangely, it all comes back 315 00:17:59,779 --> 00:18:02,213 to the expansion of the universe 316 00:18:02,215 --> 00:18:05,383 and one crucial detail in that process. 317 00:18:08,054 --> 00:18:09,153 There was a brief moment 318 00:18:09,155 --> 00:18:11,289 in the very early history of the universe 319 00:18:11,291 --> 00:18:15,326 where its expansion accelerated hugely. 320 00:18:15,328 --> 00:18:17,395 This acceleration is called inflation, 321 00:18:17,397 --> 00:18:20,832 and in a brief moment, the universe itself expanded 322 00:18:20,834 --> 00:18:23,534 at multiple times the speed of light. 323 00:18:25,805 --> 00:18:29,440 Narrator: Inflation was a formative moment 324 00:18:29,442 --> 00:18:31,008 for our universe. 325 00:18:31,010 --> 00:18:32,477 By the time it stopped, 326 00:18:32,479 --> 00:18:35,513 the universe's basic characteristics were set. 327 00:18:37,917 --> 00:18:39,851 There are these fundamental constants 328 00:18:39,853 --> 00:18:42,587 that describe the phenomena in our universe, 329 00:18:42,589 --> 00:18:46,491 the fundamentals of matter and light and space-time. 330 00:18:48,128 --> 00:18:49,794 Narrator: But some scientists think 331 00:18:49,796 --> 00:18:52,396 there could be regions of the greater universe 332 00:18:52,398 --> 00:18:54,866 where inflation has never stopped. 333 00:18:57,137 --> 00:19:02,240 The idea is the greater universe is expanding at an insane speed, 334 00:19:02,242 --> 00:19:05,409 but here and there, occasionally a little region 335 00:19:05,411 --> 00:19:10,748 will stop inflating and just expand at the normal rate. 336 00:19:10,750 --> 00:19:12,750 Inflation can end somewhere 337 00:19:12,752 --> 00:19:15,019 and that gives rise to the universe we live in, 338 00:19:15,021 --> 00:19:18,956 while inflation continues somewhere else. 339 00:19:18,958 --> 00:19:21,058 Narrator: Parts of the greater universe 340 00:19:21,060 --> 00:19:22,360 that continued to inflate 341 00:19:22,362 --> 00:19:25,630 would be left with different laws of physics. 342 00:19:27,267 --> 00:19:29,333 This incredibly violent inflation process 343 00:19:29,335 --> 00:19:33,371 is actually monkeyed with the very fabric of space itself, 344 00:19:33,373 --> 00:19:35,273 so that a lot of the things that we were taught 345 00:19:35,275 --> 00:19:39,977 that are laws of physics are different there. 346 00:19:39,979 --> 00:19:41,279 Carroll: So in an essence, 347 00:19:41,281 --> 00:19:43,181 inflation gives us a very natural way 348 00:19:43,183 --> 00:19:46,083 to make this patchwork quilt of different parts of the universe 349 00:19:46,085 --> 00:19:48,953 where things seem different. 350 00:19:48,955 --> 00:19:52,089 So what we could imagine is a super large-scale structure 351 00:19:52,091 --> 00:19:55,660 where there's different regions of the universe, domains, 352 00:19:55,662 --> 00:19:58,963 and each domain has different local laws of physics. 353 00:20:01,935 --> 00:20:03,467 Narrator: These different parts of the universe 354 00:20:03,469 --> 00:20:07,538 are separated by frontiers called domain walls. 355 00:20:11,344 --> 00:20:14,178 We have similar boundaries on earth. 356 00:20:14,180 --> 00:20:17,014 Whenever you have something that can be in many different states, 357 00:20:17,016 --> 00:20:19,884 you can end up with domain wall. 358 00:20:19,886 --> 00:20:25,523 If our fish swimming around in the arctic near an iceberg, 359 00:20:25,525 --> 00:20:27,225 there will be a domain boundary 360 00:20:27,227 --> 00:20:29,493 between the water being in the liquid state 361 00:20:29,495 --> 00:20:33,297 where I am and the solid state inside the ice. 362 00:20:33,299 --> 00:20:36,867 So a domain wall is just a wall between two domains. 363 00:20:36,869 --> 00:20:38,836 If it's water, this could be ice, 364 00:20:38,838 --> 00:20:39,904 this could be liquid. 365 00:20:39,906 --> 00:20:41,339 If you're talking about space, 366 00:20:41,341 --> 00:20:43,908 this could be a kind of space maybe you can live in. 367 00:20:43,910 --> 00:20:47,712 This could be a kind of space where you don't want to be. 368 00:20:47,714 --> 00:20:51,616 Narrator: Crossing a domain wall would be very bad news 369 00:20:51,618 --> 00:20:54,385 for anyone who dared to try. 370 00:20:54,387 --> 00:20:57,021 Cross that domain wall and the laws of physics 371 00:20:57,023 --> 00:20:59,323 could change dramatically. 372 00:20:59,325 --> 00:21:01,325 The number of dimensions could change. 373 00:21:01,327 --> 00:21:03,928 If we were somehow able to travel to places in the universe 374 00:21:03,930 --> 00:21:05,329 where the laws of physics are different, 375 00:21:05,331 --> 00:21:07,698 we would die 376 00:21:07,700 --> 00:21:10,501 because all of the chemistry going on in our bodies 377 00:21:10,503 --> 00:21:15,106 depends very, very sensitively on the laws of physics. 378 00:21:15,108 --> 00:21:19,043 So you could just dissipate like thanos snap and you're gone. 379 00:21:21,047 --> 00:21:22,747 Narrator: Domain walls might be the closest 380 00:21:22,749 --> 00:21:25,383 we get to locating an edge in the universe. 381 00:21:26,953 --> 00:21:29,320 Depends on how you define the edge. 382 00:21:29,322 --> 00:21:32,690 If it is the realm where the laws of our universe operate, 383 00:21:32,692 --> 00:21:36,627 then these domain walls are in essence the age of the universe. 384 00:21:40,767 --> 00:21:42,700 Narrator: But this is all just theory. 385 00:21:46,572 --> 00:21:48,406 If we ever really are to work out 386 00:21:48,408 --> 00:21:50,875 what the true size and shape of the universe is, 387 00:21:50,877 --> 00:21:54,545 we're going to have to look for clues that are close to us. 388 00:21:54,547 --> 00:21:58,049 Narrator: Clues that could answer the ultimate question, 389 00:21:58,051 --> 00:22:01,952 how big is the rest of the greater universe 390 00:22:01,954 --> 00:22:04,822 and could it go on forever? 391 00:22:13,533 --> 00:22:20,404 ♪ 392 00:22:20,406 --> 00:22:22,807 narrator: For tens of thousands of years, 393 00:22:22,809 --> 00:22:28,145 mankind has gazed in wonder at the vastness of the cosmos, 394 00:22:28,147 --> 00:22:32,483 but just how extensive is it? 395 00:22:32,485 --> 00:22:34,185 If we could answer that question, 396 00:22:34,187 --> 00:22:39,357 it might help us to understand our place in the universe. 397 00:22:39,359 --> 00:22:41,692 One of the fundamental questions in science 398 00:22:41,694 --> 00:22:44,995 is how big is the universe? 399 00:22:44,997 --> 00:22:47,732 O'dowd: To answer the question, "how big is the universe?" 400 00:22:47,734 --> 00:22:51,068 we have to answer the question, "what shape is the universe?" 401 00:22:51,070 --> 00:22:53,003 and by shape, I mean geometry. 402 00:22:53,005 --> 00:22:57,775 I mean, how is the universe curved on its largest scales? 403 00:22:57,777 --> 00:23:00,177 Sutter: If we are to discover that the universe 404 00:23:00,179 --> 00:23:04,215 does have some sort of geometric curvature, 405 00:23:04,217 --> 00:23:09,754 then this might imply that it wraps around in on itself 406 00:23:09,756 --> 00:23:12,390 over incredibly large distances. 407 00:23:12,392 --> 00:23:16,127 And that if you could travel in one direction long enough, 408 00:23:16,129 --> 00:23:18,129 you would end up at your starting point. 409 00:23:18,131 --> 00:23:19,797 Another version is that the universe 410 00:23:19,799 --> 00:23:22,633 is more like an infinite flat plane. 411 00:23:22,635 --> 00:23:25,136 Okay? No curvature at all. 412 00:23:25,138 --> 00:23:27,671 The further you travel, well, the further you get 413 00:23:27,673 --> 00:23:30,541 and you never get back to where you started. 414 00:23:32,779 --> 00:23:34,378 Narrator: To work out the shape of something, 415 00:23:34,380 --> 00:23:37,481 we would normally just step back and take a look. 416 00:23:37,483 --> 00:23:42,753 But clearly moving outside the universe is a nonstarter. 417 00:23:42,755 --> 00:23:44,155 Bullock: You can't jump on a rocket 418 00:23:44,157 --> 00:23:47,391 and fly a thousand times larger than our cosmic horizon 419 00:23:47,393 --> 00:23:49,226 and see what the shape of the universe is. 420 00:23:49,228 --> 00:23:51,128 We just can't do that. 421 00:23:51,130 --> 00:23:54,165 Thaller: Our human perspective on the larger universe 422 00:23:54,167 --> 00:23:55,466 is so limited. 423 00:23:55,468 --> 00:23:57,601 So if we want to figure out what the larger shape 424 00:23:57,603 --> 00:23:59,603 and scale of the universe is, 425 00:23:59,605 --> 00:24:01,939 we're going to have to be very clever indeed. 426 00:24:04,777 --> 00:24:06,577 Narrator: One way to be clever 427 00:24:06,579 --> 00:24:08,979 is to think of the geometry of the universe 428 00:24:08,981 --> 00:24:11,749 in its simplest terms. 429 00:24:11,751 --> 00:24:14,084 Bullock: When we talk about the geometry of the universe, 430 00:24:14,086 --> 00:24:16,620 we really are talking about geometry. 431 00:24:16,622 --> 00:24:19,790 In order to do geometry, you have to take measures. 432 00:24:19,792 --> 00:24:21,992 You need a cosmic ruler to do this, 433 00:24:21,994 --> 00:24:24,395 and it turns out there's a great cosmic ruler 434 00:24:24,397 --> 00:24:27,231 known as baryon acoustic oscillations. 435 00:24:29,669 --> 00:24:32,436 Narrator: Baryonic acoustic oscillations are ripples 436 00:24:32,438 --> 00:24:36,173 in the cosmic microwave background, 437 00:24:36,175 --> 00:24:38,642 the oldest light in the universe. 438 00:24:40,947 --> 00:24:42,680 As the universe expanded, 439 00:24:42,682 --> 00:24:47,284 these ripples were imprinted in space in a uniform way. 440 00:24:48,754 --> 00:24:50,654 They provide a cosmic ruler 441 00:24:50,656 --> 00:24:53,858 to measure vast distances over time, 442 00:24:53,860 --> 00:24:56,193 so we can gauge if the universe expands 443 00:24:56,195 --> 00:24:59,897 in curved space or over a flat plane. 444 00:25:02,802 --> 00:25:04,835 Bullock: When we use these cosmic rulers 445 00:25:04,837 --> 00:25:07,104 to try to back out the shape of the universe, 446 00:25:07,106 --> 00:25:09,340 we're sure to a few percent accuracy 447 00:25:09,342 --> 00:25:12,943 that the universe is flat. 448 00:25:12,945 --> 00:25:15,079 Narrator: If the universe is flat, 449 00:25:15,081 --> 00:25:17,848 we could set off traveling into the cosmos 450 00:25:17,850 --> 00:25:20,551 and continue traveling forever. 451 00:25:22,655 --> 00:25:26,357 There may be no edge to our universe 452 00:25:26,359 --> 00:25:30,928 because a flat universe can be an infinite universe. 453 00:25:30,930 --> 00:25:32,963 Now we're thinking of the universe as something 454 00:25:32,965 --> 00:25:34,832 that really does go on forever, 455 00:25:34,834 --> 00:25:38,869 that the stars and galaxies never have an end, 456 00:25:38,871 --> 00:25:42,973 and how can something truly infinite really exist? 457 00:25:42,975 --> 00:25:47,711 Infinity is weird because it's a concept of, 458 00:25:47,713 --> 00:25:50,047 because it's endless. 459 00:25:50,049 --> 00:25:52,783 What does that mean? Who knows? I don't know. 460 00:25:55,488 --> 00:25:59,056 Plait: Infinity is a concept more than anything else. 461 00:25:59,058 --> 00:26:01,025 Our brains aren't evolved for that. 462 00:26:01,027 --> 00:26:03,794 We evolved living in the plains. 463 00:26:03,796 --> 00:26:06,430 We were apes looking for food. 464 00:26:06,432 --> 00:26:09,433 We weren't evolved to think about the universe 465 00:26:09,435 --> 00:26:12,169 and all of this stuff. 466 00:26:12,171 --> 00:26:14,572 Oluseyi: I just can't stop contemplating this stuff. 467 00:26:14,574 --> 00:26:17,474 The idea of infinity and these large numbers 468 00:26:17,476 --> 00:26:19,510 and even the tininess of everything. 469 00:26:19,512 --> 00:26:20,978 It's nuts. 470 00:26:20,980 --> 00:26:24,114 Yeah, thinking about infinity makes my head hurt a little bit. 471 00:26:28,421 --> 00:26:31,589 Narrator: An infinite universe has profound implications 472 00:26:31,591 --> 00:26:34,291 for understanding our place in the cosmos. 473 00:26:36,362 --> 00:26:39,296 It guarantees we're not alone. 474 00:26:40,900 --> 00:26:42,933 Carroll: But if the universe is infinite, 475 00:26:42,935 --> 00:26:44,868 then there could be an infinite number of galaxies 476 00:26:44,870 --> 00:26:49,306 that have planets with life, an infinite number without life, 477 00:26:49,308 --> 00:26:51,642 then because life did appear here on earth, 478 00:26:51,644 --> 00:26:53,077 it's physically possible, 479 00:26:53,079 --> 00:26:54,612 therefore, it will definitely happen 480 00:26:54,614 --> 00:26:57,081 elsewhere in the universe. 481 00:26:57,083 --> 00:26:58,415 Narrator: In a flat universe, 482 00:26:58,417 --> 00:27:03,387 alien life could come in an infinite number of forms, 483 00:27:03,389 --> 00:27:07,625 but there is an altogether stranger guarantee. 484 00:27:07,627 --> 00:27:10,728 If the universe has no edge, this means that 485 00:27:10,730 --> 00:27:14,665 things that seem like they are impossible become possible. 486 00:27:17,370 --> 00:27:20,237 Every possible arrangement of matter, 487 00:27:20,239 --> 00:27:23,240 every possible history. 488 00:27:23,242 --> 00:27:27,711 A galaxy of a solar system of a planet like earth 489 00:27:27,713 --> 00:27:28,979 is possible 490 00:27:28,981 --> 00:27:32,583 and is happening right now in parallel to us 491 00:27:32,585 --> 00:27:35,052 somewhere over there. 492 00:27:36,589 --> 00:27:39,490 Freese: So that means that there has to be another place 493 00:27:39,492 --> 00:27:42,960 that has a galaxy just like ours 494 00:27:42,962 --> 00:27:44,862 and it would have an earth just like ours. 495 00:27:44,864 --> 00:27:49,700 It would have people who would have another version of you, 496 00:27:49,702 --> 00:27:51,168 another version of me. 497 00:27:51,170 --> 00:27:55,372 It's 100% guaranteed that there is another max tegmark out there 498 00:27:55,374 --> 00:27:58,776 having exactly this conversation and in fact many of them. 499 00:28:00,846 --> 00:28:03,547 Narrator: An infinite universe full of max tegmarks 500 00:28:03,549 --> 00:28:05,616 may be a strange concept, 501 00:28:05,618 --> 00:28:08,152 but what's truly mind-bending is understanding 502 00:28:08,154 --> 00:28:11,221 the physics of a flat universe. 503 00:28:11,223 --> 00:28:14,558 If the universe is infinite and it's expanding, 504 00:28:14,560 --> 00:28:20,197 what's it expanding into and what did it expand from? 505 00:28:20,199 --> 00:28:24,635 Was there ever an edge to the universe? 506 00:28:24,637 --> 00:28:28,806 Fortunately, the answer is that it doesn't make sense 507 00:28:28,808 --> 00:28:30,774 to ask that question. 508 00:28:30,776 --> 00:28:33,077 Everything is expanding, 509 00:28:33,079 --> 00:28:36,113 including the universe that we exist within. 510 00:28:36,115 --> 00:28:39,083 So in fact, it's not expanding into anything 511 00:28:39,085 --> 00:28:40,684 because it is everything. 512 00:28:40,686 --> 00:28:45,656 ♪ 513 00:28:45,658 --> 00:28:47,758 narrator: To help understand what's going on 514 00:28:47,760 --> 00:28:49,359 in an infinite universe, 515 00:28:49,361 --> 00:28:51,595 we need to go back to the big bang. 516 00:28:54,266 --> 00:28:57,601 We want to think of the big bang as an explosion in space, 517 00:28:57,603 --> 00:28:59,336 like it happened someplace, 518 00:28:59,338 --> 00:29:02,239 but there wasn't any place before the big bang. 519 00:29:02,241 --> 00:29:05,509 Space existed inside of the big bang itself. 520 00:29:05,511 --> 00:29:07,778 So it's not an explosion in space, 521 00:29:07,780 --> 00:29:10,280 it's an explosion of space. 522 00:29:10,282 --> 00:29:12,349 We're sometimes told that at the big bang, 523 00:29:12,351 --> 00:29:15,786 the universe started out very, very small and then got big, 524 00:29:15,788 --> 00:29:19,723 but how can a finite point become infinite? 525 00:29:19,725 --> 00:29:21,759 Well, if the universe is infinite, 526 00:29:21,761 --> 00:29:25,529 then it was also infinite at the big bang. 527 00:29:25,531 --> 00:29:27,831 This is a tough thing to think about. 528 00:29:27,833 --> 00:29:29,299 Think about it this way. 529 00:29:29,301 --> 00:29:31,902 In an infinite universe, the galaxies go on forever 530 00:29:31,904 --> 00:29:35,572 and now there's a great distance between every galaxy. 531 00:29:35,574 --> 00:29:36,607 But once upon a time, 532 00:29:36,609 --> 00:29:38,108 the galaxies were closer together, 533 00:29:38,110 --> 00:29:40,711 say half their current distance apart, 534 00:29:40,713 --> 00:29:42,079 but they still went on forever. 535 00:29:42,081 --> 00:29:45,749 The universe was still infinite. 536 00:29:45,751 --> 00:29:47,184 Narrator: In a flat universe, 537 00:29:47,186 --> 00:29:51,855 space was infinite from the beginning. 538 00:29:51,857 --> 00:29:54,358 There was never a single point in space 539 00:29:54,360 --> 00:29:56,994 where the big bang happened. 540 00:29:56,996 --> 00:30:00,597 It happened everywhere. 541 00:30:00,599 --> 00:30:05,602 An infinite universe offers infinite possibilities 542 00:30:05,604 --> 00:30:08,038 but no edge to space. 543 00:30:11,177 --> 00:30:14,111 But there may be another kind of edge, 544 00:30:14,113 --> 00:30:18,649 one that will only reveal itself if the universe dies. 545 00:30:30,930 --> 00:30:37,401 ♪ 546 00:30:37,403 --> 00:30:42,339 narrator: We live in an infinite and expanding universe. 547 00:30:42,341 --> 00:30:46,009 Space has no edge. It goes on forever. 548 00:30:46,011 --> 00:30:51,014 ♪ 549 00:30:51,016 --> 00:30:55,919 but there could be a different kind of edge to our universe, 550 00:30:55,921 --> 00:30:59,056 an edge of time. 551 00:30:59,058 --> 00:31:01,191 The universe seems to have begun 552 00:31:01,193 --> 00:31:03,260 13.8 billion years ago in the past, 553 00:31:03,262 --> 00:31:06,363 so there's some inclination, some impression, 554 00:31:06,365 --> 00:31:08,165 that it's finite in time. 555 00:31:08,167 --> 00:31:12,035 What we call the big bang is, as far as we understand it, 556 00:31:12,037 --> 00:31:14,638 a beginning, a start of the universe. 557 00:31:14,640 --> 00:31:16,907 The universe has a finite age. 558 00:31:16,909 --> 00:31:20,644 Now, does it have an edge in the future? 559 00:31:20,646 --> 00:31:22,546 Narrator: We used to think that time would someday 560 00:31:22,548 --> 00:31:25,148 come to a catastrophic end, 561 00:31:25,150 --> 00:31:28,518 along with the planets, galaxies, 562 00:31:28,520 --> 00:31:30,754 and all life in the universe. 563 00:31:33,425 --> 00:31:35,692 If we know there's a big bang, if we know the universe started, 564 00:31:35,694 --> 00:31:38,528 it expanded and cooled, it's very natural to wonder 565 00:31:38,530 --> 00:31:40,764 whether or not someday the expansion will stop, 566 00:31:40,766 --> 00:31:44,001 reverse, and come back, and that's a big crunch. 567 00:31:44,003 --> 00:31:49,105 ♪ 568 00:31:49,107 --> 00:31:50,340 narrator: In a big crunch, 569 00:31:50,342 --> 00:31:53,777 our expanding universe would begin to contract. 570 00:31:53,779 --> 00:31:56,680 Stars and planets would smash into each other. 571 00:31:58,851 --> 00:32:02,119 Galaxies would collide, 572 00:32:02,121 --> 00:32:05,389 and all of the life left in space would be compressed 573 00:32:05,391 --> 00:32:08,926 with all the other matter into a singularity. 574 00:32:08,928 --> 00:32:17,567 ♪ 575 00:32:17,569 --> 00:32:20,270 if this theory is true, then the universe 576 00:32:20,272 --> 00:32:25,742 would have both a beginning and an end of time. 577 00:32:25,744 --> 00:32:27,945 If we live in a universe that will expand, 578 00:32:27,947 --> 00:32:30,881 stop expanding, and then go back into a crunch, 579 00:32:30,883 --> 00:32:34,284 then it has, in effect, two edges. 580 00:32:34,286 --> 00:32:38,655 Narrator: But there's a much stranger possibility. 581 00:32:38,657 --> 00:32:41,692 Perhaps the end is but a beginning, 582 00:32:41,694 --> 00:32:43,794 where the universe is a oscillating universe. 583 00:32:43,796 --> 00:32:45,696 It has a big bang like beginning, 584 00:32:45,698 --> 00:32:47,431 it expands to a maximum size, 585 00:32:47,433 --> 00:32:49,433 and then goes back into a big crunch 586 00:32:49,435 --> 00:32:51,768 and does that over and over. 587 00:32:53,505 --> 00:32:55,539 Narrator: We could be residents of a universe 588 00:32:55,541 --> 00:32:58,709 created from the ashes of another, 589 00:32:58,711 --> 00:33:03,413 a single universe in a stream of bouncing universes... 590 00:33:06,552 --> 00:33:11,455 ...Each full of galaxies, planets, and life. 591 00:33:11,457 --> 00:33:14,992 But our most recent observations of the universe suggest 592 00:33:14,994 --> 00:33:17,928 a big crunch isn't in the cards. 593 00:33:20,099 --> 00:33:23,567 Once again, dark energy is key. 594 00:33:26,238 --> 00:33:30,340 For a while we didn't know if the expansion of the universe 595 00:33:30,342 --> 00:33:32,676 was going to slow, stop, 596 00:33:32,678 --> 00:33:35,312 and reverse itself because of gravity. 597 00:33:35,314 --> 00:33:36,713 There are all these galaxies in the universe 598 00:33:36,715 --> 00:33:38,849 and they're pulling on each other by their gravity. 599 00:33:38,851 --> 00:33:40,517 And if the expansion isn't fast enough, 600 00:33:40,519 --> 00:33:43,153 that gravity might be strong enough to stop the expansion 601 00:33:43,155 --> 00:33:44,921 and re-collapse the universe. 602 00:33:44,923 --> 00:33:46,223 Now with dark energy, 603 00:33:46,225 --> 00:33:48,658 we know that there's no way that can happen. 604 00:33:48,660 --> 00:33:51,194 The universe is going to expand forever 605 00:33:51,196 --> 00:33:55,032 because dark energy is pumping it full of acceleration. 606 00:33:55,034 --> 00:33:57,534 In order for there to be a big crunch, 607 00:33:57,536 --> 00:34:01,138 our understanding of dark energy would have to change a lot. 608 00:34:01,140 --> 00:34:03,740 That is, dark energy would have to be extremely weird 609 00:34:03,742 --> 00:34:07,911 and turn off in some very funny way for the universe 610 00:34:07,913 --> 00:34:10,914 to suddenly stop expanding and re-collapse. 611 00:34:13,252 --> 00:34:14,885 Narrator: Without a big crunch, 612 00:34:14,887 --> 00:34:19,256 there is no future edge to time. 613 00:34:19,258 --> 00:34:20,957 Carroll: The universe is not only expanding, 614 00:34:20,959 --> 00:34:22,692 but it's being driven by dark energy 615 00:34:22,694 --> 00:34:26,063 to expand faster and faster and the dark energy 616 00:34:26,065 --> 00:34:28,532 doesn't dilute away, as far as we can tell. 617 00:34:28,534 --> 00:34:31,001 So the simplest idea is that the universe 618 00:34:31,003 --> 00:34:34,738 will simply continue to expand eternally toward the future. 619 00:34:36,909 --> 00:34:41,711 Narrator: Just like space, time will go on forever. 620 00:34:41,713 --> 00:34:46,116 That might sound like a better fate for life and the universe, 621 00:34:46,118 --> 00:34:47,751 but it's not. 622 00:34:50,022 --> 00:34:53,056 Freese: One of the consequences of this dark energy 623 00:34:53,058 --> 00:34:56,026 that's causing the acceleration of the universe 624 00:34:56,028 --> 00:35:01,598 is that we eventually are headed towards the big chill. 625 00:35:01,600 --> 00:35:05,302 I should say, "we're eventually headed towards the big chill." 626 00:35:05,304 --> 00:35:08,438 so the universe is getting colder and colder 627 00:35:08,440 --> 00:35:11,441 and things are getting more and more spread out. 628 00:35:11,443 --> 00:35:13,743 So the accelerated and continual 629 00:35:13,745 --> 00:35:16,012 and forever expansion of our universe 630 00:35:16,014 --> 00:35:19,716 might make for a frankly depressing end to time itself. 631 00:35:19,718 --> 00:35:23,253 The ultimate entropy-based heat death of the universe 632 00:35:23,255 --> 00:35:27,858 where you would walk out and see no stars in the sky, 633 00:35:27,860 --> 00:35:29,526 see absolutely nothing. 634 00:35:29,528 --> 00:35:32,596 There will come one day when the very last star in the universe 635 00:35:32,598 --> 00:35:35,432 just fizzles out and that is it. 636 00:35:38,303 --> 00:35:40,303 Narrator: In the future, 637 00:35:40,305 --> 00:35:44,074 space will be a cold, dark and infinite void, 638 00:35:44,076 --> 00:35:47,844 where time goes on forever. 639 00:35:47,846 --> 00:35:49,746 There will be nothing to do 640 00:35:49,748 --> 00:35:53,216 but suffer in the eternal expanse. 641 00:35:53,218 --> 00:35:56,887 It's our inevitable fate that there's no future edge 642 00:35:56,889 --> 00:36:00,824 of time in the universe. 643 00:36:00,826 --> 00:36:04,127 But even if there isn't an edge to the universe, 644 00:36:04,129 --> 00:36:09,866 could there be edges within the universe? 645 00:36:09,868 --> 00:36:12,402 If you wanted to visit the edge of the universe, 646 00:36:12,404 --> 00:36:15,839 then go find your nearest black hole and jump on in 647 00:36:15,841 --> 00:36:18,008 because that's a one-way trip. 648 00:36:18,010 --> 00:36:21,811 Narrator: If you cross this edge, you will never return. 649 00:36:29,254 --> 00:36:33,757 ♪ 650 00:36:33,759 --> 00:36:36,159 [ applause ] 651 00:36:36,161 --> 00:36:38,295 narrator: April 2019, 652 00:36:38,297 --> 00:36:40,497 an international team of astronomers 653 00:36:40,499 --> 00:36:42,866 makes a special announcement. 654 00:36:42,868 --> 00:36:46,469 And we are delighted to be able to report to you today 655 00:36:46,471 --> 00:36:51,541 that we have seen and taken a picture of a black hole. 656 00:36:51,543 --> 00:36:52,809 Here it is. 657 00:36:52,811 --> 00:36:54,411 [ applause ] 658 00:36:54,413 --> 00:36:58,982 ♪ 659 00:36:58,984 --> 00:37:02,252 narrator: It's a picture of a supermassive black hole 660 00:37:02,254 --> 00:37:07,324 at the center of the m87 galaxy 54 million light-years away. 661 00:37:09,161 --> 00:37:14,130 It may also be the first image of an edge in the universe. 662 00:37:15,400 --> 00:37:18,001 Black holes create a really interesting scenario 663 00:37:18,003 --> 00:37:22,906 when we think about space and the universe having edges. 664 00:37:22,908 --> 00:37:27,844 Narrator: The edge between space outside and inside a black hole 665 00:37:27,846 --> 00:37:31,114 is called the event horizon. 666 00:37:31,116 --> 00:37:32,582 Bullock: The event horizon of a black hole 667 00:37:32,584 --> 00:37:36,152 is a region within which, once you cross inside, 668 00:37:36,154 --> 00:37:38,755 the gravitational tug is so strong 669 00:37:38,757 --> 00:37:43,393 that even light cannot escape, which means nothing can escape 670 00:37:43,395 --> 00:37:46,296 once you cross inside the event horizon. 671 00:37:46,298 --> 00:37:48,665 So that really is sort of an edge 672 00:37:48,667 --> 00:37:51,234 because it really does create a boundary. 673 00:37:51,236 --> 00:37:56,306 ♪ 674 00:37:56,308 --> 00:37:59,709 narrator: The event horizon is not a physical barrier in space. 675 00:38:01,913 --> 00:38:04,314 Event horizon is an edge of the part of the universe 676 00:38:04,316 --> 00:38:06,283 we can visit, but it's not an edge 677 00:38:06,285 --> 00:38:09,185 in the sense that there's anything there. 678 00:38:09,187 --> 00:38:10,553 You would just pass right through it 679 00:38:10,555 --> 00:38:12,722 if you actually got right up to that place. 680 00:38:12,724 --> 00:38:15,158 So it's sort of a conceptual boundary 681 00:38:15,160 --> 00:38:19,129 between two different parts of the universe. 682 00:38:19,131 --> 00:38:22,532 Narrator: If we sent a man to probe into a black hole, 683 00:38:22,534 --> 00:38:24,301 it would be a one-way trip. 684 00:38:26,738 --> 00:38:31,541 The event horizons of black holes are a sort of edge 685 00:38:31,543 --> 00:38:34,611 because, once you pass through an event horizon, 686 00:38:34,613 --> 00:38:38,214 you are cut off from the rest of the universe. 687 00:38:38,216 --> 00:38:40,784 You can never go back out. 688 00:38:40,786 --> 00:38:45,155 You are outside of our universe. 689 00:38:45,157 --> 00:38:46,990 Once you've crossed inside that region, 690 00:38:46,992 --> 00:38:50,360 you are never coming back out, and that's an edge. 691 00:38:52,197 --> 00:38:54,197 Narrator: Once inside the black hole, 692 00:38:54,199 --> 00:38:58,301 the probe would be in a separate part of space, 693 00:38:58,303 --> 00:39:02,339 cut off from the rest of the universe. 694 00:39:02,341 --> 00:39:04,374 Sutter: Falling through the event horizon of a black hole 695 00:39:04,376 --> 00:39:07,644 is like jumping over the edge of a cliff. 696 00:39:07,646 --> 00:39:10,580 You can see the edge and you can see the edge go by, 697 00:39:10,582 --> 00:39:13,149 and then when you're at the bottom, you can look up 698 00:39:13,151 --> 00:39:15,618 and see what's happening at the top of the cliff, 699 00:39:15,620 --> 00:39:17,354 but you can never go back. 700 00:39:19,491 --> 00:39:21,891 Narrator: At the bottom of this black-hole cliff 701 00:39:21,893 --> 00:39:24,694 sits a singularity, 702 00:39:24,696 --> 00:39:30,567 a region of space where the laws of physics go off the rails. 703 00:39:30,569 --> 00:39:33,403 Deep toward that singularity could be as surprising 704 00:39:33,405 --> 00:39:36,339 as you might imagine and yet still a possibility. 705 00:39:36,341 --> 00:39:38,808 If you map the space-time around a black hole 706 00:39:38,810 --> 00:39:40,377 in a very particular way, 707 00:39:40,379 --> 00:39:44,347 there emerges a sort of mirror universe, a parallel universe, 708 00:39:44,349 --> 00:39:46,015 on the other side of the black hole, 709 00:39:46,017 --> 00:39:50,787 identical to our own and traversable by the black hole. 710 00:39:50,789 --> 00:39:56,059 ♪ 711 00:39:56,061 --> 00:40:01,197 narrator: So black holes are not just edges to our universe, 712 00:40:01,199 --> 00:40:06,569 they may also be gateways to other universes. 713 00:40:06,571 --> 00:40:08,138 Bullock: It's highly conjecture, 714 00:40:08,140 --> 00:40:12,375 but if there's ever going to be a space, or region, 715 00:40:12,377 --> 00:40:15,545 where you're making connections with, say, some other universe, 716 00:40:15,547 --> 00:40:18,314 a black hole, in principle, could be a portal to that. 717 00:40:22,354 --> 00:40:23,753 Narrator: But it's highly unlikely 718 00:40:23,755 --> 00:40:24,954 that anyone will ever want 719 00:40:24,956 --> 00:40:29,592 to venture beyond an event horizon to find out, 720 00:40:29,594 --> 00:40:33,029 and our pursuits of the other edges in the cosmos 721 00:40:33,031 --> 00:40:35,965 offer little hope either. 722 00:40:35,967 --> 00:40:41,137 We can never travel beyond the cosmic event horizon. 723 00:40:41,139 --> 00:40:44,340 We will never be able to see beyond the edge 724 00:40:44,342 --> 00:40:47,744 of our observable universe. 725 00:40:47,746 --> 00:40:51,915 So can we ever hope to discover the true edge 726 00:40:51,917 --> 00:40:56,186 of the greater universe or find out if it even has one? 727 00:40:58,023 --> 00:41:00,423 My feeling is that probably we should not think 728 00:41:00,425 --> 00:41:03,226 about edges for the universe. 729 00:41:03,228 --> 00:41:05,929 Everything you've ever seen in your life is finite, 730 00:41:05,931 --> 00:41:10,099 it has an inside and the outside, it has an edge. 731 00:41:10,101 --> 00:41:11,901 The universe might not be like that. 732 00:41:11,903 --> 00:41:13,369 It's probably not like that. 733 00:41:13,371 --> 00:41:16,506 There's probably no sense in which the universe has an edge. 734 00:41:19,544 --> 00:41:22,111 We used to think that the ultimate limits 735 00:41:22,113 --> 00:41:24,948 on the future life were set by nature, 736 00:41:24,950 --> 00:41:26,249 we couldn't get off the planet, 737 00:41:26,251 --> 00:41:29,452 or there was nothing beyond our solar system. 738 00:41:29,454 --> 00:41:31,921 Now we realized we have this vast, 739 00:41:31,923 --> 00:41:35,258 vast cosmos out there and that the ultimate limits 740 00:41:35,260 --> 00:41:38,461 are actually simply our own imagination 741 00:41:38,463 --> 00:41:41,231 and our ability to do great things with it 742 00:41:41,233 --> 00:41:42,932 rather than self-destruct. 743 00:41:42,934 --> 00:41:46,069 Our future destiny is in our own hands 744 00:41:46,071 --> 00:41:49,873 and I find that very empowering. 745 00:41:49,875 --> 00:41:52,208 Thaller: It is beautifully frustrating 746 00:41:52,210 --> 00:41:53,943 to realize how limited we are, 747 00:41:53,945 --> 00:41:56,613 to realize that we're probably never going to get a true view 748 00:41:56,615 --> 00:41:58,515 of the real extent of the universe. 749 00:41:58,517 --> 00:42:00,583 We should keep an open mind, we should be humble, 750 00:42:00,585 --> 00:42:02,452 but I think that we should give up on the idea 751 00:42:02,454 --> 00:42:03,887 that things should have edges 752 00:42:03,889 --> 00:42:05,355 because that's what we're familiar with. 753 00:42:05,357 --> 00:42:08,124 The universe is something special. 754 00:42:08,126 --> 00:42:12,896 Sutter: What matters to us, and will only ever matter to us, 755 00:42:12,898 --> 00:42:15,298 is the observable universe 756 00:42:15,300 --> 00:42:18,001 because that's the limit of what we can see 757 00:42:18,003 --> 00:42:19,903 and that is the limit of what we can know. 758 00:42:19,905 --> 00:42:22,171 So there is an edge to the universe, 759 00:42:22,173 --> 00:42:24,173 there's an edge to what we can know.