Original subtitles

First there was light, visible light.

Then, we viewed the universe in radio waves and X-rays.

Ever since there's been astronomy,

we've been looking at different kinds of light

and opening up the universe a little bit more of the time.

But then in 2015, like, the roof came off.

Something happened that changed everything,

the ability to see waves in space and time itself.

Gravitational waves.

They help us roll back the clock to the dawn of time,

discover epic cosmic collisions,

on make Earth-shaking discoveries.

Gravitational waves are the biggest game changer

since the invention of the telescope.

We have a completely new universe to view now.

A new exploration of space is just beginning.

Long ago, 17 billion light-years away,

a cataclysmic showdown plays out.

Two black holes locked together

in a deadly cosmic dance.

Black holes are unimaginably dense objects with gravity

so intense that if you get too close to them,

you're gone.

Their immense gravitational pull causes

them to spiral towards each other.

When black holes collide, they don't just run

into each other.

They're in orbit about each other.

So what we're talking about is an inspiralling orbit

that goes faster and faster and faster and faster.

Until they finally collide in a fatal embrace.

But astronomers don't see a thing.

The problem with observing colliding black holes is all

about the name, black holes, they give off no light.

How can astronomers see something that

no telescope can detect?

Across the universe,

extraordinary events take place.

But we sometimes miss them, because we rely on light.

Now, astronomers have a new toolkit that's

revealing the cosmos in a totally different way...

...using the very fabric of our universe

we call spacetime.

Everything with mass, like stars,

planets, and black holes, all curve this fabric.

The more massive the object,

the bigger the distortion of spacetime.

The classical analogy is

this stretched rubber sheet, right?

And, like, a mass, like, the sun is, like, a ball on this sheet,

and it distorts and warps the sheet

into this valley, right?

And if you roll a marble across it like the marble is

a planet, the marble will be

pulled into orbit around the ball because

of the curvature of the sheet.

But that's only half the picture.

If an object has mass and is accelerating through

spacetime, it creates ripples

in that fabric of spacetime,

and we call these gravitational waves.

Gravitational waves give us vital clues

about distant objects that we can't see.

The more massive the object that produces them and the faster

it's moving, the bigger the ripples.

These ripples pass through planets, stars, and galaxies

with ease.

When a gravitational wave passes through an object like

a star or a planet or a person,

it stretches and compresses them,

like with this tennis ball.

Now, if you're close to a powerful source of

gravitational waves,

like merging supermassive black holes,

those waves are incredibly strong, and they're capable of

actually destroying a planet.

But like the ripples on a pond,

their strength and size diminishes over distance.

The farther away you are, the weaker they get.

And when they're hundreds of millions of light-years away,

they're actually smaller than the size of an atom.

So, to listen for gravitational waves,

scientists built the most sensitive measuring device on

the planet.

This is LIGO,

the Laser Interferometer Gravitational-Wave Observatory,

two enormous detectors located almost 2,000 miles

apart in Louisiana and Washington state.

Each sensor has L-shaped arms, measuring 2.5 miles.

Inside the LIGO detectors,

inside these concrete tunnels, there is a laser system.

It's called an interferometer, so light comes in from

a laser beam and is split into two paths.

Normally, the lengths of the two beams are the same.

That changes when gravitational waves

hit the beams.

When a gravitational wave passes through,

it changes the distance that light travels along these arms,

so one arm effectively gets longer, and the other one

gets shorter.

The length of those two beams varies just ever so slightly,

and the very sensitive apparatus in LIGO is able to pick that up.

With this ultra-sensitive laser system,

LIGO picks up distortions in spacetime, narrower than

one millionth of the diameter of an atom.

Just that feat,

just the fact that we were able to build

a detector to detect gravitational waves

is just mind-boggling.

All of a sudden now, we were listening

to the faintest whispers of the universe.

In 2015, LIGO picked up a whisper that had

been traveling towards Earth for over a billion years.

Its source? Two colliding stellar black holes.

Watching two black holes spiral in and merge...

That's not something we can do using optical

telescopes or X-ray telescopes or anything like that.

But with LIGO, we could actually detect that event.

Now, scientists can paint

accurate pictures of invisible objects.

You can tell you're looking at black holes.

You can get their masses, you can get their distance.

There's a phenomenal amount of information in that wave.

The colliding black holes are

the most massive LIGO has ever detected.

One is 66 times the mass of our sun,

the other, 85 times the mass of our sun.

As two black holes are spiraling in,

they are moving faster and faster

as they get closer and closer.

That means that the gravitational waves

they're emitting have a higher and higher frequency.

So as time goes on, the pitch gets higher.

- So it goes... - ooop!

Ooop!

Zhhhrp!

When they finally merge, they create a giant.

By analyzing that data,

it's possible to establish that the new black hole from

the merger of these two original black holes weighs as much as

something like 140 times the mass of our sun.

It's really difficult to overstate

the importance of gravitational wave detection.

It's like adding on an entirely new sense...

All of a sudden, there's a brand-new way

to explore the rest of the universe.

Invisible cosmic collisions are just

the beginning of what gravitational wave

astronomy can reveal to us.

Now, scientists are using gravitational waves

to revisit other long-standing mysteries,

like what causes the brightest explosions

in the cosmos?

This is not an everyday car crash.

This is the most dramatic event that

you're ever gonna see in our universe.

Across the universe,

strange bursts of light puzzle astronomers.

For just a fraction of a second,

they shine more than a trillion times brighter

than the sun... Then, they vanish.

These brief flashes of light are known

as gamma-ray bursts or GRBs for short,

and they're such a mystery, because they are

insanely energetic, and we don't know what causes them.

For decades, these short gamma-ray bursts

have been an enigma.

No explanation was off limits, no matter how wild.

Is it a supernova?

Is it on alien civilization saying hello?

You know, we just don't know.

In August 2017, the Fermi Gamma-ray Telescope

detected another short gamma-ray burst,

but this one was different,

So a gamma-ray burst went off 130 million light-years away,

and it actually produced a ripple in space and time

that LIGO could detect.

Gravitational waves could help

finally reveal what causes

one of the brightest explosions in the universe.

LIGOS data suggests the culprit could be two

massive objects spiraling towards each other

and colliding.

But based on the gravitational wave data,

these two objects were too small to be black holes.

They had to be something else.

Not black holes, but the ultra dense

cores of collapsed stars called neutron stars.

A neutron star is what's left over

after a massive star collapses in on itself.

It's very, very dense, because it took all, essentially,

the mass of the core and contracted it into a really,

really small radius.

As the dense neutron stars spiral ever closer,

the gravitational wave signal gets stronger and stronger,

until they collide, releasing

an epic burst of gravitational waves.

Because they're not black holes,

light can get out.

And if you smash two things together at these kind of

absolutely massive speeds,

there's a huge amount of energy involved.

Energy we detected both as

invisible gravitational waves and visible light.

Could this light be a mysterious

and ultra-powerful gamma-ray burst?

How 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?

Neutron stars have powerful magnetic fields

that trap particles of gas and dust.

During a collision,

the swirling magnetic fields twist up,

building up more and more energy.

You have lots of little particles of matter that are

trying to keep up with these rapidly spinning magnetic

fields... that starts swooshing them round until they reach

pretty much the speed of light, and eventually,

they're kind of shot out of the remnant in a tight beam.

The beam is a gamma-ray burst,

but they're not always easy to detect.

If the jet coming out is pointed right at you,

then you see this extremely high energy event,

the gamma-ray burst.

If it's not pointed at us,

we might miss it.

Fortunately, the gravitational waves

show us where to look.

Following the gamma-ray burst,

we spotted a strange red cloud, evidence of a heavy

element factory.

After the initial collision,

there is a shell of debris moving outwards,

but then, high-energy neutrons come slamming into this

material and start to build heavier elements,

one after another.

We can see the gold, we can see the potassium,

we can see the plutonium being created

before our very eyes.

The neutron star collision

produced huge quantities of heavy elements,

blasting out enough gold and platinum to weigh more than 10

times the mass of the Earth,

solving a long-standing mystery.

We knew that

supernova explosions did create some

of the heavier elements.

But from everything we've observed about supernova,

they don't happen often enough to really populate a galaxy

with all of the heavier elements that we observed.

This was the missing piece.

The gold on your wedding ring,

the gold in your jewelry,

was formed and forged from

a titanic collision before the Earth even existed.

The combination of gravitational waves

and telescopes

proves that neutron star collisions create

precious metals

and cause super-bright gamma-ray bursts.

When you can measure a gravitational wave signal

and a light signal like a gamma-ray burst,

you get a whole new way to solve complicated,

intertwined physical processes.

It's like you're watching a symphony on mute,

and then you hit that button, and the sound comes on,

and it's just a completely different picture.

The sounds of the cosmos don't just reveal collisions.

It turns out, we can use gravitational waves to help us

understand some of the biggest mysteries of the cosmos.

Gravitational waves are a new way

to listen to the universe, revealing unseen,

epic cosmic events and adding vital details to our picture

of the cosmos.

Every new way we figure out to probe the universe is

a good thing, and detecting gravitational waves,

it's a new dimension to being able to study the universe.

It's like... it's like having a new sense.

This new sense could be just what astronomers need

to answer some of the biggest questions in physics,

like, "What is the speed of gravity?"

And, "Does it travel at the universe's speed limit?"

One of the things we learn early in science is that

the universe has an absolute speed limit,

which is the speed of light in a vacuum,

which is 186,000 miles per second.

Light from the sun takes eight minutes

and 20 seconds to reach Earth.

So, if the sun disappeared,

we wouldn't miss its light immediately.

But how quickly would we notice its missing gravity?

The first thing that we'd notice is nothing.

Things would seem very normal, but then they wouldn't.

There would be nothing curving space where Earth is located,

and so Earth would take off in a straight line,

moving at the same speed at which it orbits the sun.

And things will get cold and lonely really, really fast.

According to Albert Einstein,

our skies would go dark, and the earth would be flung into

deep space at exactly the same time.

It's a foundation of his famous Theory of Relativity,

still the most complete theory of how our universe works.

Einstein's theory of relativity has been

a fantastic theory.

It explains so many things for us, including gravity.

But when we look out at the universe,

there are many mysteries, there are things that are quite hard

to explain.

At the top of the list...

The mystery of our expanding universe.

There is something pushing outward that is

making that expansion rate ever and ever faster.

Astronomers call this something dark energy.

It accounts for 70% of the total energy in the universe.

Einstein's models of the universe need dark energy

to work, but we have no idea

what it is.

Dark energy is not something we actually understand.

It's kind of a placeholder term

for something we don't understand.

And so people naturally are looking for better theories,

theories that are a bit like Einstein's theory

but just go that bit further and explain

some of these things that we don't currently understand.

One way to excise dark energy

is with a new theory of gravity,

one where the speed of gravitational waves

is different from the speed of light.

There are some so-called

non-Einsteinian theories for

the structure of spacetime itself that don't actually

require dark energy.

For example, if gravity doesn't propagate through

spacetime at the same speed that light does,

you could find models that don't actually require

dark energy... it could be a clean, simple, albeit very,

very profound solution to this underlying problem.

In order to overthrow Einstein

and eliminate dark energy,

the speeds of light and gravity must be different.

We know the speed of light.

So how do we test the speed of gravity?

In order to test the speed of gravity,

you need to have a system that emits both

gravitational waves and light.

The colliding neutron stars detected by LIGO

in 2017 are part of the solution.

The collision released a flash of light,

along with a burst of gravitational waves.

But the universe threw a curveball.

The light signal arrived 1.7 seconds

after the gravitational wave signal.

Does that mean gravitational waves

travel slightly faster than light?

Albert Einstein predicted that gravitational waves

would move at the speed of light.

So what if Albert Einstein was wrong?

I know, sounds crazy, right?

That's like almost as crazy as me being wrong, right?

But if Einstein was wrong, that's one thing.

But a bigger problem is that we'd have to rethink

our physics.

Before we do that,

let's take a closer look at the neutron star

collision site.

It's surrounded by a shroud of gas and dust.

Light is made of particles called photons,

which scatter when they hit obstacles.

But gravitational waves pass through anything.

They pass right through everything like it's not there.

Light, on the other hand,

was slowed down by interactions with that matter.

It didn't just escape immediately

like the gravitational wave signal did.

The debris gave the gravitational waves

a head start by slowing the light.

So gravitational waves and light do,

in fact, travel at the same speed.

Einstein was right.

This one event ruled out the other theories of

gravity that are competing with Einstein's theory,

things that people have been working on all their life

and overnight, it's gone.

Thanks to gravitational waves,

dark energy remains our best explanation for why

the universe's expansion is accelerating.

Maybe dark energy isn't what we think it is, and maybe

tomorrow, or maybe next year,

or maybe next decade or next century,

- we will discover that. - Gravitational waves are a huge

step forward in our effort to understand the universe,

and I mean everything.

Space, time, matter, dark energy.

We have a completely new universe to view now.

Now astronomers want to use

gravitational waves to answer another mystery.

What happens when supermassive black holes collide?

We first detected gravitational waves in 2015.

Since then, they've revealed colliding

black holes across the universe.

Prior to LIGO going online,

we never witnessed black hole collisions directly,

but now that we can witness them with our observatories,

we're finding them pretty regularly.

We're seeing gravitational waves come

across the LIGO experiment

left and right.

But LIGO has only been listening for gravitational

waves from black holes

on the smaller end of the cosmic scale.

When 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, and then large all the way up to extra-large

going from, like, a million

to a billion times as much as the sun.

These supermassive black holes

lurk at the hearts of galaxies.

When Galaxies merge, supermassive black holes

should merge, too.

But even though we see galaxies colliding

across the universe,

we've never seen two supermassive black holes

collide, because they have too

much orbital energy to get close enough to merge.

That orbital energy has to go somewhere,

and what supermassive black holes do is they throw out

stars that are around the core of the galaxy.

But when they get sufficiently close, there are just no more

stars to throw out,

and so the idea is, they can't merge.

So there's a problem.

How is it that they managed to bridge that gap

and finally spiral in?

The only way to understand if supermassive

black holes merge is by looking

at their gravitational wave signal.

Two supermassive black holes

merging should release a burst of gravitational waves

millions of times more powerful

than a stellar mass black hole merger.

But LIGO won't hear a thing.

The problem with using LIGO to detect the merger

of supermassive black holes is actually a scale of time.

One wave, as these things move around each other very slowly,

would take over 10 years to go by, just one wave.

In 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.

Vibrations from earthquakes,

weather, or even nearby traffic

prevent LIGO from listening for a decade, just to hear one wave.

But there may be another way to

detect gravitational waves from supermassive black holes,

using a strange type of dead star called a pulsar.

A pulsar is a kind of

neutron star that is rapidly spinning

and has a beam of radiation that makes

wide circles across the sky.

And 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.

Pulsars are the best timekeepers in the universe,

but passing gravitational waves make them miss a beat.

What 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?

By monitoring dozens of pulsars,

Chiara Mingarelli and a team of astronomers

have created a galaxy-sized gravitational wave detector.

It's called a pulsar timing array.

You can really look for deviations in those arrival

times over decades,

almost like a tsunami warning system to show you when

a gravitational wave is passing by.

After 12 years, the team detected

the same change in a number of pulsars.

These pulsars are all

thousands of light-years apart.

If you think about it,

it's difficult to make a signal that's the same

in all of these pulsars.

This has to be this common signal from something like

a gravitational wave event.

The signal the team detected wasn't created

by just two supermassive black holes colliding.

It's evidence of gravitational waves from hundreds of pairs of

supermassive black holes,

all in different stages of merging.

Because 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, all of them.

They all create this gravitational wave background

that we're just starting to see the first signs of now.

Astronomers predict this gravitational

wave background fills our universe.

If the signal the team detected is confirmed,

it's proof that supermassive black holes do merge.

The next step is to observe that as it happens.

It 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.

Gravitational waves reveal the hidden workings

of the cosmos.

They reach the farthest corners of our universe.

Now, astronomers are using gravitational

waves to look back in time.

They'll let us see all the way back

to the earliest moments of our Big Bang.

13.8 billion years ago,

the universe sparks into life.

The tiny speck of energy expands and cools.

The infant cosmos is a fog of tiny particles of matter.

Over time, the particles form atoms of hydrogen and helium.

The fog clears, and the first light races across

the universe.

We call that light the cosmic microwave background.

The cosmic microwave background is simply

the most distant light we can see.

So, looking at it give us baby pictures of our universe

the way it looked 400,000 years after a big bang.

What happened before these baby pictures

remains a mystery.

The leading theory is that in the very first second

of the Big Bang,

our infant universe had a growth spurt.

Scientists call this idea inflation.

In a billionth of a billionth of a billionth of a second,

our universe grew a billion, billion, billion,

billion, billion, billion times bigger.

That is the mother of all growth spurts... it laid

the foundations for the entire cosmos that we know today.

Inflation is just a theory,

but there may be a way to prove it happened.

Scientists think that during that brief moment of

cosmic expansion,

inflation stretched tiny fluctuations of gravity.

That 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.

Scientists call these theoretical ripples through

the early universe primordial gravitational waves.

When they were first released,

these were deafening.

But 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.

Scientists searched for signs of these very weak,

primordial gravitational waves in the cosmic

microwave background.

And in 2014,

a teen, using their purpose-built microwave array

in Antarctica called BICEP,

found a strange swirling pattern.

When they saw those swirls, they saw those patterns,

they thought they had seen

the signature of primordial gravitational waves.

Now this is really the conclusive

evidence that inflation had to have happened.

The results were exciting,

but there was a glitch.

This amazement lasted

for a few months until cracks started appearing in this,

and gradually, it all collapsed.

The signal, thought to be proof of

primordial gravitational waves and the theory of inflation,

turned out to be a case of mistaken identity.

As 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, and the dust itself can affect

the light and mimic what

the primordial gravitational waves can do.

The primordial gravitational wave signal

turned out to be mainly clouds of dust

floating through space.

That's how BICEP bit the dust.

BICEP failed to detect

primordial gravitational waves.

Can LIGO do any better?

Unfortunately, LIGO can't help us

in observing primordial gravitational waves.

It can't even observe supermassive black holes

at the centers of galaxies.

It is designed to observe in a particular

frequency range.

Primordial 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.

But scientists hope that an ambitious project

called LISA will.

Not on Earth, but from 30 million miles above.

LISA is like LIGO,

but bigger and in space.

Or the Laser Interferometer Space Antenna, will be a system

of three satellites arranged in a giant triangular formation,

1.5 million miles apart.

If a gravitational wave passes through them

and changes that distance,

they can detect that... because the satellites are so much

farther apart, a very low frequency wave

can make a detectable change.

LIGO wouldn't be able to see that, but LISA could.

As well as listening for low frequency

gravitational wave sources, like supermassive black

hole mergers,

LISA will listen for primordial gravitational

waves from the dawn of time.

If it detects them,

we will know that the infant universe inflated.

Inflation has explained almost everything

we measure in modern cosmology.

It's an incredibly successful theory.

The icing on the cake would be if we could

also discover these gravitational waves

that it's supposed to have created.

From the Big Bang to the most massive black holes,

the universe talks to us using gravitational waves.

Just like with telescopes, we're using gravitational waves to

look at different types of objects...

Neutron star mergers and black hole mergers... and learn more

about the universe around us.

They could even reveal

the most elusive substance in the universe...

Dark matter.

If anything's gonna help us understand

the nature of dark matter,

it might just be gravitational waves.

Across the universe,

an invisible substance holds galaxies together.

Without it, they would fly apart.

The Milky Way should've dispersed long ago,

and the Magellanic clouds right in front of us are

exactly the same.

These things should be just shedding stars

left and right as they fly off this rotating galaxy.

Instead, they're not. They're holding together.

There are motions in the stars that we just cannot

account for unless there's something holding

the whole thing together.

We call this mysterious substance dark matter.

It doesn't interact with light, so we can't see it.

But we cannot ignore it.

From the motions of stars inside of galaxies to

the motions of galaxies inside of clusters

to the very structure of the universe itself,

we see evidence for dark matter everywhere we look.

We think dark matter makes up

85% of the matter in the universe.

But because we can't see dark matter with telescopes,

we know very little about it.

While we know that it's there,

we haven't actually answered the question of what it is

or how it interacts or why it's there or how it's created.

So you have to be really creative

if you want to go after this stuff

and really understand what's it made out of?

One creative theory suggests that black holes

make up dark matter,

not the regular stellar mass black holes that LIGO detects,

or the supermassive black holes that

lurk at the center of galaxies

but tiny, primordial black holes born during the period of

rapid expansion in the first moments of the Big Bang.

Primordial black holes could be

potential explanations for what we call dark matter.

And if there's enough of them, they can hold an entire

galaxy together.

We don't know if primordial black holes exist,

but gravitational waves could change that.

When you form a primordial black hole,

you send out a burst of gravitational waves

that, in principle, carries on traveling through the universe,

and you might be able to detect it

still today.

The problem is that these things would have emitted

gravitational waves at a frequency that is not

detectable by LIGO.

And so it's very hard to discern whether or not they

are plentiful enough to actually serve as a compelling

dark matter candidate.

If primordial black holes do exist,

they still might not explain all the dark matter in

the universe.

They might be working with another type

of dark matter to hold galaxies together.

The upcoming LISA mission may fill in the blanks.

What we call dark matter could be simple.

It could just be made of one thing that absolutely floods

the universe, or it can be made of

many different things that all work together to combine

to make this effect.

Is dark matter all primordial black holes?

Is it something else that we haven't thought of yet?

Gravitational waves could provide those answers.

The detection of tiny gravitational waves generated

by primordial black holes

will be a huge advance in our understanding of dark matter.

With gravitational wave astronomy,

we're seeing things that we have never seen before.

So who knows

what we're gonna see as we continue to look out into space?

We've been able to see dozens of black holes

merge, two neutron stars merging,

and discovered from that merger that neutron stars

can make platinum and gold.

From thinking that we would never be able to see

gravitational waves to seeing gravitational wave signals

happen on the regular... It's just crazy.

Already, we've heard epic explosions.

We've identified the brightest lights in the cosmos,

and we have solved some of the biggest mysteries

in astronomy.

But that is just the beginning.

Right now is a golden age in astronomy.

Think of the time that you're living in... the first detection

of gravitational waves by LIGO was only a couple of years ago.

You were here of the birth of

this entirely new view of the universe.

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