How the Universe Works - S06E10 - Mystery of Spacetime
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00:00What is our universe made of?
00:05It's the biggest unanswered question in science.
00:08Despite the name, space is not an empty space at all.
00:13Space itself is something.
00:16There's a hidden structure and a force that exists within space itself.
00:22Space-time is something absolutely real, it's absolutely fundamental, it's really part of
00:26the fundamental architecture.
00:28A force that connects everything in our universe.
00:32It's an active player in the game of life.
00:37It underpins our reality, tying together all of space and time since the very beginning.
00:44We call it space-time.
00:48It's everything, space-time is what the universe really is.
00:55Space-time is how our universe works, but what exactly is it and how does it control
01:01our past, present and future?
01:24We can't see it, we can't touch it, but without space-time we wouldn't be here.
01:35Space-time is the fabric of our reality, it shapes and governs our lives.
01:39If we want to understand the story of the universe, it's absolutely crucial we understand
01:44how space-time behaves.
01:48Space-time has been active since the beginning of everything and is the key to the evolution
01:53of everything.
01:56We have to understand space-time in order to understand the history of the universe,
02:00to understand how the universe began, how it evolved and what's going to happen in the
02:04future.
02:06The story of space-time is the story of our universe.
02:11To know how the story evolves and how it will end, we need to go back to the very beginning.
02:20To a time when there was nothing, no stars, no space, a time before there was time.
02:30Then all of a sudden...
02:37Our entire universe was born in the Big Bang.
02:41It started in a instantaneous moment where from nothing our universe was created.
02:47The very definition of the moment of the Big Bang is that space and time were created
02:52at that instant.
02:54It is, as far as we currently know, the coming into existence of space and time itself.
03:03The infant universe, a tiny speck of energy and space-time materialized from nowhere.
03:11Then the universe suddenly expanded.
03:15The idea of inflation is that a very tiny region in an incredibly short amount of time,
03:20far shorter than a second, grew by many, many, many orders of magnitude.
03:26So imagine myself suddenly becoming the size of a galaxy.
03:31In a fraction of a second, the universe grew from smaller than the size of an atom to the
03:37size of a cricket ball.
03:40In cosmic terms, it's similar to a grain of sand growing almost to the size of the
03:45observable universe.
03:48The universe at the instant of inflation actually expanded faster than the speed of light.
03:53It seems to be a violation of everything you've heard in physics.
03:56You may be thinking, hey, hey, hey, Mr. Astronomy Guy, nothing can move faster than the speed
04:00of light.
04:01It turns out that's kind of true.
04:04But the rule is nothing can move through the universe faster than the speed of light.
04:08But inflation, it's space itself that is expanding.
04:12So there is no violation, there is no paradox.
04:20Inflating fast, the universe went through a phenomenal growth spurt.
04:26At the moment of the Big Bang, space-time was this entity that was flying out in all
04:31directions.
04:32It was space itself that was expanding.
04:35However, the universe didn't expand evenly.
04:39One spot in the universe was ever so slightly more dense than a spot right next to it.
04:44And we're talking about a tiny, tiny fraction of a percent.
04:47One part in a hundred thousand, but that was enough.
04:52Fluctuations in expanding space-time created areas with higher density.
04:59Inflation made these high-density regions larger, which enabled our universe to take
05:04shape.
05:08When parts of the universe didn't inflate quite the same way as others, all of a sudden
05:13things could start to come together.
05:18As the universe cooled, energy turned into matter.
05:24In the denser regions, that matter started to clump together, and crucially, these regions
05:30had more mass than others.
05:35Mass bends space-time, so anything that is made of matter bends space-time, and the more
05:41matter you have in one place, the more you bend it.
05:43In fact, I'm bending space-time right now.
05:46When I flex, I bend it even more because of my incredibly high muscle density.
05:50I don't bend at the maximum, I don't want to destroy the Earth and the solar system,
05:54but you know, it's an effect, it's a real thing.
05:56Space isn't constant, it's not something that is always the same everywhere.
06:00It actually bends, curves, it warps, depending on the matter inside of it.
06:11We'd see the curving grid of space-time moving and reacting to objects within it.
06:17And we'd feel the curving of space-time as the force we call gravity.
06:25Gravity is different from all the other forces.
06:27It is intimately connected with the curvature of space-time.
06:31Something that can bend space and time has gravity.
06:35That's what gravity is, the bending of space and time itself.
06:41It's hard to visualize this, but a good analogy is a trapeze artist and their safety net.
06:51You can imagine a trapeze artist falling into a net on purpose.
06:54That net is flat and looks like a nice, orderly, evenly spaced grid.
06:58But when they fall into it, they distort that grid.
07:01Well, that's a lot like space.
07:04If you have matter in space, it warps the framework.
07:07When the trapeze artist is resting in the net, they're bending that space-time grid
07:12a little bit.
07:13If you had two trapeze artists in there, double the mass in roughly the same volume, you would
07:18get a bigger dip, you have a bigger distortion.
07:21And that's how space-time works.
07:27More mass equals a bigger curve in space-time, which equals more gravity.
07:32But understanding the nature of gravity and space-time is no easy thing.
07:38It's an idea developed by one of the greatest minds ever.
07:44Einstein had the idea that space itself is something, something that can be bent, something
07:50that can be stretched, that we are all bound together by space-time.
07:57Einstein says that space and time have a geometry, they have a life of their own, they have dynamics.
08:06These dynamics are what we call gravity.
08:09The more dense the region of matter, the greater the gravity, the deeper the curve.
08:16This connection is the foundation of our physical reality.
08:20It's the interaction between matter and energy and space-time that created the universe that
08:25we see around us today.
08:27However, that doesn't mean we fully understand it.
08:31There's much more that we don't know, and that's frustrating.
08:35With the laws of physics, I can talk about how space-time behaves, but it does appear
08:39to be something that stretches, that contracts, and that gravity is the embodiment of space-time.
08:46Born in the Big Bang, space, time and energy combined to create our infant universe.
08:56These basic materials were the foundations.
09:01But how did we get to the incredible complex structures we see today?
09:05How did space-time build our majestic universe?
09:16Our entire universe was created in the Big Bang 13.8 billion years ago.
09:22Everything came from nothing.
09:26Yet the modern universe is a complex mosaic of matter.
09:32When we mark the Big Bang, we mark it as the Big Bang of the universe.
09:38Yet the modern universe is a complex mosaic of matter.
09:44When we marvel through our telescopes at the fantastic structure of our universe and its galaxies,
09:50you've got to ask, where did that come from?
09:53Matter in the universe arranges itself on a vast cosmic web.
09:57Galaxies and galaxies' clusters are strung out on sheets and filaments.
10:04It seems this intricate web is organised by a cosmic architect.
10:09Space-time.
10:11It shaped everything from planets to galaxies, atoms to cities.
10:19The universe is made of space-time.
10:21Whatever the substance is, time and space bound together,
10:24that's expanding and creating the universe we see around us.
10:27It's everything. Space-time is what the universe really is.
10:33It's a hard concept to grasp and even harder to visualise.
10:40Scientists observe the universe in different wavelengths of light.
10:44This is the sun, invisible light, X-ray and ultraviolet.
10:52Now imagine if we could see it in the space-time spectrum.
10:56We would see space-time distorting as objects move through it.
11:02Space-time can warp and push things around.
11:05It can expand and pull things apart.
11:08But it's the shape of space-time that dictates how we experience it.
11:14Imagine you're in your car. You go up hills and you go down hills.
11:18So the shape of Earth's surface determines how you travel across Earth's surface.
11:23In the same way, the geometry of space-time
11:26determines how light and matter move through space-time.
11:33The rules are simple.
11:36Any object with mass tells space-time how to curve.
11:40The curvature of space-time tells matter how to move.
11:47Because the shape of space-time tells matter how to move,
11:52what we call gravity, this means that gravity and the shape of space-time
11:57tells matter how to clump together and form larger and larger structures.
12:04But at the beginning, the space-time landscape was very different to today's.
12:10So the very first matter started to change the shape of space-time.
12:16So this space right here has a tiny bit more matter in it than this over here.
12:21Wherever there was a little bit of extra mass,
12:24that would bend space a little bit more.
12:27Well, if you're bending space a little bit more,
12:29then more mass would collect there.
12:32In the early universe, these denser regions of matter
12:36created deeper curves in space-time.
12:39And as the mass gets bigger, as stuff falls into that well,
12:43it gets deeper and deeper and deeper and attracts more stuff,
12:46and it's just a runaway process.
12:48Gravity increased, pulling in more and more matter.
12:53It got more dense, got more dense, got more dense,
12:56and then before you know it, you've got a star,
12:58and you've got a bunch of stars, and you start to make a galaxy.
13:01And these stars evolved and began forming large structures.
13:04They sort of burned through all their nuclear fuel and exploded,
13:07and they made all the heavier elements.
13:09And with time, we got down to having things like planets,
13:14atmospheres, people, all the things that we care about today.
13:20All of this started out as energy fluctuations in expanding space-time.
13:25These at first very tiny fluctuations
13:28became these gigantic structures that we actually see today.
13:32And over billions of years, that material began to coalesce
13:35into individual galaxies, stars, planets, and you.
13:40Fluctuations in the expansion of space-time
13:43laid out the pattern of the universe.
13:48The curvature of space-time
13:50controlled the evolution of everything we see today.
13:55If space-time didn't have that property of bringing mass together,
13:59then all we would be is a thin haze of hydrogen gas,
14:02not a very interesting universe at all.
14:04But if space-time didn't curve because of matter inside of it,
14:08the universe would be a really weird place.
14:11I mean, there'd be no gravity.
14:13There'd be nothing to make things stick together.
14:16No force of gravity means no stars, no planets, and no people.
14:24We owe our existence to space-time.
14:28But even scientists struggle to understand it.
14:32I wish I knew what space-time is.
14:35We know things about space-time,
14:37but at the same time we feel like we know almost nothing about space-time.
14:50But sometimes scientists get a glimpse
14:53of this elusive puppet master in action.
14:57In 2016, astronomers witnessed a strange optical phenomenon,
15:02a weird circle of light similar to a cosmic halo.
15:07This is actually what you would see in the sky
15:10if your eyes were as sensitive as a telescope.
15:12They're real.
15:13This is not some artefact of how we adjust the images.
15:16Something is actually bending space and time itself into a lens.
15:22That something is a red galaxy,
15:25which is over 7 billion light-years away from Earth.
15:30It's bending the light from a blue galaxy,
15:33which should be hidden behind it.
15:35It's called gravitational lensing.
15:40Gravitational lenses are caused by objects with huge masses,
15:44say, clusters of galaxies, that distort space and time so much
15:48that when light comes from farther objects
15:51and has to pass around the galaxy clusters, the light bends.
15:55It really is a true warp in space-time.
15:59Mass from the foreground galaxy creates curves in space-time,
16:03which we know as gravity.
16:05Light follows those curves and is warped,
16:09so it bends around the galaxy.
16:12Massive objects, like clusters of galaxies,
16:16can bend the path of light through space-time.
16:20A lot like a piece of glass can bend the path of light.
16:24So when we look at a distant galaxy,
16:27as the light passes through a galaxy cluster,
16:29we see multiple images of the same galaxy.
16:33We see arcs and circles,
16:35as if that galaxy cluster were made of glass.
16:39We are seeing the warping of space-time
16:42literally played out in front of our very eyes.
16:47Gravitational lensing gives us a way of seeing the effects of space-time on light.
16:52However, it's only an indirect observation of space-time.
17:01Is there another way of experiencing space-time right here on Earth?
17:08Not everything that happens in space can be seen.
17:11Sometimes you have to listen for it as well.
17:19Believe it or not, space is a material much like this iron sheet.
17:23And like this iron, space can distort.
17:26If I put a very heavy weight on this sheet of metal,
17:29its shape is going to change and it's going to distort.
17:31Amazingly, space can carry waves, and so can this iron sheet.
17:35But to get this sheet waving, you need something really powerful.
17:39Something like me and my hammer.
17:48Did you see those waves travel through that iron sheet?
17:52Well, waves pass through space in exactly the same way.
17:56We call these gravitational waves.
18:01Gravitational waves are vibrations from cosmic events
18:04transmitted through the material of space-time.
18:09To set off waves in space,
18:11you need the biggest, baddest, most powerful events in the universe.
18:15Something like the collision of two black holes.
18:18When two black holes collide,
18:21the energy released sends shockwaves through space-time across the universe.
18:30By the time they reach Earth, they're so small they're almost immeasurable.
18:35But in 2015,
18:38scientists at the LIGO Observatory made a groundbreaking observation.
18:43They had detected ripples in space-time.
18:47Gravitational waves.
18:51Rumours began flying, but it became clear after a while
18:54that this was indeed the first direct detection of gravitational waves
18:58seen by man-made instruments on the Earth.
19:01When we discovered gravitational waves,
19:04it had been so long that we'd been waiting for signals.
19:07Not only did most of us not believe it,
19:10I went so far as to be so sceptical
19:13as to look into all kinds of conspiracy theories for ways it could be fake.
19:16When I saw this data, I still think back on it now,
19:20and the emotional impact it has on me.
19:23The only thing comparable is when I saw my daughter's face
19:26for the first time after she had been born.
19:29It was that kind of an emotional impact,
19:32just having all of this thing that we had worked for
19:35coming to fruition in one moment.
19:38It's mind-blowing.
19:43And we can actually hear these gravitational waves.
19:47Part of what makes this so amazing,
19:50it's a bit of a coincidence, but it's a really cool coincidence,
19:53is that the signals that LIGO actually measures
19:56are in the same frequency band
19:59as the sounds that the human ear is sensitive to.
20:03We can hear the waves change frequency
20:06as the two black holes get closer and collide.
20:13It's a swoop up in frequency that sounds like...
20:18What we were hearing in that...
20:21were two black holes that are orbiting around one another
20:24and then coming together. That was it.
20:28Listening to ripples in space-time
20:31has given us a powerful new tool to investigate the universe.
20:36We are now hearing things in gravity for the first time.
20:39It's a sense that we have never been able to apply to the universe,
20:43and we're beginning to learn what is out there.
20:46The observation of gravitational waves from black holes
20:49is one of the most significant findings in astronomy by anyone.
20:53in the recent hundred years.
20:56It's hard to overstate the importance of gravitational wave astronomy.
21:00Much like when Galileo first pointed his telescope at the stars
21:03to see something new, we now have an entirely new window into the universe.
21:11Gravitational lensing and gravitational waves
21:14offer us an insight into the complex relationship
21:17between gravity and space.
21:24However, what about the other half of the equation?
21:28Time.
21:30But it turns out, you know, it's space-time.
21:33Gravity not only distorts space, it actually distorts time, too.
21:38We think of time as something that can't be changed.
21:41It simply flows ahead at a constant rate.
21:44But that's not the universe we find ourselves in.
21:47In some crazy circumstances,
21:50time might even, according to you, stop.
21:54It might even be possible to travel through time
21:58and go back to the future.
22:01SPACE IS THE FINAL FRONTIER
22:19For science fiction fans, space is the final frontier.
22:25For scientists, exploring time is a much bigger challenge.
22:31Originally, we thought of time as the same thing as the sun rising and setting.
22:36But now we've come to realise that time is a more fundamental concept than that.
22:43Time isn't just something that passes.
22:46Time is an essential part of our universe.
22:50It's part of the fabric of space-time.
22:55The Big Bang was the beginning of space and time.
22:59Since then, space has been expanding and time has been ticking forward.
23:05It's been doing this for 13.8 billion years,
23:09creating the universe we see today.
23:13There's sort of two sides of the same coin.
23:16You can't have time without space or space without time.
23:19So when matter influences space-time,
23:22it's not just creating deformations in space,
23:25it's also affecting the flow of time.
23:28This is where space-time becomes really cool.
23:31Just as gravity bends space,
23:34it also distorts the flow of time.
23:38This isn't how we perceive time.
23:40This is actually the rate at which time flows.
23:43Very massive objects can warp and twist space-time itself.
23:47So not only is space distorted,
23:49but time itself can slow down or even stop.
23:53The stronger the gravity, the greater the distortion.
23:58What has the most gravity?
24:00A black hole.
24:03Out in space, near a strong gravitational tug from a black hole,
24:07clocks can do funny things.
24:09And this is where things start getting really interesting.
24:13Around a black hole, space-time warps and twists,
24:17slowing time down.
24:20Scientists dream of sending a probe there
24:23to test their hypothesis.
24:25There's a famous way of thinking about this called the twin paradox,
24:29where two twins are born at exactly the same time,
24:32so they're the same age,
24:34but one of them zips very, very close to a black hole,
24:37hangs out a while, and then comes back.
24:39If I had an identical twin who stayed back on Earth
24:42while I flew near a black hole,
24:44when we had our daily video phone calls,
24:47he would see me go,
24:49and I would see him say,
24:51Oh, my goodness, I'm a little bit concerned about you,
24:53because you're talking so funny.
24:55We would literally notice that time is running at a different pace
24:58for the other one.
25:01The closer to the black hole, the slower time passes.
25:05If instead of coming back home,
25:07I accidentally fell backwards into the black hole,
25:10my twin back on Earth would see me slow down even more and go,
25:13Oh!
25:16And completely grind the hull,
25:18and seem frozen on the event horizon.
25:21Time appears to stand still.
25:25I would just have this sinking feeling
25:27that I would never be able to come home again.
25:31But if the twin could escape from the black hole,
25:34he would be returning to the future.
25:37Maybe it's only been a few days or weeks
25:40experienced by the one that travelled to the black hole,
25:43while the other is, you know, grey-haired
25:46and has grandkids by now,
25:48has lived decades here on Earth.
25:54The black hole warps space-time so much
25:57that the ultimate science fiction fantasy
26:00would become a reality.
26:04Time travel is a staple of science fiction,
26:07and we know that time travel into the past
26:10appears to be ruled out in our universe.
26:13But time travel into the future is totally acceptable.
26:21Time travel isn't possible just yet.
26:24But space-time has a very real effect on our daily lives.
26:29It controls how we age.
26:34The key to different rates and flows of time is gravity.
26:40If you experience a different gravitational environment,
26:43you will have a different flow of time.
26:47As I climb up these stairs,
26:49and I put myself further away from the mass of the Earth,
26:52my own clock runs a little bit faster.
26:56If you go down closer to the surface,
26:58the more your clock slows down.
27:02We have sensitive enough clocks
27:05that we can measure this different flow of time.
27:10Exaggerate this effect,
27:12and we would see the flow of time change in front of us.
27:16Those closer to the Earth would appear slowed down.
27:20Those higher up, the opposite.
27:23And this means the wealthy in their penthouses
27:26actually age faster than people on the ground.
27:31This is a mind-blowing concept, but it's reality.
27:37Earth's gravity even controls time high above the planet.
27:4120,200 kilometers up,
27:44there's an array of global positioning satellites
27:47crucial to the navigation systems in our cars and mobile phones.
27:51We here on Earth use the global positioning system
27:54as a way of getting around.
27:56Most people these days would be lost
27:58if they have to go more than about a kilometer from their house
28:01unless they have their GPS app on their phone to tell them where to go.
28:05The GPS receiver in your mobile phone
28:07bounces signals over four satellites
28:10to figure out exactly where you are.
28:13It's an exercise in precision timing.
28:18On board each satellite is an atomic clock.
28:23The weaker gravity in orbit means these satellite clocks
28:26tick fractionally faster than those on the ground.
28:31If we didn't know to correct for the fact
28:34that our clocks and our satellites move at different rates,
28:36the GPS system here on Earth would not work.
28:39It would actually lose accuracy at such a rate
28:42that the entire global positioning system
28:44would become useless in less than an hour.
28:46We correct for that every moment of every day.
28:51Think about this as you go through your everyday life.
28:54Every time you go up a flight of stairs,
28:56you're actually experiencing a slightly different flow of time.
29:01But what exactly do we mean by flow of time?
29:07In space, I can go anywhere I want.
29:09I can go left or I can go right.
29:11But time, it appears I can only go from the past to the future.
29:17So if you were to play a movie backwards,
29:20it would be immediately obvious that it's flowing backwards.
29:25Our perception is that time only runs in one direction,
29:29from the past to the present to the future.
29:34But some of the most fundamental laws of nature
29:36work both forwards and backwards.
29:39They're time-reversible.
29:43We don't fully understand why time has an arrow,
29:46why it has a flow from the past to the future,
29:49why the past is different from the future
29:52and why we only experience it in the present.
29:55The fundamental aspects of time and why there's an arrow
29:58is one of the biggest unanswered questions in modern physics.
30:03What's even more confusing is that all points in time are equal.
30:09In space-time, there's no difference between past, present and future.
30:17There is nothing magical about now.
30:20Past, present and future is just illusory talk.
30:24The future is no more special as a part of space-time
30:28than what's in front of me.
30:31It's similar to watching this programme or a film.
30:35You might not know how it will end,
30:38but you do know that the end already exists.
30:42If we think of life as a movie, then space-time is the entire DVD.
30:46And if this particular moment in my life is a scene in the movie,
30:50then, of course, the past that happened before that scene
30:55and the rest of the film, the ending, it's all in there.
30:59They all exist exactly to the same extent.
31:04Understanding the true nature of time
31:07is key to understanding how the universe works.
31:11There's got to be physics behind it,
31:13and we're trying to figure out what it is.
31:16Without understanding that, we can't understand the origins of the universe.
31:22Space-time has controlled every phase of the universe's evolution since its birth.
31:28Now, we're discovering space-time will also dictate
31:32how the universe will die.
31:47Our universe started with a bang 13.8 billion years ago,
31:52and ever since, it's been expanding.
31:57However, will this expansion last forever,
32:00or will our universe come to a violent end?
32:17For almost 100 years, we've now known that the universe is expanding.
32:21Everything in the universe is expanding away from everything else.
32:25This can be tested by measuring light from exploding stars.
32:30Type 1A supernovas all explode with the same brightness,
32:35so scientists can accurately work out their distance from Earth.
32:41For decades, astronomers have measured this light
32:44and stretched by expanding space-time.
32:49The universe is expanding, and there's matter in it.
32:52That matter has gravity, and that is distorting the curvature of space-time.
32:56So it made sense to us that as the universe expanded,
32:59all of the matter in the universe would hold on to each other gravitationally.
33:04If there's enough matter in the universe,
33:06it can actually pull on itself enough that the expansion gets slower.
33:11But in 1998, astronomers took new measurements
33:15and made a sensational discovery.
33:19People had expected it to be slowing down, to decelerate,
33:22but instead they found the opposite.
33:24The expansion is accelerating.
33:28If expansion was slowing, then these distant lights should seem brighter.
33:33Instead, they were dimmer.
33:36They were getting further away, much faster than expected.
33:40And this could only mean one thing.
33:44That expansion is getting faster. It's accelerating every day.
33:50This discovery turned our understanding of the universe upside down.
33:56For the first seven billion years of the universe,
33:59the rate at which the universe was expanding was going slower and slower.
34:03But then something crazy happened.
34:06It was as if gravity had become the opposite.
34:10Instead of attracting the galaxies, it was almost as if it was pushing them apart.
34:15That's a very surprising result. We're still struggling to understand it.
34:21Was gravity losing its power?
34:24Or was something else pushing space apart?
34:27There's another ingredient in our universe,
34:30an ingredient that behaves very oddly.
34:34The mysterious quantity called dark energy.
34:39Similar to space-time, dark energy is all around us.
34:43It can't be seen, but it makes up 70% of the stuff in our universe.
34:49But what is it exactly?
34:52Dark simply means dark.
34:55Dark simply means that we have no idea what it is.
34:58We don't know what form it is in.
35:00Something is pouring energy into the universe, causing it to accelerate.
35:04We don't know what it is.
35:05No clue whatsoever.
35:07We don't understand it.
35:08That's the greatest mystery out there today.
35:13Dark energy behaves in mysterious ways.
35:18Ordinary matter is attractive.
35:21Dark energy is repulsive.
35:23That's why it's causing an acceleration.
35:26Ordinary matter feels gravity. It comes together.
35:29But this stuff doesn't.
35:31Is dark energy a new force in the universe?
35:35Or, like gravity, could it come from space-time itself?
35:40Dark energy may very well be a property of space-time.
35:44It may be that space itself has an energy,
35:47and it's this energy that's driving it to accelerate in its expansion.
35:50Dark energy is a thing.
35:52We don't really know exactly what it is,
35:54but it will have a huge effect on the future changes in the universe.
35:59So what will happen if dark energy continues to keep accelerating
36:03the expansion of our space-time universe?
36:07Because of the presence of dark energy,
36:09it'll expand faster and faster and faster,
36:12which means the universe is going to become a lonelier and lonelier place to be.
36:17All the galaxies are accelerating away from each other.
36:19The universe gets dimmer and dimmer and colder and colder.
36:24Everything gets darker and more desolate.
36:26And right now, that is the leading candidate for what's going to happen in our future.
36:31Eventually, our space-time universe will freeze.
36:35The Big Freeze is the ultimate endgame of the universe as we know it.
36:41It is an ugly fate. It's a depressing fate.
36:44But luckily for us, it's not until an unimaginably long time from now.
36:52Trillions of years from now, the universe could end in a Big Freeze.
37:01But a 2017 study hinted at an even more frightening possibility.
37:08Dark energy might be getting stronger.
37:14One horrible scenario for the ultimate fate of the universe
37:19is if dark energy eventually grows so strong enough
37:23that it can overwhelm the gravitational attraction of a galaxy itself.
37:27It's even able to rip black holes apart.
37:32The very fabric that holds everything together could be ripped apart.
37:35This idea is called the Big Rip.
37:38First clusters, then galaxies such as our own Milky Way will be torn apart.
37:45Then our solar system will break up.
37:48And in the final half hour of the universe, the Earth will explode.
37:54In the final second, atoms will vaporize.
37:59Everything in the universe would individually be torn apart by the expansion of space.
38:05We don't understand dark energy.
38:07Is it constant? Is it getting stronger? Is it getting weaker?
38:11At this stage, we simply don't know.
38:20So it seems that space-time not only controlled the birth of our universe,
38:24but will also dictate its death.
38:30And we're now discovering these two events may be linked.
38:35A link that reveals a flaw in our understanding of the Big Bang.
38:40All the galaxies, all the planets and stars, all of this matter
38:44was compressed into a tiny volume, shrunk down to infinitely small size.
38:52We call this tiny, infinitely dense point a singularity.
38:59Singularities are predicted by the general theory of relativity.
39:03But the universe is also governed by another set of rules.
39:10Quantum mechanics.
39:14Quantum mechanics is our description of the subatomic realm,
39:19of fundamental particles and fields and forces and how they interact.
39:26Quantum mechanics states that nothing can be infinitely small or dense.
39:31So singularities can't exist.
39:36Singularity is a bit where everything kind of goes to hell
39:39because the densities become infinite, gravitational forces become infinite.
39:43Things just sort of break down there and the equations sort of stop making sense.
39:49But no singularity would mean no Big Bang as we understand it.
39:53So then how did the universe spark into existence?
39:59Scientists now think they have an answer.
40:01A solution that works with both general relativity and quantum mechanics.
40:09Quantum space-time.
40:13A successful theory of quantum space-time should answer the question
40:16of what really happened in the earliest moments of our universe.
40:19Hopefully, the correct quantum theory of gravity won't have any singularities.
40:24It will replace the Big Bang with something else.
40:27If we only knew what that something else was,
40:30we might have a clue as to how and why the universe began.
40:35In a quantum space-time Big Bang,
40:38there was no singularity bursting from nothing.
40:42Instead, the universe formed from the remnants of the Big Bang.
40:46Instead, the universe formed from the remnants of another dying universe.
40:53It's possible that before the Big Bang there was still a universe,
40:56there was still space and time,
40:58but rather than expanding, the universe was contracting.
41:04Perhaps universes don't end in rips or freezes.
41:07Perhaps they collapse.
41:11An ancient universe expands,
41:14but then begins to collapse under its own gravity,
41:17crunching space-time down to a speck.
41:25But instead of forming a singularity,
41:28space-time explodes once again.
41:31As matter gets more dense, then as the matter crunches down,
41:35this force will push everything back out.
41:40Then the universe would bounce and reignite
41:43in a new round of expansion, a new Big Bang.
41:46What we perceive as the Big Bang is the aftermath of that bounce.
41:54This suggests that the space-time that dictates our lives today
41:58comes from the collapse of an old universe,
42:01and we live in an infinite space-time cycle
42:04of birth, death and rebirth.
42:09A bouncing space-time universe.
42:13If we live in a bouncing universe, it's very plausible
42:16the universe is infinitely old,
42:18that it's gone through an infinite series of bounces
42:21and there is no absolute beginning.
42:23It could be that we are just one iteration
42:26of an infinite number of cycles in the lifetime of some meta-universe.
42:33We barely understand space-time.
42:36Perhaps we will never understand it completely.
42:40But one thing is clear.
42:42Without space-time, we would not be here.
42:47Space-time is something absolutely real.
42:50It's really part of the fundamental architecture,
42:53the furniture of reality.
42:57To really understand the ultimate fate of our cosmos,
43:00it's not enough just to look more with our telescopes.
43:03We also have to understand the basic nature of space-time.
43:10I think the biggest thing to take out of all of this
43:14is that the universe is weird.
43:20You hear about these very weird things that we talk about
43:23when we study the universe and cosmology and relativity.
43:26It sounds like it could be all made up.
43:28But trust, it's not.
43:33You really are right now living in a far more complex and beautiful universe
43:37than the human mind can comprehend.