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00:00Islands have edges, planets have edges, even galaxies have edges, but what about the universe?
00:20As explorers, as curious humans that we are, we're obsessed with boundaries and limits
00:27and we want to know, does the whole thing, the universe, have a limit?
00:33Does the universe have an edge?
00:35Well, the answer is yes and no.
00:38It depends on what you mean by edge.
00:41The edge of what we can see, the edge of where we can go, or the edge of reality itself.
00:54Looking out to the edge of the universe is tremendously important to understand our place
00:58in the universe itself.
01:00We're talking about our universe.
01:02We're talking about the thing that we exist within, the most fundamental thing there is.
01:06We're driven to understand it.
01:08There is always a desire to push the knowledge to the edge.
01:12So can we ever find the edge of the universe?
01:21In 2016, the Hubble Space Telescope turned toward a dark patch of sky in the constellation
01:40Ursa Major.
01:46It captured an image of an indistinct blob of light.
01:51The glow is from a distant galaxy called GNZ 11.
02:02It's the most distant galaxy we've ever observed.
02:08But is it the edge of the universe?
02:12The universe all around us is filled with galaxies.
02:16So it's kind of natural to say, would there be a final galaxy if you traveled far enough
02:20away?
02:21Would you finally be at the very last galaxy in the universe looking out into empty space?
02:28That's a difficult question to answer because there is a limit to how far we can see.
02:37It all comes down to the speed of light and the age of the universe.
02:44The key to understanding the edge of the universe is that light travels very, very fast, but
02:51not infinitely fast.
02:53It takes time for it to get from one place in the universe to the other.
02:56You open the curtains, light fills the room, it doesn't seem to travel at all.
03:01But over the vast distances of the universe, you actually notice this travel time.
03:07Even the sun, 93 million miles away, the light takes eight minutes to get to us.
03:12When you look out at the stars, we start to think of distance in terms of light years
03:16because it takes years for the light to get from those stars to us.
03:20And then when you look at galaxies, then you're talking about millions or billions of light
03:24years.
03:28When we look at light from Galaxy GNZ 11, we're seeing light emitted 13.4 billion years
03:36ago.
03:42You can't really even find a galaxy too much farther away than that because the universe
03:46is only 13.8 billion years old and it takes a certain amount of time for galaxies to even
03:52form.
03:53So we're not going to find too many more galaxies farther away than this.
03:56If things are far enough away, there is no way that light can get to us in the age of
04:00the universe.
04:02What this means is there's a hard limit to the edge of the universe that we can see and
04:07this is set by the age of the universe.
04:10GNZ 11 sparked into life early in the history of the universe, just 400 million years after
04:19the Big Bang.
04:21Before that, there were no stars to send out light.
04:26If you look in any direction at all, you get all the way back to when there were no
04:30stars, no galaxies, nothing but very, very hot gas.
04:34And that sort of forms a shell around us.
04:39That outer shell is the cosmic microwave background.
04:44It's the oldest light in the universe.
04:48The echo of the birth of the universe, the Big Bang.
04:54The edge of our universe, the very furthest thing that we can see, is one of the earliest
04:59relics of the formation of the universe itself.
05:01That is the cosmic microwave background.
05:05We call this the edge of our observable universe.
05:11So we have an observable universe, but beyond that, even if there are things out there,
05:15there's no way we can see them because the light just could not have gotten to us by now.
05:25As the name states, the observable universe is simply the part of the universe we can
05:30see.
05:32We can think of the observable universe sort of like a spotlight centered on wherever you're
05:36standing right now.
05:38And you can see to the edge of your spotlight and not beyond.
05:42But if you move a little bit to the left, a little bit to the right, your observable
05:48universe actually moves with you.
05:54For someone living in galaxy GNZ 11, a totally different part of the universe would be observable.
06:02So that distant galaxy is at the edge of our observable universe.
06:08And we are at the edge of their observable universe.
06:13We have different spotlights.
06:15One of the wonderful things to think about is that there are other spheres around other
06:18galaxies.
06:19There are other aliens looking up into the sky tonight, wondering what the true extent
06:23of the universe is.
06:27The true extent of our universe doesn't end with galaxy GNZ 11.
06:36But when astronomers use the Hubble Space Telescope to measure the distance to GNZ 11,
06:44they find something shocking.
06:48It's 32 billion light years away, three times further than thought possible.
06:57So if nothing can travel faster than light, and we measure the distance to this galaxy,
07:03how can it be 32 billion light years away?
07:08There hasn't been enough time in the history of the universe for light from GNZ 11 to reach
07:13us.
07:14There must be some mistake here, right?
07:19At this point, your brain is probably thinking of leaping out of your skull and running around
07:23screaming.
07:24Trust me.
07:25I know.
07:26I'm an astronomer.
07:27I've been doing this my whole life.
07:29And this stuff twists my imagination up.
07:32It's really hard to grasp this.
07:36How do we see a galaxy that's 32 billion light years away and only 13.4 billion years old?
07:44GNZ 11 is further away than it should be because something strange is going on with our universe.
07:55It's expanding.
07:57If the universe is expanding, then where does its edge lie?
08:03And can we ever reach it?
08:1513.8 billion years ago, a speck of energy burst into life.
08:24We call it the Big Bang.
08:27Space and time pushed out in all directions.
08:31Ever since, our universe has expanded.
08:36But the way it's expanding makes finding an edge a major challenge.
08:43The universe is expanding and expands according to a very simple law.
08:48That the farther away a galaxy is from us, the faster it appears to be receding away
08:52from us.
08:55The furthest galaxies are moving at very high speeds.
08:59The most distant galaxy we've ever spotted, GNZ 11, seems to have moved 32 billion light
09:06years away from us in just 13.4 billion years.
09:13That's faster than the speed of light.
09:17We can measure the speeds with which galaxies are moving away from us.
09:21And many, many galaxies are moving away from us at speeds faster than the speed of light.
09:27This sounds like it's breaking the law, right?
09:29There's this idea that you've all been told that relativity says nothing goes faster than
09:33the speed of light.
09:34Okay, you've been lied to.
09:38Space itself can do what it wants.
09:40It makes the rules.
09:41It can break the rules.
09:44That rule applies to matter, not to space itself.
09:47Space can expand at whatever rate it wants.
09:50A simple way to think of this expansion law is imagine standing on an infinite rubber
09:55sheet that stretches all the way out into the distance.
09:58And you're standing on the same place.
09:59You can mark it with a little X.
10:02Now all the sheet expands in every direction.
10:06So if it expands by a factor of two, another galaxy that was, say, one foot away from you
10:11is now two feet away from you as we stretch the sheet.
10:14But another galaxy was 10 feet away from you.
10:17Expand that by a factor of two, and now it's 20 feet away from you.
10:21So in the same amount of time, one galaxy moved one foot, where another galaxy moved
10:2610 feet.
10:27So the more stuff there is, the more elastic between you and another galaxy, the more it
10:32seems to expand away from you.
10:37Expansion means our observable universe stretches for a colossal 46 billion light-years in all
10:44directions, 92 billion light-years across, and getting bigger by the second.
10:58This number is so incomprehensibly large that it's difficult to wrap your brain around.
11:04There are trillions of galaxies within this volume.
11:08It's staggering.
11:09It's so much larger than anything we're familiar with.
11:13If we were to travel to the edge of the observable universe, we would enter even more unfamiliar
11:20territory.
11:22Imagine we're in an ultra-fast spaceship.
11:26We leave the solar system, then the Milky Way.
11:32As we travel deeper into intergalactic space, things start to get really weird.
11:41For every million light-years we go from the Milky Way, the galaxies move away from
11:47us at around 13 miles per second faster.
11:54We have to accelerate just to keep up, but the galaxies keep on moving, always beyond
12:01our reach.
12:16Imagine you're a sprinter on a racetrack.
12:18If you're running towards the finish line, it may take you a few seconds to cross it.
12:22But now imagine that that finish line is moving away from you.
12:26If it's moving away from you at the same speed you're running, you'll never reach it.
12:29And if it's moving faster than the runner, then even faster runners won't reach it.
12:35And that's sort of what we're seeing here with the universe.
12:39Beyond a certain distance, galaxies are racing away from us faster than the speed of light.
12:46It's a line called the cosmic even horizon.
12:52And 97% of galaxies we see in the observable universe are beyond this line and unreachable,
13:00including GN-Z11.
13:01They're sort of teasing us to say, look at me, what a nice piece of real estate, but
13:08we know even if we started going there now, we could never reach them.
13:15Anything that has crossed the cosmic even horizon is out of our reach forever.
13:21But that's not the full picture, because the expansion rate of the universe is changing.
13:28A little over 20 years ago, astronomers discovered that the current rate of the universe's expansion
13:33is accelerating.
13:35It's speeding up.
13:36Everyone thought that the expansion of the universe should be slowing down because of
13:40gravity.
13:42Galaxies have mass, they have visible matter, dark matter.
13:45Light has energy as well, all known forms of energy.
13:50Slow down the expansion of the universe because they act in an attractive gravitational way.
13:57Imagine our surprise when we found the universe wasn't slowing down, it wasn't even going
14:02at a constant speed, it's speeding up with time.
14:08That was a real jaw dropper.
14:12Astronomers discovered that distant galaxies were speeding away from us faster than the
14:17laws of physics predicted.
14:22That is one of the most bizarre things you can imagine.
14:26I know it doesn't sound like much, but imagine holding a rock in front of you and letting
14:30it go, and instead of falling down, it falls up.
14:34That is as big of a shock as what the astronomers got.
14:38We did not expect the universe to be accelerating its expansion.
14:44We suspect a mysterious force is at work.
14:49Dark energy.
14:52Dark energy is what we think is pushing the universe apart, causing this accelerating
14:57expansion.
14:59And the origin and true physical nature of dark energy is a big mystery.
15:07Thanks to dark energy, more and more galaxies are crossing the cosmic event horizon and
15:13leaving the observable universe.
15:18These galaxies are lost to us forever.
15:25There are galaxies that we can see today that in a few million years, say, we won't be able
15:30to see because the edge of the observable universe has basically moved in closer than
15:35that galaxy.
15:36That's going to happen all the time, and in a trillion years or something like that, all
15:41these galaxies that we see in our sky will be completely invisible because they'll be
15:45beyond the edge of the universe.
15:50So eventually, every last galaxy will be so far away from us that light cannot reach us
15:57through that expanding space.
15:59It's almost as if you're driving through a dark desert in your car, and the very, very
16:03last town that ever exists has gone over the horizon, and there'll never be any light again.
16:12We can see less and less of the universe as we go into the future.
16:16What a strange thought.
16:18So that means we should build all the telescopes we can now.
16:24There's a limit to the universe we can see, even with the most advanced telescopes.
16:31But what lies beyond is one of the biggest mysteries in astronomy.
16:38The greater universe could be stranger than our wildest imagination.
16:46When you stand on the beach and you look at the horizon, and you kind of think, oh, what
16:50beautiful lands are there beyond the horizon, things I've never imagined before.
16:56It's so natural, it's so human to ask, what lies beyond that?
17:01What is the true extent of the universe?
17:08The observable universe contains trillions of galaxies.
17:14It's about 92 billion light years across.
17:20But astronomers believe this isn't the full extent of the universe.
17:28What we don't know is how much of the universe is our observable universe.
17:33It could be a tiny microscopic speck of this much more vast universe.
17:37We just don't know.
17:40We have no idea how much stuff there is outside the observable universe, but because, by definition,
17:46it's outside the observable universe, we really don't know right now.
17:51So what is out there?
17:53One theory is that space outside the observable universe is pretty much the same as our own
17:59cosmic neighborhood.
18:03It's just more universe.
18:04It's just like here.
18:05It's just far enough away that we can't see it.
18:08So it's not like there's bizarre places where time runs backwards or aliens have two heads.
18:12Well, yeah, maybe.
18:16But further out in the deepest parts of the greater universe, all bets are off.
18:24We expect that as you go sort of twice or three times beyond the observable universe,
18:29it's probably very similar to the universe we inhabit.
18:32But if you go a thousand times or a million times farther, who knows what you might see?
18:39It might be very, very different if we go far enough away.
18:43Strangely, it all comes back to the expansion of the universe and one crucial detail in
18:50that process.
18:52There was a brief moment in the very early history of the universe where its expansion
18:59accelerated hugely.
19:02This acceleration is called inflation, and in a brief moment, the universe itself expanded
19:07at multiple times the speed of light.
19:13Inflation was a formative moment for our universe.
19:17By the time it stopped, the universe's basic characteristics were set.
19:25There are these fundamental constants that describe the phenomena in our universe.
19:29The fundamentals of matter and light and space-time.
19:35But some scientists think there could be regions of the greater galaxy where inflation has
19:40never stopped.
19:44The idea is the greater universe is expanding at an insane speed, but here and there, occasionally,
19:51a little region will stop inflating and just expand at the normal rate.
19:58Inflation can end somewhere, and that gives rise to the universe we live in, while inflation
20:02continues somewhere else.
20:06Parts of the greater universe that continue to inflate would be left with different laws
20:11of physics.
20:14These incredibly violent inflation processes actually monkeyed with the very fabric of
20:18space itself, so that a lot of the things that we were taught are laws of physics are
20:23different there.
20:27So in essence, inflation gives us a very natural way to make this patchwork quilt of different
20:32parts of the universe where things seem different.
20:36So what we can imagine is a super large-scale structure where there's different regions
20:40of the universe, domains, then each domain has different local laws of physics.
20:48These different parts of the universe are separated by frontiers called domain walls.
20:58We have similar boundaries on Earth.
21:01Whenever you have something that can be in many different states, you can end up with
21:04domain wall.
21:06If I were a fish swimming around in the Arctic near an iceberg, there would be a domain boundary
21:14between the water being in the liquid state where I am and the solid state inside the
21:18ice.
21:19So a domain wall is just a wall between two domains.
21:23If it's water, this could be ice, this could be liquid.
21:26If we're talking about space, this could be a kind of space maybe you can live in.
21:30This could be the kind of space where you don't want to be.
21:35Crossing a domain wall would be very bad news for anyone who dared to try.
21:41Cross that domain wall and the laws of physics could change dramatically.
21:46The number of dimensions could change.
21:48If we were somehow able to travel to places in the universe where the laws of physics
21:51are different, we would die.
21:55Because all of the chemistry going on in our bodies depends very, very sensitively on the
21:59laws of physics.
22:02So you could just dissipate like Thanos snapped and you're gone.
22:07Domain walls might be the closest we get to locating an edge in the universe.
22:14Depends on how you define the edge.
22:16If it is the realm where the laws of our universe operate, then these domain walls are in essence
22:22the edge of the universe.
22:27But this is all just theory.
22:33If we ever really are to work out what the true size and shape of the universe is, we're
22:37going to have to look for clues that are close to us.
22:42Clues that could answer the ultimate question, how big is the rest of the greater universe?
22:49And could it go on forever?
22:58For tens of thousands of years, mankind has gazed in wonder at the vastness of the cosmos.
23:06But just how extensive is it?
23:10If we could answer that question, it might help us to understand our place in the universe.
23:17One of the fundamental questions in science is, how big is the universe?
23:23To answer the question, how big is the universe?
23:26We have to answer the question, what shape is the universe?
23:29And by shape, I mean geometry.
23:31I mean, how is the universe curved on its largest scales?
23:36If we are to discover that the universe does have some sort of geometric curvature, then
23:42this might imply that it wraps around in on itself over incredibly large distances, and
23:50that if you could travel in one direction long enough, you would end up at your starting
23:55point.
23:56Another version is that the universe is more like an infinite flat plane, okay, no curvature
24:02at all.
24:03The further you travel, well, the further you get.
24:06And you never get back to where you started.
24:11To work out the shape of something, we would normally just step back and take a look.
24:16But clearly, moving outside the universe is a non-starter.
24:19We can't jump on a rocket and fly a thousand times larger than our cosmic horizon and see
24:26what the shape of the universe is.
24:27We just can't do that.
24:29Our human perspective on the larger universe is so limited.
24:33So if we want to figure out what the larger shape and scale of the universe is, we're
24:38going to have to be very clever indeed.
24:43One way to be clever is to think of the geometry of the universe in its simplest terms.
24:49When we talk about the geometry of the universe, we really are talking about geometry.
24:55In order to do geometry, you have to take measures.
24:58You need a cosmic ruler to do this.
25:00And it turns out there's a great cosmic ruler known as baryonic acoustic oscillations.
25:08Baryonic acoustic oscillations are ripples in the cosmic microwave background, the oldest
25:15light in the universe.
25:19As the universe expanded, these ripples were imprinted in space in a uniform way.
25:26We provide a cosmic ruler to measure vast distances over time so we can gauge if the
25:32universe expands in curved space or over a flat plane.
25:41When we use these cosmic rulers to try to back out the shape of the universe, we're
25:45sure to a few percent accuracy that the universe is flat.
25:51If the universe is flat, we could set off into the cosmos and continue traveling forever.
26:01There may be no edge to our universe because a flat universe can be infinite.
26:09Now we're thinking of the universe as something that really does go on forever, that the stars
26:14and galaxies never have an end.
26:17And how can something truly infinite really exist?
26:21Infinity is weird because we, it's a concept of, because it's endless.
26:28What does that mean?
26:29Who knows?
26:30I don't know.
26:33Infinity is a concept more than anything else.
26:37Our brains aren't evolved for that.
26:39We evolved living in the plains.
26:41We were apes looking for food.
26:44You know, we weren't evolved to think about the universe and all of this stuff.
26:50I just can't stop contemplating this stuff.
26:52The idea of infinity and these large numbers and even the tininess of everything.
26:57It's nuts.
26:58Yeah, thinking about infinity makes my head hurt a little bit.
27:06An infinite universe has profound implications for understanding our place in the cosmos.
27:14It guarantees we are not alone.
27:18If the universe is infinite, then there could be an infinite number of galaxies that have
27:23planets with life, an infinite number without life.
27:27Then because life did appear here on Earth, it's physically possible, therefore it will
27:31definitely happen elsewhere in the universe.
27:35In a flat universe, alien life could come in an infinite number of forms.
27:41But there is an altogether stranger guarantee.
27:45If the universe has no edge, this means that things that seem like they are impossible
27:51become possible.
27:55Every possible arrangement of matter, every possible history of a galaxy, of a solar system,
28:04of a planet like Earth is possible and is happening right now in parallel to us somewhere
28:11over there.
28:15So that means that there has to be another place that has a galaxy just like ours, and
28:21it would have an Earth just like ours, it would have people, it would have another version
28:26of you, another version of me.
28:29It's a hundred percent guaranteed that there is another Max Tegmark out there having exactly
28:34this conversation, and in fact many of them.
28:38An infinite universe full of Max Tegmarks may be a strange concept, but what's truly
28:44mind-bending is understanding the physics of a flat universe.
28:49If the universe is infinite, and it's expanding, what is it expanding into, and what did it
28:56expand from, and was there ever an edge to the universe?
29:03Unfortunately the answer is that it doesn't make sense to ask that question.
29:10Everything is expanding, including the universe that we exist within.
29:14So in fact it's not expanding into anything, because it is everything.
29:23To help understand what's going on in an infinite universe, we need to go back to the Big Bang.
29:32We want to think of the Big Bang as an explosion in space, like it happened someplace.
29:37But there wasn't any place before the Big Bang.
29:40Space existed inside of the Big Bang itself.
29:43So it's not an explosion in space, it's an explosion of space.
29:48We're sometimes told that at the Big Bang the universe started out very, very small
29:52and then got big, but how can a finite point become infinite?
29:58Well, if the universe is infinite, then it was also infinite at the Big Bang.
30:03This is a tough thing to think about.
30:05Think about it this way, in an infinite universe the galaxies go on forever, and now there's
30:11a great distance between every galaxy, but once upon a time the galaxies were closer
30:15together, say half their current distance apart.
30:19But they still went on forever, the universe was still infinite.
30:23In a flat universe, space was infinite from the beginning.
30:29There was never a single point in space where the Big Bang happened.
30:35It happened everywhere.
30:39An infinite universe offers infinite possibilities, but no edge to space.
30:49But there may be another kind of edge, one that will only reveal itself if the universe
30:55dies.
31:06We live in an infinite and expanding universe.
31:11Space has no edge, it goes on forever.
31:20But there could be a different kind of edge to our universe, an edge of time.
31:28The universe seems to have begun 13.8 billion years ago in the past, so there's some inclination,
31:35some impression that it's finite in time.
31:37What we call the Big Bang is, as far as we understand it, a beginning, a start of the
31:42universe.
31:43The universe has a finite edge.
31:46Now does it have an edge in the future?
31:49We used to think that time would someday come to a catastrophic end, along with the planets,
31:57galaxies and all life in the universe.
32:02If we know there's a Big Bang, if we know the universe started, expanded and cooled,
32:06it's very natural to wonder whether or not someday the expansion will stop, reverse and
32:10come back.
32:11And that's a big crunch.
32:18In a big crunch, our expanding universe will begin to contract.
32:23Stars and planets will smash into each other.
32:28Galaxies will collide.
32:31And all of the life left in space will be compressed with all other matter into a singularity.
32:46If this theory is true, then the universe will have a beginning and an end of time.
32:55If we live in a universe that will expand, stop expanding and then go back into a crunch,
33:00then it has in effect two edges.
33:03But there's a much stranger possibility.
33:07Perhaps the end is but a beginning.
33:10Well, the universe is an oscillating universe.
33:13It has a Big Bang-like beginning.
33:15It expands to a maximum size and then goes back into a big crunch and does that over
33:19and over.
33:22We could be residents of a universe created from the ashes of another.
33:28A single universe in a stream of bouncing universes.
33:35Each full of galaxies, planets and life.
33:40But our most recent observations of the universe suggest a big crunch isn't in the cards.
33:49Once again, dark energy is key.
33:56It's pushing our universe apart.
33:59The universe is being dominated right now by dark energy.
34:02And because the dark energy doesn't seem to be fading away, it's easy to imagine eternal
34:06expansion toward the future.
34:10Dark energy seems to be overpowering the large-scale effects of gravity.
34:18But some theories suggest that this strange force could switch sides.
34:24It's possible that the currently repulsive dark energy will someday change sign and become
34:31gravitationally attractive.
34:33If that's true, and if there's enough of this energy, or if it grows in magnitude, then
34:39the universe would slow down, eventually come to a stop, and then reverse its motion and
34:46collapse in on a big crunch.
34:50So in that case, despite the current acceleration, the universe could ultimately reverse its
34:55motion and collapse in on itself.
34:59Dark energy really is little more than a word for our ignorance.
35:04And until we can figure out more about what this stuff is, we will not know our ultimate
35:09destiny and whether things are going to keep flying apart or come crashing together or
35:15what.
35:19But most scientists think a big crunch is unlikely.
35:24For a while, we didn't know if the expansion of the universe was going to slow, stop, and
35:31reverse itself because of gravity.
35:33There are all these galaxies in the universe and they're pulling on each other by their
35:36gravity.
35:37And if the expansion isn't fast enough, that gravity might be strong enough to stop the
35:40expansion and re-collapse the universe.
35:43Now with dark energy, we know that there's no way that can happen.
35:46The universe is going to expand forever because dark energy is pumping it full of acceleration.
35:53In order for there to be a big crunch, our understanding of dark energy would have to
35:57change a lot.
35:59That is, dark energy would have to be extremely weird and turn off in some very funny way
36:05for the universe to suddenly stop expanding and re-collapse.
36:11Without a big crunch, there is no future edge to time.
36:17The universe is not only expanding, but it's being driven by dark energy to expand faster
36:21and faster.
36:22And the dark energy doesn't dilute away, as far as we can tell.
36:26So the simplest idea is that the universe will simply continue to expand eternally toward
36:31the future.
36:35Just like space, time would go on forever.
36:40It may sound like a better fate for life in the universe, but it's not.
36:48One of the consequences of this dark energy that's causing the acceleration of the universe
36:54is that we eventually are headed towards the big chill.
36:58I should say, we're eventually headed towards the big chill.
37:02So the universe is getting colder and colder and things are getting more and more spread
37:08out.
37:09So the accelerated and continual and forever expansion of our universe might make for a
37:14frankly depressing end to time itself.
37:17The ultimate entropy-based heat death of the universe, where you would walk out and see
37:23no stars in the sky.
37:26See absolutely nothing.
37:27There will come one day when the very last star in the universe just fizzles out and
37:32that is it.
37:36In the future, space will be a cold, dark, infinite void where time goes on forever.
37:44There will be nothing to do but suffer in the internal expanse.
37:51It's our inevitable fate if there's no future edge of time in the universe.
37:58But even if there isn't an edge to the universe, could there be edges within the universe?
38:10April 2019, an international team of astronomers makes a special announcement.
38:17And we are delighted to be able to report to you today that we have seen and taken a
38:22picture of a black hole.
38:25Here it is.
38:33It's a picture of a supermassive black hole at the center of the M87 galaxy, 54 million
38:40light years away.
38:43It may also be the first image of an edge in the universe.
38:49Black holes create a really interesting scenario when we think about space and the universe
38:54having edges.
38:57This edge between space outside and inside a black hole is called the event horizon.
39:05The event horizon of a black hole is a region within which, once you cross inside, the gravitational
39:11tug is so strong that even light cannot escape, which means nothing can escape once you cross
39:18inside the event horizon.
39:20So that really is sort of an edge, because it really does create a boundary.
39:30The event horizon isn't a physical barrier in space.
39:35The event horizon is an edge of the part of the universe we can visit, but it's not an
39:40edge in the sense that there's anything there.
39:43You would just pass right through it if you actually got right up to that place.
39:47So it's sort of a conceptual boundary between two different parts of the universe.
39:52If we did send a manned probe to a black hole, it would be a one-way trip.
40:01The event horizons of black holes are a sort of edge, because once you pass through an
40:07event horizon, you are cut off from the rest of the universe.
40:12You can never go back out.
40:14You are outside of our universe.
40:19Once you've crossed inside that region, you are never coming back out, and that's an edge.
40:26Once inside the black hole, the probe would be in a separate part of space, cut off from
40:33the rest of the universe.
40:36Falling through the event horizon of a black hole is like jumping over the edge of a cliff.
40:41You can see the edge, and you can see the edge go by, and then when you're at the bottom,
40:46you can look up and see what's happening at the top of the cliff, but you can never go
40:50back.
40:53At the bottom of this black hole cliff sits a singularity, a region of space where the
41:00laws of physics go off the rails.
41:04Deep toward that singularity could be as surprising as you might imagine, and yet still a possibility.
41:10If you map the spacetime around a black hole in a very particular way, there emerges a
41:15sort of mirror universe, a parallel universe on the other side of the black hole, identical
41:20to our own and traversable via the black hole.
41:30So black holes are not just edges to our universe.
41:35They may also be gateways to other universes.
41:40It's highly conjectured, but if there's ever going to be a space where, or a region, where
41:46you're making connections with, say, some other universe, a black hole in principle
41:51could be a portal to that.
41:56But it's highly unlikely anyone will ever want to venture beyond an event horizon.
42:04And our pursuit of the other edges in the cosmos offers little hope.
42:09We can never travel beyond the cosmic event horizon, and we will never be able to see
42:17beyond the edge of our observable universe.
42:22So can we ever hope to discover the true edge of the greater universe, or find out if it
42:29even has one?
42:32My feeling is that probably we should not think about edges for the universe.
42:37Everything you've ever seen in your life is finite.
42:40It has an inside and an outside.
42:41It has an edge.
42:43The universe might not be like that.
42:46It's probably not like that.
42:47There's probably no sense in which the universe has an edge.
42:53We used to think that the ultimate limits on the future life were set by nature.
42:59We couldn't get off the planet, or there was nothing beyond the solar system.
43:03Now we realize we have this vast, vast cosmos out there, and that the ultimate limits are
43:09actually simply our own imagination and our ability to do great things with it rather
43:15than self-destruct.
43:17Our future destiny is in our own hands.
43:20And I find that very empowering.
43:22It is beautifully frustrating to realize how limited we are.
43:28To realize that we're probably never going to get a true view of the real extent of the
43:32universe.
43:33We should keep an open mind.
43:34We should be humble.
43:35But I think that we should give up on the idea that things should have edges because
43:38that's what we're familiar with.
43:39The universe is something special.
43:42What matters to us, and will only ever matter to us, is the observable universe.
43:49Because that's the limit of what we can see, and that is the limit of what we can know.
43:54So there is an edge to the universe.
43:56There's an edge to what we can know.