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Across the universe,
an endless war rages,
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a bitter struggle
between invisible forces.
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00:00:15,000 --> 00:00:17,170
We've seen this
cosmic battle go on
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for the past 14 billion years.
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00:00:19,800 --> 00:00:23,300
Like two navies, fighting it out
in the ocean of space time.
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Dark matter...
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And dark energy
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00:00:29,450 --> 00:00:32,510
battling for control
of the universe.
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00:00:32,520 --> 00:00:35,020
They've shaped the entire
history of the universe.
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They're not about to stop now.
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00:00:37,650 --> 00:00:41,060
Shadowy forces
dictating our past,
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our present, and our future.
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00:00:43,960 --> 00:00:45,890
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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It's taken over
the universe and, eventually,
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00:00:53,300 --> 00:00:55,240
it may well
destroy the universe.
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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, 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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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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To encapsulate that
in one sentence,
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dark matter is attractive,
dark energy is repulsive.
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Since the dawn of time,
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two forces slugging it out
for control of the universe.
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00:03:05,000 --> 00:03:06,300
Dark matter and dark energy
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00:03:06,300 --> 00:03:08,270
are locked
in this epic struggle.
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00:03:08,270 --> 00:03:11,070
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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00:03:21,480 --> 00:03:26,520
It's a struggle that started
13.8 billion years ago
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in the cosmic furnace
of the big bang.
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00:03:31,960 --> 00:03:36,600
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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00:04:02,030 --> 00:04:03,590
after the big bang,
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00:04:03,590 --> 00:04:06,290
around the time
that normal matter formed,
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00:04:06,300 --> 00:04:07,830
and the processes that created
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00:04:07,830 --> 00:04:09,700
the normal matter
we know all about.
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00:04:09,700 --> 00:04:13,330
Similar processes probably
created the dark matter.
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00:04:15,140 --> 00:04:16,870
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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00:04:36,830 --> 00:04:40,460
planets, stars, and us...
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00:04:40,460 --> 00:04:42,660
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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00:05:04,690 --> 00:05:08,590
But the dark universe
does leave clues.
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It's like a crime scene.
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00:05:10,160 --> 00:05:12,630
You know that a crime
has been committed,
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00:05:12,630 --> 00:05:15,300
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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There are several contenders
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for what dark matter could be.
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00:05:30,980 --> 00:05:34,150
It could be normal matter
that we just don't see,
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00:05:34,150 --> 00:05:36,250
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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00:05:39,220 --> 00:05:42,720
It could be
a species of particle,
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like an electron,
like a proton, 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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00:05:58,910 --> 00:06:03,480
Wimps may not interact strongly
with other matter or light,
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00:06:03,480 --> 00:06:06,250
but they do exert
a gravitational pull.
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00:06:08,790 --> 00:06:11,950
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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00:06:20,860 --> 00:06:24,200
full of new particles
of both visible matter
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00:06:24,200 --> 00:06:27,300
and invisible dark matter.
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00:06:27,300 --> 00:06:30,970
But another force was present...
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00:06:30,970 --> 00:06:34,040
Dark energy.
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00:06:34,040 --> 00:06:37,710
Dark energy has existed
since the time of the big bang.
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Dark energy was always there.
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00:06:39,580 --> 00:06:41,720
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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00:06:44,320 --> 00:06:45,420
was always there.
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Our understanding of
what that dark energy is
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is very limited.
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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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00:07:00,970 --> 00:07:05,470
We're not sure
what dark energy is.
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00:07:05,480 --> 00:07:08,740
Dark energy is just a fancy
name for our ignorance.
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00:07:08,750 --> 00:07:12,450
Dark energy is nothing
more than a placeholder name
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00:07:12,450 --> 00:07:15,080
for this enormous gap
in our understanding
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00:07:15,080 --> 00:07:16,520
of how the cosmos works.
128
00:07:16,520 --> 00:07:19,420
But we don't understand
it at all.
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00:07:19,420 --> 00:07:21,560
It's true.
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00:07:23,390 --> 00:07:25,960
One potential answer
to what dark energy
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00:07:25,960 --> 00:07:31,600
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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We used to think of
space as just boring emptiness.
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00:07:38,310 --> 00:07:41,710
But now, I think it's healthier
to think of space
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00:07:41,710 --> 00:07:44,380
as a kind of substance.
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00:07:44,380 --> 00:07:48,280
A substance that carries
a strange type of energy.
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00:07:48,280 --> 00:07:49,880
Every small region of space
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00:07:49,890 --> 00:07:52,250
has a little bit
of energy in it,
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00:07:52,260 --> 00:07:55,120
just associated
with the vacuum itself.
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00:07:55,120 --> 00:07:59,490
Vacuum energy is the idea
that vacuum isn't empty,
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that there is something there.
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00:08:01,100 --> 00:08:04,570
There's an energy in it
with a kind of antigravity.
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00:08:06,970 --> 00:08:08,900
Perhaps this vacuum energy
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00:08:08,910 --> 00:08:12,840
that pushes against gravity
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00:08:12,840 --> 00:08:16,040
is the mysterious dark energy.
146
00:08:16,050 --> 00:08:19,750
We simply don't know.
147
00:08:19,750 --> 00:08:23,580
We're not sure that the dark
energy is the vacuum energy.
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00:08:23,590 --> 00:08:27,760
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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00:08:31,130 --> 00:08:33,390
Dark energy and dark matter,
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00:08:33,400 --> 00:08:36,930
forged in the intense heat
of the big bang,
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00:08:36,930 --> 00:08:41,840
opposing forces,
one attractive, one repulsive.
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00:08:41,840 --> 00:08:44,910
But together,
over 13.8 billions years,
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00:08:44,910 --> 00:08:48,610
they will shape
the history of the universe.
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00:09:08,230 --> 00:09:10,530
The story of
the universe is dominated
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00:09:10,530 --> 00:09:15,340
by two powerful
opposing forces...
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Dark matter and dark energy.
158
00:09:18,740 --> 00:09:21,610
For 13.8 billions years,
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00:09:21,610 --> 00:09:25,550
they've battled it out
for control of the cosmos.
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00:09:25,550 --> 00:09:27,750
Dark matter and dark energy
are out there,
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00:09:27,750 --> 00:09:30,150
and they've shaped the entire
history of the universe.
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00:09:30,150 --> 00:09:33,250
Our universe is actually
the balance between dark forces.
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00:09:33,260 --> 00:09:35,660
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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00:09:39,960 --> 00:09:44,100
After the big bang,
the infant universe was small,
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00:09:44,100 --> 00:09:47,440
intensely hot
and intensely dense.
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Dark matter, the force bent on
bringing things together, 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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00:10:03,950 --> 00:10:05,720
When things were
closer together,
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00:10:05,720 --> 00:10:08,860
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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The environment was
also tough for normal matter.
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It was so hot, intense radiation
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00:10:19,900 --> 00:10:25,510
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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00:10:35,020 --> 00:10:37,120
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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00:10:50,670 --> 00:10:52,330
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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00:11:02,350 --> 00:11:05,910
the force that brings things
together, intervened.
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00:11:07,620 --> 00:11:10,220
Dark matter doesn't talk
to radiation,
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00:11:10,220 --> 00:11:11,390
doesn't talk to light.
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00:11:11,390 --> 00:11:12,990
Can do whatever it wants.
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It starts clumping together.
195
00:11:16,360 --> 00:11:19,060
Radiation
pushes normal matter apart,
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00:11:19,060 --> 00:11:22,800
stopping it
from forming dense regions.
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00:11:22,800 --> 00:11:27,070
But photons simply pass straight
through the dark matter,
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00:11:27,070 --> 00:11:32,740
allowing it to clump and fall
into dense pockets or wells.
199
00:11:32,740 --> 00:11:36,910
The dark matter begins to clump
together gravitationally,
200
00:11:36,910 --> 00:11:39,610
and this means that the matter
is going to fall into
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00:11:39,620 --> 00:11:42,850
those dark matter wells.
202
00:11:42,850 --> 00:11:45,750
Over time,
more and more regular matter
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00:11:45,760 --> 00:11:48,860
is pulled into
the dark matter wells.
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00:11:48,860 --> 00:11:50,560
The regions that have
a little bit more stuff,
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00:11:50,560 --> 00:11:52,030
gravity makes them bigger,
206
00:11:52,030 --> 00:11:54,860
and the regions that have
less stuff, those expand more.
207
00:11:54,860 --> 00:11:58,630
So you have little pockets
of slight extra matter,
208
00:11:58,630 --> 00:12:00,840
have more and more
and more matter over time.
209
00:12:02,870 --> 00:12:05,170
Gravity-rich pockets
of dark matter
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00:12:05,170 --> 00:12:08,240
pull particles
of regular matter together.
211
00:12:10,110 --> 00:12:13,450
Gradually, they form
giant clouds of hydrogen
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00:12:13,450 --> 00:12:16,820
and helium gas.
213
00:12:16,820 --> 00:12:21,490
Dark matter has laid the
foundation stones of the cosmos.
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00:12:21,490 --> 00:12:25,730
This force may be dark,
but it's highly creative.
215
00:12:29,300 --> 00:12:34,370
And now, 180 million years
after the big bang,
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00:12:34,370 --> 00:12:35,870
everything is in place
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00:12:35,870 --> 00:12:39,310
for the next dark matter
construction milestone...
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00:12:39,310 --> 00:12:42,780
The creation of stars.
219
00:12:42,780 --> 00:12:44,150
We know stars,
220
00:12:44,150 --> 00:12:46,350
in the very early universe
at the edge of time,
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00:12:46,350 --> 00:12:48,920
had to form from
the collapse of gas clouds
222
00:12:48,920 --> 00:12:50,380
under their own gravity.
223
00:12:53,290 --> 00:12:54,960
But there's a problem.
224
00:12:54,960 --> 00:13:00,490
The clouds of hydrogen in the
infant universe can't collapse,
225
00:13:00,500 --> 00:13:04,630
and no collapse means no stars.
226
00:13:04,630 --> 00:13:07,230
The gas in the early
universe has a lot of pressure,
227
00:13:07,240 --> 00:13:09,400
and this pressure
keeps it from collapsing.
228
00:13:09,410 --> 00:13:10,670
The dark matter
doesn't experience
229
00:13:10,670 --> 00:13:12,040
that kind of pressure.
230
00:13:12,040 --> 00:13:13,510
So the dark matter can clump up
231
00:13:13,510 --> 00:13:17,280
and make sites
for structure formation.
232
00:13:17,280 --> 00:13:19,450
So dark matter comes
to the rescue,
233
00:13:19,450 --> 00:13:23,080
creating regions
of higher gravity,
234
00:13:23,090 --> 00:13:25,190
dragging in hydrogen gas,
235
00:13:25,190 --> 00:13:29,660
forcing the clouds to get
denser and denser,
236
00:13:29,660 --> 00:13:34,460
creating the conditions
for collapse and then creation.
237
00:13:40,870 --> 00:13:44,310
So it's only when the gravity
of the dark matter
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00:13:44,310 --> 00:13:46,710
overwhelms the pressure of gases
239
00:13:46,710 --> 00:13:50,880
that the gases can collapse
and turn into stars.
240
00:13:50,880 --> 00:13:52,280
As soon as the gas cools down,
241
00:13:52,280 --> 00:13:53,980
it can fall into those
potential wells
242
00:13:53,980 --> 00:13:55,920
that the dark matter created,
243
00:13:55,920 --> 00:13:58,620
almost like little nurseries
for stars,
244
00:13:58,620 --> 00:14:00,650
and they start
forming in earnest.
245
00:14:02,660 --> 00:14:05,230
Dark matter provides
a boost of gravity
246
00:14:05,230 --> 00:14:10,800
to kick-start hydrogen into
constructing the first stars,
247
00:14:10,800 --> 00:14:14,670
stars that are the seeds
of the first galaxies.
248
00:14:14,670 --> 00:14:17,370
So it's dark matter
that would have coalesced
249
00:14:17,370 --> 00:14:20,710
in the early universe
and grown from there,
250
00:14:20,710 --> 00:14:22,640
and then the luminous
component of the universe,
251
00:14:22,650 --> 00:14:24,980
the things that we think of
as being the universe itself,
252
00:14:24,980 --> 00:14:26,510
like stars and galaxies,
253
00:14:26,520 --> 00:14:28,920
would have just been along
for the ride.
254
00:14:28,920 --> 00:14:34,260
Without the presence of dark
matter to seed structures,
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00:14:34,260 --> 00:14:39,060
there wasn't enough time in the
early universe to form galaxies,
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00:14:39,060 --> 00:14:42,930
which means you and me
have to thank dark matter
257
00:14:42,930 --> 00:14:45,170
for our existence.
258
00:14:45,170 --> 00:14:47,270
And dark matter now begins
259
00:14:47,270 --> 00:14:51,910
a much more ambitious
architectural project...
260
00:14:51,910 --> 00:14:56,210
To shape the entire
universe itself,
261
00:14:56,210 --> 00:15:00,450
to build the biggest structure
ever constructed...
262
00:15:00,450 --> 00:15:02,020
The cosmic web.
263
00:15:20,440 --> 00:15:23,270
The war between
dark matter and dark energy
264
00:15:23,270 --> 00:15:26,010
has been raging
since the birth of time.
265
00:15:29,080 --> 00:15:33,880
But in the early years,
it's a one-sided contest.
266
00:15:33,880 --> 00:15:36,220
In the early universe, the only
thing that really mattered
267
00:15:36,220 --> 00:15:39,790
was the dark matter
and the normal matter.
268
00:15:39,790 --> 00:15:41,890
Dark matter,
269
00:15:41,890 --> 00:15:45,690
the force that brings things
together,
270
00:15:45,700 --> 00:15:47,360
is in the driver's seat.
271
00:15:50,970 --> 00:15:55,040
Dark energy, the force
that pulls things apart,
272
00:15:55,040 --> 00:15:57,040
is the underdog.
273
00:15:57,040 --> 00:15:59,610
We've seen this
cosmic battle go on
274
00:15:59,610 --> 00:16:01,710
for the past 14 billion years.
275
00:16:01,710 --> 00:16:06,810
Fortunately for us, the dark
energy got off to a slow start.
276
00:16:06,820 --> 00:16:10,220
Meanwhile, dark matter
is busy at work,
277
00:16:10,220 --> 00:16:12,890
building the universe.
278
00:16:12,890 --> 00:16:17,760
Not only does it trigger
the birth of the first stars,
279
00:16:17,760 --> 00:16:19,990
it embarks on an even
more formidable
280
00:16:20,000 --> 00:16:22,960
construction project...
281
00:16:22,970 --> 00:16:26,470
The cosmic web.
282
00:16:26,470 --> 00:16:30,540
There's this large-scale
structure of filaments
283
00:16:30,540 --> 00:16:32,770
that galaxies seem to form on,
284
00:16:32,780 --> 00:16:35,540
and that's what we call
the cosmic web.
285
00:16:35,540 --> 00:16:38,450
And we can trace the formation
of this cosmic web
286
00:16:38,450 --> 00:16:41,280
all the way back
to the early universe.
287
00:16:41,280 --> 00:16:43,050
Now this is such a huge
structure,
288
00:16:43,050 --> 00:16:44,750
we don't think there's time
in the universe
289
00:16:44,750 --> 00:16:47,350
for matter's gravity alone
to do this.
290
00:16:47,360 --> 00:16:49,520
There must have been
an underlying scaffold
291
00:16:49,530 --> 00:16:51,260
of dark matter.
292
00:16:51,260 --> 00:16:53,590
The dark matter started
forming into these filaments,
293
00:16:53,600 --> 00:16:55,600
and when the universe
cooled enough,
294
00:16:55,600 --> 00:16:57,300
normal matter
could start to stream
295
00:16:57,300 --> 00:17:00,400
into this gravitational
attraction of the dark matter.
296
00:17:00,400 --> 00:17:02,000
That became the scaffolding
297
00:17:02,000 --> 00:17:05,870
on which this large-scale
structure was built.
298
00:17:05,880 --> 00:17:08,810
The filaments of
dark matter joined together,
299
00:17:08,810 --> 00:17:12,650
drawing in more
and more hydrogen gas.
300
00:17:12,650 --> 00:17:16,820
Dense clouds of gas build up at
the junctions of the filaments,
301
00:17:16,820 --> 00:17:19,890
the point where gravity
is at its strongest.
302
00:17:22,930 --> 00:17:24,860
Slowly and surely,
303
00:17:24,860 --> 00:17:29,400
a familiar-looking structure
starts to take shape.
304
00:17:29,400 --> 00:17:30,730
If you've ever gone outside,
305
00:17:30,730 --> 00:17:34,540
and you can see a spider web
covered in dew,
306
00:17:34,540 --> 00:17:37,370
that's kind of like
what happened with the universe.
307
00:17:37,370 --> 00:17:38,910
In this case, the spider web
308
00:17:38,910 --> 00:17:40,810
is the structure
of the dark matter.
309
00:17:40,810 --> 00:17:43,340
It's all of these filaments,
and the moisture in the air
310
00:17:43,350 --> 00:17:44,880
is what condenses around them,
311
00:17:44,880 --> 00:17:48,720
just like the normal matter
fell into the dark matter web
312
00:17:48,720 --> 00:17:51,790
to form these gigantic
structures in the universe.
313
00:17:51,790 --> 00:17:55,390
Dark matter,
the universe's master builder,
314
00:17:55,390 --> 00:17:59,690
succeeds in stitching
together a cosmic web.
315
00:17:59,700 --> 00:18:04,700
This will be the framework
for the entire universe.
316
00:18:04,700 --> 00:18:07,630
And so it is dark matter
that would choreograph
317
00:18:07,640 --> 00:18:10,140
and sculpt the shape
of the universe itself.
318
00:18:10,140 --> 00:18:12,110
One of the amazing things
about dark matter is,
319
00:18:12,110 --> 00:18:14,340
without it, we wouldn't be here.
320
00:18:14,340 --> 00:18:16,840
It's hard to imagine how you
could have structure
321
00:18:16,850 --> 00:18:19,750
in the universe
without dark matter.
322
00:18:19,750 --> 00:18:22,380
Galaxies,
and then galactic clusters,
323
00:18:22,380 --> 00:18:25,190
form at the junctions
of the filaments.
324
00:18:25,190 --> 00:18:30,690
Slowly, but surely, the universe
begins to take shape.
325
00:18:30,690 --> 00:18:33,730
When we look at this structure
over a cosmic scale,
326
00:18:33,730 --> 00:18:35,860
we see that it looks
kind of like a sponge.
327
00:18:35,870 --> 00:18:39,400
You see voids with galaxies
all over the edges of them.
328
00:18:39,400 --> 00:18:41,570
That is the structure
that was formed
329
00:18:41,570 --> 00:18:45,540
by the dark matter
in the early universe.
330
00:18:45,540 --> 00:18:47,670
Dark matter is the thing
that enabled,
331
00:18:47,680 --> 00:18:49,680
that provided enough gravity
332
00:18:49,680 --> 00:18:53,350
for the initial seeds of
structure formation to coalesce,
333
00:18:53,350 --> 00:18:55,520
for galaxies themselves to form.
334
00:18:57,320 --> 00:18:58,950
And, of course,
without galaxies,
335
00:18:58,950 --> 00:19:01,020
there are no stars,
and there's no planets,
336
00:19:01,020 --> 00:19:02,920
and there's no us.
337
00:19:02,930 --> 00:19:04,530
For 9 billion years,
338
00:19:04,530 --> 00:19:08,030
dark matter orchestrates the
construction of the universe.
339
00:19:10,000 --> 00:19:13,000
In these, the first battles
of the cosmos,
340
00:19:13,000 --> 00:19:17,040
this constructive force
is the clear victor.
341
00:19:17,040 --> 00:19:19,610
For the time being,
the dark matter has won.
342
00:19:19,610 --> 00:19:21,480
Galaxies continue to form.
343
00:19:21,480 --> 00:19:23,880
Clusters of galaxies
are getting bigger over time.
344
00:19:27,080 --> 00:19:30,080
But dark matter's success
in building up the universe
345
00:19:30,090 --> 00:19:33,420
sets in motion
its potential downfall.
346
00:19:37,830 --> 00:19:41,700
As the cosmic web evolves
into a more complex structure...
347
00:19:44,430 --> 00:19:48,970
...Gaps form between
the filaments, the cosmic voids.
348
00:19:50,740 --> 00:19:55,510
The cosmic voids formed because
other, more dense regions
349
00:19:55,510 --> 00:20:00,380
of the universe gravitationally
stole material away from them.
350
00:20:00,380 --> 00:20:02,420
So the dense parts
of the universe
351
00:20:02,420 --> 00:20:07,050
accumulated more matter at the
expense of the less dense parts,
352
00:20:07,060 --> 00:20:10,190
which then became voids.
353
00:20:10,190 --> 00:20:14,390
And lurking
in these voids, dark energy.
354
00:20:14,400 --> 00:20:16,000
Since the dawn of time,
355
00:20:16,000 --> 00:20:19,730
it's been waiting
for its opportunity.
356
00:20:19,740 --> 00:20:23,770
Now it's preparing an offensive
that may help it
357
00:20:23,770 --> 00:20:26,140
conquer the universe.
358
00:20:26,140 --> 00:20:27,840
In the very earliest times,
359
00:20:27,840 --> 00:20:29,480
the dark matter dominated
everything.
360
00:20:29,480 --> 00:20:32,810
It was the big brother pushing
the little brother around.
361
00:20:32,820 --> 00:20:34,250
But in the long run,
362
00:20:34,250 --> 00:20:37,020
the dark energy is going
to overpower dark matter,
363
00:20:37,020 --> 00:20:39,390
and so the relationship
is entirely flipped.
364
00:20:41,020 --> 00:20:43,890
We used to think
the fate of the cosmos itself
365
00:20:43,890 --> 00:20:45,290
depended on dark matter,
366
00:20:45,290 --> 00:20:47,890
and it turns out
that's not the case at all.
367
00:20:47,900 --> 00:20:51,930
The fate of the universe
depends entirely on dark energy.
368
00:20:51,930 --> 00:20:54,170
The long reign of dark matter
369
00:20:54,170 --> 00:20:56,170
may be coming to an end.
370
00:20:56,170 --> 00:20:58,910
Dark energy,
the great destroyer,
371
00:20:58,910 --> 00:21:01,910
is hoping to take control.
372
00:21:01,910 --> 00:21:06,350
This destructive force
has one overriding aim...
373
00:21:06,350 --> 00:21:09,550
To tear the universe apart.
374
00:21:25,100 --> 00:21:28,140
Our universe is at war,
375
00:21:28,140 --> 00:21:32,510
a relentless conflict
between dark forces.
376
00:21:32,510 --> 00:21:38,450
For the first 9 billion years,
dark energy is subjugated.
377
00:21:38,450 --> 00:21:42,720
Dark matter has the upper hand.
378
00:21:42,720 --> 00:21:45,050
When the universe
emerged from the big bang,
379
00:21:45,050 --> 00:21:47,650
the dark energy played no role.
380
00:21:47,660 --> 00:21:48,820
It was insignificant.
381
00:21:50,630 --> 00:21:53,160
But at some stage
in the 14 billion years
382
00:21:53,160 --> 00:21:58,330
since the big bang,
these roles became reversed.
383
00:21:58,330 --> 00:22:02,640
Dark energy came to be
the more powerful force.
384
00:22:02,640 --> 00:22:05,770
The question was, when?
385
00:22:05,770 --> 00:22:09,640
The answer came at the end
of the 20th century.
386
00:22:09,650 --> 00:22:12,850
So it was an amazing
breakthrough, really important.
387
00:22:14,080 --> 00:22:15,750
In 1999,
388
00:22:15,750 --> 00:22:18,920
scientists measure
the expansion of the universe.
389
00:22:20,420 --> 00:22:23,390
What they find shocks them.
390
00:22:23,390 --> 00:22:28,160
They expect the speed
of expansion to be decreasing.
391
00:22:28,160 --> 00:22:31,370
In fact,
it's actually increasing
392
00:22:31,370 --> 00:22:34,770
and getting faster all the time.
393
00:22:34,770 --> 00:22:38,170
The data indicate that for about
the first 9 billion years,
394
00:22:38,170 --> 00:22:39,610
it was slowing down.
395
00:22:39,610 --> 00:22:42,010
But then,
in the past 5 billion years,
396
00:22:42,010 --> 00:22:46,250
it started accelerating
faster and faster.
397
00:22:46,250 --> 00:22:49,050
Alex Filippenko
was part of the team
398
00:22:49,050 --> 00:22:52,520
that made this
explosive discovery.
399
00:22:52,520 --> 00:22:53,950
It befuddled us.
400
00:22:53,960 --> 00:22:56,990
This isn't how nature
was supposed to be behaving,
401
00:22:56,990 --> 00:22:59,060
and, in fact,
initially we thought
402
00:22:59,060 --> 00:23:00,460
that there was something wrong
403
00:23:00,460 --> 00:23:03,360
with either the observations
or the measurements.
404
00:23:03,370 --> 00:23:05,830
I didn't believe it
for the longest time.
405
00:23:05,830 --> 00:23:07,400
When the first data came out,
I'm like,
406
00:23:07,400 --> 00:23:09,470
"nah, I don't believe this,
no way."
407
00:23:09,470 --> 00:23:10,940
but it's in the data.
408
00:23:10,940 --> 00:23:12,740
It's there.
You can't escape it.
409
00:23:12,740 --> 00:23:16,480
This is as shocking as if you
held up a rock, let go of it,
410
00:23:16,480 --> 00:23:17,780
and it went up into the air.
411
00:23:20,820 --> 00:23:22,450
Five billion years ago,
412
00:23:22,450 --> 00:23:26,390
galaxies started moving apart
faster than before.
413
00:23:26,390 --> 00:23:29,490
The question is, why?
414
00:23:29,490 --> 00:23:32,790
What could be causing that?
Well, one thing is clear.
415
00:23:32,800 --> 00:23:35,530
It must be getting some
extra energy from somewhere.
416
00:23:35,530 --> 00:23:37,860
There is one main contender
417
00:23:37,870 --> 00:23:41,540
for what may be supplying
this extra energy,
418
00:23:41,540 --> 00:23:44,440
a force with repulsive gravity,
419
00:23:44,440 --> 00:23:47,840
a force
that pushes things apart.
420
00:23:47,840 --> 00:23:50,340
This is what astronomers
call dark energy.
421
00:23:50,350 --> 00:23:53,180
It's this mysterious
repulsive force
422
00:23:53,180 --> 00:23:55,180
that we know exists
in the universe,
423
00:23:55,180 --> 00:23:58,220
and we have no idea what it is.
424
00:23:58,220 --> 00:24:01,820
Physicists may not
agree on what dark energy is,
425
00:24:01,820 --> 00:24:03,260
but there is a consensus
426
00:24:03,260 --> 00:24:08,330
on where this repulsive force
has the most influence...
427
00:24:08,330 --> 00:24:13,200
In the regions between galaxies
and galaxy clusters,
428
00:24:13,200 --> 00:24:15,200
the cosmic voids.
429
00:24:15,200 --> 00:24:19,710
They're actually filled
to the brim with dark energy.
430
00:24:19,710 --> 00:24:21,410
The first time dark energy
is really going to make
431
00:24:21,410 --> 00:24:24,180
its mark in the universe
is going to be the time
432
00:24:24,180 --> 00:24:26,880
when the first cosmic voids
begin to appear.
433
00:24:28,950 --> 00:24:33,250
We see dark energy's effects
throughout the universe.
434
00:24:33,260 --> 00:24:36,420
But when we look into
the cosmic voids,
435
00:24:36,430 --> 00:24:39,590
which are the most empty regions
of our universe,
436
00:24:39,600 --> 00:24:42,430
this is where dark energy
is strongest.
437
00:24:45,270 --> 00:24:47,530
Dark energy
is the repulsive force
438
00:24:47,540 --> 00:24:49,300
pushing things apart.
439
00:24:49,300 --> 00:24:52,710
It prefers the voids
where gravity is weak.
440
00:24:54,580 --> 00:24:57,010
These are areas where
there's a lot less dark matter,
441
00:24:57,010 --> 00:24:59,410
and because the overall density
is low,
442
00:24:59,410 --> 00:25:01,410
that's where the dark energy
starts to peek out
443
00:25:01,420 --> 00:25:04,220
and can really drive
those voids to expand.
444
00:25:04,220 --> 00:25:07,450
So the expansion and
acceleration of the universe
445
00:25:07,460 --> 00:25:10,190
are driven by the dark energy
in those regions.
446
00:25:11,630 --> 00:25:14,900
Dark energy pushes thing apart,
447
00:25:14,900 --> 00:25:17,800
things that get in its way,
448
00:25:17,800 --> 00:25:21,600
things like the cosmic web.
449
00:25:21,600 --> 00:25:26,970
Dark matter and normal matter
are also in its path
450
00:25:26,980 --> 00:25:30,610
and are bulldozed out
across the cosmos.
451
00:25:30,610 --> 00:25:34,880
Slowly, but surely, the balance
between dark energy
452
00:25:34,880 --> 00:25:37,380
and dark matter is changing.
453
00:25:39,920 --> 00:25:41,690
Imagine you have
a giant swimming pool,
454
00:25:41,690 --> 00:25:44,260
and at the very bottom
there's a puddle of water
455
00:25:44,260 --> 00:25:45,490
with a splash of whiskey.
456
00:25:45,490 --> 00:25:47,330
So you have sort of a strong
whiskey drink
457
00:25:47,330 --> 00:25:49,460
down at the bottom of your pool.
458
00:25:49,460 --> 00:25:52,970
But now you start dumping water
into your pool, no more whiskey,
459
00:25:52,970 --> 00:25:54,970
and it begins to get diluted
and diluted,
460
00:25:54,970 --> 00:25:57,840
and eventually, you just have
a swimming pool full of water,
461
00:25:57,840 --> 00:25:59,670
with one shot of whiskey
mixed in.
462
00:25:59,680 --> 00:26:01,170
That's not a very strong drink.
463
00:26:01,180 --> 00:26:03,310
It's basically a water
swimming pool.
464
00:26:03,310 --> 00:26:05,580
That's pretty much happening
with the dark energy.
465
00:26:05,580 --> 00:26:07,210
At first,
it's a one-to-one mixture
466
00:26:07,220 --> 00:26:08,920
of dark matter and dark energy.
467
00:26:08,920 --> 00:26:11,350
But in the long run,
it's all dark energy
468
00:26:11,350 --> 00:26:13,390
and pretty much
no dark matter left over.
469
00:26:15,490 --> 00:26:17,990
The forces of dark energy
470
00:26:17,990 --> 00:26:20,530
are on an unstoppable March,
471
00:26:20,530 --> 00:26:22,530
picking up more and more power
472
00:26:22,530 --> 00:26:26,200
from the vast scale
of the cosmic voids.
473
00:26:26,200 --> 00:26:28,970
Dark energy is intrinsically
very weak.
474
00:26:28,970 --> 00:26:30,870
There's very little dark energy
475
00:26:30,870 --> 00:26:34,840
and this repulsive effect
in every cubic centimeter.
476
00:26:34,840 --> 00:26:36,610
But the universe is vast.
477
00:26:36,610 --> 00:26:38,850
Space is big.
478
00:26:38,850 --> 00:26:42,650
So cumulatively, all this
small amount of stuff
479
00:26:42,650 --> 00:26:45,220
adds up to a very large amount,
480
00:26:45,220 --> 00:26:48,890
and over a scale encompassing
the entire universe,
481
00:26:48,890 --> 00:26:51,460
the dark energy dominates.
482
00:26:53,330 --> 00:26:55,930
But it's been
a very long process.
483
00:26:55,930 --> 00:26:57,630
After the big bang,
484
00:26:57,630 --> 00:27:01,330
dark matter dominates
for the first 9 billion years.
485
00:27:03,570 --> 00:27:06,040
Then, 5 billion years ago,
486
00:27:06,040 --> 00:27:08,980
dark energy starts
to get the upper hand.
487
00:27:08,980 --> 00:27:12,750
It causes the expansion
of the universe to accelerate
488
00:27:12,750 --> 00:27:16,480
and the space in the voids
to grow more rapidly.
489
00:27:16,490 --> 00:27:20,420
As the space expands, there's
more and more dark energy
490
00:27:20,420 --> 00:27:22,190
because you have a bigger space.
491
00:27:22,190 --> 00:27:26,760
It sort of creates itself
with the expansion of space.
492
00:27:26,760 --> 00:27:29,400
Dark energy has
a sneaky way of taking over
493
00:27:29,400 --> 00:27:30,930
because it causes the space
494
00:27:30,930 --> 00:27:32,630
to stretch out
and get twice as big.
495
00:27:32,630 --> 00:27:36,170
So now there's twice
as much dark energy.
496
00:27:36,170 --> 00:27:39,640
Dark energy
just can't stop pushing,
497
00:27:39,640 --> 00:27:42,410
causing the empty space
of the voids
498
00:27:42,410 --> 00:27:44,210
to continuously expand.
499
00:27:47,150 --> 00:27:50,720
As the universe expands
because of dark energy,
500
00:27:50,720 --> 00:27:53,190
more and more dark energy
is being created.
501
00:27:53,190 --> 00:27:54,750
Dark energy is
definitely gaining
502
00:27:54,760 --> 00:27:56,520
the upper hand on dark matter.
503
00:27:56,530 --> 00:28:01,560
It was always there, but it took
over compared to other stuff.
504
00:28:03,100 --> 00:28:06,470
Eventually creating
enough energy to supercharge
505
00:28:06,470 --> 00:28:08,570
the expansion of the universe.
506
00:28:10,940 --> 00:28:13,040
This acceleration continues.
507
00:28:13,040 --> 00:28:15,910
The universe is getting bigger
and bigger,
508
00:28:15,910 --> 00:28:19,450
and it's all powered
by the forces of repulsion,
509
00:28:19,450 --> 00:28:20,850
dark energy.
510
00:28:20,850 --> 00:28:25,850
And for the universe,
that could be very bad news.
511
00:28:25,850 --> 00:28:30,190
If that's the case, dark energy
may destroy the universe.
512
00:28:30,190 --> 00:28:31,660
It will get stronger
and stronger
513
00:28:31,660 --> 00:28:34,890
until it literally rips apart
the fabric of space-time.
514
00:28:49,780 --> 00:28:51,710
Dark matter and dark energy
515
00:28:51,710 --> 00:28:56,450
have been battling each other
for 13.8 billion years.
516
00:28:56,450 --> 00:29:01,190
For the first 9 billion years,
dark matter dominates.
517
00:29:01,190 --> 00:29:04,790
Dark matter exerts
positive gravity,
518
00:29:04,790 --> 00:29:09,930
but pulling everything together
leads to one inevitable outcome.
519
00:29:09,930 --> 00:29:14,130
If the universe was totally
dominated by matter,
520
00:29:14,140 --> 00:29:16,800
eventually our expansion
would slow down,
521
00:29:16,810 --> 00:29:20,040
glide to a stop,
and then turn around
522
00:29:20,040 --> 00:29:24,040
and collapse into
a small, dense state
523
00:29:24,050 --> 00:29:29,120
from where it came from,
an event we call the big crunch.
524
00:29:29,120 --> 00:29:30,880
During the big crunch,
525
00:29:30,890 --> 00:29:34,620
gravity would play havoc
with the cosmos.
526
00:29:34,620 --> 00:29:38,390
Galaxies
would be dragged together.
527
00:29:38,390 --> 00:29:42,500
Stars and planets
would smash into each other.
528
00:29:42,500 --> 00:29:46,230
The universe would collapse
in a blazing inferno
529
00:29:46,230 --> 00:29:48,900
of superdense matter and energy.
530
00:29:52,940 --> 00:29:56,010
Fortunately, none of this
will probably happen.
531
00:29:59,080 --> 00:30:03,250
Scientists have now dismissed
the possibility of a big crunch.
532
00:30:06,350 --> 00:30:08,760
We don't face that,
because we have a universe
533
00:30:08,760 --> 00:30:11,120
filled with dark energy.
534
00:30:11,130 --> 00:30:14,160
Dark energy is causing the
universe to do something else,
535
00:30:14,160 --> 00:30:16,930
something it would prefer
not to do.
536
00:30:16,930 --> 00:30:20,770
It is accelerating the expansion
of the universe.
537
00:30:20,770 --> 00:30:23,300
As dark energy gets stronger,
538
00:30:23,310 --> 00:30:26,610
it supercharges this expansion.
539
00:30:26,610 --> 00:30:32,410
The presence of dark energy
is like a high-octane additive
540
00:30:32,410 --> 00:30:34,210
into a gas tank,
541
00:30:34,220 --> 00:30:37,850
where a car isn't
just coasting along.
542
00:30:37,850 --> 00:30:40,150
It's boosting along,
543
00:30:40,160 --> 00:30:42,990
and that's what's happening
with our universe.
544
00:30:42,990 --> 00:30:45,960
If that's correct,
this supercharged expansion
545
00:30:45,960 --> 00:30:50,000
will continue
for the next few billion years.
546
00:30:50,000 --> 00:30:54,430
The repulsive force of dark
energy will become invincible,
547
00:30:54,440 --> 00:31:00,140
and such unbridled power
will come with a high cost.
548
00:31:00,140 --> 00:31:03,640
I know of three ultimate
doomsday weapons.
549
00:31:03,650 --> 00:31:07,410
One is Galactus"
ultimate nullifier.
550
00:31:07,420 --> 00:31:11,280
Another is the infinity stones
in the hands of Thanos.
551
00:31:11,290 --> 00:31:13,150
But those two are make-believe.
552
00:31:13,150 --> 00:31:15,290
The real one is dark energy.
553
00:31:15,290 --> 00:31:18,660
It's the real ultimate
universe destroyer.
554
00:31:20,730 --> 00:31:23,760
The universe may pay
the ultimate price
555
00:31:23,770 --> 00:31:28,370
for this ever-increasing
expansion driven by dark energy.
556
00:31:28,370 --> 00:31:29,870
And if that continues forever,
557
00:31:29,870 --> 00:31:33,070
the future of the universe
is very, very simple.
558
00:31:33,070 --> 00:31:35,510
It continues to expand,
to accelerate.
559
00:31:35,510 --> 00:31:37,410
Everything moves apart
from everything else,
560
00:31:37,410 --> 00:31:40,510
and the universe becomes empty.
561
00:31:40,520 --> 00:31:42,350
There'll be nothing left
562
00:31:42,350 --> 00:31:46,020
but cold, desolate,
empty space forever.
563
00:31:47,690 --> 00:31:49,590
Dark energy will finally win
564
00:31:49,590 --> 00:31:52,260
the long war with dark matter.
565
00:31:52,260 --> 00:31:55,190
But the result won't be pretty.
566
00:31:55,200 --> 00:32:00,700
This future universe
will be cold, dark, and empty.
567
00:32:00,700 --> 00:32:02,500
Eventually it's going
to get so cold
568
00:32:02,500 --> 00:32:05,040
that really nothing
can happen in it.
569
00:32:05,040 --> 00:32:06,710
It's the big chill.
570
00:32:06,710 --> 00:32:10,180
It gets colder and colder
and darker and darker.
571
00:32:10,180 --> 00:32:13,550
Everything will fade out,
and though it began with a bang,
572
00:32:13,550 --> 00:32:15,010
it's going to die in a whimper.
573
00:32:15,020 --> 00:32:19,050
The universe flatlines
because of dark energy.
574
00:32:19,050 --> 00:32:22,560
The universe dies
in a big chill.
575
00:32:22,560 --> 00:32:24,860
Galaxies are so far apart,
576
00:32:24,860 --> 00:32:29,130
they're distant islands
in a sea of darkness.
577
00:32:29,130 --> 00:32:32,300
Gradually,
the galaxies dies, too.
578
00:32:32,300 --> 00:32:36,970
Star birth stops,
and the universe fades away.
579
00:32:39,010 --> 00:32:40,770
Or maybe not.
580
00:32:40,780 --> 00:32:45,450
There's another,
far more violent scenario.
581
00:32:45,450 --> 00:32:50,780
Here, dark energy just goes
from strength to strength.
582
00:32:50,790 --> 00:32:54,720
It could be that dark energy
is so strong
583
00:32:54,720 --> 00:32:58,020
that it will multiply
upon itself
584
00:32:58,030 --> 00:33:00,390
as the universe gets bigger.
585
00:33:00,400 --> 00:33:05,060
This is a process that
we call phantom dark energy.
586
00:33:05,070 --> 00:33:09,770
Phantom energy is
dark energy on steroids.
587
00:33:09,770 --> 00:33:13,640
It multiplies uncontrollably
in the voids,
588
00:33:13,640 --> 00:33:16,040
tearing at the fabric
of the universe
589
00:33:16,040 --> 00:33:20,610
in a process called the big rip.
590
00:33:20,620 --> 00:33:24,950
Dark energy is weird enough,
but imagine the possibility
591
00:33:24,950 --> 00:33:28,420
that there is more and more
of it as time goes on,
592
00:33:28,420 --> 00:33:32,530
and it's called phantom energy,
and in that case,
593
00:33:32,530 --> 00:33:35,700
it would rip everything apart,
even black holes.
594
00:33:39,900 --> 00:33:42,600
It will start to rip apart
galaxies themselves...
595
00:33:45,670 --> 00:33:47,710
...Rip apart solar systems...
596
00:33:51,580 --> 00:33:56,680
...Rip apart people,
rip apart atoms...
597
00:33:56,680 --> 00:33:57,880
Rip apart nuclei.
598
00:34:01,160 --> 00:34:03,790
Until finally,
space itself is pulled apart.
599
00:34:05,060 --> 00:34:08,390
The universe, as we know
it, will be destroyed,
600
00:34:08,400 --> 00:34:11,800
but the big rip may not be
the end of everything.
601
00:34:15,840 --> 00:34:20,740
There will be no normal matter
and no dark matter.
602
00:34:20,740 --> 00:34:22,840
And with nothing
left to conquer,
603
00:34:22,840 --> 00:34:27,480
phantom energy may use its
powers to become a creator,
604
00:34:27,480 --> 00:34:29,250
triggering a rebirth.
605
00:34:30,990 --> 00:34:35,420
Eventually, when you get to this
ultimate stage of emptiness,
606
00:34:35,420 --> 00:34:36,890
because of the phantom energy,
607
00:34:36,890 --> 00:34:38,690
we're actually able to turn
the universe around
608
00:34:38,690 --> 00:34:41,190
and get it to collapse again,
609
00:34:41,200 --> 00:34:44,130
and then go through
a series of bounces.
610
00:34:44,130 --> 00:34:45,800
So we call it
the phantom bounce.
611
00:34:47,370 --> 00:34:49,240
With this phantom bounce,
612
00:34:49,240 --> 00:34:51,740
universal life energy left
in this dead universe
613
00:34:51,740 --> 00:34:54,370
starts to collapse.
614
00:34:54,380 --> 00:34:56,440
And eventually
it becomes hotter and hotter
615
00:34:56,440 --> 00:34:59,480
and denser and denser,
and then the fiery inferno
616
00:34:59,480 --> 00:35:02,950
eventually pushes you back out
into another big bang,
617
00:35:02,950 --> 00:35:05,490
and this just keeps
going on indefinitely.
618
00:35:07,160 --> 00:35:10,490
So the destructive
and repulsive dark energy
619
00:35:10,490 --> 00:35:16,230
spawns a force that becomes
the ultimate universe recycler.
620
00:35:16,230 --> 00:35:18,600
The end state of our universe
621
00:35:18,600 --> 00:35:20,870
would lead you back
into another cycle,
622
00:35:20,870 --> 00:35:22,970
a whole new big bang
from the beginning.
623
00:35:28,140 --> 00:35:31,980
In the end,
dark energy may kill the cosmos,
624
00:35:31,980 --> 00:35:35,210
or it may create a new one.
625
00:35:35,220 --> 00:35:36,820
Dark energy is mysterious.
626
00:35:36,820 --> 00:35:38,350
Dark energy is unknown.
627
00:35:38,350 --> 00:35:41,720
Dark energy is going to do
whatever it feels like.
628
00:35:41,720 --> 00:35:43,720
Maybe dark energy will go away.
629
00:35:43,730 --> 00:35:45,830
Maybe dark energy will decay
630
00:35:45,830 --> 00:35:48,930
and become a flood
of new matter and radiation.
631
00:35:48,930 --> 00:35:51,800
Maybe dark matter
will get stronger.
632
00:35:51,800 --> 00:35:52,930
We don't know.
633
00:35:56,270 --> 00:35:58,840
For now, we think dark energy
634
00:35:58,840 --> 00:36:02,180
will determine the fate
of the universe.
635
00:36:02,180 --> 00:36:05,950
But all of our evidence
is speculative.
636
00:36:05,950 --> 00:36:07,810
What if we have it all wrong?
637
00:36:07,820 --> 00:36:11,620
What if there is
no dark universe at all?
638
00:36:29,840 --> 00:36:31,900
A large part of
our understanding
639
00:36:31,910 --> 00:36:33,510
of the universe's past,
640
00:36:33,510 --> 00:36:37,240
present, and future is based
on educated guesswork
641
00:36:37,250 --> 00:36:39,780
about two invisible forces...
642
00:36:42,080 --> 00:36:45,690
...Dark matter and dark energy.
643
00:36:45,690 --> 00:36:49,090
But it's pure speculation.
644
00:36:49,090 --> 00:36:53,590
Perhaps dark matter
and dark energy don't exist.
645
00:36:53,600 --> 00:36:56,300
There's not new stuff
in the universe.
646
00:36:56,300 --> 00:36:57,530
Anything is possible.
647
00:36:57,530 --> 00:36:58,970
Dark energy, in particular,
648
00:36:58,970 --> 00:37:01,670
might not be real,
so maybe there's something else
649
00:37:01,670 --> 00:37:03,370
that could be pushing
the universe apart.
650
00:37:03,370 --> 00:37:05,040
So we could absolutely be wrong
651
00:37:05,040 --> 00:37:07,270
about dark energy
and dark matter.
652
00:37:07,280 --> 00:37:08,980
Maybe they don't exist.
653
00:37:08,980 --> 00:37:11,980
Maybe tomorrow we'll discover
that our understanding
654
00:37:11,980 --> 00:37:14,710
was wrong all along.
655
00:37:14,720 --> 00:37:16,750
That's an awful lot of maybes.
656
00:37:16,750 --> 00:37:19,050
Let's add one more.
657
00:37:19,050 --> 00:37:21,650
When it comes to finding
answers,
658
00:37:21,660 --> 00:37:24,260
maybe we're looking
in the wrong place.
659
00:37:26,360 --> 00:37:29,730
One possibility is
that there are other universes
660
00:37:29,730 --> 00:37:34,500
out there pulling outward,
so to speak, on our universe.
661
00:37:34,500 --> 00:37:35,900
That might be the answer.
662
00:37:35,900 --> 00:37:38,570
But most theoretical physicists
and astrophysicists
663
00:37:38,570 --> 00:37:41,640
these days think
that dark energy is real
664
00:37:41,640 --> 00:37:44,740
because that seems to be
the simplest explanation
665
00:37:44,750 --> 00:37:47,580
for a wide variety
of observations.
666
00:37:49,120 --> 00:37:52,750
No one really knows
what dark energy is made of.
667
00:37:52,750 --> 00:37:56,290
Maybe the answer lies
in the past.
668
00:37:56,290 --> 00:37:58,520
The best theory for dark energy
we have right now
669
00:37:58,530 --> 00:38:00,830
is the simplest one
and the oldest one,
670
00:38:00,830 --> 00:38:03,260
and that's the idea that
it's a cosmological constant.
671
00:38:05,170 --> 00:38:07,400
Albert Einstein
came up with the idea
672
00:38:07,400 --> 00:38:12,310
of a cosmological constant
in 1917.
673
00:38:12,310 --> 00:38:16,180
He suggested that space
has its own energy,
674
00:38:16,180 --> 00:38:19,410
energy that can affect
the way the universe expands.
675
00:38:21,880 --> 00:38:24,950
When Edwin Hubble proved
the universe is expanding,
676
00:38:24,950 --> 00:38:27,950
Einstein thought
the cosmological constant
677
00:38:27,960 --> 00:38:30,090
was his biggest blunder.
678
00:38:30,090 --> 00:38:33,890
But observations that the
expansion of the universe
679
00:38:33,900 --> 00:38:35,430
is accelerating
680
00:38:35,430 --> 00:38:38,960
reveal Einstein
was right all along.
681
00:38:38,970 --> 00:38:42,940
Well, here we are.
We've reintroduced the idea.
682
00:38:42,940 --> 00:38:46,770
So Einstein's biggest blunder
may have actually
683
00:38:46,770 --> 00:38:49,310
conceptually been
his greatest triumph.
684
00:38:52,850 --> 00:38:56,220
But to understand the true
nature of the dark universe,
685
00:38:56,220 --> 00:39:00,550
we may need to re-evaluate what
we think we know about gravity.
686
00:39:02,820 --> 00:39:05,560
When we're trying to understand
dark energy and dark matter,
687
00:39:05,560 --> 00:39:09,400
there's a chance that just our
fundamental theories of gravity
688
00:39:09,400 --> 00:39:13,800
are wrong, that general
relativity isn't quite right.
689
00:39:13,800 --> 00:39:16,170
Einstein's theory
of general relativity
690
00:39:16,170 --> 00:39:18,470
explains how gravity works,
691
00:39:18,470 --> 00:39:21,140
how stars orbit in galaxies,
692
00:39:21,140 --> 00:39:23,610
and planets orbit stars.
693
00:39:25,480 --> 00:39:28,410
Some scientists wonder
if altering this theory
694
00:39:28,420 --> 00:39:32,690
will help us understand
the dark universe.
695
00:39:32,690 --> 00:39:34,720
So you need not
absolutely believe
696
00:39:34,720 --> 00:39:37,920
that there is something
actually called dark matter.
697
00:39:37,930 --> 00:39:39,890
You need to only understand
that there is something
698
00:39:39,890 --> 00:39:42,760
in the universe
which behaves like dark matter.
699
00:39:42,760 --> 00:39:45,730
For example, you could
effectively mimic the behavior
700
00:39:45,730 --> 00:39:50,000
of dark matter by modifying
our current theory of gravity.
701
00:39:50,000 --> 00:39:51,940
But successfully modifying
702
00:39:51,940 --> 00:39:56,510
Einstein's theories on gravity
is a big challenge.
703
00:39:56,510 --> 00:39:58,810
Einstein's equations
are very robust.
704
00:39:58,810 --> 00:40:02,420
You don't fluff around
with Einstein with impunity.
705
00:40:05,120 --> 00:40:07,390
For decades,
theoretical physicists
706
00:40:07,390 --> 00:40:10,520
have toyed
with Einstein's equations,
707
00:40:10,530 --> 00:40:14,760
looking for ways to explain
dark matter and dark energy,
708
00:40:14,760 --> 00:40:17,200
or make them go away.
709
00:40:17,200 --> 00:40:19,670
As yet, no one has managed.
710
00:40:19,670 --> 00:40:22,200
The dark universe persists.
711
00:40:23,740 --> 00:40:26,170
I think that the best
description of the observations
712
00:40:26,170 --> 00:40:29,380
we have today
is that dark matter exists.
713
00:40:29,380 --> 00:40:31,780
It's out there,
as well as dark energy.
714
00:40:31,780 --> 00:40:33,750
I think dark energy exists.
715
00:40:33,750 --> 00:40:35,380
I think dark energy is real,
716
00:40:35,380 --> 00:40:38,450
but I must admit that sometimes,
at 3 o'clock in the morning,
717
00:40:38,450 --> 00:40:41,790
I wake up screaming,
worried that, in fact,
718
00:40:41,790 --> 00:40:44,760
we've settled
on the wrong answer,
719
00:40:44,760 --> 00:40:46,830
and that in
a couple hundred years,
720
00:40:46,830 --> 00:40:48,760
they're going to be
laughing at us.
721
00:40:51,700 --> 00:40:54,900
Until then, our
observations tell us the battle
722
00:40:54,900 --> 00:40:59,540
between dark matter and dark
energy has shaped the universe.
723
00:41:02,940 --> 00:41:06,350
Dark matter dictated the past,
724
00:41:06,350 --> 00:41:09,650
built the galaxies,
the stars, and the planets.
725
00:41:11,520 --> 00:41:14,620
Dark energy will determine
its future,
726
00:41:14,620 --> 00:41:18,060
potentially tearing
the universe apart.
727
00:41:18,060 --> 00:41:21,530
Just because we can't see dark
matter and dark energy directly
728
00:41:21,530 --> 00:41:24,360
doesn't mean they have not
had a profound effect
729
00:41:24,370 --> 00:41:26,700
on the evolution
of the entire universe.
730
00:41:26,700 --> 00:41:30,040
The dark universe was there at
the beginning of the universe,
731
00:41:30,040 --> 00:41:32,510
shaping it, and actually
creating the conditions
732
00:41:32,510 --> 00:41:36,180
for us to be here, and it's
taken over the universe.
733
00:41:36,180 --> 00:41:39,550
And, eventually, it may well
destroy the universe.
734
00:41:41,180 --> 00:41:43,020
Our universe may be dominated
735
00:41:43,020 --> 00:41:47,490
by the long struggle between
dark matter and dark energy.
736
00:41:47,490 --> 00:41:51,360
But all of this conflict
has led to a creative outcome,
737
00:41:51,360 --> 00:41:55,690
an outcome for which
we should all be grateful.
738
00:41:55,700 --> 00:41:59,870
The name dark matter suggests
that it's something nefarious
739
00:41:59,870 --> 00:42:02,270
and somehow bad for us,
but actually it's turned out
740
00:42:02,270 --> 00:42:04,840
that dark matter
is very much our friend.
741
00:42:04,840 --> 00:42:07,410
Because if it weren't
for the dark matter,
742
00:42:07,410 --> 00:42:08,710
we wouldn't be here.
743
00:42:09,910 --> 00:42:11,240
There's a wonderful irony
744
00:42:11,250 --> 00:42:13,110
to calling it the dark universe,
745
00:42:13,110 --> 00:42:15,010
because now we're actually
beginning to shed light
746
00:42:15,020 --> 00:42:16,520
on how the universe began,
747
00:42:16,520 --> 00:42:18,950
how the largest structures
in the universe evolved.
748
00:42:18,950 --> 00:42:20,820
We wouldn't be here
without this dark universe.
749
00:42:20,820 --> 00:42:22,320
It's not dark at all.
750
00:42:22,320 --> 00:42:24,160
It's shedding light
on our own reality.
59926
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