All language subtitles for How.the.Universe.Works.Series.7.07of10.Battle.of.the.Dark.Universe.1080p.HDTV.x264.AAC.MVGroup.org.eng

Across the universe, an endless war rages,

a bitter struggle between invisible forces.

We've seen this cosmic battle go on

for the past 14 billion years.

Like two navies, fighting it out in the ocean of space time.

Dark matter...

And dark energy

battling for control of the universe.

They've shaped the entire history of the universe.

They're not about to stop now.

Shadowy forces dictating our past,

our present, and our future.

The dark universe is kind of a puppet master

behind the scenes,

guiding the universe that we can see.

It's taken over the universe and, eventually,

it may well destroy the universe.

Captions paid for by discovery communications

As we look out across the universe,

we see nebulas,

stars,

and planets,

all constructed from visible matter.

But what we see is just a small part of the cosmos.

The rest is invisible,

unknown...

The dark universe.

Dark universe is a very common phrase to describe our universe,

because it turns out most of our cosmos is dark.

Dark not as in night.

Dark as in, doesn't interact with light,

and dark as in, we don't really understand it.

All the objects in our daily experience...

our bodies, the air, the chair that you're sitting in,

the planets and stars,

all of that only adds up to 5 percent of the universe.

It's the other 95 percent that is the dark stuff,

the dark universe.

The dark universe is made of two forces...

dark matter and dark energy.

In a way, the dark matter and dark energy

sort of oppose each other.

Dark matter has positive gravity that pulls things together,

and dark energy has repulsive gravity

that pushes things apart.

Freese: To encapsulate that in one sentence,

dark matter is attractive, dark energy is repulsive.

Since the dawn of time,

two forces slugging it out for control of the universe.

Dark matter and dark energy

are locked in this epic struggle.

The dark matter is trying to bring things together.

The dark energy is trying to drive everything apart.

So it's basically a battle. Who's going to win?

It's a struggle that started

13.8 billion years ago

in the cosmic furnace of the big bang.

The infant universe was a super hot ball of intense radiation,

but suddenly...

It started to transform.

It cooled and expanded,

triggering the birth of the regular universe

and the dark universe.

Scientists believe that both dark matter and dark energy

formed in the first moments of the big bang.

It would have formed probably just fractions of a second

after the big bang,

around the time that normal matter formed,

and the processes that created

the normal matter we know all about.

Similar processes probably created the dark matter.

During these first microseconds,

the universe was a hot, dense ball of matter and energy.

They're two sides of the same coin.

Matter can convert into energy...

And energy can convert directly into matter,

visible particles of regular matter forming atoms,

planets, stars, and us...

and other particles.

They are invisible.

They are dark matter.

One of the big mysteries that we as astronomers have to solve

is what this dark matter is.

We just don't know.

The idea of a type of matter that you can't see

and that acts differently than normal matter

is sort of out there.

It's sort of weird.

But the dark universe does leave clues.

It's like a crime scene.

You know that a crime has been committed,

even though you don't know the perpetrator yet.

We see the hints. We see the signs.

We see the signals that something funny

is happening in our universe,

even though we don't know exactly what's causing it.

There are several contenders

for what dark matter could be.

It could be normal matter that we just don't see,

or it could be some sort of exotic matter,

a particle of some sort that we haven't detected yet.

Sutter: It could be a species of particle,

like an electron, like a proton,

like a quark,

but a special kind that doesn't interact with light.

One of the favorite possible models of dark matter

are wimps, weakly interacting massive particles.

Wimps may not interact strongly

with other matter or light,

but they do exert a gravitational pull.

So they're the best candidates for the particles of dark matter

that formed in the big bang.

The early universe was intensely hot and dense,

full of new particles of both visible matter

and invisible dark matter.

But another force was present...

dark energy.

Dark energy has existed since the time of the big bang.

Dark energy was always there.

We don't know how much of it there was,

but it's possible that the same amount of dark energy

was always there.

Our understanding of what that dark energy is

is very limited.

If you were to ask a roomful of 10 theoretical physicists

on the nature of dark energy,

you'd get about 12 different answers.

We're not sure what dark energy is.

Dark energy is just a fancy name for our ignorance.

Dark energy is nothing more than a placeholder name

for this enormous gap in our understanding

of how the cosmos works.

But we don't understand it at all.

It's true.

One potential answer to what dark energy

actually is may be found in so-called empty space.

Could this be the source of dark energy?

We used to think of space as just boring emptiness.

But now, I think it's healthier to think of space

as a kind of substance.

A substance that carries

a strange type of energy.

Every small region of space

has a little bit of energy in it,

just associated with the vacuum itself.

Vacuum energy is the idea that vacuum isn't empty,

that there is something there.

There's an energy in it with a kind of antigravity.

Perhaps this vacuum energy

that pushes against gravity

is the mysterious dark energy.

We simply don't know.

We're not sure that the dark energy is the vacuum energy.

It could be a new type of energy that permeates all of space.

This is what we're trying to measure now.

Dark energy and dark matter,

forged in the intense heat of the big bang,

opposing forces, one attractive, one repulsive.

But together, over 13.8 billions years,

they will shape the history of the universe.

The story of the universe is dominated

by two powerful opposing forces...

dark matter and dark energy.

For 13.8 billions years,

they've battled it out for control of the cosmos.

Dark matter and dark energy are out there,

and they've shaped the entire history of the universe.

Our universe is actually the balance between dark forces.

Dark matter is trying to draw everything together,

and dark energy is trying to rip everything apart.

After the big bang, the infant universe was small,

intensely hot and intensely dense.

Dark matter, the force bent on bringing things together,

thrived.

But in this compressed space, dark energy,

the force trying to drive things apart, had no room to act.

When things were closer together,

the density of matter and radiation was bigger,

so big that the dark energy didn't matter.

The environment was also tough for normal matter.

It was so hot, intense radiation

prevented visible matter from bunching together to form atoms.

If any normal matter tried to clump together

through gravity or some other force,

this energy would just basically blast it apart.

In the very early universe,

when our universe was a lot smaller

and a lot hotter and a lot denser,

matter, normal matter, tried to collect together,

wanted to join the party.

But it was prevented from doing so

because there was also radiation that would throw it out.

Unable to stick together,

normal visible matter sped out

across the infant cosmos in a blizzard of particles.

But then dark matter,

the force that brings things together, intervened.

Dark matter doesn't talk to radiation,

doesn't talk to light.

Can do whatever it wants.

It starts clumping together.

Radiation pushes normal matter apart,

stopping it from forming dense regions.

But photons simply pass straight through the dark matter,

allowing it to clump and fall into dense pockets or wells.

The dark matter begins to clump together gravitationally,

and this means that the matter is going to fall into

those dark matter wells.

Over time, more and more regular matter

is pulled into the dark matter wells.

The regions that have a little bit more stuff,

gravity makes them bigger,

and the regions that have less stuff, those expand more.

So you have little pockets of slight extra matter,

have more and more and more matter over time.

Gravity-rich pockets of dark matter

pull particles of regular matter together.

Gradually, they form giant clouds of hydrogen

and helium gas.

Dark matter has laid the foundation stones of the cosmos.

This force may be dark, but it's highly creative.

And now, 180 million years after the big bang,

everything is in place

for the next dark matter construction milestone...

the creation of stars.

We know stars,

in the very early universe at the edge of time,

had to form from the collapse of gas clouds

under their own gravity.

But there's a problem.

The clouds of hydrogen in the infant universe can't collapse,

and no collapse means no stars.

Bullock: The gas in the early universe has a lot of pressure,

and this pressure keeps it from collapsing.

The dark matter doesn't experience

that kind of pressure.

So the dark matter can clump up

and make sites for structure formation.

So dark matter comes to the rescue,

creating regions of higher gravity,

dragging in hydrogen gas,

forcing the clouds to get denser and denser,

creating the conditions for collapse and then creation.

So it's only when the gravity of the dark matter

overwhelms the pressure of gases

that the gases can collapse and turn into stars.

Bullock: As soon as the gas cools down,

it can fall into those potential wells

that the dark matter created,

almost like little nurseries for stars,

and they start forming in earnest.

Dark matter provides a boost of gravity

to kick-start hydrogen into constructing the first stars,

stars that are the seeds of the first galaxies.

So it's dark matter that would have coalesced

in the early universe and grown from there,

and then the luminous component of the universe,

the things that we think of as being the universe itself,

like stars and galaxies,

would have just been along for the ride.

Without the presence of dark matter to seed structures,

there wasn't enough time in the early universe to form galaxies,

which means you and me have to thank dark matter

for our existence.

And dark matter now begins

a much more ambitious architectural project...

to shape the entire universe itself,

to build the biggest structure ever constructed...

the cosmic web.

The war between dark matter and dark energy

has been raging since the birth of time.

But in the early years, it's a one-sided contest.

In the early universe, the only thing that really mattered

was the dark matter and the normal matter.

Dark matter,

the force that brings things together,

is in the driver's seat.

Dark energy, the force that pulls things apart,

is the underdog.

We've seen this cosmic battle go on

for the past 14 billion years.

Fortunately for us, the dark energy got off to a slow start.

Meanwhile, dark matter is busy at work,

building the universe.

Not only does it trigger the birth of the first stars,

it embarks on an even more formidable

construction project...

the cosmic web.

There's this large-scale structure of filaments

that galaxies seem to form on,

and that's what we call the cosmic web.

And we can trace the formation of this cosmic web

all the way back to the early universe.

Now this is such a huge structure,

we don't think there's time in the universe

for matter's gravity alone to do this.

There must have been an underlying scaffold

of dark matter.

The dark matter started forming into these filaments,

and when the universe cooled enough,

normal matter could start to stream

into this gravitational attraction of the dark matter.

That became the scaffolding

on which this large-scale structure was built.

The filaments of dark matter joined together,

drawing in more and more hydrogen gas.

Dense clouds of gas build up at the junctions of the filaments,

the point where gravity is at its strongest.

Slowly and surely,

a familiar-looking structure starts to take shape.

If you've ever gone outside,

and you can see a spider web covered in dew,

that's kind of like what happened with the universe.

In this case, the spider web

is the structure of the dark matter.

It's all of these filaments, and the moisture in the air

is what condenses around them,

just like the normal matter fell into the dark matter web

to form these gigantic structures in the universe.

Dark matter, the universe's master builder,

succeeds in stitching together a cosmic web.

This will be the framework for the entire universe.

And so it is dark matter that would choreograph

and sculpt the shape of the universe itself.

One of the amazing things about dark matter is,

without it, we wouldn't be here.

It's hard to imagine how you could have structure

in the universe without dark matter.

Galaxies, and then galactic clusters,

form at the junctions of the filaments.

Slowly, but surely, the universe begins to take shape.

When we look at this structure over a cosmic scale,

we see that it looks kind of like a sponge.

You see voids with galaxies all over the edges of them.

That is the structure that was formed

by the dark matter in the early universe.

Dark matter is the thing that enabled,

that provided enough gravity

for the initial seeds of structure formation to coalesce,

for galaxies themselves to form.

And, of course, without galaxies,

there are no stars, and there's no planets,

and there's no us.

For 9 billion years,

dark matter orchestrates the construction of the universe.

In these, the first battles of the cosmos,

this constructive force is the clear victor.

For the time being, the dark matter has won.

Galaxies continue to form.

Clusters of galaxies are getting bigger over time.

But dark matter's success in building up the universe

sets in motion its potential downfall.

As the cosmic web evolves into a more complex structure...

Gaps form between the filaments, the cosmic voids.

The cosmic voids formed because other, more dense regions

of the universe gravitationally stole material away from them.

So the dense parts of the universe

accumulated more matter at the expense of the less dense parts,

which then became voids.

And lurking in these voids, dark energy.

Since the dawn of time,

it's been waiting for its opportunity.

Now it's preparing an offensive that may help it

conquer the universe.

In the very earliest times,

the dark matter dominated everything.

It was the big brother pushing the little brother around.

But in the long run,

the dark energy is going to overpower dark matter,

and so the relationship is entirely flipped.

We used to think the fate of the cosmos itself

depended on dark matter,

and it turns out that's not the case at all.

The fate of the universe depends entirely on dark energy.

The long reign of dark matter

may be coming to an end.

Dark energy, the great destroyer,

is hoping to take control.

This destructive force has one overriding aim...

to tear the universe apart.

Our universe is at war,

a relentless conflict between dark forces.

For the first 9 billion years, dark energy is subjugated.

Dark matter has the upper hand.

Frenk: When the universe emerged from the big bang,

the dark energy played no role.

It was insignificant.

But at some stage in the 14 billion years

since the big bang, these roles became reversed.

Dark energy came to be the more powerful force.

The question was, when?

The answer came at the end of the 20th century.

So it was an amazing breakthrough, really important.

In 1999,

scientists measure the expansion of the universe.

What they find shocks them.

They expect the speed of expansion to be decreasing.

In fact, it's actually increasing

and getting faster all the time.

The data indicate that for about the first 9 billion years,

it was slowing down.

But then, in the past 5 billion years,

it started accelerating faster and faster.

Alex filippenko was part of the team

that made this explosive discovery.

It befuddled us.

This isn't how nature was supposed to be behaving,

and, in fact, initially we thought

that there was something wrong

with either the observations or the measurements.

I didn't believe it for the longest time.

When the first data came out, I'm like,

"nah, I don't believe this, no way."

but it's in the data.

It's there. You can't escape it.

This is as shocking as if you held up a rock, let go of it,

and it went up into the air.

Five billion years ago,

galaxies started moving apart faster than before.

The question is, why?

What could be causing that? Well, one thing is clear.

It must be getting some extra energy from somewhere.

There is one main contender

for what may be supplying this extra energy,

a force with repulsive gravity,

a force that pushes things apart.

This is what astronomers call dark energy.

It's this mysterious repulsive force

that we know exists in the universe,

and we have no idea what it is.

Physicists may not agree on what dark energy is,

but there is a consensus

on where this repulsive force has the most influence...

in the regions between galaxies and galaxy clusters,

the cosmic voids.

They're actually filled to the brim with dark energy.

The first time dark energy is really going to make

its mark in the universe is going to be the time

when the first cosmic voids begin to appear.

We see dark energy's effects throughout the universe.

But when we look into the cosmic voids,

which are the most empty regions of our universe,

this is where dark energy is strongest.

Dark energy is the repulsive force

pushing things apart.

It prefers the voids where gravity is weak.

Bullock: These are areas where there's a lot less dark matter,

and because the overall density is low,

that's where the dark energy starts to peek out

and can really drive those voids to expand.

So the expansion and acceleration of the universe

are driven by the dark energy in those regions.

Dark energy pushes thing apart,

things that get in its way,

things like the cosmic web.

Dark matter and normal matter are also in its path

and are bulldozed out across the cosmos.

Slowly, but surely, the balance between dark energy

and dark matter is changing.

Imagine you have a giant swimming pool,

and at the very bottom there's a puddle of water

with a splash of whiskey.

So you have sort of a strong whiskey drink

down at the bottom of your pool.

But now you start dumping water into your pool, no more whiskey,

and it begins to get diluted and diluted,

and eventually, you just have a swimming pool full of water,

with one shot of whiskey mixed in.

That's not a very strong drink.

It's basically a water swimming pool.

That's pretty much happening with the dark energy.

At first, it's a one-to-one mixture

of dark matter and dark energy.

But in the long run, it's all dark energy

and pretty much no dark matter left over.

The forces of dark energy

are on an unstoppable March,

picking up more and more power

from the vast scale of the cosmic voids.

Dark energy is intrinsically very weak.

There's very little dark energy

and this repulsive effect in every cubic centimeter.

But the universe is vast.

Space is big.

So cumulatively, all this small amount of stuff

adds up to a very large amount,

and over a scale encompassing the entire universe,

the dark energy dominates.

But it's been a very long process.

After the big bang,

dark matter dominates for the first 9 billion years.

Then, 5 billion years ago,

dark energy starts to get the upper hand.

It causes the expansion of the universe to accelerate

and the space in the voids to grow more rapidly.

As the space expands, there's more and more dark energy

because you have a bigger space.

It sort of creates itself with the expansion of space.

Dark energy has a sneaky way of taking over

because it causes the space

to stretch out and get twice as big.

So now there's twice as much dark energy.

Dark energy just can't stop pushing,

causing the empty space of the voids

to continuously expand.

As the universe expands because of dark energy,

more and more dark energy is being created.

Dark energy is definitely gaining

the upper hand on dark matter.

It was always there, but it took over compared to other stuff.

Eventually creating enough energy to supercharge

the expansion of the universe.

This acceleration continues.

The universe is getting bigger and bigger,

and it's all powered by the forces of repulsion,

dark energy.

And for the universe, that could be very bad news.

If that's the case, dark energy may destroy the universe.

It will get stronger and stronger

until it literally rips apart the fabric of space-time.

Dark matter and dark energy

have been battling each other for 13.8 billion years.

For the first 9 billion years, dark matter dominates.

Dark matter exerts positive gravity,

but pulling everything together leads to one inevitable outcome.

If the universe was totally dominated by matter,

eventually our expansion would slow down,

glide to a stop, and then turn around

and collapse into a small, dense state

from where it came from, an event we call the big crunch.

During the big crunch,

gravity would play havoc with the cosmos.

Galaxies would be dragged together.

Stars and planets would smash into each other.

The universe would collapse in a blazing inferno

of superdense matter and energy.

Fortunately, none of this will probably happen.

Scientists have now dismissed the possibility of a big crunch.

Sutter: We don't face that, because we have a universe

filled with dark energy.

Dark energy is causing the universe to do something else,

something it would prefer not to do.

It is accelerating the expansion of the universe.

As dark energy gets stronger,

it supercharges this expansion.

The presence of dark energy is like a high-octane additive

into a gas tank,

where a car isn't just coasting along.

It's boosting along,

and that's what's happening with our universe.

If that's correct, this supercharged expansion

will continue for the next few billion years.

The repulsive force of dark energy will become invincible,

and such unbridled power will come with a high cost.

I know of three ultimate doomsday weapons.

One is galactus' ultimate nullifier.

Another is the infinity stones in the hands of thanos.

But those two are make-believe.

The real one is dark energy.

It's the real ultimate universe destroyer.

The universe may pay the ultimate price

for this ever-increasing expansion driven by dark energy.

And if that continues forever,

the future of the universe is very, very simple.

It continues to expand, to accelerate.

Everything moves apart from everything else,

and the universe becomes empty.

There'll be nothing left

but cold, desolate, empty space forever.

Dark energy will finally win

the long war with dark matter.

But the result won't be pretty.

This future universe will be cold, dark, and empty.

Eventually it's going to get so cold

that really nothing can happen in it.

It's the big chill.

It gets colder and colder and darker and darker.

Everything will fade out, and though it began with a bang,

it's going to die in a whimper.

The universe flatlines because of dark energy.

The universe dies in a big chill.

Galaxies are so far apart,

they're distant islands in a sea of darkness.

Gradually, the galaxies dies, too.

Star birth stops, and the universe fades away.

Or maybe not.

There's another, far more violent scenario.

Here, dark energy just goes from strength to strength.

It could be that dark energy is so strong

that it will multiply upon itself

as the universe gets bigger.

This is a process that we call phantom dark energy.

Phantom energy is dark energy on steroids.

It multiplies uncontrollably in the voids,

tearing at the fabric of the universe

in a process called the big rip.

Dark energy is weird enough, but imagine the possibility

that there is more and more of it as time goes on,

and it's called phantom energy, and in that case,

it would rip everything apart, even black holes.

It will start to rip apart galaxies themselves...

Rip apart solar systems...

Rip apart people,

rip apart atoms...

Rip apart nuclei.

Until finally, space itself is pulled apart.

The universe, as we know it,

will be destroyed,

but the big rip may not be the end of everything.

There will be no normal matter and no dark matter.

And with nothing left to conquer,

phantom energy may use its powers to become a creator,

triggering a rebirth.

Eventually, when you get to this ultimate stage of emptiness,

because of the phantom energy,

we're actually able to turn the universe around

and get it to collapse again,

and then go through a series of bounces.

So we call it the phantom bounce.

With this phantom bounce,

universal life energy left in this dead universe

starts to collapse.

Freese: And eventually it becomes hotter and hotter

and denser and denser, and then the fiery inferno

eventually pushes you back out into another big bang,

and this just keeps going on indefinitely.

So the destructive and repulsive dark energy

spawns a force that becomes the ultimate universe recycler.

The end state of our universe

would lead you back into another cycle,

a whole new big bang from the beginning.

In the end, dark energy may kill the cosmos,

or it may create a new one.

Dark energy is mysterious.

Dark energy is unknown.

Dark energy is going to do whatever it feels like.

Maybe dark energy will go away.

Maybe dark energy will decay

and become a flood of new matter and radiation.

Maybe dark matter will get stronger.

We don't know.

For now, we think dark energy

will determine the fate of the universe.

But all of our evidence is speculative.

What if we have it all wrong?

What if there is no dark universe at all?

A large part of our understanding

of the universe's past,

present, and future is based on educated guesswork

about two invisible forces...

Dark matter and dark energy.

But it's pure speculation.

Perhaps dark matter and dark energy don't exist.

There's not new stuff in the universe.

Anything is possible.

Dark energy, in particular,

might not be real, so maybe there's something else

that could be pushing the universe apart.

So we could absolutely be wrong

about dark energy and dark matter.

Maybe they don't exist.

Maybe tomorrow we'll discover that our understanding

was wrong all along.

That's an awful lot of maybes.

Let's add one more.

When it comes to finding answers,

maybe we're looking in the wrong place.

Filippenko: One possibility is that there are other universes

out there pulling outward, so to speak, on our universe.

That might be the answer.

But most theoretical physicists and astrophysicists

these days think that dark energy is real

because that seems to be the simplest explanation

for a wide variety of observations.

No one really knows what dark energy is made of.

Maybe the answer lies in the past.

The best theory for dark energy we have right now

is the simplest one and the oldest one,

and that's the idea that it's a cosmological constant.

Albert einstein came up with the idea

of a cosmological constant in 1917.

He suggested that space has its own energy,

energy that can affect the way the universe expands.

When edwin hubble proved the universe is expanding,

einstein thought the cosmological constant

was his biggest blunder.

But observations that the expansion of the universe

is accelerating

reveal einstein was right all along.

Filippenko: Well, here we are. We've reintroduced the idea.

So einstein's biggest blunder may have actually

conceptually been his greatest triumph.

But to understand the true nature of the dark universe,

we may need to re-evaluate what we think we know about gravity.

When we're trying to understand dark energy and dark matter,

there's a chance that just our fundamental theories of gravity

are wrong, that general relativity isn't quite right.

Einstein's theory of general relativity

explains how gravity works,

how stars orbit in galaxies,

and planets orbit stars.

Some scientists wonder if altering this theory

will help us understand the dark universe.

So you need not absolutely believe

that there is something actually called dark matter.

You need to only understand that there is something

in the universe which behaves like dark matter.

For example, you could effectively mimic the behavior

of dark matter by modifying our current theory of gravity.

But successfully modifying

einstein's theories on gravity is a big challenge.

Einstein's equations are very robust.

You don't fluff around with einstein with impunity.

For decades, theoretical physicists

have toyed with einstein's equations,

looking for ways to explain dark matter and dark energy,

or make them go away.

As yet, no one has managed.

The dark universe persists.

I think that the best description of the observations

we have today is that dark matter exists.

It's out there, as well as dark energy.

I think dark energy exists.

I think dark energy is real,

but I must admit that sometimes, at 3 o'clock in the morning,

I wake up screaming, worried that, in fact,

we've settled on the wrong answer,

and that in a couple hundred years,

they're going to be laughing at us.

Until then, our observations tell us the battle

between dark matter and dark energy has shaped the universe.

Dark matter dictated the past,

built the galaxies, the stars, and the planets.

Dark energy will determine its future,

potentially tearing the universe apart.

Just because we can't see dark matter and dark energy directly

doesn't mean they have not had a profound effect

on the evolution of the entire universe.

The dark universe was there at the beginning of the universe,

shaping it, and actually creating the conditions

for us to be here, and it's taken over the universe.

And, eventually, it may well destroy the universe.

Our universe may be dominated

by the long struggle between dark matter and dark energy.

But all of this conflict has led to a creative outcome,

an outcome for which we should all be grateful.

The name dark matter suggests that it's something nefarious

and somehow bad for us, but actually it's turned out

that dark matter is very much our friend.

Because if it weren't for the dark matter,

we wouldn't be here.

There's a wonderful irony

to calling it the dark universe,

because now we're actually beginning to shed light

on how the universe began,

how the largest structures in the universe evolved.

We wouldn't be here without this dark universe.

It's not dark at all.

It's shedding light on our own reality.