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�

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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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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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-- Captions by vitac --
 www.vitac.com

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captions paid for by
discovery communications

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�

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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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�

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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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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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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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�

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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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...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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�

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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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...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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�

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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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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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�

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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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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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It's true.

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�

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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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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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�

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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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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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But then dark matter,

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the force that brings things
together, intervened.

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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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Narrator: Gravity-rich pockets
of dark matter

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pull particles
of regular matter together.

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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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�

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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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had to form from
the collapse of gas clouds

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under their own gravity.

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�

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narrator:
But there's a problem.

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

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and no collapse
means no stars.

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

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and this pressure
keeps it from collapsing.

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The dark matter
doesn't experience

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that kind of pressure.

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So the dark matter can clump up

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and make sites
for structure formation.

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Narrator: So dark matter comes
to the rescue,

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creating regions
of higher gravity,

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dragging in hydrogen gas,

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forcing the clouds to get
denser and denser,

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creating the conditions
for collapse and then creation.

256
00:13:37,551 --> 00:13:40,885
�

257
00:13:40,887 --> 00:13:44,289
so it's only when the gravity
of the dark matter

258
00:13:44,291 --> 00:13:46,692
overwhelms the pressure of gases

259
00:13:46,694 --> 00:13:50,895
that the gases can collapse
and turn into stars.

260
00:13:50,897 --> 00:13:52,364
Bullock: As soon as the gas
cools down,

261
00:13:52,366 --> 00:13:54,031
it can fall into those
potential Wells

262
00:13:54,033 --> 00:13:55,901
that the dark matter created,

263
00:13:55,903 --> 00:13:58,636
almost like little nurseries
for stars,

264
00:13:58,638 --> 00:14:00,705
and they start
forming in earnest.

265
00:14:00,707 --> 00:14:02,707
�

266
00:14:02,709 --> 00:14:05,310
narrator: Dark matter provides
a boost of gravity

267
00:14:05,312 --> 00:14:10,848
to kick-start hydrogen into
constructing the first stars,

268
00:14:10,850 --> 00:14:14,719
stars that are the seeds
of the first galaxies.

269
00:14:14,721 --> 00:14:17,456
Tremblay: So it's dark matter
that would have coalesced

270
00:14:17,458 --> 00:14:20,792
in the early universe
and grown from there,

271
00:14:20,794 --> 00:14:22,660
and then the luminous
component of the universe,

272
00:14:22,662 --> 00:14:25,063
the things that we think of
as being the universe itself,

273
00:14:25,065 --> 00:14:26,531
like stars and galaxies,

274
00:14:26,533 --> 00:14:28,933
would have just been along
for the ride.

275
00:14:28,935 --> 00:14:34,272
Without the presence of dark
matter to seed structures,

276
00:14:34,274 --> 00:14:39,077
there wasn't enough time in the
early universe to form galaxies,

277
00:14:39,079 --> 00:14:42,947
which means you and me
have to thank dark matter

278
00:14:42,949 --> 00:14:45,150
for our existence.

279
00:14:45,152 --> 00:14:47,352
Narrator:
And dark matter now begins

280
00:14:47,354 --> 00:14:51,956
a much more ambitious
architectural project --

281
00:14:51,958 --> 00:14:56,294
to shape the entire
universe itself,

282
00:14:56,296 --> 00:15:00,498
to build the biggest structure
ever constructed --

283
00:15:00,500 --> 00:15:02,033
the cosmic web.

284
00:15:02,035 --> 00:15:06,171
�

285
00:15:16,983 --> 00:15:20,452
�

286
00:15:20,454 --> 00:15:23,355
narrator: The war between
dark matter and dark energy

287
00:15:23,357 --> 00:15:25,991
has been raging
since the birth of time.

288
00:15:29,062 --> 00:15:33,865
But in the early years,
it's a one-sided contest.

289
00:15:33,867 --> 00:15:36,267
In the early universe, the only
thing that really mattered

290
00:15:36,269 --> 00:15:39,804
was the dark matter
and the normal matter.

291
00:15:39,806 --> 00:15:41,873
Narrator: Dark matter,

292
00:15:41,875 --> 00:15:45,743
the force that brings things
together,

293
00:15:45,745 --> 00:15:47,345
is in the driver's seat.

294
00:15:50,951 --> 00:15:55,086
Dark energy, the force
that pulls things apart,

295
00:15:55,088 --> 00:15:57,055
is the underdog.

296
00:15:57,057 --> 00:15:59,691
We've seen this
cosmic battle go on

297
00:15:59,693 --> 00:16:01,759
for the past 14 billion years.

298
00:16:01,761 --> 00:16:06,898
Fortunately for us, the dark
energy got off to a slow start.

299
00:16:06,900 --> 00:16:10,235
Narrator: Meanwhile, dark matter
is busy at work,

300
00:16:10,237 --> 00:16:12,904
building the universe.

301
00:16:12,906 --> 00:16:17,842
Not only does it trigger
the birth of the first stars,

302
00:16:17,844 --> 00:16:20,044
it embarks on an even
more formidable

303
00:16:20,046 --> 00:16:23,014
construction project --

304
00:16:23,016 --> 00:16:26,451
the cosmic web.

305
00:16:26,453 --> 00:16:30,588
Straughn:
There's this large-scale
structure of filaments

306
00:16:30,590 --> 00:16:32,790
that galaxies
seem to form on,

307
00:16:32,792 --> 00:16:35,527
and that's what we call
the cosmic web.

308
00:16:35,529 --> 00:16:38,530
And we can trace the formation
of this cosmic web

309
00:16:38,532 --> 00:16:41,266
all the way back
to the early universe.

310
00:16:41,268 --> 00:16:43,067
Now this is such a huge
structure,

311
00:16:43,069 --> 00:16:44,736
we don't think there's time
in the universe

312
00:16:44,738 --> 00:16:47,405
for matter's gravity alone
to do this.

313
00:16:47,407 --> 00:16:49,608
There must have been
an underlying scaffold

314
00:16:49,610 --> 00:16:51,343
of dark matter.

315
00:16:51,345 --> 00:16:53,611
Plait: The dark matter started
forming into these filaments,

316
00:16:53,613 --> 00:16:55,613
and when the universe
cooled enough,

317
00:16:55,615 --> 00:16:57,282
normal matter
could start to stream

318
00:16:57,284 --> 00:17:00,418
into this gravitational
attraction of the dark matter.

319
00:17:00,420 --> 00:17:02,020
That became the scaffolding

320
00:17:02,022 --> 00:17:05,890
on which this large-scale
structure was built.

321
00:17:05,892 --> 00:17:08,826
Narrator: The filaments of
dark matter joined together,

322
00:17:08,828 --> 00:17:12,631
drawing in more
and more hydrogen gas.

323
00:17:12,633 --> 00:17:16,834
Dense clouds of gas build up at
the junctions of the filaments,

324
00:17:16,836 --> 00:17:19,904
the point where gravity
is at its strongest.

325
00:17:19,906 --> 00:17:22,908
�

326
00:17:22,910 --> 00:17:24,909
slowly and surely,

327
00:17:24,911 --> 00:17:29,447
a familiar-looking structure
starts to take shape.

328
00:17:29,449 --> 00:17:30,781
If you've ever gone outside,

329
00:17:30,783 --> 00:17:34,519
and you can see a spider web
covered in dew,

330
00:17:34,521 --> 00:17:37,388
that's kind of like
what happened with the universe.

331
00:17:37,390 --> 00:17:38,990
In this case, the spider web

332
00:17:38,992 --> 00:17:40,792
is the structure
of the dark matter.

333
00:17:40,794 --> 00:17:43,328
It's all of these filaments,
and the moisture in the air

334
00:17:43,330 --> 00:17:44,862
is what condenses around them,

335
00:17:44,864 --> 00:17:48,733
just like the normal matter
fell into the dark matter web

336
00:17:48,735 --> 00:17:51,803
to form these gigantic
structures in the universe.

337
00:17:51,805 --> 00:17:55,406
Narrator: Dark matter,
the universe's master builder,

338
00:17:55,408 --> 00:17:59,744
succeeds in stitching
together a cosmic web.

339
00:17:59,746 --> 00:18:04,683
This will be the framework
for the entire universe.

340
00:18:04,685 --> 00:18:07,619
And so it is dark matter
that would choreograph

341
00:18:07,621 --> 00:18:10,221
and sculpt the shape
of the universe itself.

342
00:18:10,223 --> 00:18:12,156
One of the amazing things
about dark matter is,

343
00:18:12,158 --> 00:18:14,359
without it, we wouldn't be here.

344
00:18:14,361 --> 00:18:16,828
It's hard to imagine how you
could have structure

345
00:18:16,830 --> 00:18:19,831
in the universe
without dark matter.

346
00:18:19,833 --> 00:18:22,433
Narrator: Galaxies,
and then galactic clusters,

347
00:18:22,435 --> 00:18:25,236
form at the junctions
of the filaments.

348
00:18:25,238 --> 00:18:30,775
Slowly, but surely, the universe
begins to take shape.

349
00:18:30,777 --> 00:18:33,778
When we look at this structure
over a cosmic scale,

350
00:18:33,780 --> 00:18:35,913
we see that it looks
kind of like a sponge.

351
00:18:35,915 --> 00:18:39,384
You see voids with galaxies
all over the edges of them.

352
00:18:39,386 --> 00:18:41,586
That is the structure
that was formed

353
00:18:41,588 --> 00:18:45,523
by the dark matter
in the early universe.

354
00:18:45,525 --> 00:18:47,659
Dark matter is the thing
that enabled,

355
00:18:47,661 --> 00:18:49,661
that provided enough gravity

356
00:18:49,663 --> 00:18:53,397
for the initial seeds of
structure formation to coalesce,

357
00:18:53,399 --> 00:18:55,600
for galaxies themselves to form.

358
00:18:57,337 --> 00:18:59,003
And, of course,
without galaxies,

359
00:18:59,005 --> 00:19:01,006
there are no stars,
and there's no planets,

360
00:19:01,008 --> 00:19:02,940
and there's no us.

361
00:19:02,942 --> 00:19:04,542
Narrator:
For 9 billion years,

362
00:19:04,544 --> 00:19:08,079
dark matter orchestrates the
construction of the universe.

363
00:19:10,016 --> 00:19:13,084
In these, the first battles
of the cosmos,

364
00:19:13,086 --> 00:19:17,022
this constructive force
is the clear Victor.

365
00:19:17,024 --> 00:19:19,624
For the time being,
the dark matter has won.

366
00:19:19,626 --> 00:19:21,492
Galaxies continue to form.

367
00:19:21,494 --> 00:19:23,895
Clusters of galaxies
are getting bigger over time.

368
00:19:27,100 --> 00:19:30,168
Narrator:
But dark matter's success
in building up the universe

369
00:19:30,170 --> 00:19:33,505
sets in motion
its potential downfall.

370
00:19:37,911 --> 00:19:41,780
As the cosmic web evolves
into a more complex structure...

371
00:19:41,782 --> 00:19:44,449
�

372
00:19:44,451 --> 00:19:48,986
...gaps form between
the filaments, the cosmic voids.

373
00:19:48,988 --> 00:19:50,788
�

374
00:19:50,790 --> 00:19:55,526
the cosmic voids formed because
other, more dense regions

375
00:19:55,528 --> 00:20:00,465
of the universe gravitationally
stole material away from them.

376
00:20:00,467 --> 00:20:02,400
So the dense parts
of the universe

377
00:20:02,402 --> 00:20:07,138
accumulated more matter at the
expense of the less dense parts,

378
00:20:07,140 --> 00:20:10,208
which then became voids.

379
00:20:10,210 --> 00:20:14,479
Narrator: And lurking
in these voids, dark energy.

380
00:20:14,481 --> 00:20:16,080
Since the dawn of time,

381
00:20:16,082 --> 00:20:19,818
it's been waiting
for its opportunity.

382
00:20:19,820 --> 00:20:23,755
Now it's preparing an offensive
that may help it

383
00:20:23,757 --> 00:20:26,224
conquer the universe.

384
00:20:26,226 --> 00:20:27,892
In the very earliest times,

385
00:20:27,894 --> 00:20:29,561
the dark matter dominated
everything.

386
00:20:29,563 --> 00:20:32,863
It was the big brother pushing
the little brother around.

387
00:20:32,865 --> 00:20:34,298
But in the long run,

388
00:20:34,300 --> 00:20:37,034
the dark energy is going
to overpower dark matter,

389
00:20:37,036 --> 00:20:39,370
and so the relationship
is entirely flipped.

390
00:20:39,372 --> 00:20:41,071
�

391
00:20:41,073 --> 00:20:43,975
plait: We used to think
the fate of the cosmos itself

392
00:20:43,977 --> 00:20:45,309
depended on dark matter,

393
00:20:45,311 --> 00:20:47,979
and it turns out
that's not the case at all.

394
00:20:47,981 --> 00:20:51,916
The fate of the universe
depends entirely on dark energy.

395
00:20:51,918 --> 00:20:54,252
Narrator:
The long reign of dark matter

396
00:20:54,254 --> 00:20:56,254
may be coming to an end.

397
00:20:56,256 --> 00:20:58,923
Dark energy,
the great destroyer,

398
00:20:58,925 --> 00:21:01,993
is hoping to take control.

399
00:21:01,995 --> 00:21:06,331
This destructive force
has one overriding aim --

400
00:21:06,333 --> 00:21:09,534
to tear the universe apart.

401
00:21:25,151 --> 00:21:28,152
Narrator:
Our universe is at war,

402
00:21:28,154 --> 00:21:32,490
a relentless conflict
between dark forces.

403
00:21:32,492 --> 00:21:38,495
For the first 9 billion years,
dark energy is subjugated.

404
00:21:38,497 --> 00:21:42,766
Dark matter has the upper hand.

405
00:21:42,768 --> 00:21:45,036
Frenk: When the universe
emerged from the big bang,

406
00:21:45,038 --> 00:21:47,705
the dark energy played no role.

407
00:21:47,707 --> 00:21:48,840
It was insignificant.

408
00:21:50,710 --> 00:21:53,210
Narrator: But at some stage
in the 14 billion years

409
00:21:53,212 --> 00:21:58,382
since the big bang,
these roles became reversed.

410
00:21:58,384 --> 00:22:02,720
Dark energy came to be
the more powerful force.

411
00:22:02,722 --> 00:22:05,790
The question was, when?

412
00:22:05,792 --> 00:22:09,660
The answer came at the end
of the 20th century.

413
00:22:09,662 --> 00:22:12,864
So it was an amazing
breakthrough, really important.

414
00:22:12,866 --> 00:22:14,131
�

415
00:22:14,133 --> 00:22:15,733
narrator: In 1999,

416
00:22:15,735 --> 00:22:19,003
scientists measure
the expansion of the universe.

417
00:22:20,407 --> 00:22:23,407
What they find shocks them.

418
00:22:23,409 --> 00:22:28,146
They expect the speed
of expansion to be decreasing.

419
00:22:28,148 --> 00:22:31,349
In fact,
it's actually increasing

420
00:22:31,351 --> 00:22:34,818
and getting faster all the time.

421
00:22:34,820 --> 00:22:38,156
The data indicate that for about
the first 9 billion years,

422
00:22:38,158 --> 00:22:39,691
it was slowing down.

423
00:22:39,693 --> 00:22:42,093
But then,
in the past 5 billion years,

424
00:22:42,095 --> 00:22:46,297
it started accelerating
faster and faster.

425
00:22:46,299 --> 00:22:49,099
Narrator: Alex filippenko
was part of the team

426
00:22:49,101 --> 00:22:52,569
that made this
explosive discovery.

427
00:22:52,571 --> 00:22:53,971
Filippenko:
It befuddled us.

428
00:22:53,973 --> 00:22:56,975
This isn't how nature
was supposed to be behaving,

429
00:22:56,977 --> 00:22:59,109
and, in fact,
initially we thought

430
00:22:59,111 --> 00:23:00,445
that there was something wrong

431
00:23:00,447 --> 00:23:03,448
with either the observations
or the measurements.

432
00:23:03,450 --> 00:23:05,850
I didn't believe it
for the longest time.

433
00:23:05,852 --> 00:23:07,385
When the first data came out,
I'm like,

434
00:23:07,387 --> 00:23:09,520
"nah, I don't believe this,
no way."

435
00:23:09,522 --> 00:23:10,922
But it's in the data.

436
00:23:10,924 --> 00:23:12,724
It's there.
You can't escape it.

437
00:23:12,726 --> 00:23:16,461
This is as shocking as if you
held up a rock, let go of it,

438
00:23:16,463 --> 00:23:17,862
and it went up into the air.

439
00:23:17,864 --> 00:23:20,864
�

440
00:23:20,866 --> 00:23:22,533
narrator:
Five billion years ago,

441
00:23:22,535 --> 00:23:26,404
galaxies started moving apart
faster than before.

442
00:23:26,406 --> 00:23:29,540
The question is, why?

443
00:23:29,542 --> 00:23:32,810
What could be causing that?
Well, one thing is clear.

444
00:23:32,812 --> 00:23:35,579
It must be getting some
extra energy from somewhere.

445
00:23:35,581 --> 00:23:37,882
Narrator:
There is one main contender

446
00:23:37,884 --> 00:23:41,619
for what may be supplying
this extra energy,

447
00:23:41,621 --> 00:23:44,455
a force with repulsive gravity,

448
00:23:44,457 --> 00:23:47,825
a force
that pushes things apart.

449
00:23:47,827 --> 00:23:50,428
This is what astronomers
call dark energy.

450
00:23:50,430 --> 00:23:53,164
It's this mysterious
repulsive force

451
00:23:53,166 --> 00:23:55,166
that we know exists
in the universe,

452
00:23:55,168 --> 00:23:58,302
and we have no idea what it is.

453
00:23:58,304 --> 00:24:01,906
Narrator: Physicists may not
agree on what dark energy is,

454
00:24:01,908 --> 00:24:03,307
but there is a consensus

455
00:24:03,309 --> 00:24:08,379
on where this repulsive force
has the most influence --

456
00:24:08,381 --> 00:24:13,184
in the regions between galaxies
and galaxy clusters,

457
00:24:13,186 --> 00:24:15,186
the cosmic voids.

458
00:24:15,188 --> 00:24:19,791
They're actually filled
to the brim with dark energy.

459
00:24:19,793 --> 00:24:21,458
The first time dark energy
is really going to make

460
00:24:21,460 --> 00:24:24,195
its Mark in the universe
is going to be the time

461
00:24:24,197 --> 00:24:26,930
when the first cosmic voids
begin to appear.

462
00:24:28,935 --> 00:24:33,270
Sutter:
We see dark energy's effects
throughout the universe.

463
00:24:33,272 --> 00:24:36,407
But when we look into
the cosmic voids,

464
00:24:36,409 --> 00:24:39,677
which are the most empty regions
of our universe,

465
00:24:39,679 --> 00:24:42,413
this is where dark energy
is strongest.

466
00:24:42,415 --> 00:24:45,282
�

467
00:24:45,284 --> 00:24:47,551
narrator: Dark energy
is the repulsive force

468
00:24:47,553 --> 00:24:49,353
pushing things apart.

469
00:24:49,355 --> 00:24:52,690
It prefers the voids
where gravity is weak.

470
00:24:54,561 --> 00:24:57,028
Bullock: These are areas where
there's a lot less dark matter,

471
00:24:57,030 --> 00:24:59,430
and because the overall density
is low,

472
00:24:59,432 --> 00:25:01,432
that's where the dark energy
starts to peek out

473
00:25:01,434 --> 00:25:04,235
and can really drive
those voids to expand.

474
00:25:04,237 --> 00:25:07,504
So the expansion and
acceleration of the universe

475
00:25:07,506 --> 00:25:10,174
are driven by the dark energy
in those regions.

476
00:25:11,711 --> 00:25:14,912
Narrator: Dark energy
pushes thing apart,

477
00:25:14,914 --> 00:25:17,782
things that get in its way,

478
00:25:17,784 --> 00:25:21,652
things like the cosmic web.

479
00:25:21,654 --> 00:25:27,058
Dark matter and normal matter
are also in its path

480
00:25:27,060 --> 00:25:30,595
and are bulldozed out
across the cosmos.

481
00:25:30,597 --> 00:25:34,866
Slowly, but surely, the balance
between dark energy

482
00:25:34,868 --> 00:25:37,468
and dark matter is changing.

483
00:25:40,006 --> 00:25:41,672
Imagine you have
a giant swimming pool,

484
00:25:41,674 --> 00:25:44,275
and at the very bottom
there's a puddle of water

485
00:25:44,277 --> 00:25:45,542
with a splash of whiskey.

486
00:25:45,544 --> 00:25:47,344
So you have sort of a strong
whiskey drink

487
00:25:47,346 --> 00:25:49,480
down at the bottom of your pool.

488
00:25:49,482 --> 00:25:53,016
But now you start dumping water
into your pool, no more whiskey,

489
00:25:53,018 --> 00:25:55,018
and it begins to get diluted
and diluted,

490
00:25:55,020 --> 00:25:57,822
and eventually, you just have
a swimming pool full of water,

491
00:25:57,824 --> 00:25:59,757
with one shot of whiskey
mixed in.

492
00:25:59,759 --> 00:26:01,159
That's not a very strong drink.

493
00:26:01,161 --> 00:26:03,294
It's basically a water
swimming pool.

494
00:26:03,296 --> 00:26:05,563
That's pretty much happening
with the dark energy.

495
00:26:05,565 --> 00:26:07,231
At first,
it's a one-to-one mixture

496
00:26:07,233 --> 00:26:08,900
of dark matter and dark energy.

497
00:26:08,902 --> 00:26:11,368
But in the long run,
it's all dark energy

498
00:26:11,370 --> 00:26:13,437
and pretty much
no dark matter left over.

499
00:26:13,439 --> 00:26:15,506
�

500
00:26:15,508 --> 00:26:17,975
narrator:
The forces of dark energy

501
00:26:17,977 --> 00:26:20,511
are on an unstoppable march,

502
00:26:20,513 --> 00:26:22,513
picking up more and more power

503
00:26:22,515 --> 00:26:26,250
from the vast scale
of the cosmic voids.

504
00:26:26,252 --> 00:26:28,986
Dark energy is intrinsically
very weak.

505
00:26:28,988 --> 00:26:30,888
There's very little dark energy

506
00:26:30,890 --> 00:26:34,859
and this repulsive effect
in every cubic centimeter.

507
00:26:34,861 --> 00:26:36,660
But the universe is vast.

508
00:26:36,662 --> 00:26:38,863
Space is big.

509
00:26:38,865 --> 00:26:42,733
So cumulatively, all this
small amount of stuff

510
00:26:42,735 --> 00:26:45,203
adds up to a very large amount,

511
00:26:45,205 --> 00:26:48,939
and over a scale encompassing
the entire universe,

512
00:26:48,941 --> 00:26:51,542
the dark energy dominates.

513
00:26:53,413 --> 00:26:55,946
Narrator: But it's been
a very long process.

514
00:26:55,948 --> 00:26:57,615
After the big bang,

515
00:26:57,617 --> 00:27:01,419
dark matter dominates
for the first 9 billion years.

516
00:27:01,421 --> 00:27:03,620
�

517
00:27:03,622 --> 00:27:06,023
then, 5 billion years ago,

518
00:27:06,025 --> 00:27:08,960
dark energy starts
to get the upper hand.

519
00:27:08,962 --> 00:27:12,763
It causes the expansion
of the universe to accelerate

520
00:27:12,765 --> 00:27:16,500
and the space in the voids
to grow more rapidly.

521
00:27:16,502 --> 00:27:20,504
As the space expands, there's
more and more dark energy

522
00:27:20,506 --> 00:27:22,173
because you have a bigger space.

523
00:27:22,175 --> 00:27:26,777
It sort of creates itself
with the expansion of space.

524
00:27:26,779 --> 00:27:29,380
Tegmark: Dark energy has
a sneaky way of taking over

525
00:27:29,382 --> 00:27:30,981
because it causes the space

526
00:27:30,983 --> 00:27:32,650
to stretch out
and get twice as big.

527
00:27:32,652 --> 00:27:36,254
So now there's twice
as much dark energy.

528
00:27:36,256 --> 00:27:39,724
Narrator: Dark energy
just can't stop pushing,

529
00:27:39,726 --> 00:27:42,459
causing the empty space
of the voids

530
00:27:42,461 --> 00:27:44,195
to continuously expand.

531
00:27:47,133 --> 00:27:50,801
As the universe expands
because of dark energy,

532
00:27:50,803 --> 00:27:53,204
more and more dark energy
is being created.

533
00:27:53,206 --> 00:27:54,739
Dark energy is
definitely gaining

534
00:27:54,741 --> 00:27:56,540
the upper hand on dark matter.

535
00:27:56,542 --> 00:28:01,546
It was always there, but it took
over compared to other stuff.

536
00:28:01,548 --> 00:28:03,146
�

537
00:28:03,148 --> 00:28:06,550
narrator: Eventually creating
enough energy to supercharge

538
00:28:06,552 --> 00:28:08,619
the expansion of the universe.

539
00:28:08,621 --> 00:28:10,922
�

540
00:28:10,924 --> 00:28:13,024
this acceleration continues.

541
00:28:13,026 --> 00:28:15,959
The universe is getting bigger
and bigger,

542
00:28:15,961 --> 00:28:19,430
and it's all powered
by the forces of repulsion,

543
00:28:19,432 --> 00:28:20,897
dark energy.

544
00:28:20,899 --> 00:28:25,836
And for the universe,
that could be very bad news.

545
00:28:25,838 --> 00:28:30,240
If that's the case, dark energy
may destroy the universe.

546
00:28:30,242 --> 00:28:31,708
It will get stronger
and stronger

547
00:28:31,710 --> 00:28:34,979
until it literally rips apart
the fabric of space-time.

548
00:28:48,127 --> 00:28:49,793
�

549
00:28:49,795 --> 00:28:51,796
narrator:
Dark matter and dark energy

550
00:28:51,798 --> 00:28:56,467
have been battling each other
for 13.8 billion years.

551
00:28:56,469 --> 00:29:01,272
For the first 9 billion years,
dark matter dominates.

552
00:29:01,274 --> 00:29:04,875
Dark matter exerts
positive gravity,

553
00:29:04,877 --> 00:29:09,947
but pulling everything together
leads to one inevitable outcome.

554
00:29:09,949 --> 00:29:14,218
If the universe was totally
dominated by matter,

555
00:29:14,220 --> 00:29:16,887
eventually our expansion
would slow down,

556
00:29:16,889 --> 00:29:20,024
glide to a stop,
and then turn around

557
00:29:20,026 --> 00:29:24,028
and collapse into
a small, dense state

558
00:29:24,030 --> 00:29:29,100
from where it came from,
an event we call the big crunch.

559
00:29:29,102 --> 00:29:30,901
Narrator:
During the big crunch,

560
00:29:30,903 --> 00:29:34,638
gravity would play havoc
with the cosmos.

561
00:29:34,640 --> 00:29:38,442
Galaxies
would be dragged together.

562
00:29:38,444 --> 00:29:42,513
Stars and planets
would smash into each other.

563
00:29:42,515 --> 00:29:46,250
The universe would collapse
in a blazing inferno

564
00:29:46,252 --> 00:29:48,919
of superdense matter
and energy.

565
00:29:52,991 --> 00:29:56,059
Fortunately, none of this
will probably happen.

566
00:29:56,061 --> 00:29:59,129
�

567
00:29:59,131 --> 00:30:03,334
scientists have now dismissed
the possibility of a big crunch.

568
00:30:03,336 --> 00:30:06,337
�

569
00:30:06,339 --> 00:30:08,739
sutter: We don't face that,
because we have a universe

570
00:30:08,741 --> 00:30:11,208
filled with dark energy.

571
00:30:11,210 --> 00:30:14,211
Dark energy is causing the
universe to do something else,

572
00:30:14,213 --> 00:30:16,947
something it would prefer
not to do.

573
00:30:16,949 --> 00:30:20,751
It is accelerating the expansion
of the universe.

574
00:30:20,753 --> 00:30:23,287
Narrator:
As dark energy gets stronger,

575
00:30:23,289 --> 00:30:26,690
it supercharges this expansion.

576
00:30:26,692 --> 00:30:32,429
The presence of dark energy
is like a high-octane additive

577
00:30:32,431 --> 00:30:34,298
into a gas tank,

578
00:30:34,300 --> 00:30:37,835
where a car isn't
just coasting along.

579
00:30:37,837 --> 00:30:40,170
It's boosting along,

580
00:30:40,172 --> 00:30:43,039
and that's what's happening
with our universe.

581
00:30:43,041 --> 00:30:46,043
Narrator: If that's correct,
this supercharged expansion

582
00:30:46,045 --> 00:30:49,980
will continue
for the next few billion years.

583
00:30:49,982 --> 00:30:54,451
The repulsive force of dark
energy will become invincible,

584
00:30:54,453 --> 00:31:00,124
and such unbridled power
will come with a high cost.

585
00:31:00,126 --> 00:31:03,660
I know of three ultimate
doomsday weapons.

586
00:31:03,662 --> 00:31:07,464
One is galactus'
ultimate nullifier.

587
00:31:07,466 --> 00:31:11,334
Another is the infinity stones
in the hands of thanos.

588
00:31:11,336 --> 00:31:13,203
But those two are make-believe.

589
00:31:13,205 --> 00:31:15,338
The real one is dark energy.

590
00:31:15,340 --> 00:31:18,742
It's the real ultimate
universe destroyer.

591
00:31:18,744 --> 00:31:20,744
�

592
00:31:20,746 --> 00:31:23,748
narrator: The universe may pay
the ultimate price

593
00:31:23,750 --> 00:31:28,385
for this ever-increasing
expansion driven by dark energy.

594
00:31:28,387 --> 00:31:29,954
Carroll:
And if that continues forever,

595
00:31:29,956 --> 00:31:33,157
the future of the universe
is very, very simple.

596
00:31:33,159 --> 00:31:35,493
It continues to expand,
to accelerate.

597
00:31:35,495 --> 00:31:37,428
Everything moves apart
from everything else,

598
00:31:37,430 --> 00:31:40,564
and the universe
becomes empty.

599
00:31:40,566 --> 00:31:42,366
There'll be nothing left

600
00:31:42,368 --> 00:31:46,103
but cold, desolate,
empty space forever.

601
00:31:46,105 --> 00:31:47,704
�

602
00:31:47,706 --> 00:31:49,639
narrator:
Dark energy will finally win

603
00:31:49,641 --> 00:31:52,309
the long war with dark matter.

604
00:31:52,311 --> 00:31:55,245
But the result
won't be pretty.

605
00:31:55,247 --> 00:32:00,784
This future universe
will be cold, dark, and empty.

606
00:32:00,786 --> 00:32:02,519
Eventually it's going
to get so cold

607
00:32:02,521 --> 00:32:05,055
that really nothing
can happen in it.

608
00:32:05,057 --> 00:32:06,790
It's the big chill.

609
00:32:06,792 --> 00:32:10,261
It gets colder and colder
and darker and darker.

610
00:32:10,263 --> 00:32:13,531
Everything will fade out,
and though it began with a bang,

611
00:32:13,533 --> 00:32:14,999
it's going to die in a whimper.

612
00:32:15,001 --> 00:32:19,069
The universe flatlines
because of dark energy.

613
00:32:19,071 --> 00:32:22,606
Narrator: The universe dies
in a big chill.

614
00:32:22,608 --> 00:32:24,942
Galaxies are so far apart,

615
00:32:24,944 --> 00:32:29,213
they're distant islands
in a sea of darkness.

616
00:32:29,215 --> 00:32:32,349
Gradually,
the galaxies dies, too.

617
00:32:32,351 --> 00:32:36,954
Star birth stops,
and the universe fades away.

618
00:32:39,091 --> 00:32:40,758
Or maybe not.

619
00:32:40,760 --> 00:32:45,429
There's another,
far more violent scenario.

620
00:32:45,431 --> 00:32:50,768
Here, dark energy just goes
from strength to strength.

621
00:32:50,770 --> 00:32:54,771
It could be that dark energy
is so strong

622
00:32:54,773 --> 00:32:58,041
that it will multiply
upon itself

623
00:32:58,043 --> 00:33:00,377
as the universe gets bigger.

624
00:33:00,379 --> 00:33:05,049
This is a process that
we call phantom dark energy.

625
00:33:05,051 --> 00:33:09,786
Narrator: Phantom energy is
dark energy on steroids.

626
00:33:09,788 --> 00:33:13,657
It multiplies uncontrollably
in the voids,

627
00:33:13,659 --> 00:33:16,059
tearing at the fabric
of the universe

628
00:33:16,061 --> 00:33:20,664
in a process
called the big rip.

629
00:33:20,666 --> 00:33:24,935
Dark energy is weird enough,
but imagine the possibility

630
00:33:24,937 --> 00:33:28,405
that there is more and more
of it as time goes on,

631
00:33:28,407 --> 00:33:32,610
and it's called phantom energy,
and in that case,

632
00:33:32,612 --> 00:33:35,745
it would rip everything apart,
even black holes.

633
00:33:39,886 --> 00:33:42,619
It will start to rip apart
galaxies themselves...

634
00:33:45,691 --> 00:33:47,757
...rip apart solar systems...

635
00:33:51,630 --> 00:33:53,230
...rip apart people,

636
00:33:53,232 --> 00:33:56,767
rip apart atoms...

637
00:33:56,769 --> 00:33:57,968
Rip apart nuclei.

638
00:34:01,173 --> 00:34:03,774
Until finally,
space itself is pulled apart.

639
00:34:05,110 --> 00:34:06,643
Narrator:
The universe, as we know it,

640
00:34:06,645 --> 00:34:08,445
will be destroyed,

641
00:34:08,447 --> 00:34:11,782
but the big rip may not be
the end of everything.

642
00:34:11,784 --> 00:34:15,852
�

643
00:34:15,854 --> 00:34:20,724
there will be no normal matter
and no dark matter.

644
00:34:20,726 --> 00:34:22,926
And with nothing
left to conquer,

645
00:34:22,928 --> 00:34:27,530
phantom energy may use its
powers to become a creator,

646
00:34:27,532 --> 00:34:29,333
triggering a rebirth.

647
00:34:31,070 --> 00:34:35,406
Eventually, when you get to this
ultimate stage of emptiness,

648
00:34:35,408 --> 00:34:36,874
because of the phantom energy,

649
00:34:36,876 --> 00:34:38,742
we're actually able to turn
the universe around

650
00:34:38,744 --> 00:34:41,211
and get it
to collapse again,

651
00:34:41,213 --> 00:34:44,147
and then go through
a series of bounces.

652
00:34:44,149 --> 00:34:45,883
So we call it
the phantom bounce.

653
00:34:45,885 --> 00:34:47,417
�

654
00:34:47,419 --> 00:34:49,285
narrator:
With this phantom bounce,

655
00:34:49,287 --> 00:34:51,755
universal life energy left
in this dead universe

656
00:34:51,757 --> 00:34:54,358
starts to collapse.

657
00:34:54,360 --> 00:34:56,493
Freese: And eventually
it becomes hotter and hotter

658
00:34:56,495 --> 00:34:59,496
and denser and denser,
and then the fiery inferno

659
00:34:59,498 --> 00:35:02,966
eventually pushes you back out
into another big bang,

660
00:35:02,968 --> 00:35:05,535
and this just keeps
going on indefinitely.

661
00:35:05,537 --> 00:35:07,237
�

662
00:35:07,239 --> 00:35:10,574
narrator: So the destructive
and repulsive dark energy

663
00:35:10,576 --> 00:35:16,313
spawns a force that becomes
the ultimate universe recycler.

664
00:35:16,315 --> 00:35:18,648
The end state of our universe

665
00:35:18,650 --> 00:35:20,917
would lead you back
into another cycle,

666
00:35:20,919 --> 00:35:22,986
a whole new big bang
from the beginning.

667
00:35:28,127 --> 00:35:32,062
Narrator: In the end,
dark energy may kill the cosmos,

668
00:35:32,064 --> 00:35:35,199
or it may create a new one.

669
00:35:35,201 --> 00:35:36,800
Dark energy is mysterious.

670
00:35:36,802 --> 00:35:38,401
Dark energy is unknown.

671
00:35:38,403 --> 00:35:41,805
Dark energy is going to do
whatever it feels like.

672
00:35:41,807 --> 00:35:43,807
Maybe dark energy will go away.

673
00:35:43,809 --> 00:35:45,875
Maybe dark energy will decay

674
00:35:45,877 --> 00:35:49,012
and become a flood
of new matter and radiation.

675
00:35:49,014 --> 00:35:51,815
Maybe dark matter
will get stronger.

676
00:35:51,817 --> 00:35:53,016
We don't know.

677
00:35:53,018 --> 00:35:56,353
�

678
00:35:56,355 --> 00:35:58,822
narrator:
For now, we think dark energy

679
00:35:58,824 --> 00:36:02,159
will determine the fate
of the universe.

680
00:36:02,161 --> 00:36:05,962
But all of our evidence
is speculative.

681
00:36:05,964 --> 00:36:07,831
What if we have it all wrong?

682
00:36:07,833 --> 00:36:11,702
What if there is
no dark universe at all?

683
00:36:11,704 --> 00:36:16,240
�

684
00:36:26,986 --> 00:36:29,853
�

685
00:36:29,855 --> 00:36:31,989
narrator: A large part of
our understanding

686
00:36:31,991 --> 00:36:33,590
of the universe's past,

687
00:36:33,592 --> 00:36:37,327
present, and future is based
on educated guesswork

688
00:36:37,329 --> 00:36:39,829
about two invisible forces --

689
00:36:42,068 --> 00:36:45,669
...dark matter and dark energy.

690
00:36:45,671 --> 00:36:49,139
But it's pure speculation.

691
00:36:49,141 --> 00:36:53,610
Perhaps dark matter
and dark energy don't exist.

692
00:36:53,612 --> 00:36:56,346
There's not new stuff
in the universe.

693
00:36:56,348 --> 00:36:57,614
Anything is possible.

694
00:36:57,616 --> 00:36:59,015
Dark energy, in particular,

695
00:36:59,017 --> 00:37:01,752
might not be real,
so maybe there's something else

696
00:37:01,754 --> 00:37:03,419
that could be pushing
the universe apart.

697
00:37:03,421 --> 00:37:05,022
Tremblay: So we could
absolutely be wrong

698
00:37:05,024 --> 00:37:07,357
about dark energy
and dark matter.

699
00:37:07,359 --> 00:37:09,025
Maybe they don't exist.

700
00:37:09,027 --> 00:37:11,962
Maybe tomorrow we'll discover
that our understanding

701
00:37:11,964 --> 00:37:14,764
was wrong all along.

702
00:37:14,766 --> 00:37:16,834
Narrator:
That's an awful lot of maybes.

703
00:37:16,836 --> 00:37:19,036
Let's add one more.

704
00:37:19,038 --> 00:37:21,704
When it comes to finding
answers,

705
00:37:21,706 --> 00:37:24,241
maybe we're looking
in the wrong place.

706
00:37:26,445 --> 00:37:29,713
Filippenko: One possibility is
that there are other universes

707
00:37:29,715 --> 00:37:34,585
out there pulling outward,
so to speak, on our universe.

708
00:37:34,587 --> 00:37:35,919
That might be the answer.

709
00:37:35,921 --> 00:37:38,588
But most theoretical physicists
and astrophysicists

710
00:37:38,590 --> 00:37:41,658
these days think
that dark energy is real

711
00:37:41,660 --> 00:37:44,794
because that seems to be
the simplest explanation

712
00:37:44,796 --> 00:37:47,664
for a wide variety
of observations.

713
00:37:47,666 --> 00:37:49,132
�

714
00:37:49,134 --> 00:37:52,802
narrator: No one really knows
what dark energy is made of.

715
00:37:52,804 --> 00:37:56,273
Maybe the answer lies
in the past.

716
00:37:56,275 --> 00:37:58,609
The best theory for dark energy
we have right now

717
00:37:58,611 --> 00:38:00,811
is the simplest one
and the oldest one,

718
00:38:00,813 --> 00:38:03,280
and that's the idea that
it's a cosmological constant.

719
00:38:03,282 --> 00:38:05,215
�

720
00:38:05,217 --> 00:38:07,417
narrator: Albert Einstein
came up with the idea

721
00:38:07,419 --> 00:38:12,289
of a cosmological constant
in 1917.

722
00:38:12,291 --> 00:38:16,160
He suggested that space
has its own energy,

723
00:38:16,162 --> 00:38:19,429
energy that can affect
the way the universe expands.

724
00:38:19,431 --> 00:38:21,965
�

725
00:38:21,967 --> 00:38:25,035
when Edwin hubble proved
the universe is expanding,

726
00:38:25,037 --> 00:38:27,971
Einstein thought
the cosmological constant

727
00:38:27,973 --> 00:38:30,106
was his biggest blunder.

728
00:38:30,108 --> 00:38:33,977
But observations that the
expansion of the universe

729
00:38:33,979 --> 00:38:35,445
is accelerating

730
00:38:35,447 --> 00:38:39,049
reveal Einstein
was right all along.

731
00:38:39,051 --> 00:38:42,986
Filippenko: Well, here we are.
We've reintroduced the idea.

732
00:38:42,988 --> 00:38:46,790
So Einstein's biggest blunder
may have actually

733
00:38:46,792 --> 00:38:49,326
conceptually been
his greatest triumph.

734
00:38:49,328 --> 00:38:52,930
�

735
00:38:52,932 --> 00:38:56,200
narrator:
But to understand the true
nature of the dark universe,

736
00:38:56,202 --> 00:39:00,603
we may need to re-evaluate what
we think we know about gravity.

737
00:39:02,874 --> 00:39:05,542
When we're trying to understand
dark energy and dark matter,

738
00:39:05,544 --> 00:39:09,479
there's a chance that just our
fundamental theories of gravity

739
00:39:09,481 --> 00:39:13,884
are wrong, that general
relativity isn't quite right.

740
00:39:13,886 --> 00:39:16,219
Narrator: Einstein's theory
of general relativity

741
00:39:16,221 --> 00:39:18,555
explains how gravity works,

742
00:39:18,557 --> 00:39:21,224
how stars orbit in galaxies,

743
00:39:21,226 --> 00:39:23,660
and planets orbit stars.

744
00:39:23,662 --> 00:39:25,495
�

745
00:39:25,497 --> 00:39:28,431
some scientists wonder
if altering this theory

746
00:39:28,433 --> 00:39:32,702
will help us understand
the dark universe.

747
00:39:32,704 --> 00:39:34,771
Tremblay: So you need not
absolutely believe

748
00:39:34,773 --> 00:39:37,974
that there is something
actually called dark matter.

749
00:39:37,976 --> 00:39:39,910
You need to only understand
that there is something

750
00:39:39,912 --> 00:39:42,846
in the universe
which behaves like dark matter.

751
00:39:42,848 --> 00:39:45,716
For example, you could
effectively mimic the behavior

752
00:39:45,718 --> 00:39:49,987
of dark matter by modifying
our current theory of gravity.

753
00:39:49,989 --> 00:39:51,988
Narrator:
But successfully modifying

754
00:39:51,990 --> 00:39:56,593
Einstein's theories on gravity
is a big challenge.

755
00:39:56,595 --> 00:39:58,795
Einstein's equations
are very robust.

756
00:39:58,797 --> 00:40:02,399
You don't fluff around
with Einstein with impunity.

757
00:40:05,137 --> 00:40:07,404
Narrator: For decades,
theoretical physicists

758
00:40:07,406 --> 00:40:10,607
have toyed
with Einstein's equations,

759
00:40:10,609 --> 00:40:14,811
looking for ways to explain
dark matter and dark energy,

760
00:40:14,813 --> 00:40:17,280
or make them go away.

761
00:40:17,282 --> 00:40:19,683
As yet, no one has managed.

762
00:40:19,685 --> 00:40:22,218
The dark universe persists.

763
00:40:23,822 --> 00:40:26,156
I think that the best
description of the observations

764
00:40:26,158 --> 00:40:29,359
we have today
is that dark matter exists.

765
00:40:29,361 --> 00:40:31,762
It's out there,
as well as dark energy.

766
00:40:31,764 --> 00:40:33,830
I think dark energy exists.

767
00:40:33,832 --> 00:40:35,365
I think dark energy is real,

768
00:40:35,367 --> 00:40:38,435
but I must admit that sometimes,
at 3 o'clock in the morning,

769
00:40:38,437 --> 00:40:41,772
I wake up screaming,
worried that, in fact,

770
00:40:41,774 --> 00:40:44,774
we've settled
on the wrong answer,

771
00:40:44,776 --> 00:40:46,910
and that in
a couple hundred years,

772
00:40:46,912 --> 00:40:48,778
they're going to be
laughing at us.

773
00:40:48,780 --> 00:40:51,715
�

774
00:40:51,717 --> 00:40:54,918
narrator: Until then, our
observations tell us the battle

775
00:40:54,920 --> 00:40:59,523
between dark matter and dark
energy has shaped the universe.

776
00:40:59,525 --> 00:41:02,992
�

777
00:41:02,994 --> 00:41:06,330
dark matter dictated the past,

778
00:41:06,332 --> 00:41:09,733
built the galaxies,
the stars, and the planets.

779
00:41:09,735 --> 00:41:11,601
�

780
00:41:11,603 --> 00:41:14,604
dark energy will determine
its future,

781
00:41:14,606 --> 00:41:18,141
potentially tearing
the universe apart.

782
00:41:18,143 --> 00:41:21,611
Just because we can't see dark
matter and dark energy directly

783
00:41:21,613 --> 00:41:24,348
doesn't mean they have not
had a profound effect

784
00:41:24,350 --> 00:41:26,749
on the evolution
of the entire universe.

785
00:41:26,751 --> 00:41:30,086
The dark universe was there at
the beginning of the universe,

786
00:41:30,088 --> 00:41:32,489
shaping it, and actually
creating the conditions

787
00:41:32,491 --> 00:41:36,225
for us to be here, and it's
taken over the universe.

788
00:41:36,227 --> 00:41:39,629
And, eventually, it may well
destroy the universe.

789
00:41:39,631 --> 00:41:41,164
�

790
00:41:41,166 --> 00:41:43,100
narrator:
Our universe may be dominated

791
00:41:43,102 --> 00:41:47,504
by the long struggle between
dark matter and dark energy.

792
00:41:47,506 --> 00:41:51,374
But all of this conflict
has led to a creative outcome,

793
00:41:51,376 --> 00:41:55,779
an outcome for which
we should all be grateful.

794
00:41:55,781 --> 00:41:59,850
The name dark matter suggests
that it's something nefarious

795
00:41:59,852 --> 00:42:02,252
and somehow bad for us,
but actually it's turned out

796
00:42:02,254 --> 00:42:04,854
that dark matter
is very much our friend.

797
00:42:04,856 --> 00:42:07,424
Because if it weren't
for the dark matter,

798
00:42:07,426 --> 00:42:08,725
we wouldn't be here.

799
00:42:09,928 --> 00:42:11,261
Thaller:
There's a wonderful irony

800
00:42:11,263 --> 00:42:13,129
to calling it
the dark universe,

801
00:42:13,131 --> 00:42:15,064
because now we're actually
beginning to shed light

802
00:42:15,066 --> 00:42:16,600
on how the universe began,

803
00:42:16,602 --> 00:42:18,935
how the largest structures
in the universe evolved.

804
00:42:18,937 --> 00:42:20,804
We wouldn't be here
without this dark universe.

805
00:42:20,806 --> 00:42:22,338
It's not dark at all.

806
00:42:22,340 --> 00:42:24,141
It's shedding light
on our own reality.

807
00:42:24,143 --> 00:42:28,512
�