All language subtitles for The Universe S02EP06 Dark Matter - Dark Energy

For thousands of years,
we have looked at the night sky...

and believed
that the illuminated stuff...

was all that made up
our universe.

Scientists now realize
it's not what shines in the light...

but what hides in the dark...

that holds the true secrets
of our sky.

There is
a mysterious dark matter...

that binds stars
and galaxies together...

and strange particles like
WIMPS, axions, and MACHOs...

might be to blame.

And there is a dark,
repulsive energy...

that is creating space
in the universe...

but driving the galaxies
further and further apart...

to a dismal fate.

Combined, dark matter
and dark energy...

make up ninety-six percent
of the universe...

and uncovering their secrets...

is like making
the one-in-a-million shot.

If uncovered, the ultimate
fate of the universe...

might be revealed.

Will it crash and burn
in a horrific collision...

of gravitational forces...

or will dark energy
tear the universe apart?

The betting is the universe
will die in ice.

Understanding these two quantities,
dark matter and dark energy...

is really fundamental
to understanding the universe.

This is a trip to the dark side
of the universe.

This is the hunt for
"Dark Matter and Dark Energy. "

Dark matter is unlike anything
we have ever encountered on Earth.

Billions of these
strange particles...

pass through everything
they encounter each second.

They are so massive in weight...

they have the power
to influence the galaxies-

how they form
and how fast they spin.

Dark matter's invisible
presence is everywhere...

or so it seems.

Science has not directly proven
dark matter particles exist.

There are many suspects,
but no answers...

and observing something
you can't see isn't easy.

It doesn't emit light,
and it doesn't absorb light.

It doesn't interact
with light at all.

Not only does it not shine...

you can't easily see it
in obscuration.

But the evidence is there.
Science knows it.

Every textbook
on the planet Earth...

says that the universe
is made out of atoms...

and some subatomic particles.

Well, all those textbooks
are wrong.

And they are going
underground to prove it.

When they hear about
this invisible matter...

called dark matter,
they say, "Bah, humbug.

"Show me proof
that dark matter exists. "

Soudan, Minnesota, 200 miles
from any major city lights...

a perfect place
for hunting dark matter...

but not for obvious reasons.

So you'd think we'd want to look
up in the sky for dark matter.

That's where dark matter
is coming from, after all.

But instead,
we're going to don our helmets...

and we're going to walk down
into this mine.

2,431 feet underground is
the Soudan National Laboratory...

an abandoned iron mine
reconfigured into a research facility.

This is just one lab of many
around the world...

going underground to shield
experiments from cosmic rays.

Each are racing
to detect dark matter-

an invisible particle that has
only been indirectly observed...

but never captured.

We've been at this for,
now, a decade...

and we have yet to see
a dark matter particle.

The hunt for dark matter
started almost a century ago.

Astronomers finally had the tools
to see deep into the night sky...

and the questions began.

So it wasn't until the 1920s...

that the technology
developed well enough...

so that we could take
the little fuzzy patches...

that people had noticed
in their telescopes...

and resolve them
and figure out what they are.

Then, Edwin Hubble
shocked the world...

and declared the universe
was bigger than just the Milky Way.

People realized that some
of these little fuzzy patches...

are separate galaxies,
just like our Milky Way galaxy.

As astronomers
discovered new galaxies...

Cal Tech professor
Fritz Zwicky looked up...

to the neighboring
Coma Cluster of galaxies...

and observed something strange.

When he measured
what the motions were...

in the Coma Cluster of galaxies...

he got an estimate for how much
mass there was in that cluster.

Then he compared it
to how much mass...

you could actually see
by looking at the galaxies.

Something didn't add up.

The galaxies were moving
too fast within the cluster...

for the amount of illuminated
stuff he could see.

By his calculations...

there should have been 160 times
more illuminated mass...

to account for the random speeds
of galaxies in the cluster.

Something else was affecting
their motions...

but what was it?

He analyzed their motions...

and concluded that the cluster
could not be stable...

unless there was a large amount
of dark matter present.

In 1933, he was one
of the first people...

to really grasp
the significance...

of the presence
of dark matter.

He called it "missing matter. "

Dark matter, an invisible mass
that was gravitationally attractive...

and was able
to affect the speeds...

of entire galaxies
in a cluster.

Revolutionary thinking?

Yes, but the discovery
was largely ignored.

I think that people
took Zwicky seriously...

but they didn't jump
to any conclusions.

This was a time
when the universe...

was just beginning
to be explored.

It was the 1920s
when we first realized...

there were galaxies
outside our own.

What we didn't know back then
was whether it was simply galaxies...

or stars or gas or dust
that we couldn't see...

or whether it was something
truly different.

Zwicky's observations were
based on measuring the mass...

in the stars and galaxies.

But how do you
weigh stuff in space?

You can't go and put
the Sun on a scale.

It's a little bit hard.

But what you can do
is you can measure...

how fast the planets
are moving around the Sun.

And the more stuff
there is in the Sun...

the faster those planets
have to move...

to stay in their orbits.

Newton and Einstein both said...

the more mass or stuff
you have in an object...

the more gravitational pull
it will have.

And the further an object is
from the center...

the slower
it should travel in orbit...

because the gravitational pull
is weaker.

According to Einstein's
general theory of relativity...

or even according
to Newtonian gravity...

all of the galaxies
are pulling on each other.

It's like the Sun's influence
on our solar system.

The mass of the Sun pulls
Mercury faster than Pluto...

because Mercury is positioned
closer to the Sun.

Likewise for a galaxy,
you would expect...

as you got further
and further away...

things are moving
more and more slowly...

to stay in their orbits.

But Zwicky didn't observe that.

Neither did a young scientist
named Vera Rubin fifty years later.

She was observing
the rotational curves...

of galaxies similar
to the Milky Way.

Like Zwicky, her observations
also seemed strange.

What Vera Rubin
actually measured was...

as you got further
and further away...

the velocity of the orbiting
gas and dust remained constant.

What Rubin observed was
as if a city were a galaxy...

and every car on the road
was a planet or star.

And despite
the amount of traffic...

every car traveled around the city
at the same speed.

This same, consistent
rotational speed...

despite the amount
of stuff, or traffic...

was exactly
what Rubin observed.

The outer parts of the galaxy
were rotating fast enough...

that there must be
a lot more mass.

Otherwise, the galaxy
would have flown apart.

The only way to resolve
this paradox...

of galaxies which spin
ten times too fast...

is to assume
that there is a halo...

a halo of invisible matter
surrounding the galaxy...

keeping the galaxy whole.

Dark matter was present
in the galaxies...

and it had enough mass to keep
the rotation speed constant.

Imagine that I am
the dark matter.

This ball is a star
orbiting me...

because my gravitational force
is keeping it in place.

But even if
you couldn't see me...

you would know
there must be something here.

Otherwise, the star would just
zoom off in a straight line.

There must be something
causing that gravity...

and that's how we know
that there must be dark matter.

Rubin estimated that there was
ten times more dark matter...

than ordinary illuminated stuff.

Since then, we've analyzed
hundreds of galaxies...

and they all have
the same pattern.

They all rotate too fast
for their own good...

and they need dark matter
to hold them together.

This time,
science paid attention...

and started to wonder:
What is dark matter?

How do you find something
that is invisible in space?

They needed to see
just where dark matter...

was hiding out
in the universe.

And even if
they couldn't see it...

science realized that
dark matter exposed itself...

by bending light
that passes through it.

It's called gravitational lensing,
and it's a virtual spotlight...

that uncovers any invisible
stuff in the universe.

What it does do
is it does what all matter does...

in that it can deflect
the light ray.

So a light ray can be deflected
in its path by dark matter.

By tracing
the battered light's path...

gravitational lensing
detected dark matter...

concentrated in the halos
of galaxies.

Gravitational lensing
proved to be infallible...

and dark matter's presence
was suddenly revealed.

This technique
of gravitational lensing...

is the most precise because
we can actually pinpoint...

not just how much
dark matter there is...

but how it's distributed
in its position on the sky.

And that's because
we can measure the distortion...

of the light rays passing
through the dark matter.

How do you know
that your glasses are there?

Because it bends light.

In the same way...

by looking at Hubble space
pictures of the universe...

and looking
at the distortion of light...

as it goes through galaxies...

we actually have maps
of dark matter.

Most of the mass of a galaxy
is from the dark matter.

The ordinary matter accumulates...

in the gravitational field
of the dark matter.

But once dark matter
came on the scene...

scientists wondered if it was
a new, undetected particle...

orjust invisible
ordinary matter.

When people
found dark matter...

everybody wanted to know,
well, what is it, you know?

And, of course,
the first answer is...

it's just the stuff
that makes up you and me...

but it's not shining.

Scientists started
to investigate objects...

in the universe
that didn't emit light.

Black holes were considered.

They don't emit light,
can draw matter to themselves...

and are detected
with gravitational lensing.

It could take a form
of black holes...

or MACHOs,
massive compact halo objects...

which are basically
dark, small stars...

that don't give off
a lot of light.

MACHOs hide out in the halo
of the Milky Way...

and are detected
by gravitational lensing.

But there weren't enough...

to account for the amount
of dark matter needed.

Failed stars like brown dwarfs
were also suspected.

They are massive enough
to make up dark matter's presence.

Whatever dark matter is,
there is way more of it...

than the ordinary matter
of stars and planets...

ten times more.

All the stuff
that you can construct...

from ordinary atoms, protons,
and neutrons and electrons...

cannot possibly be enough...

to account for the total
amount of matter...

that you see in galaxies
and clusters.

Scientists continued
to present new suspects...

as the search continued
for dark matter.

Previously discovered
exotic particles...

like neutrinos,
were reconsidered.

Like dark matter, neutrinos
are passing through the Earth...

millions of particles at a time...

but they are too light...

to account for dark matter's
effect on gravity...

and scientists can recreate
neutrinos in particle colliders.

They also come from the Sun.

Axions is also another possible
dark matter candidate.

They were invented
to explain a particular glitch...

in one of the particle
physics theories.

They would be extremely light...

so you search for them
in a completely different way...

than what we're doing,
but there would be...

They would also be
very numerous.

And so they could possibly
be the dark matter.

Axions are very light...

and are believed
to have been created...

at the moment of the Big Bang,
just like dark matter.

But theories suggest that
they can change to protons...

while dark matter is stable.

After exhausting
all the usual suspects...

many scientists believe
dark matter is a new, exotic particle...

unlike anything on Earth...

and billions are passing
through us every second.

Up until the discovery
of dark matter...

scientists believed
the universe...

was made only of protons,
neutrons, and electrons...

the stuff everything
on Earth is made of.

And we know it has some mass.

And we're left with something
that we've not yet detected.

But to be a perfect
dark matter candidate...

it must have certain
physical properties...

and none of the usual suspects
were fitting the crime.

So we know the dark matter
is some ponderous substance...

we know that it's not
moving too quickly...

and we know
that we can't see it.

Dark matter particles are
not traveling at the speed of light...

and they don't interact
with you and me...

or anything pretty well, and
that's why it's been so difficult...

to track down these particles.

And it doesn't interact
with ordinary matter...

except through gravity.

If I had some dark matter
in my hand...

it would have weight.

But first it would dissolve
right through my fingers.

There aren't any candidates
for cold dark matter...

within what we call
the standard model particle physics.

Like an invisible man
passing through walls...

dark matter is passing through Earth
billions of particles at a time...

never colliding
with ordinary matter.

So the most popular idea
for what the dark matter could be...

is something called a WIMP.

WIMPs are
weakly interacting massive particles.

They have not been detected,
but their characteristics...

match the perfect
dark matter candidate.

At the Soudan Laboratory...

the Fermilab team
has gone underground...

braving thousands of bats...

to try and capture
a WIMP particle.

This is called
the cryogenic dark matter search.

CDMS is the acronym for it.

This was an iron ore mine
until 1962 when it shut down.

We're half a mile underground.

Fermilab has designed a machine
that, at subzero temperatures...

can detect a dark matter
particle passing through it...

and its sensor
is made of germanium...

a dense metal
jam-packed with atoms.

It's a very pure block
of germanium.

It's got on the surface of it...

a pattern of tiny little
thermometers, basically...

that are able to detect
when a particle passes...

through this hockey puck-sized
chunk of germanium.

Dark matter is streaming
right through us right now...

without doing anything.

Very occasionally, it will bump
into the nucleus of an atom.

And that's the signature
that we're hoping to see.

To get a clean
dark matter hit...

Fermilab needed to filter out
junk from space.

We would get
so many particles...

that it would be really trying
to sift a needle in a haystack.

Fermilab's experiment
picks up all matter...

that passes
through this detector.

The less junk in the air...

the easier it will be to detect
a dark matter particle.

Because dark matter
doesn't interact easily...

with regular protons
and electrons...

Fermilab has frozen
the germanium pucks...

to near subzero temperatures.

If a dark matter particle comes
through and hits a nucleus...

it will actually change the temperature
of the crystal very slightly...

and so we're looking
for that tiny change...

of temperature
in the crystal...

to signal that a dark matter
particle has passed by.

Sixteen germanium pucks...

sit in a chamber
inside a clean room.

So we're suited up...

we're about to go
into the experimental room.

It's a Class 10,000 clean room.

That's why we're all suited up
so we don't carry in any dust...

because that would cause
background for the experiment.

So let's go inside.

So we have
scintillation counters...

that are catching
any cosmic-ray particles...

that get all the way down
underground here.

So right here is what keeps
our experiment cold...

a tiny little bit
above absolute zero.

This is the dilution refrigerator.

So way inside here...

are the germanium
and silicon detectors...

so we're just waiting for a WIMP,
a dark matter particle...

to get down to this depth...

and hit one of those germanium
and silicon crystals...

that's buried way inside
all the shields.

Fermilab has been visiting the mine
for nearly two years...

trying to capture dark matter.

This is far harder
than it sounds.

Although billions of particles are
passing through Earth at one time...

it's a one-in-a-million shot...

dark matter will interact
with ordinary matter.

Getting a dark matter particle
to hit a germanium atom...

is like an archer
trying to hit a bull's eye...

when the target is
a mile away.

All these green lights
indicate particles...

passing through the germanium
and silicon detectors...

that we saw downstairs.

These are almost certainly
all background particles.

But maybe it's buried in there
someplace as a WIMP...

but we won't know
until we analyze the data.

Hunting dark matter is tedious.

Each day, the Fermilab team
reports to the underground lab...

analyzes data, and perfects
their ping-pong game...

while waiting for the one hit
that will prove dark matter exists.

But for all this effort
and waiting around...

no dark matter has been detected
by Fermilab or by anyone else.

Unfortunately, we've seen...

precisely zero
dark matter particles so far.

Any day now,
we may have the announcement...

that physicists have captured
dark matter in a bottle.

We have a hypothesis.

It certainly seems to explain
the universe we live in.

But the plain fact
is we haven't yet detected...

this cold dark matter particle.

Finding dark matter will not
only give us proof of its existence...

but might also answer
the other big questions of space.

Detecting dark matter directly...

will give us a window
onto what was going on...

1/10,000th of a second
after the Big Bang.

If scientists can discover
what dark matter is...

they might also discover how
the universe behaved early in its life.

Dark matter is not only
a mysterious quantity in the universe...

but also it's fundamental
to our, you know...

why we're here, in fact.

It would be difficult
to form galaxies...

and, hence, you know,
the solar system...

and, hence, life on Earth.

The story of dark matter
and dark energy...

can't be told without going back
to the beginning of time...

to the moment of the Big Bang
when space didn't exist.

There was no center
where you can point to.

And that's exactly the analogy
for the Big Bang.

There was no direction
in the sky...

from which
all the galaxies were expanding.

From this moment of nothing
to a violent explosion...

space was created, and the universe
began to grow from a seed.

Particles formed in a nuclear fusion
of gas and energy.

Ordinary matter was reacting
with other ordinary matter.

The early universe was
in fact a nuclear reactor...

when it was one minute old.

And 380,000 years later,
bits of particles began to cluster...

creating the seeds from which
stars and galaxies would later form.

Bigger lumps grow to form
yet bigger lumps...

and so gravity
is a slowly pulling force...

bringing objects together.

What scientists now realize was
that at the moment of the Big Bang...

dark matter was created.

And it played a critical role
in helping ordinary matter...

clump together to form
stars and planets.

Like steel girders used
on a building site...

dark matter's slow-moving particles
acted like scaffolding...

upon which ordinary matter
could attach itself.

We believe that, because it's cold
and doesn't interact very much...

that dark matter
was pulled together by gravity...

very slowly over time...

and actually formed the seeds...

around which normal matter
coalesced into galaxies.

It's like a cosmic web,
like a spider's web...

where there are strands
of dark matter...

and where these strands intersect,
like a scaffolding pattern.

So in a sense, the dark matter
is the framework.

It's providing the scaffolding...

for the shining galaxies
that we can easily see.

They are really like
the Christmas tree lights.

They're not
the actual Christmas tree.

They're the things that are visible
from very, very far away.

But the reality of the galaxy
is a big halo...

most of which you don't see.

You see the shiny bits
that are stars and planets...

that have accumulated
at the center of that large halo...

which is mostly dark matter.

Scientists have long wondered
why galaxies formed...

in seemingly random patterns
across space.

Now scientists know it's because
of dark matter's gravitational pull.

The universe is not uniform at all,
but has voids.

It has clumps, it seems
to have bubble-like regions.

Well, we now believe
it's due to dark matter.

In the last year,
astronomers have been able...

to take their theory
one step further...

and create
a detailed 3-D map...

of dark matter in the universe,
using gravitational lensing.

And Einstein said that gravity
affects everything...

just like gravity
is caused by everything.

So one of the things that
is affected by gravity is light itself.

Because when light goes through
dark matter, it bends...

just the way light bends
when it goes through glass.

And light doesn't discriminate
between ordinary matter and dark.

Both types of matter batter light's path
as it travels through galaxies.

Like plotting a course
on a map...

astronomers have traced
thousands of light sources...

as they pass
through dark matter.

It has given science
the most accurate picture yet...

of where dark matter
hides in space.

We can compare that map
of the dark matter...

with where the galaxies are,
and, lo and behold...

we find that the dark matter
is acting as the skeleton.

It is the backbone around which
the visible material is clumping.

By mapping the universe, astronomers
can also look back in time...

and predict how much matter
was created at the Big Bang.

So if you know how lumpy
the universe appears now...

and when it was
half its current size...

and when it was half
its current size before that...

you can infer the total amount
of stuff in the universe.

It gives us another
very nice way of matching on...

to what we believe is
the total amount of dark matter.

It's estimated dark matter...

makes up twenty-three percent
of the universe...

while ordinary matter
makes up only four percent.

You need a lot of dark matter...

to account for the total
amount of gravity...

that exists in these
clusters and galaxies.

But what makes up the final
seventy-three percent of the universe?

Scientists were shocked
to discover...

a new mysterious dark energy
was dominating space.

And its repulsive energy
is driving the galaxies apart.

Science always assumed...

that although the universe
continues to grow in size...

it would eventually slow
in its expansion...

or perhaps even collapse
on itself.

Gravity would overcome
any momentum it had.

But while measuring the
expansion history of the universe...

scientists
were shocked to realize...

that the universe
wasn't slowing down.

It was speeding up...

and a grim fate awaited
any living thing.

The universe will disintegrate...

and temperatures
will become so cold...

that any intelligent life
will freeze to death.

From the moment
the Big Bang created the universe...

space has been expanding
and never stopping...

carrying galaxies
along for the ride.

The space in between
the galaxies is expanding.

The galaxies are not expanding.

The Earth is not expanding.

The solar system
is not expanding.

Edwin Hubble first discovered...

galaxies were moving away
from the Milky Way in 1929...

by realizing
the more distant galaxies...

move faster away from us
than the nearby ones.

He realized he could measure
their velocities...

by studying their wavelengths
through a prism.

It's called
measuring the redshift...

and is still used
to measure distances in space.

And he found that, in fact...

the greater the distance
of a galaxy right now...

the greater the speed
with which it's moving away from us.

That is, the greater its redshift.

A few years ago...

astronomers decided
to use redshift measurements...

to measure the expansion
history of the universe.

But how do you measure...

the entire expansion history
of the universe?

How do you travel back
twelve billion years in time?

We have the capability
of going back in time directly...

to observe the past.

So, much in the same way
as a geologist, you know...

looks at layers
in the Grand Canyon...

and as he goes down
into lower and lower layers...

looking back in history.

If you look at progressively
more distant galaxies...

you're looking at them
as they were...

at progressively greater
times in the past.

To measure expansion history,
scientists used Type 1 a supernovas...

as their standard candle.

One example of a standard candle
might be a 100-watt light bulb.

You could have
a bunch of these things...

sitting around in your room
at different distances from you.

Then the more distant ones
will appear fainter...

and the more nearby ones
will appear brighter.

Type 1 a supernovas are
always consistently brilliant...

no matter where
they occur in space.

A supernova is
the colossal explosion of a star...

at the end of its life.

It just goes kabam.

And it occurs when a dying star,
known as a white dwarf...

goes through a nuclear runaway...

and just literally blows itself
to smithereens.

We find the Type 1 a supernovae
in very distant galaxies...

so they look really faint.

And then we compare them
with Type 1 a supernovae...

in nearby galaxies
whose distance were-

Distances were measured
using Cepheid variables...

or some other technique.

Using these Type 1 a supernovas...

two different teams
set out in the 1990s...

to measure the deceleration
rate of the universe.

But to capture supernovas
as they occur...

astronomers had to put
the universe on surveillance.

You can compare this a little bit
to perhaps surveying a casino.

So all these cameras
are on all the time...

and most of the time, they don't
find much of anything interesting.

But, occasionally, they find
what they're looking for.

You have to look
at lots of galaxies.

So what we did is
we took large telescopes...

with cameras
that have wide fields of view...

about as wide as, say,
the width of the full Moon.

And we took many snapshots of space
using this wide-field camera.

And each snapshot contains
tens of thousands of galaxies.

And by comparing the apparent
brightness of the distant Type 1 a's...

with those of the nearby Type 1 a's
in nearby galaxies...

we can get the distance
of the distant galaxies...

and, hence, the amount of time...

that we're looking back
in the history of the universe.

After the two teams
studied the results...

of sixty Type 1 a supernovas...

scientists were shocked
at their results.

The universe
wasn't slowing down.

Its expansion was speeding up.

We all expected that expansion
to be slowing down with time...

because, after all, all of the galaxies
are pulling on one another.

We were so confident
we were going to measure the rate...

at which the universe
was slowing down...

and then we found, of course,
a negative answer.

The universe
is not slowing down.

It's speeding up...

and this was just
a big mystery.

That is really, really weird.

You know, we expected to measure
some amount of slowdown...

and instead,
it's expanding more quickly.

That's like the wrong sign, right?

We were really afraid...

that we had gotten completely
the wrong answer.

We rechecked our measurements.

We checked the analysis.

A bunch of people
on each of the two teams...

did the measurements
and the analysis independently...

and kept getting
the same result.

One of the greatest shocks
in the world of cosmology...

just in the last few years
has been the realization...

that our universe
is accelerating.

This repulsive force
driving the universe...

was called dark energy,
an invisible energy...

that was nothing anyone
expected or understood.

It suggests that over
the largest distances in the universe...

there's a repulsive effect
that dominates over gravity.

And dark energy
was creating space...

taking galaxies
along for the ride.

This energy that appears
to fill the universe...

and stretch the expansion
of the universe...

faster and faster with time
is now known as dark energy.

So here I throw the apple,
and, initially, it's decelerating.

And then dark energy makes it
accelerate away from me.

So you throw the apple...

and it just zooms away
faster and faster with time.

Like the apple forever
traveling into space...

galaxies are being carried away
as more space is created.

So if you could imagine,
you know...

a classroom
populated by chairs...

and those chairs are
slowly getting further apart...

from one another
within a classroom...

because if all the chairs
are being stretched apart...

like in the expanding universe...

no matter which chair
you sit on...

you will find all chairs
are moving away from you.

So the chairs
are not really expanding.

In fact, the chairs
are all the same size, really.

What's happening is that
the room is getting bigger...

the sort of the space
in between the chairs...

is being stretched apart.

More space is being created
in between the galaxies.

So you have individual galaxies
remaining a constant size...

in a universe where all of space
is getting bigger and bigger.

Dark energy is very different
from dark matter.

It doesn't clump up
like galaxies do in clusters...

or like stars do in galaxies.

Instead, it appears
to be pretty uniform.

And we find
the same amount of acceleration...

no matter which direction
we look at.

It's probably smooth...

although some people believe
there may be structure...

in its distribution
and its influence.

Dark energy is the energy
of the vacuum...

the energy of nothing.

Even nothingness has energy.

And it's pushing the galaxies apart,
creating a runaway universe.

It appears dark energy
and dark matter...

have been at war with one another
since the beginning of time.

Science believes
dark energy was created...

along with dark matter
at the moment of the Big Bang.

It has always existed
in the universe...

but gravitational forces
of dark matter kept it in check...

slowing down
the expansion of space...

during the first
nine billion years of time.

This changed
five billion years ago...

when the universe
grew big enough...

so that dark matter was dispersed
throughout the universe...

and dark energy wasn't
so affected by dark matter's pull.

As a result,
the universe began to expand...

at an accelerated rate.

Dark energy is a constant term
that was probably very insignificant...

when the universe was hot
and dense in the beginning...

and so it doesn't really matter...

whether dark energy
is there or not.

It is there, but it's-
It just plays no role at all.

And then, as the universe gets cooler
and less dense and bigger...

so gravity becomes
less important...

and then dark energy
takes over.

It's a property of space that
we don't yet fully understand.

As the universe expanded,
astronomers realized dark energy...

won its struggle
with dark matter...

and started the acceleration
five billion years ago.

So there came a time,
about five billion years ago...

when the dark energy
started dominating...

over the attractive matter
in the universe.

So, in a sense, if you plot
force versus time...

the gravitational attraction
is declining with time...

the repulsion
is increasing with time...

and about five billion years ago,
the two curves crossed...

and that's when the universe started
accelerating in its expansion.

Dark energy is fundamental
to understand...

because it tells us
where the universe is going.

What's the fate of the universe?

Is it going to expand forever
and get cold and dark?

Or is there, you know,
some end in sight?

Dark energy now drives
the expansion of the universe...

and it doesn't seem
to be stopping.

The repulsive effect
of the dark energy increased...

because the more space
there is between galaxies...

the greater
is the cumulative effect...

of the dark energy,
the repulsive effect.

And individual galaxies seem
destined to a lonely existence.

So it looks as if this is
the end of everything.

Surprisingly,
the theory of dark energy...

was proposed
and discarded long ago...

from one of physics'
greatest minds.

He called it
his biggest blunder.

His name was Einstein,
and he might have been...

onto the greatest discovery
of the 21 st century...

eighty years
before anyone had a clue.

Science now understands...

that dark energy is creating
and expanding space.

But in the early 20th century...

astronomers believed
the universe was only as big...

as the Milky Way
and would never grow in size.

But Einstein had just formed
his theory of relativity...

and decided to test it
on the static universe.

But as hard as Einstein tried...

he could not balance his equation
to equal a static universe.

His calculations
showed a universe...

that must either expand
or contract.

He realized that if you had
a universe that was smooth...

that was uniformly
distributed with stuff...

his theory
unambiguously predicted...

that it should either be
expanding or contracting.

So Einstein proposed
a repulsive vacuum energy...

that would hold the universe
in balance with attractive gravity.

He called it
his cosmological constant...

a constant energy that would
hold the universe in balance.

He introduced
the cosmological constant...

or dark energy,
to hold the universe static.

When Hubble announced
space was expanding...

suddenly, Einstein's cosmological
constant seemed irrelevant...

and he labeled it
his biggest blunder.

Now it turns out
that dark energy...

a concept that he threw away
back in the 1920s, is, in fact...

the dominant force
blowing the universe apart.

Einstein's so-called blunder...

will eventually determine
whether or not...

the universe dies in fire or ice.

And the betting is
the universe will die in ice.

In trying to survey
how the universe behaves...

Einstein had erroneously
predicted dark energy...

and what is the total makeup
of the universe.

The total amount of stuff in
the ordinary matter and dark matter...

is not enough to account
for the curvature of space...

that we observe.

Like looking across
a horizon on Earth...

the size of the universe
is so great...

the curvature of space
appears flat.

Add ordinary matter, dark matter,
and dark energy together...

that makes a prediction
for the curvature of space.

And that prediction
comes spot on.

You get the right answer.

Our satellite data
now has revealed the fact...

that seventy-three percent
of the matter-energy content...

of the universe
is dark energy.

Dark energy, which was
once Einstein's blunder...

is now known to be
the dominant force in the universe.

His blunders
are our great discoveries.

Scientists are at the very
beginning of understanding...

what effect dark energy
will have on the fate of the universe.

Ideally, we'd like to measure...

how dark energy is behaving
as the universe ages.

Eventually, when the dark energy...

completely dominates
over dark matter...

the universe will enter a stage
known as exponential expansion.

For every given unit of time,
it'll double in size...

And unless the dark energy
changes sign someday...

and becomes
gravitationally attractive...

the fate of the universe
is to expand forever...

more and more quickly with time.

We don't understand
if the vacuum energy is driving...

the acceleration of the universe,
why it has the amount it does.

That is one
of the deepest puzzles...

remaining
in theoretical physics today.

Trillions of years from now...

it's going to be
a very lonely place.

We'll look up in the sky...

and the skies
will be practically dark.

The oceans will freeze over...

and it looks as if this is
the death of all intelligent life.

It looks as if dark energy
and the laws of physics...

are a death warrant
to all intelligent life in the universe.

In discovering dark matter
and dark energy...

science is one step closer
to defining the theory of everything:

One equation that will define
the entire workings of the universe.

Once we have
the theory of everything...

we'll be able to answer
some of the deepest questions...

ever since man and women
first looked at the heavens.

This could be
the crowning achievement...

of 2,000 years of investigation
into the laws of nature...

ever since the Greeks
asked the question...

"What are things made of?"

For now, dark matter
and dark energy...

continue to be the greatest
cosmological questions...

of the 21 st century.

It's certainly frustrating.

Yes, I mean, it's humbling,
too, to know that...

you know, all that we know
about physics...

is restricted to normal matter...

and yet there's all this other
dark matter and dark energy...

that we really understand
very little about.

It's the beginning of a new era
and the mysteries...

of the dark side
of the universe.

It's the Wild West
as far as particle astrophysics...

which is what we call this field.

It's a property of space
that we don't yet fully understand.