How the Universe Works - S09E10 - Gravitational Waves Revealed
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00:00First, there was light, visible light.
00:07Then we viewed the universe in radio waves and x-rays.
00:13Ever since there's been astronomy, we've been looking at different kinds of light and opening
00:19up the universe a little bit more at a time.
00:24But then in 2015, like the roof came off.
00:29Something happened that changed everything.
00:31The ability to see waves in space and time itself.
00:36Gravitational waves.
00:37They help us roll back the clock to the dawn of time, discover epic cosmic collisions,
00:45and make earth-shaking discoveries.
00:49Gravitational waves are the biggest game changer since the invention of the telescope.
00:53We have a completely new universe to view now.
00:57A new exploration of space is just beginning.
01:13Long ago, 17 billion light years away, a cataclysmic showdown plays out.
01:20Two black holes locked together in a deadly cosmic dance.
01:26Black holes are unimaginably dense objects with gravity so intense that if you get too
01:32close to them, you're gone.
01:39Their immense gravitational pull causes them to spiral towards each other.
01:46When black holes collide, they don't just run into each other.
01:49They're in orbit about each other.
01:52So what we're talking about is an in-spiraling orbit.
01:55And it goes faster and faster and faster and faster until they finally collide in a
02:01fatal embrace.
02:08But astronomers don't see a thing.
02:11The problem with observing colliding black holes is all about the name.
02:15Black holes, they give off no light.
02:17How can astronomers see something that no telescope can detect?
02:23Across the universe, extraordinary events take place.
02:29But we sometimes miss them because we rely on light.
02:34Now astronomers have a new tool kit that's revealing the cosmos in a totally different
02:40way.
02:44Using the very fabric of our universe we call space-time.
02:52Everything with mass, like stars, planets, and black holes, all curve this fabric.
03:00The more massive the object, the bigger the distortion of space-time.
03:05The classical analogy is this stretched rubber sheet, right?
03:09And like a mass, like the sun is like a ball on this sheet and it distorts and warps the
03:14sheet into this valley, right?
03:16And if you roll a marble across it, like the marble is a planet, the marble will be pulled
03:21into orbit around the ball because of the curvature of the sheet.
03:29But that's only half the picture.
03:31If an object has mass and is accelerating through space-time, it creates ripples in
03:37that fabric of space-time, and we call these gravitational waves.
03:43Gravitational waves give us vital clues about distant objects that we can't see.
03:50The more massive the object that produces them, and the faster it's moving, the bigger
03:55the ripples.
03:58These ripples pass through planets, stars, and galaxies with ease.
04:05When a gravitational wave passes through an object like a star or a planet or a person,
04:10it stretches and compresses them, like with this tennis ball.
04:15Now, if you're close to a powerful source of gravitational waves, like merging supermassive
04:20black holes, those waves are incredibly strong, and they're capable of actually destroying
04:24a planet.
04:27But like the ripples on a pond, their strength and size diminishes over distance.
04:34The farther away you are, the weaker they get.
04:37And when they're hundreds of millions of light years away, they're actually smaller than
04:41the size of an atom.
04:43So to listen for gravitational waves, scientists built the most sensitive measuring device
04:48on the planet.
04:55This is LIGO, the Laser Interferometer Gravitational-Wave Observatory.
05:03Two enormous detectors located almost 2,000 miles apart in Louisiana and Washington State.
05:12Each sensor has L-shaped arms measuring two and a half miles.
05:19Inside the LIGO detectors, inside these concrete tunnels, there is a laser system.
05:25It's called an interferometer.
05:28So light comes in from a laser beam and is split into two paths.
05:35Normally, the lengths of the two beams are the same.
05:41That changes when gravitational waves hit the beams.
05:47When a gravitational wave passes through, it changes the distance that light travels
05:52along these arms.
05:53So one arm effectively gets longer, and the other one gets shorter.
05:59The length of those two beams varies just ever so slightly, and the very sensitive apparatus
06:05in LIGO is able to pick that up.
06:09With this ultra-sensitive laser system, LIGO picks up distortions in space-time narrower
06:16than one millionth of the diameter of an atom.
06:20Just that feat, just the fact that we were able to build a detector to detect gravitational
06:25waves is just mind-boggling.
06:29All of a sudden now, we were listening to the faintest whispers of the universe.
06:34In 2015, LIGO picked up a whisper that had been traveling towards Earth for over a billion
06:41years.
06:44Its source?
06:45Two colliding stellar black holes.
06:49Watching two black holes spiral in and merge, that's not something we can do using optical
06:54telescopes or x-ray telescopes or anything like that.
06:57But with LIGO, we could actually detect that event.
07:11Now scientists can paint accurate pictures of invisible objects.
07:18You can tell.
07:19You're looking at black holes.
07:20You can get their masses.
07:22You can get their distance.
07:23There's a phenomenal amount of information in that wave.
07:29The colliding black holes are the most massive LIGO has ever detected.
07:34One is 66 times the mass of our sun.
07:38The other, 85 times the mass of our sun.
07:43As two black holes are spiraling in, they are moving faster and faster as they get closer
07:47and closer.
07:49That means that the gravitational waves they are emitting have a higher and higher frequency.
07:54So as time goes on, the pitch gets higher.
07:57So it goes oooop, oooop, zzzzzzoop.
08:08When they finally merge, they create a giant.
08:12By analysing that data, it's possible to establish that the new black hole, from the
08:19merger of these two original black holes, weighs as much as something like 140 times
08:25the mass of our sun.
08:29It's really difficult to overstate the importance of gravitational wave detection.
08:32It's like adding on an entirely new sense.
08:35All of a sudden, there's a brand new way to observe a black hole.
08:39All of a sudden, there's a brand new way to explore the rest of the universe.
08:46Invisible cosmic collisions are just the beginning of what gravitational wave astronomy can reveal to us.
08:54Now, scientists are using gravitational waves to revisit other long-standing mysteries.
09:01Like, what causes the brightest explosions in the cosmos?
09:07This is not an everyday car crash.
09:09This is the most dramatic event that you're ever going to see in our universe.
09:23Across the universe, strange bursts of light puzzle astronomers.
09:30For just a fraction of a second, they shine more than a trillion times brighter than the sun.
09:36Then, they vanish.
09:40These brief flashes of light are known as gamma ray bursts, or GRBs for short.
09:46And they're such a mystery because they are insanely energetic, and we don't know what causes them.
09:55For decades, these short gamma ray bursts have been an enigma.
10:01No explanation was off-limits, no matter how wild.
10:05Is it a supernova? Is it an alien civilization saying hello? You know, we just don't know.
10:13In August 2017, the Fermi Gamma Ray Telescope detected another short gamma ray burst.
10:22But this one was different.
10:25So, a gamma ray burst went off 130 million light years away.
10:29And it actually produced a ripple in space and time that LIGO could detect.
10:34Gravitational waves could help finally reveal what causes one of the brightest explosions in the universe.
10:44LIGO's data suggests the culprit could be two massive objects spiraling towards each other and colliding.
10:52But based on the gravitational wave data, these two objects were too small to be black holes.
10:59They had to be something else.
11:02Not black holes, but the ultra-dense cores of collapsed stars, called neutron stars.
11:10A neutron star is what's left over after a massive star collapses in on itself.
11:16It's very, very dense because it took all, essentially, the mass of the core and contracted it into a really, really small radius.
11:32As the dense neutron stars spiral ever closer, the gravitational wave signal gets stronger and stronger.
11:40Until they collide, releasing an epic burst of gravitational waves.
11:48Because they're not black holes, light can get out.
11:53And if you smash two things together at these kind of absolutely massive speeds, there's a huge amount of energy involved.
12:01Energy we detected both as invisible gravitational waves and visible light.
12:10Could this light be a mysterious and ultra-powerful gamma-ray burst?
12:17How could these colliding dead stars be associated with gamma-ray bursts, which are in fact the most energetic explosions we see in the entire universe?
12:26Neutron stars have powerful magnetic fields that trap particles of gas and dust.
12:33During a collision, the swirling magnetic fields twist up, building up more and more energy.
12:42You have lots of little particles of matter that are trying to keep up with these rapidly spinning magnetic fields.
12:49That starts swooshing them around until they reach pretty much the speed of light, and eventually they're kind of shot out of the remnant in a tight beam.
13:01The beam is a gamma-ray burst.
13:04But they're not always easy to detect.
13:08If the jet coming out is pointed right at you, then you see this extremely high-energy event, the gamma-ray burst.
13:15If it's not pointed at us, we might miss it.
13:20Fortunately, the gravitational waves show us where to look.
13:30Following the gamma-ray burst, we spotted a strange red cloud, evidence of a heavy element factory.
13:38After the initial collision, there is a shell of debris moving outwards.
13:43But then high-energy neutrons come slamming into this material and start to build heavier elements one after another.
13:55We can see the gold. We can see the potassium.
14:00We can see the plutonium being created before our eyes.
14:05The neutron star collision produced huge quantities of heavy elements,
14:11blasting out enough gold and platinum to weigh more than ten times the mass of the Earth, solving a long-standing mystery.
14:19We knew that supernova explosions did create some of the heavier elements,
14:25but from everything we've observed about supernovae, there's no way to tell.
14:30This was the missing piece.
14:33The gold on your wedding ring, the gold in your jewelry,
14:39was formed and forged from a titanic collision before the Earth even existed.
14:46The combination of supernovae and supernovae,
14:51and the fact that they're so close to each other,
14:54proves that neutron star collisions create precious metals
14:59and cause super-bright gamma-ray bursts.
15:05When you can measure a gravitational wave signal and a light signal like a gamma-ray burst,
15:12you get a whole new way to solve complicated problems.
15:16When you can measure a gravitational wave signal and a light signal like a gamma-ray burst,
15:22you get a whole new way to solve complicated, intertwined physical processes.
15:29It's like you're watching a symphony on mute, and then you hit that button and the sound comes on,
15:35and it's just a completely different picture.
15:41The sounds of the cosmos don't just reveal collisions.
15:47It turns out we can use gravitational waves to help us understand some of the biggest mysteries of the cosmos.
15:58Every new way we figure out to probe the universe is a good thing.
16:03And detecting gravitational waves, it's a new dimension to being able to study the universe.
16:08It's like having a new sense.
16:11This new sense could be just what astronomers need to answer some of the biggest questions in physics.
16:19Like, what is the speed of gravity?
16:23And does it travel at the universe's speed limit?
16:27One of the things we learned early in science is that the universe has an absolute speed limit,
16:33which is the speed of light in a vacuum, which is 186,000 miles per second.
16:39Light from the sun takes 8 minutes and 20 seconds to reach Earth.
16:44So, if the sun disappeared, we wouldn't miss its light immediately.
16:51But how quickly would we notice its missing gravity?
16:56The first thing that we'd notice is nothing.
16:59Things would seem very normal, but then they wouldn't.
17:03There would be nothing curving space where Earth is located,
17:08and so Earth would take off in a straight line, moving at the same speed at which it orbits the sun.
17:14And things would get cold and lonely really, really fast.
17:21According to Albert Einstein, our skies would go dark,
17:25and the Earth would be flung into deep space at exactly the same time.
17:29It's a foundation of his famous theory of relativity,
17:33still the most complete theory of how our universe works.
17:38Einstein's theory of relativity has been a fantastic theory.
17:42It explains so many things for us, including gravity.
17:46But when we look out at the universe, there are many mysteries,
17:50there are things that are quite hard to explain.
17:52At the top of the list, the mystery of our expanding universe.
17:59There is something pushing outward that is making that expansion rate ever and ever faster.
18:06Astronomers call this something, dark energy.
18:12It accounts for 70% of the total mass of the universe.
18:16Dark energy.
18:19It accounts for 70% of the total energy in the universe.
18:27Einstein's models of the universe need dark energy to work,
18:31but we have no idea what it is.
18:36Dark energy is not something we actually understand.
18:40It's kind of a placeholder term for something we don't understand.
18:44And so people naturally are looking for better theories.
18:48Theories that are a bit like Einstein's theory,
18:51but just go that bit further and explain some of these things that we don't currently understand.
18:58One way to excise dark energy is with a new theory of gravity.
19:03One where the speed of gravitational waves is different from the speed of light.
19:07There are some so-called non-Einsteinian theories for the structure of spacetime itself
19:12that don't actually require dark energy.
19:15For example, if gravity doesn't propagate through spacetime at the same speed that light does,
19:20you could find models that don't actually require dark energy.
19:24It could be a clean, simple, albeit very, very profound solution to this underlying problem.
19:32In order to overthrow Einstein and eliminate dark energy,
19:35the speeds of light and gravity must be different.
19:40We know the speed of light.
19:43So how do we test the speed of gravity?
19:47In order to test the speed of gravity,
19:50you need to have a system that emits both gravitational waves and light.
19:55The colliding neutron stars detected by LIGO in 2017 are part of the solution.
20:00The collision released a flash of light along with a burst of gravitational waves.
20:10But the universe threw a curveball.
20:14The light signal arrived 1.7 seconds after the gravitational wave signal.
20:20Does that mean gravitational waves travel slightly faster than light?
20:25Albert Einstein predicted that gravitational waves would move at the speed of light.
20:30So what if Albert Einstein was wrong?
20:33I know, sounds crazy, right?
20:35That's like almost as crazy as me being wrong, right?
20:38But if Einstein was wrong, that's one thing.
20:42But a bigger problem is that we'd have to rethink our physics.
20:47Einstein predicted that gravitational waves would move at the speed of light.
20:51But a bigger problem is that we'd have to rethink our physics.
20:56Before we do that, let's take a closer look at the neutron star collision site.
21:02It's surrounded by a shroud of gas and dust.
21:07Light is made of particles called photons, which scatter when they hit obstacles.
21:13But gravitational waves pass through anything.
21:18They pass right through everything like it's not there.
21:21Light, on the other hand, was slowed down by interactions with that matter.
21:26It didn't just escape immediately like the gravitational wave signal did.
21:32The debris gave the gravitational waves a head start by slowing the light.
21:37So gravitational waves and light do in fact travel at the same speed.
21:43Einstein was right.
21:46This one event ruled out the other theories of gravity that are competing with Einstein's theory.
21:53Things that people have been working on all their life, and overnight it's gone.
21:59Thanks to gravitational waves, dark energy remains our best explanation for why the universe's expansion is accelerating.
22:09Maybe dark energy isn't what we think it is.
22:11And maybe tomorrow, or maybe next year, or maybe next decade, or next century, we will discover that.
22:17Gravitational waves are a huge step forward in our effort to understand the universe.
22:23And I mean everything. Space, time, matter, dark energy.
22:28We have a completely new universe to view now.
22:34Now, astronomers want to use gravitational waves to answer another mystery.
22:39What happens when supermassive black holes collide?
22:52We first detected gravitational waves in 2015.
22:56Since then, they've revealed colliding black holes across the universe.
23:01Prior to LIGO going online, we'd never witnessed black hole collisions directly.
23:07But now that we can witness them with our observatories, we're finding them pretty regularly.
23:13We're seeing gravitational waves come across the LIGO experiment left and right.
23:20But LIGO has only been listening for gravitational waves from black holes on the smaller end of the cosmic scale.
23:27When we look at the cosmic zoo of black holes out there, we find small ones weighing, you know, 10, maybe 30 times as much as the sun.
23:37And then large, all the way up to extra large, going from like a million to a billion times as much as the sun.
23:43These supermassive black holes lurk at the hearts of galaxies.
23:49When galaxies merge, supermassive black holes collide.
23:54When galaxies merge, supermassive black holes should merge too.
24:03But even though we see galaxies colliding across the universe, we've never seen two supermassive black holes collide.
24:13Because they have too much orbital energy to get close enough to merge.
24:23That orbital energy has to go somewhere.
24:27And what supermassive black holes do is they throw out stars that are around the core of the galaxy.
24:33But when they get sufficiently close, there are just no more stars to throw out.
24:38And so the idea is they can't merge.
24:41So there's a problem. How is it that they managed to bridge that gap and finally spiral in?
24:47The only way to understand if supermassive black holes merge is by looking at their gravitational wave signal.
24:55Two supermassive black holes merging should release a burst of gravitational waves millions of times more powerful than a stellar mass black hole merger.
25:08But LIGO won't hear a thing.
25:11The problem with using LIGO to detect the merger of supermassive black holes is actually a scale of time.
25:20One wave, as these things move around each other very slowly, would take over 10 years to go by. Just one wave.
25:28In order to detect a gravitational wave with periods of decades, you also need an experiment that can be extremely stable over that amount of time.
25:38Vibrations from earthquakes, weather, or even nearby traffic prevent LIGO from listening for a decade, just to hear one wave.
25:50But there may be another way to detect gravitational waves from supermassive black holes, using a strange type of dead star called a pulsar.
26:01A pulsar is a kind of neutron star that is rapidly spinning and has a beam of radiation that makes wide circles across the sky.
26:12And when that flash of circle washes over the planet Earth, we get a little beep, a little beep, we get pulses of radiation, hence pulsar.
26:24Pulsars are the best timekeepers in the universe.
26:27But passing gravitational waves make them miss a beat.
26:31What if we noticed that the frequency of a pulsar was shifting very, very slowly, year to year to year, over 10 years or more, just slightly getting a little bit longer, as space itself was changing between us and the pulsar?
26:46By monitoring dozens of pulsars, Chiara Mingherelli and a team of astronomers have created a galaxy-sized gravitational wave detector.
26:57It's called a pulsar timing array.
27:02You can really look for deviations in those arrival times over decades, almost like a tsunami warning system.
27:09To show you when a gravitational wave is passing by.
27:15After 12 years, the team detected the same change in a number of pulsars.
27:21These pulsars are all thousands of light years apart.
27:25If you think about it, it's difficult to make a signal that's the same in all of these pulsars.
27:31This has to be this common signal from something like a gravitational wave event.
27:39The signal the team detected wasn't created by just two supermassive black holes colliding.
27:47It's evidence of gravitational waves from hundreds of pairs of supermassive black holes.
27:52Because it takes so long for one of these individual binary systems to merge, there could be thousands, if not millions, of these signals all being emitted at the same time.
28:03All of them. They all create this gravitational wave.
28:07Because it takes so long for one of these individual binary systems to merge, there could be thousands, if not millions, of these signals all being emitted at the same time.
28:20All of them. They all create this gravitational wave background that we're just starting to see the first signs of now.
28:28Astronomers predict this gravitational wave background fills our universe.
28:36If the signal the team detected is confirmed, it's proof that supermassive black holes do merge.
28:44The next step is to observe that as it happens.
28:49It would be a dream to see two supermassive black holes merging, emitting gravitational waves, and also being able to point a telescope at them and to see the physics of how they merge.
29:01Gravitational waves reveal the hidden workings of the cosmos.
29:06They reach the farthest corners of our universe.
29:11Now, astronomers are using gravitational waves to look back in time.
29:17They'll let us see all the way back to the earliest moments of our Big Bang.
29:23The Big Bang
29:3513.8 billion years ago, the universe sparks into life.
29:43The tiny speck of energy expands and cools.
29:48The infant cosmos is a fog of tiny particles of matter.
29:54Over time, the particles form atoms of hydrogen and helium.
30:00The fog clears and the first light races across the universe.
30:05We call that light the cosmic microwave background.
30:10The cosmic microwave background is simply the most distant light we can see.
30:14So, looking at it gives us baby pictures of our universe the way it looked 400,000 years after our Big Bang.
30:21What happened before these baby pictures remains a mystery.
30:27The leading theory is that in the very first second of the Big Bang, our infant universe had a growth spurt.
30:37Scientists call this idea inflation.
30:41In a billionth of a billionth of a billionth of a second, our universe grew a billion, billion, billion, billion, billion times bigger.
30:54That is the mother of all growth spurts.
30:57It laid the foundations for the entire cosmos that we know today.
31:08Inflation is just a theory.
31:11But there may be a way to prove it happened.
31:15Scientists think that during that brief moment of cosmic expansion, inflation stretched tiny fluctuations of gravity.
31:23That is such a violent process that it actually causes ripples and distortions in the very shape and fabric of space itself, which we can see today as gravitational waves.
31:34Scientists call these theoretical ripples through the early universe primordial gravitational waves.
31:42When they were first released, these were deafening.
31:47But in the billions of years since, our universe has grown bigger and colder, and these gravitational waves have diluted so that they barely even exist today.
31:58Scientists searched for signs of these very weak primordial gravitational waves in the cosmic microwave background.
32:07And in 2014, a team using their purpose-built microwave array in Antarctica, called BICEP, found a strange swirling pattern.
32:18When they saw those swirls, they saw those patterns, they thought they had seen the signature of primordial gravitational waves.
32:26Now, this is really the conclusive evidence that inflation had to have happened.
32:34The results were exciting, but there was a glitch.
32:37This amazement lasted for a few months until cracks started appearing in this, and gradually it all collapsed.
32:47The signal thought to be proof of primordial gravitational waves, and the theory of inflation, turned out to be a case of mistaken identity.
32:57As this light from the ancient universe, from the cosmic microwave background, travels through the universe, it had to travel through dust before reaching our detectors.
33:07And the dust itself can affect the light, and mimic what the primordial gravitational waves can do.
33:16The primordial gravitational waves seem to be the cause of all of this.
33:21What can primordial gravitational waves do?
33:25The primordial gravitational wave signal turned out to be, mainly, clouds of dust floating through space.
33:35That's how BICEP bit the dust.
33:39BICEP failed to detect primordial gravitational waves.
33:44Can LIGO do any better?
33:47Everything changed in 2015, with that announcement of the first detection of gravitational waves.
33:55That is one of the great triumphs in all of science.
33:58We have broken through the electromagnetic window on our universe, and we've moved into the gravitational wave.
34:05Ladies and gentlemen, we have detected gravitational waves. We did it.
34:13They were jumping up and down, they were crying, they were smiling.
34:17The first LIGO detection of two black holes spiraling in and merging, and emitting these gravitational waves, was a very big deal.
34:27This was the very first time gravitational waves had ever been directly detected.
34:31And we've been trying for like half a century.
34:34Einstein predicted this in the early 20th century, and it took us literally 100 years before we could detect them for the first time.
34:40I'm getting goosebumps just thinking about that. We really had found a gravitational wave.
34:46Unfortunately, LIGO can't help us in observing primordial gravitational waves.
34:51It can't even observe supermassive black holes in the centers of galaxies.
34:55It is designed to observe in a particular frequency range.
34:58Primordial gravitational waves are at such a low frequency, in such a low amplitude, that there is no hope of LIGO being able to detect them.
35:10But scientists hope that an ambitious project called LISA will.
35:19Not on Earth, but from 30 million miles above.
35:25LISA is like LIGO, but bigger and in space.
35:29LISA, or the Laser Interferometer Space Antenna, will be a system of three satellites, arranged in a giant triangular formation, 1.5 million miles apart.
35:44If a gravitational wave passes through them and changes that distance, they can detect that.
35:49Because the satellites are so much farther apart, a very low frequency wave can make a detectable change.
35:55LIGO wouldn't be able to see that, but LISA could.
35:59As well as listening for low frequency gravitational wave sources, like supermassive black hole mergers,
36:05LISA will listen for primordial gravitational waves from the dawn of time.
36:11If it detects them, we will know that the infant universe inflated.
36:18Inflation has explained almost everything in the universe.
36:22Inflation has explained almost everything we measure in modern cosmology.
36:28It's an incredibly successful theory.
36:31The icing on the cake would be if we could also discover these gravitational waves that it's supposed to have created.
36:40From the Big Bang to the most massive black holes, the universe talks to us using gravitational waves.
36:48Just like with telescopes, we're using gravitational waves to look at different types of objects.
36:55Neutron star mergers and black hole mergers and learn more about the universe around us.
37:03They could even reveal the most elusive substance in the universe, dark matter.
37:10If anything's going to help us understand the nature of dark matter, it's going to have to do with black holes.
37:17It might just be gravitational waves.
37:25Across the universe, an invisible substance holds galaxies together.
37:30Without it, they would fly apart.
37:35The Milky Way should have dispersed long ago.
37:38And the Magellanic clouds right in front of us are exactly the same.
37:41These things should be just shedding stars left and right as they fly off this rotating galaxy.
37:45Instead, they're not. They're holding together.
37:48There are motions in the stars that we just cannot account for unless there's something holding the whole thing together.
37:56We call this mysterious substance dark matter.
38:00It doesn't interact with light, so we can't see it.
38:04But we cannot ignore it.
38:07From the motions of stars inside of galaxies, to the motions of galaxies inside of clusters,
38:14to the very structure of the universe itself, we see evidence for dark matter everywhere we look.
38:25We think dark matter makes up 85% of the matter in the universe.
38:30But because we can't see dark matter with telescopes, we know very little about it.
38:36While we know that it's there, we haven't actually answered the question of what it is,
38:43or how it interacts, or why it's there, or how it's created.
38:47So you have to be really creative if you want to go after this stuff and really understand what's it made out of.
38:53One creative theory suggests that black holes make up dark matter.
38:58Not the regular stellar-mass black holes that LIGO detects,
39:03or the supermassive black holes that lurk at the center of galaxies,
39:08but tiny, primordial black holes, born during the period of rapid expansion in the first moments of time.
39:16Primordial black holes could be potential explanations for what we call dark matter.
39:22And if there's enough of them, they can hold an entire galaxy together.
39:26We don't know if primordial black holes exist.
39:31But gravitational waves could change that.
39:35When you form a primordial black hole, it's not just a black hole.
39:40But gravitational waves could change that.
39:44When you form a primordial black hole, you send out a burst of gravitational waves
39:49that, in principle, carries on traveling through the universe and you might be able to detect it still today.
39:55The problem is that these things would have emitted gravitational waves at a frequency that is not detectable by LIGO.
40:02And so it's very hard to discern whether or not there are plentiful enough
40:05to actually serve as a compelling dark matter candidate.
40:11But some scientists have a radical idea.
40:15Could LIGO have already spotted them?
40:18LIGO's detection of black holes more than 50 times the mass of our sun could provide tantalizing evidence.
40:27We expect stellar mass black holes to be up to maybe 10 or 20 times the mass of the sun.
40:35We know that they are born from the deaths of massive stars.
40:39And so the mass of the parent star sets a limit to how big the black holes can be.
40:46Some scientists are wondering, could these black holes already detected by LIGO
40:52in fact be primordial black holes grown up?
40:56Could we have already detected a form of dark matter?
41:01It's possible that tiny black holes could have been formed in the early universe
41:08and then slowly over time they snack on gas and dust
41:13so that over the course of billions of years they become 10 to 50 times the mass of the sun.
41:21And there's one more piece of evidence.
41:24Because stars themselves are spinning, black holes that are born from stars must also spin
41:32because you can't get rid of the spin.
41:34But the black holes that LIGO discovered merging didn't have a lot of spin.
41:39And that's a very curious situation.
41:42Even more curious is the fact that primordial black holes
41:46born from collapsing space-time in the early universe shouldn't spin either.
41:52Is it possible we've detected a grown-up primordial black hole?
41:57It's definitely a speculative idea.
42:00But on the other hand, physics definitely turns up very weird things from time to time.
42:06So you can certainly say stranger things have happened.
42:10If primordial black holes do exist, they still might not explain all the dark matter in the universe.
42:18They might be working with another type of dark matter to hold galaxies together.
42:24The upcoming LISA mission may fill in the blanks.
42:29What we call dark matter could be simple.
42:32It could just be made of one thing that absolutely floods the universe.
42:36Or it could be made of many different things that all work together to combine to make this effect.
42:42Is dark matter all primordial black holes?
42:45Is it something else that we haven't thought of yet?
42:47Gravitational waves could provide those answers.
42:52The detection of tiny gravitational waves generated by primordial black holes
42:58will be a huge advance in our understanding of dark matter.
43:02With gravitational wave astronomy, we're seeing things that we have never seen before.
43:08So who knows what we're going to see as we continue to look out into space.
43:12We've been able to see dozens of black holes merge.
43:16Two neutron stars merging and discovered from that merger that neutron stars can make platinum and gold.
43:26From thinking that we would never be able to see gravitational waves
43:31to seeing gravitational wave signals happen on the regular, it's just crazy.
43:36Already, we've heard epic explosions.
43:41We've identified the brightest lights in the cosmos.
43:46And we have solved some of the biggest mysteries in astronomy.
43:51But that is just the beginning.
43:55Right now is a golden age in astronomy.
43:58Think of the time that you're living in.
44:00The first detection of gravitational waves by LIGO was only a few years ago.
44:03You were here at the birth of this entirely new view of the universe.