All language subtitles for How.The.Universe.Works.S09E10.Gravitational.Waves.Revealed.1080p.AMZN.WEB-DL.DDP2.0.H.264-alfaHD_track4_[eng]

NARRATOR: 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 at a 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...

..and 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.

(EXPLOSION)

Long ago,
17 billion light years away,

a cataclysmic showdown plays out.

Two black holes lock 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 are in orbit about each other.

So, what we're talking about
is an in-spiralling orbit

that goes faster and faster
and faster and faster...

..until they finally collide
in a fatal embrace.

(EXPLOSION)

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 tool kit

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 the sheet

and it distorts and warps the sheet
into this valley, right.

And if you roll a marble across it,
like the marble's 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're listening

to the faintest whispers
of the universe.

In 2015, LIGO picked up a whisper

that had been travelling
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 are 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 spiralling in,

they are moving faster and faster
as they get closer and closer.

That means that the gravitational
waves they are emitting

have a higher and higher frequency.

So, as time goes on,
the pitch gets higher.

So, it goes oohrup!

Oohrup! Oohrup!

(BEEPING)

When they finally merge,
they create a giant.

By analysing 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 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
an alien civilisation 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.

LIGO's data suggests the culprit
could be two massive objects,

spiralling 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,

a 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.

PAUL: 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 ten 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 observe.

This was the missing piece.

The gold on your wedding ring,

the gold in your jewellery

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.

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 mi/s.

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 curve ball.

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'll 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'd 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, ten,
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 manage 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,

will take over ten 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 ten 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.

(EXPLOSION)

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 gives us
baby pictures of our universe,

the way it looked 400,000 years
after our 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 team, 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 to appear 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?

Everything changed in 2015

with that announcement
of first detection

of gravitational waves.

That is one of the great triumphs
in all of science.

We have broken through
the electromagnetic window
on our universe.

And we've moved
into the gravitational wave.

Ladies and gentlemen,

we have detected
gravitational waves.

We did it. (APPLAUSE)

They were jumping up and down,
they were crying,
they were smiling.

The first LIGO detection
of two black holes
spiralling in and merging

and emitting these
gravitational waves

was a very big deal.

This was the very first time
gravitational waves

had ever been directly detected.

And we've been trying
for, like, half a century.

Einstein predicted this
in the early 20th century.

And it took us literally 100 years

before we could detect them
for the first time.

I'm getting goosebumps
just thinking about that.

We really had found
a gravitational wave.

HAKEEM:
Unfortunately, LIGO can't help us

in observing primordial
gravitational waves.

It can't even observe
supermassive black holes
at the centres of galaxies.

It is designed to observe
in a particular frequency range.

Primordial gravitational waves
are at such a low frequency

and 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.

LISA, 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 will 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 have
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 centre 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.

But 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
travelling 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.

But some scientists
have a radical idea.

Could LIGO have
already spotted that?

LIGO's detection of black holes,

more than 50 times
the mass of our Sun,

could provide tantalising evidence.

We expect stellar mass black holes

to be up to maybe 10 or 20 times
the mass of the sun.

We know that they are born
from the deaths of massive stars,

and so the mass of the parent star

sets a limit to how big
the black holes can be.

Some scientists are wondering,

could these black holes
already detected by LIGO in fact

be primordial black holes grown up?

Could we have already detected
a form of dark matter?

It's possible that tiny black holes

could have been formed
in the early universe.

And then slowly over time,
they snack on gas and dust

so that over the course
of billions of years,

they become 10 to 50 times
the mass of the sun.

And there's one more
piece of evidence.

Because stars themselves
are spinning,

black holes that are born
from stars must also spin

because you can't
get rid of the spin.

But the black holes
that LIGO discovered merging

didn't have a lot of spin.

And that's a very curious
situation.

Even more curious, is the fact
that primordial black holes

born from collapsing spacetime
in the early universe

shouldn't spin either.

Is it possible we've detected
a grown-up primordial black hole?

It's definitely a speculative idea.

But on the other hand,
physics definitely turns up

very weird things
from time to time,

so you can certainly say
stranger things have happened.

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 could 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 have 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.

MICHELLE: 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 at the birth

of this entirely new view
of the universe.

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