All language subtitles for How the Universe Works s09e10 Gravitational Waves Revealed.eng

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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