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narrator: Across the universe,
an endless war rages,

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a bitter struggle
between invisible forces.

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Tegmark: We've seen this
cosmic battle go on

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for the past 14 billion years.

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

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[ crackling ]

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narrator:
Dark matter...

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And dark energy

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battling for control
of the universe.

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They've shaped the entire
history of the universe.

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They're not about to stop now.

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Narrator: Shadowy forces
dictating our past,

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our present, and our future.

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Thaller: The dark universe
is kind of a puppet master

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behind the scenes,

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guiding the universe
that we can see.

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Oluseyi: It's taken over
the universe and, eventually,

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it may well
destroy the universe.

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[ rumbling ]

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[ crackling, bursting ]

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-- captions by vitac --
 www.Vitac.Com

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captions paid for by
discovery communications

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narrator: As we look out
across the universe,

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we see nebulas,

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stars,

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and planets,

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all constructed
from visible matter.

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

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The rest is invisible,

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unknown...

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The dark universe.

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

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because it turns out
most of our cosmos is dark.

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Dark not as in night.

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Dark as in,
doesn't interact with light,

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

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All the objects
in our daily experience --

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

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the planets and stars,

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

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

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the dark universe.

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narrator: The dark universe
is made of two forces --

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dark matter and dark energy.

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In a way, the dark matter
and dark energy

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sort of oppose each other.

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[ whooshing ]

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dark matter has positive gravity
that pulls things together,

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and dark energy
has repulsive gravity

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that pushes things apart.

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Freese: To encapsulate that
in one sentence,

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dark matter is attractive,
dark energy is repulsive.

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Narrator:
Since the dawn of time,

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two forces slugging it out
for control of the universe.

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Dark matter and dark energy

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are locked
in this epic struggle.

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The dark matter is trying
to bring things together.

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The dark energy is trying
to drive everything apart.

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

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[ rumbling ]

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narrator:
It's a struggle that started

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13.8 billion years ago

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in the cosmic furnace
of the big bang.

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

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but suddenly...

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[ bursting ]
...It started to transform.

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It cooled and expanded,

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triggering the birth
of the regular universe

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and the dark universe.

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Scientists believe that both
dark matter and dark energy

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formed in the first moments
of the big bang.

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It would have formed probably
just fractions of a second

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after the big bang,

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around the time
that normal matter formed,

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and the processes that created

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the normal matter
we know all about.

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Similar processes probably
created the dark matter.

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narrator: During these
first microseconds,

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

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They're two sides
of the same coin.

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Matter can convert
into energy...

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[ crackling ]

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...And energy can convert
directly into matter,

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visible particles of regular
matter forming atoms,

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planets, stars, and us --

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and other particles.

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They are invisible.

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They are dark matter.

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

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is what this dark matter is.

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We just don't know.

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

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and that acts differently
than normal matter

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is sort of out there.

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It's sort of weird.

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Narrator: But the dark universe
does leave clues.

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It's like a crime scene.

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You know that a crime
has been committed,

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even though you don't know
the perpetrator yet.

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We see the hints.
We see the signs.

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We see the signals
that something funny

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is happening
in our universe,

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

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Narrator:
There are several contenders

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for what dark matter could be.

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

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or it could be some
sort of exotic matter,

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

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Sutter: It could be
a species of particle,

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like an electron,
like a proton,

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like a quark,

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

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One of the favorite possible
models of dark matter

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are wimps, weakly interacting
massive particles.

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narrator: Wimps may not
interact strongly

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with other matter or light,

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but they do exert
a gravitational pull.

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[ crackling ]

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

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that formed in the big bang.

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the early universe was
intensely hot and dense,

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full of new particles
of both visible matter

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and invisible dark matter.

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But another force
was present --

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dark energy.

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

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Freese:
Dark energy was always there.

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

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

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was always there.

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[ rumbling ]

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narrator: Our understanding of
what that dark energy is

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is very limited.

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Sutter:
If you were to ask a roomful of 10 theoretical physicists

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on the nature of dark energy,

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you'd get about
12 different answers.

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We're not sure
what dark energy is.

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Dark energy is just a fancy
name for our ignorance.

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Dark energy is nothing
more than a placeholder name

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for this enormous gap
in our understanding

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of how the cosmos works.

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But we don't understand
it at all.

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[ chuckles ] it's true.

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narrator: One potential answer
to what dark energy

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

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Could this be the source
of dark energy?

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Tegmark: We used to think of
space as just boring emptiness.

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

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as a kind of substance.

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Narrator:
A substance that carries

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a strange type of energy.

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Every small region of space

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has a little bit
of energy in it,

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just associated
with the vacuum itself.

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

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that there is something there.

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

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[ crackling ]

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narrator:
Perhaps this vacuum energy

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that pushes against gravity

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is the mysterious dark energy.

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We simply don't know.

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

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

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This is what we're
trying to measure now.

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Narrator:
Dark energy and dark matter,

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forged in the intense heat
of the big bang,

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opposing forces,
one attractive, one repulsive.

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But together,
over 13.8 billions years,

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they will shape
the history of the universe.

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[ rumbling ]

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[ rumbling ]

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narrator: The story of
the universe is dominated

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by two powerful
opposing forces --

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dark matter and dark energy.

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For 13.8 billions years,

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

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Dark matter and dark energy
are out there,

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

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Our universe is actually
the balance between dark forces.

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Dark matter is trying to draw
everything together,

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and dark energy is trying
to rip everything apart.

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Narrator: After the big bang,
the infant universe was small,

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intensely hot
and intensely dense.

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[ whooshing, rumbling ]

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dark matter, the force bent
on bringing things together,

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thrived.

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But in this compressed space,
dark energy,

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

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When things were
closer together,

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the density of matter
and radiation was bigger,

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so big that the dark energy
didn't matter.

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Narrator: The environment was
also tough for normal matter.

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It was so hot,
intense radiation

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prevented visible matter from
bunching together to form atoms.

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If any normal matter
tried to clump together

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through gravity
or some other force,

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this energy would just
basically blast it apart.

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In the very early universe,

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when our universe
was a lot smaller

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and a lot hotter
and a lot denser,

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matter, normal matter,
tried to collect together,

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wanted to join the party.

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But it was prevented
from doing so

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because there was also radiation
that would throw it out.

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Narrator:
Unable to stick together,

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normal visible matter
sped out

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across the infant cosmos
in a blizzard of particles.

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[ crackling ]

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00:11:00,227 --> 00:11:02,327
but then dark matter,

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the force that brings things
together, intervened.

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[ crackling ]

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00:11:07,601 --> 00:11:10,201
dark matter doesn't talk
to radiation,

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doesn't talk to light.

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Can do whatever it wants.

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It starts clumping together.

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Narrator: Radiation
pushes normal matter apart,

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stopping it
from forming dense regions.

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But photons simply pass straight
through the dark matter,

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

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The dark matter begins to clump
together gravitationally,

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and this means that the matter
is going to fall into

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those dark matter wells.

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Narrator: Over time,
more and more regular matter

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is pulled into
the dark matter wells.

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The regions that have
a little bit more stuff,

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gravity makes them bigger,

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

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So you have little pockets
of slight extra matter,

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have more and more
and more matter over time.

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[ rumbling ]

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00:12:02,856 --> 00:12:05,156
narrator: Gravity-rich pockets
of dark matter

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00:12:05,158 --> 00:12:08,226
pull particles
of regular matter together.

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[ whistling, rumbling ]

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gradually, they form
giant clouds of hydrogen

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and helium gas.

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Dark matter has laid the
foundation stones of the cosmos.

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

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and now, 180 million years
after the big bang,

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everything is in place

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for the next dark matter
construction milestone --

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the creation of stars.

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We know stars,

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in the very early universe
at the edge of time,

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00:12:46,333 --> 00:12:48,900
had to form from
the collapse of gas clouds

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under their own gravity.

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00:12:53,273 --> 00:12:54,939
narrator:
But there's a problem.

259
00:12:54,941 --> 00:13:00,478
The clouds of hydrogen in the
infant universe can't collapse,

260
00:13:00,480 --> 00:13:04,616
and no collapse
means no stars.

261
00:13:04,618 --> 00:13:07,218
Bullock: The gas in the early
universe has a lot of pressure,

262
00:13:07,220 --> 00:13:09,387
and this pressure
keeps it from collapsing.

263
00:13:09,389 --> 00:13:10,655
The dark matter
doesn't experience

264
00:13:10,657 --> 00:13:12,023
that kind of pressure.

265
00:13:12,025 --> 00:13:13,491
So the dark matter can clump up

266
00:13:13,493 --> 00:13:17,262
and make sites
for structure formation.

267
00:13:17,264 --> 00:13:19,430
Narrator: So dark matter comes
to the rescue,

268
00:13:19,432 --> 00:13:23,067
creating regions
of higher gravity,

269
00:13:23,069 --> 00:13:25,170
dragging in hydrogen gas,

270
00:13:25,172 --> 00:13:29,641
forcing the clouds to get
denser and denser,

271
00:13:29,643 --> 00:13:34,445
creating the conditions
for collapse and then creation.

272
00:13:34,447 --> 00:13:37,448
[ rumbling ]

273
00:13:37,450 --> 00:13:40,852



274
00:13:40,854 --> 00:13:44,289
so it's only when the gravity
of the dark matter

275
00:13:44,291 --> 00:13:46,691
overwhelms the pressure of gases

276
00:13:46,693 --> 00:13:50,862
that the gases can collapse
and turn into stars.

277
00:13:50,864 --> 00:13:52,263
Bullock: As soon as the gas
cools down,

278
00:13:52,265 --> 00:13:53,965
it can fall into those
potential wells

279
00:13:53,967 --> 00:13:55,900
that the dark matter created,

280
00:13:55,902 --> 00:13:58,603
almost like little nurseries
for stars,

281
00:13:58,605 --> 00:14:00,638
and they start
forming in earnest.

282
00:14:00,640 --> 00:14:02,640



283
00:14:02,642 --> 00:14:05,210
narrator: Dark matter provides
a boost of gravity

284
00:14:05,212 --> 00:14:10,782
to kick-start hydrogen into
constructing the first stars,

285
00:14:10,784 --> 00:14:14,652
stars that are the seeds
of the first galaxies.

286
00:14:14,654 --> 00:14:17,355
Tremblay: So it's dark matter
that would have coalesced

287
00:14:17,357 --> 00:14:20,692
in the early universe
and grown from there,

288
00:14:20,694 --> 00:14:22,627
and then the luminous
component of the universe,

289
00:14:22,629 --> 00:14:24,963
the things that we think of
as being the universe itself,

290
00:14:24,965 --> 00:14:26,497
like stars and galaxies,

291
00:14:26,499 --> 00:14:28,900
would have just been along
for the ride.

292
00:14:28,902 --> 00:14:34,239
Without the presence of dark
matter to seed structures,

293
00:14:34,241 --> 00:14:39,043
there wasn't enough time in the
early universe to form galaxies,

294
00:14:39,045 --> 00:14:42,914
which means you and me
have to thank dark matter

295
00:14:42,916 --> 00:14:45,149
for our existence.

296
00:14:45,151 --> 00:14:47,252
Narrator:
And dark matter now begins

297
00:14:47,254 --> 00:14:51,890
a much more ambitious
architectural project --

298
00:14:51,892 --> 00:14:56,194
to shape the entire
universe itself,

299
00:14:56,196 --> 00:15:00,431
to build the biggest structure
ever constructed --

300
00:15:00,433 --> 00:15:02,000
the cosmic web.

301
00:15:02,002 --> 00:15:06,170



302
00:15:12,646 --> 00:15:16,948
[ rumbling ]

303
00:15:16,950 --> 00:15:20,418



304
00:15:20,420 --> 00:15:23,254
narrator: The war between
dark matter and dark energy

305
00:15:23,256 --> 00:15:25,990
has been raging
since the birth of time.

306
00:15:25,992 --> 00:15:29,060
[ rumbling ]

307
00:15:29,062 --> 00:15:33,865
but in the early years,
it's a one-sided contest.

308
00:15:33,867 --> 00:15:36,200
In the early universe, the only
thing that really mattered

309
00:15:36,202 --> 00:15:39,771
was the dark matter
and the normal matter.

310
00:15:39,773 --> 00:15:41,873
Narrator: Dark matter,

311
00:15:41,875 --> 00:15:45,677
the force that brings things
together,

312
00:15:45,679 --> 00:15:47,345
is in the driver's seat.

313
00:15:47,347 --> 00:15:50,949
[ rumbling ]

314
00:15:50,951 --> 00:15:55,019
dark energy, the force
that pulls things apart,

315
00:15:55,021 --> 00:15:57,021
is the underdog.

316
00:15:57,023 --> 00:15:59,590
We've seen this
cosmic battle go on

317
00:15:59,592 --> 00:16:01,693
for the past 14 billion years.

318
00:16:01,695 --> 00:16:06,798
Fortunately for us, the dark
energy got off to a slow start.

319
00:16:06,800 --> 00:16:10,201
Narrator: Meanwhile, dark matter
is busy at work,

320
00:16:10,203 --> 00:16:12,870
building the universe.

321
00:16:12,872 --> 00:16:17,742
Not only does it trigger
the birth of the first stars,

322
00:16:17,744 --> 00:16:19,978
it embarks on an even
more formidable

323
00:16:19,980 --> 00:16:22,947
construction project --

324
00:16:22,949 --> 00:16:26,451
the cosmic web.

325
00:16:26,453 --> 00:16:30,521
Straughn:
There's this large-scale structure of filaments

326
00:16:30,523 --> 00:16:32,757
that galaxies
seem to form on,

327
00:16:32,759 --> 00:16:35,526
and that's what we call
the cosmic web.

328
00:16:35,528 --> 00:16:38,429
And we can trace the formation
of this cosmic web

329
00:16:38,431 --> 00:16:41,265
all the way back
to the early universe.

330
00:16:41,267 --> 00:16:43,034
Now this is such a huge
structure,

331
00:16:43,036 --> 00:16:44,736
we don't think there's time
in the universe

332
00:16:44,738 --> 00:16:47,338
for matter's gravity alone
to do this.

333
00:16:47,340 --> 00:16:49,507
There must have been
an underlying scaffold

334
00:16:49,509 --> 00:16:51,242
of dark matter.

335
00:16:51,244 --> 00:16:53,578
Plait: The dark matter started
forming into these filaments,

336
00:16:53,580 --> 00:16:55,580
and when the universe
cooled enough,

337
00:16:55,582 --> 00:16:57,281
normal matter
could start to stream

338
00:16:57,283 --> 00:17:00,385
into this gravitational
attraction of the dark matter.

339
00:17:00,387 --> 00:17:01,986
That became the scaffolding

340
00:17:01,988 --> 00:17:05,857
on which this large-scale
structure was built.

341
00:17:05,859 --> 00:17:08,793
Narrator: The filaments of
dark matter joined together,

342
00:17:08,795 --> 00:17:12,630
drawing in more
and more hydrogen gas.

343
00:17:12,632 --> 00:17:16,801
Dense clouds of gas build up at
the junctions of the filaments,

344
00:17:16,803 --> 00:17:19,871
the point where gravity
is at its strongest.

345
00:17:19,873 --> 00:17:22,907



346
00:17:22,909 --> 00:17:24,842
slowly and surely,

347
00:17:24,844 --> 00:17:29,380
a familiar-looking structure
starts to take shape.

348
00:17:29,382 --> 00:17:30,715
If you've ever gone outside,

349
00:17:30,717 --> 00:17:34,519
and you can see a spider web
covered in dew,

350
00:17:34,521 --> 00:17:37,355
that's kind of like
what happened with the universe.

351
00:17:37,357 --> 00:17:38,890
In this case, the spider web

352
00:17:38,892 --> 00:17:40,792
is the structure
of the dark matter.

353
00:17:40,794 --> 00:17:43,327
It's all of these filaments,
and the moisture in the air

354
00:17:43,329 --> 00:17:44,862
is what condenses around them,

355
00:17:44,864 --> 00:17:48,699
just like the normal matter
fell into the dark matter web

356
00:17:48,701 --> 00:17:51,769
to form these gigantic
structures in the universe.

357
00:17:51,771 --> 00:17:55,373
Narrator: Dark matter,
the universe's master builder,

358
00:17:55,375 --> 00:17:59,677
succeeds in stitching
together a cosmic web.

359
00:17:59,679 --> 00:18:04,682
This will be the framework
for the entire universe.

360
00:18:04,684 --> 00:18:07,618
And so it is dark matter
that would choreograph

361
00:18:07,620 --> 00:18:10,121
and sculpt the shape
of the universe itself.

362
00:18:10,123 --> 00:18:12,090
One of the amazing things
about dark matter is,

363
00:18:12,092 --> 00:18:14,325
without it, we wouldn't be here.

364
00:18:14,327 --> 00:18:16,828
It's hard to imagine how you
could have structure

365
00:18:16,830 --> 00:18:19,730
in the universe
without dark matter.

366
00:18:19,732 --> 00:18:22,366
Narrator: Galaxies,
and then galactic clusters,

367
00:18:22,368 --> 00:18:25,169
form at the junctions
of the filaments.

368
00:18:25,171 --> 00:18:30,675
Slowly, but surely, the universe
begins to take shape.

369
00:18:30,677 --> 00:18:33,711
When we look at this structure
over a cosmic scale,

370
00:18:33,713 --> 00:18:35,847
we see that it looks
kind of like a sponge.

371
00:18:35,849 --> 00:18:39,383
You see voids with galaxies
all over the edges of them.

372
00:18:39,385 --> 00:18:41,552
That is the structure
that was formed

373
00:18:41,554 --> 00:18:45,523
by the dark matter
in the early universe.

374
00:18:45,525 --> 00:18:47,658
Dark matter is the thing
that enabled,

375
00:18:47,660 --> 00:18:49,660
that provided enough gravity

376
00:18:49,662 --> 00:18:53,331
for the initial seeds of
structure formation to coalesce,

377
00:18:53,333 --> 00:18:55,500
for galaxies themselves to form.

378
00:18:55,502 --> 00:18:57,301
[ rumbling ]

379
00:18:57,303 --> 00:18:58,936
and, of course,
without galaxies,

380
00:18:58,938 --> 00:19:01,005
there are no stars,
and there's no planets,

381
00:19:01,007 --> 00:19:02,907
and there's no us.

382
00:19:02,909 --> 00:19:04,509
Narrator:
For 9 billion years,

383
00:19:04,511 --> 00:19:08,012
dark matter orchestrates the
construction of the universe.

384
00:19:08,014 --> 00:19:09,981
[ rumbling ]

385
00:19:09,983 --> 00:19:12,984
in these, the first battles
of the cosmos,

386
00:19:12,986 --> 00:19:17,021
this constructive force
is the clear victor.

387
00:19:17,023 --> 00:19:19,590
For the time being,
the dark matter has won.

388
00:19:19,592 --> 00:19:21,459
Galaxies continue to form.

389
00:19:21,461 --> 00:19:23,861
Clusters of galaxies
are getting bigger over time.

390
00:19:23,863 --> 00:19:27,064
[ rumbling ]

391
00:19:27,066 --> 00:19:30,067
narrator:
But dark matter's success in building up the universe

392
00:19:30,069 --> 00:19:33,404
sets in motion
its potential downfall.

393
00:19:33,406 --> 00:19:37,808
[ whooshing, rumbling ]

394
00:19:37,810 --> 00:19:41,679
as the cosmic web evolves
into a more complex structure...

395
00:19:41,681 --> 00:19:44,415



396
00:19:44,417 --> 00:19:48,953
...Gaps form between
the filaments, the cosmic voids.

397
00:19:48,955 --> 00:19:50,721



398
00:19:50,723 --> 00:19:55,493
the cosmic voids formed because
other, more dense regions

399
00:19:55,495 --> 00:20:00,364
of the universe gravitationally
stole material away from them.

400
00:20:00,366 --> 00:20:02,400
So the dense parts
of the universe

401
00:20:02,402 --> 00:20:07,038
accumulated more matter at the
expense of the less dense parts,

402
00:20:07,040 --> 00:20:10,174
which then became voids.

403
00:20:10,176 --> 00:20:14,378
Narrator: And lurking
in these voids, dark energy.

404
00:20:14,380 --> 00:20:15,980
Since the dawn of time,

405
00:20:15,982 --> 00:20:19,717
it's been waiting
for its opportunity.

406
00:20:19,719 --> 00:20:23,754
Now it's preparing an offensive
that may help it

407
00:20:23,756 --> 00:20:26,123
conquer the universe.

408
00:20:26,125 --> 00:20:27,825
In the very earliest times,

409
00:20:27,827 --> 00:20:29,460
the dark matter dominated
everything.

410
00:20:29,462 --> 00:20:32,797
It was the big brother pushing
the little brother around.

411
00:20:32,799 --> 00:20:34,232
But in the long run,

412
00:20:34,234 --> 00:20:37,001
the dark energy is going
to overpower dark matter,

413
00:20:37,003 --> 00:20:39,370
and so the relationship
is entirely flipped.

414
00:20:39,372 --> 00:20:41,005



415
00:20:41,007 --> 00:20:43,874
plait: We used to think
the fate of the cosmos itself

416
00:20:43,876 --> 00:20:45,276
depended on dark matter,

417
00:20:45,278 --> 00:20:47,878
and it turns out
that's not the case at all.

418
00:20:47,880 --> 00:20:51,916
The fate of the universe
depends entirely on dark energy.

419
00:20:51,918 --> 00:20:54,151
Narrator:
The long reign of dark matter

420
00:20:54,153 --> 00:20:56,153
may be coming to an end.

421
00:20:56,155 --> 00:20:58,889
Dark energy,
the great destroyer,

422
00:20:58,891 --> 00:21:01,892
is hoping to take control.

423
00:21:01,894 --> 00:21:06,330
This destructive force
has one overriding aim --

424
00:21:06,332 --> 00:21:09,533
to tear the universe apart.

425
00:21:09,535 --> 00:21:13,204
[ rumbling ]

426
00:21:19,612 --> 00:21:22,647
[ rumbling ]

427
00:21:25,084 --> 00:21:28,119
narrator:
Our universe is at war,

428
00:21:28,121 --> 00:21:32,490
a relentless conflict
between dark forces.

429
00:21:32,492 --> 00:21:38,429
For the first 9 billion years,
dark energy is subjugated.

430
00:21:38,431 --> 00:21:42,700
Dark matter has the upper hand.

431
00:21:42,702 --> 00:21:45,036
Frenk: When the universe
emerged from the big bang,

432
00:21:45,038 --> 00:21:47,638
the dark energy played no role.

433
00:21:47,640 --> 00:21:48,806
It was insignificant.

434
00:21:48,808 --> 00:21:50,608
[ rumbling ]

435
00:21:50,610 --> 00:21:53,144
narrator: But at some stage
in the 14 billion years

436
00:21:53,146 --> 00:21:58,316
since the big bang,
these roles became reversed.

437
00:21:58,318 --> 00:22:02,620
Dark energy came to be
the more powerful force.

438
00:22:02,622 --> 00:22:05,756
The question was, when?

439
00:22:05,758 --> 00:22:09,627
The answer came at the end
of the 20th century.

440
00:22:09,629 --> 00:22:12,830
So it was an amazing
breakthrough, really important.

441
00:22:12,832 --> 00:22:14,065



442
00:22:14,067 --> 00:22:15,733
narrator: In 1999,

443
00:22:15,735 --> 00:22:18,903
scientists measure
the expansion of the universe.

444
00:22:18,905 --> 00:22:20,404
[ whooshing ]

445
00:22:20,406 --> 00:22:23,374
what they find shocks them.

446
00:22:23,376 --> 00:22:28,145
They expect the speed
of expansion to be decreasing.

447
00:22:28,147 --> 00:22:31,349
In fact,
it's actually increasing

448
00:22:31,351 --> 00:22:34,752
and getting faster all the time.

449
00:22:34,754 --> 00:22:38,155
The data indicate that for about
the first 9 billion years,

450
00:22:38,157 --> 00:22:39,590
it was slowing down.

451
00:22:39,592 --> 00:22:41,992
But then,
in the past 5 billion years,

452
00:22:41,994 --> 00:22:46,230
it started accelerating
faster and faster.

453
00:22:46,232 --> 00:22:49,033
Narrator: Alex filippenko
was part of the team

454
00:22:49,035 --> 00:22:52,503
that made this
explosive discovery.

455
00:22:52,505 --> 00:22:53,938
Filippenko:
It befuddled us.

456
00:22:53,940 --> 00:22:56,974
This isn't how nature
was supposed to be behaving,

457
00:22:56,976 --> 00:22:59,043
and, in fact,
initially we thought

458
00:22:59,045 --> 00:23:00,444
that there was something wrong

459
00:23:00,446 --> 00:23:03,347
with either the observations
or the measurements.

460
00:23:03,349 --> 00:23:05,816
I didn't believe it
for the longest time.

461
00:23:05,818 --> 00:23:07,385
When the first data came out,
I'm like,

462
00:23:07,387 --> 00:23:09,453
"nah, I don't believe this,
no way."

463
00:23:09,455 --> 00:23:10,921
but it's in the data.

464
00:23:10,923 --> 00:23:12,723
It's there.
You can't escape it.

465
00:23:12,725 --> 00:23:16,460
This is as shocking as if you
held up a rock, let go of it,

466
00:23:16,462 --> 00:23:17,762
and it went up into the air.

467
00:23:17,764 --> 00:23:20,798



468
00:23:20,800 --> 00:23:22,433
narrator:
Five billion years ago,

469
00:23:22,435 --> 00:23:26,370
galaxies started moving apart
faster than before.

470
00:23:26,372 --> 00:23:29,473
The question is, why?

471
00:23:29,475 --> 00:23:32,777
What could be causing that?
Well, one thing is clear.

472
00:23:32,779 --> 00:23:35,513
It must be getting some
extra energy from somewhere.

473
00:23:35,515 --> 00:23:37,848
Narrator:
There is one main contender

474
00:23:37,850 --> 00:23:41,519
for what may be supplying
this extra energy,

475
00:23:41,521 --> 00:23:44,422
a force with repulsive gravity,

476
00:23:44,424 --> 00:23:47,825
a force
that pushes things apart.

477
00:23:47,827 --> 00:23:50,327
This is what astronomers
call dark energy.

478
00:23:50,329 --> 00:23:53,164
It's this mysterious
repulsive force

479
00:23:53,166 --> 00:23:55,166
that we know exists
in the universe,

480
00:23:55,168 --> 00:23:58,202
and we have no idea what it is.

481
00:23:58,204 --> 00:24:01,806
Narrator: Physicists may not
agree on what dark energy is,

482
00:24:01,808 --> 00:24:03,240
but there is a consensus

483
00:24:03,242 --> 00:24:08,312
on where this repulsive force
has the most influence --

484
00:24:08,314 --> 00:24:13,184
in the regions between galaxies
and galaxy clusters,

485
00:24:13,186 --> 00:24:15,186
the cosmic voids.

486
00:24:15,188 --> 00:24:19,690
They're actually filled
to the brim with dark energy.

487
00:24:19,692 --> 00:24:21,392
The first time dark energy
is really going to make

488
00:24:21,394 --> 00:24:24,161
its mark in the universe
is going to be the time

489
00:24:24,163 --> 00:24:26,864
when the first cosmic voids
begin to appear.

490
00:24:26,866 --> 00:24:28,933
[ whooshing ]

491
00:24:28,935 --> 00:24:33,237
sutter:
We see dark energy's effects throughout the universe.

492
00:24:33,239 --> 00:24:36,407
But when we look into
the cosmic voids,

493
00:24:36,409 --> 00:24:39,577
which are the most empty regions
of our universe,

494
00:24:39,579 --> 00:24:42,413
this is where dark energy
is strongest.

495
00:24:42,415 --> 00:24:45,249



496
00:24:45,251 --> 00:24:47,518
narrator: Dark energy
is the repulsive force

497
00:24:47,520 --> 00:24:49,286
pushing things apart.

498
00:24:49,288 --> 00:24:52,690
It prefers the voids
where gravity is weak.

499
00:24:52,692 --> 00:24:54,558
[ rumbling ]

500
00:24:54,560 --> 00:24:56,994
bullock: These are areas where
there's a lot less dark matter,

501
00:24:56,996 --> 00:24:59,396
and because the overall density
is low,

502
00:24:59,398 --> 00:25:01,398
that's where the dark energy
starts to peek out

503
00:25:01,400 --> 00:25:04,201
and can really drive
those voids to expand.

504
00:25:04,203 --> 00:25:07,438
So the expansion and
acceleration of the universe

505
00:25:07,440 --> 00:25:10,174
are driven by the dark energy
in those regions.

506
00:25:10,176 --> 00:25:11,609
[ whooshing ]

507
00:25:11,611 --> 00:25:14,879
narrator: Dark energy
pushes thing apart,

508
00:25:14,881 --> 00:25:17,781
things that get in its way,

509
00:25:17,783 --> 00:25:21,585
things like the cosmic web.

510
00:25:21,587 --> 00:25:26,957
Dark matter and normal matter
are also in its path

511
00:25:26,959 --> 00:25:30,594
and are bulldozed out
across the cosmos.

512
00:25:30,596 --> 00:25:34,865
Slowly, but surely, the balance
between dark energy

513
00:25:34,867 --> 00:25:37,368
and dark matter is changing.

514
00:25:37,370 --> 00:25:39,904
[ whooshing ]

515
00:25:39,906 --> 00:25:41,672
imagine you have
a giant swimming pool,

516
00:25:41,674 --> 00:25:44,241
and at the very bottom
there's a puddle of water

517
00:25:44,243 --> 00:25:45,476
with a splash of whiskey.

518
00:25:45,478 --> 00:25:47,311
So you have sort of a strong
whiskey drink

519
00:25:47,313 --> 00:25:49,446
down at the bottom of your pool.

520
00:25:49,448 --> 00:25:52,950
But now you start dumping water
into your pool, no more whiskey,

521
00:25:52,952 --> 00:25:54,952
and it begins to get diluted
and diluted,

522
00:25:54,954 --> 00:25:57,821
and eventually, you just have
a swimming pool full of water,

523
00:25:57,823 --> 00:25:59,657
with one shot of whiskey
mixed in.

524
00:25:59,659 --> 00:26:01,158
That's not a very strong drink.

525
00:26:01,160 --> 00:26:03,294
It's basically a water
swimming pool.

526
00:26:03,296 --> 00:26:05,563
That's pretty much happening
with the dark energy.

527
00:26:05,565 --> 00:26:07,197
At first,
it's a one-to-one mixture

528
00:26:07,199 --> 00:26:08,899
of dark matter and dark energy.

529
00:26:08,901 --> 00:26:11,335
But in the long run,
it's all dark energy

530
00:26:11,337 --> 00:26:13,370
and pretty much
no dark matter left over.

531
00:26:13,372 --> 00:26:15,472



532
00:26:15,474 --> 00:26:17,975
narrator:
The forces of dark energy

533
00:26:17,977 --> 00:26:20,511
are on an unstoppable March,

534
00:26:20,513 --> 00:26:22,513
picking up more and more power

535
00:26:22,515 --> 00:26:26,183
from the vast scale
of the cosmic voids.

536
00:26:26,185 --> 00:26:28,953
Dark energy is intrinsically
very weak.

537
00:26:28,955 --> 00:26:30,854
There's very little dark energy

538
00:26:30,856 --> 00:26:34,825
and this repulsive effect
in every cubic centimeter.

539
00:26:34,827 --> 00:26:36,594
But the universe is vast.

540
00:26:36,596 --> 00:26:38,829
Space is big.

541
00:26:38,831 --> 00:26:42,633
So cumulatively, all this
small amount of stuff

542
00:26:42,635 --> 00:26:45,202
adds up to a very large amount,

543
00:26:45,204 --> 00:26:48,872
and over a scale encompassing
the entire universe,

544
00:26:48,874 --> 00:26:51,442
the dark energy dominates.

545
00:26:51,444 --> 00:26:53,310
[ rumbling ]

546
00:26:53,312 --> 00:26:55,913
narrator: But it's been
a very long process.

547
00:26:55,915 --> 00:26:57,615
After the big bang,

548
00:26:57,617 --> 00:27:01,318
dark matter dominates
for the first 9 billion years.

549
00:27:01,320 --> 00:27:03,554



550
00:27:03,556 --> 00:27:06,023
then, 5 billion years ago,

551
00:27:06,025 --> 00:27:08,959
dark energy starts
to get the upper hand.

552
00:27:08,961 --> 00:27:12,730
It causes the expansion
of the universe to accelerate

553
00:27:12,732 --> 00:27:16,467
and the space in the voids
to grow more rapidly.

554
00:27:16,469 --> 00:27:20,404
As the space expands, there's
more and more dark energy

555
00:27:20,406 --> 00:27:22,172
because you have a bigger space.

556
00:27:22,174 --> 00:27:26,744
It sort of creates itself
with the expansion of space.

557
00:27:26,746 --> 00:27:29,380
Tegmark: Dark energy has
a sneaky way of taking over

558
00:27:29,382 --> 00:27:30,914
because it causes the space

559
00:27:30,916 --> 00:27:32,616
to stretch out
and get twice as big.

560
00:27:32,618 --> 00:27:36,153
So now there's twice
as much dark energy.

561
00:27:36,155 --> 00:27:39,623
Narrator: Dark energy
just can't stop pushing,

562
00:27:39,625 --> 00:27:42,393
causing the empty space
of the voids

563
00:27:42,395 --> 00:27:44,194
to continuously expand.

564
00:27:44,196 --> 00:27:47,131
[ rumbling ]

565
00:27:47,133 --> 00:27:50,701
as the universe expands
because of dark energy,

566
00:27:50,703 --> 00:27:53,170
more and more dark energy
is being created.

567
00:27:53,172 --> 00:27:54,738
Dark energy is
definitely gaining

568
00:27:54,740 --> 00:27:56,507
the upper hand on dark matter.

569
00:27:56,509 --> 00:28:01,545
It was always there, but it took
over compared to other stuff.

570
00:28:01,547 --> 00:28:03,080



571
00:28:03,082 --> 00:28:06,450
narrator: Eventually creating
enough energy to supercharge

572
00:28:06,452 --> 00:28:08,552
the expansion of the universe.

573
00:28:08,554 --> 00:28:10,921



574
00:28:10,923 --> 00:28:13,023
this acceleration continues.

575
00:28:13,025 --> 00:28:15,893
The universe is getting bigger
and bigger,

576
00:28:15,895 --> 00:28:19,430
and it's all powered
by the forces of repulsion,

577
00:28:19,432 --> 00:28:20,831
dark energy.

578
00:28:20,833 --> 00:28:25,836
And for the universe,
that could be very bad news.

579
00:28:25,838 --> 00:28:30,174
If that's the case, dark energy
may destroy the universe.

580
00:28:30,176 --> 00:28:31,642
It will get stronger
and stronger

581
00:28:31,644 --> 00:28:34,878
until it literally rips apart
the fabric of space-time.

582
00:28:34,880 --> 00:28:37,381
[ rumbling ]

583
00:28:43,823 --> 00:28:48,125
[ rumbling ]

584
00:28:48,127 --> 00:28:49,760



585
00:28:49,762 --> 00:28:51,695
narrator:
Dark matter and dark energy

586
00:28:51,697 --> 00:28:56,433
have been battling each other
for 13.8 billion years.

587
00:28:56,435 --> 00:29:01,171
For the first 9 billion years,
dark matter dominates.

588
00:29:01,173 --> 00:29:04,775
Dark matter exerts
positive gravity,

589
00:29:04,777 --> 00:29:09,913
but pulling everything together
leads to one inevitable outcome.

590
00:29:09,915 --> 00:29:14,118
If the universe was totally
dominated by matter,

591
00:29:14,120 --> 00:29:16,787
eventually our expansion
would slow down,

592
00:29:16,789 --> 00:29:20,023
glide to a stop,
and then turn around

593
00:29:20,025 --> 00:29:24,027
and collapse into
a small, dense state

594
00:29:24,029 --> 00:29:29,099
from where it came from,
an event we call the big crunch.

595
00:29:29,101 --> 00:29:30,868
Narrator:
During the big crunch,

596
00:29:30,870 --> 00:29:34,605
gravity would play havoc
with the cosmos.

597
00:29:34,607 --> 00:29:38,375
Galaxies
would be dragged together.

598
00:29:38,377 --> 00:29:42,479
Stars and planets
would smash into each other.

599
00:29:42,481 --> 00:29:46,216
The universe would collapse
in a blazing inferno

600
00:29:46,218 --> 00:29:48,886
of superdense matter
and energy.

601
00:29:48,888 --> 00:29:52,923
[ rumbling ]

602
00:29:52,925 --> 00:29:55,993
fortunately, none of this
will probably happen.

603
00:29:55,995 --> 00:29:59,062



604
00:29:59,064 --> 00:30:03,233
scientists have now dismissed
the possibility of a big crunch.

605
00:30:03,235 --> 00:30:06,336



606
00:30:06,338 --> 00:30:08,739
sutter: We don't face that,
because we have a universe

607
00:30:08,741 --> 00:30:11,108
filled with dark energy.

608
00:30:11,110 --> 00:30:14,144
Dark energy is causing the
universe to do something else,

609
00:30:14,146 --> 00:30:16,914
something it would prefer
not to do.

610
00:30:16,916 --> 00:30:20,751
It is accelerating the expansion
of the universe.

611
00:30:20,753 --> 00:30:23,287
Narrator:
As dark energy gets stronger,

612
00:30:23,289 --> 00:30:26,590
it supercharges this expansion.

613
00:30:26,592 --> 00:30:32,396
The presence of dark energy
is like a high-octane additive

614
00:30:32,398 --> 00:30:34,198
into a gas tank,

615
00:30:34,200 --> 00:30:37,835
where a car isn't
just coasting along.

616
00:30:37,837 --> 00:30:40,137
It's boosting along,

617
00:30:40,139 --> 00:30:42,973
and that's what's happening
with our universe.

618
00:30:42,975 --> 00:30:45,943
Narrator: If that's correct,
this supercharged expansion

619
00:30:45,945 --> 00:30:49,980
will continue
for the next few billion years.

620
00:30:49,982 --> 00:30:54,418
The repulsive force of dark
energy will become invincible,

621
00:30:54,420 --> 00:31:00,123
and such unbridled power
will come with a high cost.

622
00:31:00,125 --> 00:31:03,627
I know of three ultimate
doomsday weapons.

623
00:31:03,629 --> 00:31:07,397
One is galactus'
ultimate nullifier.

624
00:31:07,399 --> 00:31:11,268
Another is the infinity stones
in the hands of thanos.

625
00:31:11,270 --> 00:31:13,136
But those two are make-believe.

626
00:31:13,138 --> 00:31:15,272
The real one is dark energy.

627
00:31:15,274 --> 00:31:18,642
It's the real ultimate
universe destroyer.

628
00:31:18,644 --> 00:31:20,711



629
00:31:20,713 --> 00:31:23,747
narrator: The universe may pay
the ultimate price

630
00:31:23,749 --> 00:31:28,352
for this ever-increasing
expansion driven by dark energy.

631
00:31:28,354 --> 00:31:29,853
Carroll:
And if that continues forever,

632
00:31:29,855 --> 00:31:33,056
the future of the universe
is very, very simple.

633
00:31:33,058 --> 00:31:35,492
It continues to expand,
to accelerate.

634
00:31:35,494 --> 00:31:37,394
Everything moves apart
from everything else,

635
00:31:37,396 --> 00:31:40,497
and the universe
becomes empty.

636
00:31:40,499 --> 00:31:42,332
There'll be nothing left

637
00:31:42,334 --> 00:31:46,003
but cold, desolate,
empty space forever.

638
00:31:46,005 --> 00:31:47,671



639
00:31:47,673 --> 00:31:49,573
narrator:
Dark energy will finally win

640
00:31:49,575 --> 00:31:52,242
the long war with dark matter.

641
00:31:52,244 --> 00:31:55,178
But the result
won't be pretty.

642
00:31:55,180 --> 00:32:00,684
This future universe
will be cold, dark, and empty.

643
00:32:00,686 --> 00:32:02,486
Eventually it's going
to get so cold

644
00:32:02,488 --> 00:32:05,022
that really nothing
can happen in it.

645
00:32:05,024 --> 00:32:06,690
It's the big chill.

646
00:32:06,692 --> 00:32:10,160
It gets colder and colder
and darker and darker.

647
00:32:10,162 --> 00:32:13,530
Everything will fade out,
and though it began with a bang,

648
00:32:13,532 --> 00:32:14,998
it's going to die in a whimper.

649
00:32:15,000 --> 00:32:19,036
The universe flatlines
because of dark energy.

650
00:32:19,038 --> 00:32:22,539
Narrator: The universe dies
in a big chill.

651
00:32:22,541 --> 00:32:24,841
Galaxies are so far apart,

652
00:32:24,843 --> 00:32:29,112
they're distant islands
in a sea of darkness.

653
00:32:29,114 --> 00:32:32,282
Gradually,
the galaxies dies, too.

654
00:32:32,284 --> 00:32:36,954
Star birth stops,
and the universe fades away.

655
00:32:38,991 --> 00:32:40,757
Or maybe not.

656
00:32:40,759 --> 00:32:45,429
There's another,
far more violent scenario.

657
00:32:45,431 --> 00:32:50,767
Here, dark energy just goes
from strength to strength.

658
00:32:50,769 --> 00:32:54,705
It could be that dark energy
is so strong

659
00:32:54,707 --> 00:32:58,008
that it will multiply
upon itself

660
00:32:58,010 --> 00:33:00,377
as the universe gets bigger.

661
00:33:00,379 --> 00:33:05,048
This is a process that
we call phantom dark energy.

662
00:33:05,050 --> 00:33:09,753
Narrator: Phantom energy is
dark energy on steroids.

663
00:33:09,755 --> 00:33:13,623
It multiplies uncontrollably
in the voids,

664
00:33:13,625 --> 00:33:16,026
tearing at the fabric
of the universe

665
00:33:16,028 --> 00:33:20,597
in a process
called the big rip.

666
00:33:20,599 --> 00:33:24,935
Dark energy is weird enough,
but imagine the possibility

667
00:33:24,937 --> 00:33:28,405
that there is more and more
of it as time goes on,

668
00:33:28,407 --> 00:33:32,509
and it's called phantom energy,
and in that case,

669
00:33:32,511 --> 00:33:35,679
it would rip everything apart,
even black holes.

670
00:33:35,681 --> 00:33:39,883
[ whooshing ]

671
00:33:39,885 --> 00:33:42,586
it will start to rip apart
galaxies themselves...

672
00:33:42,588 --> 00:33:45,655
[ rumbling ]

673
00:33:45,657 --> 00:33:47,691
...Rip apart solar systems...

674
00:33:47,693 --> 00:33:51,561
[ rumbling ]

675
00:33:51,563 --> 00:33:53,230
...Rip apart people,

676
00:33:53,232 --> 00:33:56,666
rip apart atoms...

677
00:33:56,668 --> 00:33:57,868
Rip apart nuclei.

678
00:33:57,870 --> 00:34:01,138
[ rumbling ]

679
00:34:01,140 --> 00:34:03,774
until finally,
space itself is pulled apart.

680
00:34:03,776 --> 00:34:05,042
[ whooshes ]

681
00:34:05,044 --> 00:34:06,610
narrator:
The universe, as we know it,

682
00:34:06,612 --> 00:34:08,378
will be destroyed,

683
00:34:08,380 --> 00:34:11,782
but the big rip may not be
the end of everything.

684
00:34:11,784 --> 00:34:15,819



685
00:34:15,821 --> 00:34:20,724
there will be no normal matter
and no dark matter.

686
00:34:20,726 --> 00:34:22,826
And with nothing
left to conquer,

687
00:34:22,828 --> 00:34:27,464
phantom energy may use its
powers to become a creator,

688
00:34:27,466 --> 00:34:29,232
triggering a rebirth.

689
00:34:29,234 --> 00:34:30,967
[ bursting ]

690
00:34:30,969 --> 00:34:35,405
eventually, when you get to this
ultimate stage of emptiness,

691
00:34:35,407 --> 00:34:36,873
because of the phantom energy,

692
00:34:36,875 --> 00:34:38,675
we're actually able to turn
the universe around

693
00:34:38,677 --> 00:34:41,178
and get it
to collapse again,

694
00:34:41,180 --> 00:34:44,114
and then go through
a series of bounces.

695
00:34:44,116 --> 00:34:45,782
So we call it
the phantom bounce.

696
00:34:45,784 --> 00:34:47,350



697
00:34:47,352 --> 00:34:49,219
narrator:
With this phantom bounce,

698
00:34:49,221 --> 00:34:51,721
universal life energy left
in this dead universe

699
00:34:51,723 --> 00:34:54,357
starts to collapse.

700
00:34:54,359 --> 00:34:56,426
Freese: And eventually
it becomes hotter and hotter

701
00:34:56,428 --> 00:34:59,463
and denser and denser,
and then the fiery inferno

702
00:34:59,465 --> 00:35:02,933
eventually pushes you back out
into another big bang,

703
00:35:02,935 --> 00:35:05,469
and this just keeps
going on indefinitely.

704
00:35:05,471 --> 00:35:07,137



705
00:35:07,139 --> 00:35:10,474
narrator: So the destructive
and repulsive dark energy

706
00:35:10,476 --> 00:35:16,213
spawns a force that becomes
the ultimate universe recycler.

707
00:35:16,215 --> 00:35:18,582
The end state of our universe

708
00:35:18,584 --> 00:35:20,851
would lead you back
into another cycle,

709
00:35:20,853 --> 00:35:22,953
a whole new big bang
from the beginning.

710
00:35:22,955 --> 00:35:28,125
[ bursting, rumbling ]

711
00:35:28,127 --> 00:35:31,962
narrator: In the end,
dark energy may kill the cosmos,

712
00:35:31,964 --> 00:35:35,198
or it may create a new one.

713
00:35:35,200 --> 00:35:36,800
Dark energy is mysterious.

714
00:35:36,802 --> 00:35:38,335
Dark energy is unknown.

715
00:35:38,337 --> 00:35:41,705
Dark energy is going to do
whatever it feels like.

716
00:35:41,707 --> 00:35:43,707
Maybe dark energy will go away.

717
00:35:43,709 --> 00:35:45,809
Maybe dark energy will decay

718
00:35:45,811 --> 00:35:48,912
and become a flood
of new matter and radiation.

719
00:35:48,914 --> 00:35:51,781
Maybe dark matter
will get stronger.

720
00:35:51,783 --> 00:35:52,916
We don't know.

721
00:35:52,918 --> 00:35:56,253



722
00:35:56,255 --> 00:35:58,822
narrator:
For now, we think dark energy

723
00:35:58,824 --> 00:36:02,159
will determine the fate
of the universe.

724
00:36:02,161 --> 00:36:05,929
But all of our evidence
is speculative.

725
00:36:05,931 --> 00:36:07,797
What if we have it all wrong?

726
00:36:07,799 --> 00:36:11,601
What if there is
no dark universe at all?

727
00:36:11,603 --> 00:36:16,239



728
00:36:22,581 --> 00:36:26,983
[ rumbling ]

729
00:36:26,985 --> 00:36:29,819



730
00:36:29,821 --> 00:36:31,888
narrator: A large part of
our understanding

731
00:36:31,890 --> 00:36:33,490
of the universe's past,

732
00:36:33,492 --> 00:36:37,227
present, and future is based
on educated guesswork

733
00:36:37,229 --> 00:36:39,763
about two invisible forces --

734
00:36:39,765 --> 00:36:42,065
[ crackling ]

735
00:36:42,067 --> 00:36:45,669
...Dark matter and dark energy.

736
00:36:45,671 --> 00:36:49,072
But it's pure speculation.

737
00:36:49,074 --> 00:36:53,577
Perhaps dark matter
and dark energy don't exist.

738
00:36:53,579 --> 00:36:56,279
There's not new stuff
in the universe.

739
00:36:56,281 --> 00:36:57,514
Anything is possible.

740
00:36:57,516 --> 00:36:58,949
Dark energy, in particular,

741
00:36:58,951 --> 00:37:01,651
might not be real,
so maybe there's something else

742
00:37:01,653 --> 00:37:03,353
that could be pushing
the universe apart.

743
00:37:03,355 --> 00:37:05,021
Tremblay: So we could
absolutely be wrong

744
00:37:05,023 --> 00:37:07,257
about dark energy
and dark matter.

745
00:37:07,259 --> 00:37:08,959
Maybe they don't exist.

746
00:37:08,961 --> 00:37:11,962
Maybe tomorrow we'll discover
that our understanding

747
00:37:11,964 --> 00:37:14,698
was wrong all along.

748
00:37:14,700 --> 00:37:16,733
Narrator:
That's an awful lot of maybes.

749
00:37:16,735 --> 00:37:19,035
Let's add one more.

750
00:37:19,037 --> 00:37:21,638
When it comes to finding
answers,

751
00:37:21,640 --> 00:37:24,241
maybe we're looking
in the wrong place.

752
00:37:26,345 --> 00:37:29,713
Filippenko: One possibility is
that there are other universes

753
00:37:29,715 --> 00:37:34,484
out there pulling outward,
so to speak, on our universe.

754
00:37:34,486 --> 00:37:35,885
That might be the answer.

755
00:37:35,887 --> 00:37:38,555
But most theoretical physicists
and astrophysicists

756
00:37:38,557 --> 00:37:41,625
these days think
that dark energy is real

757
00:37:41,627 --> 00:37:44,728
because that seems to be
the simplest explanation

758
00:37:44,730 --> 00:37:47,564
for a wide variety
of observations.

759
00:37:47,566 --> 00:37:49,099



760
00:37:49,101 --> 00:37:52,736
narrator: No one really knows
what dark energy is made of.

761
00:37:52,738 --> 00:37:56,273
Maybe the answer lies
in the past.

762
00:37:56,275 --> 00:37:58,508
The best theory for dark energy
we have right now

763
00:37:58,510 --> 00:38:00,810
is the simplest one
and the oldest one,

764
00:38:00,812 --> 00:38:03,246
and that's the idea that
it's a cosmological constant.

765
00:38:03,248 --> 00:38:05,148



766
00:38:05,150 --> 00:38:07,384
narrator: Albert einstein
came up with the idea

767
00:38:07,386 --> 00:38:12,289
of a cosmological constant
in 1917.

768
00:38:12,291 --> 00:38:16,159
He suggested that space
has its own energy,

769
00:38:16,161 --> 00:38:19,396
energy that can affect
the way the universe expands.

770
00:38:19,398 --> 00:38:21,865



771
00:38:21,867 --> 00:38:24,934
when edwin hubble proved
the universe is expanding,

772
00:38:24,936 --> 00:38:27,937
einstein thought
the cosmological constant

773
00:38:27,939 --> 00:38:30,073
was his biggest blunder.

774
00:38:30,075 --> 00:38:33,877
But observations that the
expansion of the universe

775
00:38:33,879 --> 00:38:35,412
is accelerating

776
00:38:35,414 --> 00:38:38,948
reveal einstein
was right all along.

777
00:38:38,950 --> 00:38:42,919
Filippenko: Well, here we are.
We've reintroduced the idea.

778
00:38:42,921 --> 00:38:46,756
So einstein's biggest blunder
may have actually

779
00:38:46,758 --> 00:38:49,292
conceptually been
his greatest triumph.

780
00:38:49,294 --> 00:38:52,829



781
00:38:52,831 --> 00:38:56,199
narrator:
But to understand the true nature of the dark universe,

782
00:38:56,201 --> 00:39:00,537
we may need to re-evaluate what
we think we know about gravity.

783
00:39:00,539 --> 00:39:02,806
[ rumbling ]

784
00:39:02,808 --> 00:39:05,542
when we're trying to understand
dark energy and dark matter,

785
00:39:05,544 --> 00:39:09,379
there's a chance that just our
fundamental theories of gravity

786
00:39:09,381 --> 00:39:13,783
are wrong, that general
relativity isn't quite right.

787
00:39:13,785 --> 00:39:16,152
Narrator: Einstein's theory
of general relativity

788
00:39:16,154 --> 00:39:18,455
explains how gravity works,

789
00:39:18,457 --> 00:39:21,124
how stars orbit in galaxies,

790
00:39:21,126 --> 00:39:23,593
and planets orbit stars.

791
00:39:23,595 --> 00:39:25,462



792
00:39:25,464 --> 00:39:28,398
some scientists wonder
if altering this theory

793
00:39:28,400 --> 00:39:32,669
will help us understand
the dark universe.

794
00:39:32,671 --> 00:39:34,704
Tremblay: So you need not
absolutely believe

795
00:39:34,706 --> 00:39:37,907
that there is something
actually called dark matter.

796
00:39:37,909 --> 00:39:39,876
You need to only understand
that there is something

797
00:39:39,878 --> 00:39:42,746
in the universe
which behaves like dark matter.

798
00:39:42,748 --> 00:39:45,715
For example, you could
effectively mimic the behavior

799
00:39:45,717 --> 00:39:49,986
of dark matter by modifying
our current theory of gravity.

800
00:39:49,988 --> 00:39:51,921
Narrator:
But successfully modifying

801
00:39:51,923 --> 00:39:56,493
einstein's theories on gravity
is a big challenge.

802
00:39:56,495 --> 00:39:58,795
Einstein's equations
are very robust.

803
00:39:58,797 --> 00:40:02,399
You don't fluff around
with einstein with impunity.

804
00:40:02,401 --> 00:40:05,101
[ rumbling ]

805
00:40:05,103 --> 00:40:07,370
narrator: For decades,
theoretical physicists

806
00:40:07,372 --> 00:40:10,507
have toyed
with einstein's equations,

807
00:40:10,509 --> 00:40:14,744
looking for ways to explain
dark matter and dark energy,

808
00:40:14,746 --> 00:40:17,180
or make them go away.

809
00:40:17,182 --> 00:40:19,649
As yet, no one has managed.

810
00:40:19,651 --> 00:40:22,185
The dark universe persists.

811
00:40:22,187 --> 00:40:23,720
[ whooshing ]

812
00:40:23,722 --> 00:40:26,156
I think that the best
description of the observations

813
00:40:26,158 --> 00:40:29,359
we have today
is that dark matter exists.

814
00:40:29,361 --> 00:40:31,761
It's out there,
as well as dark energy.

815
00:40:31,763 --> 00:40:33,730
I think dark energy exists.

816
00:40:33,732 --> 00:40:35,365
I think dark energy is real,

817
00:40:35,367 --> 00:40:38,435
but I must admit that sometimes,
at 3 o'clock in the morning,

818
00:40:38,437 --> 00:40:41,771
I wake up screaming,
worried that, in fact,

819
00:40:41,773 --> 00:40:44,741
we've settled
on the wrong answer,

820
00:40:44,743 --> 00:40:46,810
and that in
a couple hundred years,

821
00:40:46,812 --> 00:40:48,745
they're going to be
laughing at us.

822
00:40:48,747 --> 00:40:51,681



823
00:40:51,683 --> 00:40:54,884
narrator: Until then, our
observations tell us the battle

824
00:40:54,886 --> 00:40:59,522
between dark matter and dark
energy has shaped the universe.

825
00:40:59,524 --> 00:41:02,926



826
00:41:02,928 --> 00:41:06,329
dark matter dictated the past,

827
00:41:06,331 --> 00:41:09,632
built the galaxies,
the stars, and the planets.

828
00:41:09,634 --> 00:41:11,501



829
00:41:11,503 --> 00:41:14,604
dark energy will determine
its future,

830
00:41:14,606 --> 00:41:18,041
potentially tearing
the universe apart.

831
00:41:18,043 --> 00:41:21,511
Just because we can't see dark
matter and dark energy directly

832
00:41:21,513 --> 00:41:24,347
doesn't mean they have not
had a profound effect

833
00:41:24,349 --> 00:41:26,683
on the evolution
of the entire universe.

834
00:41:26,685 --> 00:41:30,019
The dark universe was there at
the beginning of the universe,

835
00:41:30,021 --> 00:41:32,489
shaping it, and actually
creating the conditions

836
00:41:32,491 --> 00:41:36,159
for us to be here, and it's
taken over the universe.

837
00:41:36,161 --> 00:41:39,529
And, eventually, it may well
destroy the universe.

838
00:41:39,531 --> 00:41:41,164



839
00:41:41,166 --> 00:41:42,999
narrator:
Our universe may be dominated

840
00:41:43,001 --> 00:41:47,470
by the long struggle between
dark matter and dark energy.

841
00:41:47,472 --> 00:41:51,341
But all of this conflict
has led to a creative outcome,

842
00:41:51,343 --> 00:41:55,678
an outcome for which
we should all be grateful.

843
00:41:55,680 --> 00:41:59,849
The name dark matter suggests
that it's something nefarious

844
00:41:59,851 --> 00:42:02,252
and somehow bad for us,
but actually it's turned out

845
00:42:02,254 --> 00:42:04,821
that dark matter
is very much our friend.

846
00:42:04,823 --> 00:42:07,390
Because if it weren't
for the dark matter,

847
00:42:07,392 --> 00:42:08,691
we wouldn't be here.

848
00:42:08,693 --> 00:42:09,893
[ rumbling ]

849
00:42:09,895 --> 00:42:11,227
thaller:
There's a wonderful irony

850
00:42:11,229 --> 00:42:13,096
to calling it
the dark universe,

851
00:42:13,098 --> 00:42:14,998
because now we're actually
beginning to shed light

852
00:42:15,000 --> 00:42:16,499
on how the universe began,

853
00:42:16,501 --> 00:42:18,935
how the largest structures
in the universe evolved.

854
00:42:18,937 --> 00:42:20,803
We wouldn't be here
without this dark universe.

855
00:42:20,805 --> 00:42:22,305
It's not dark at all.

856
00:42:22,307 --> 00:42:24,140
It's shedding light
on our own reality.

857
00:42:24,142 --> 00:42:28,511



858
00:42:28,561 --> 00:42:33,111
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