NARRATOR: For
thousands of years, we have looked at the
night sky and believed that the illuminated stuff was
all that made up our universe. Scientists now realize it's
not what shines in the light but what hides in
the dark that holds the true secrets of our sky. There is a mysterious
dark matter that binds stars and
galaxies together and strange particles like
WIMPs, axions, and MACHOs might be to blame. And there is a dark,
repulsive energy that is creating
space in the universe but driving the galaxies
further and further apart to a dismal fate. Combined, dark matter
and dark energy make up 96% of the universe. And uncovering their
secrets is like making the one-in-a-million shot. If uncovered, the ultimate
fate of the universe might be revealed. Will it crash and burn
in a horrific collision of gravitational forces? Or will dark energy
tear the universe apart? The betting is the
universe will die in ice. Understanding these two
quantities, dark matter and dark energy, is really
fundamental to understanding the universe. NARRATOR: This is a trip to
the dark side of the universe. This is the hunt for dark
matter and dark energy. [music playing] Dark matter is
unlike anything we have ever encountered on Earth. Billions of these
strange particles pass through everything
they encounter each second. They are so massive in
weight, they have the power to influence the galaxies-- how they form and
how fast they spin. Dark matter's invisible presence
is everywhere or so it seems. Science has not directly proven
dark matter particles exist. There are many suspects
but no answers. And observing something
you can't see isn't easy. It doesn't emit light,
and it doesn't absorb light. It doesn't interact
with light at all. Not only does it not
shine, you can't easily see it in obscuration. NARRATOR: But the
evidence is there. Science knows it. Every textbook
on the planet Earth says that the universe
is made out of atoms and some subatomic particles. Well, all those
textbooks are wrong. NARRATOR: And they are going
underground to prove it. MICHIO KAKU: When they hear
about this invisible matter called dark matter,
they say, bah, humbug. Show me proof that
dark matter exists. NARRATOR: Soudan, Minnesota,
200 miles from any major city lights-- a perfect place for
hunting dark matter but not for obvious reasons. Well, you'd think
we'd want to look up in the sky for dark matter. That's where dark matter's
coming from after all. But instead, we're going
to don our helmets. And we're going to walk
down into this mine. NARRATOR: 2,431 feet underground
is the Soudan National Laboratory, an abandoned
iron mine reconfigured into a research facility. This is just one lab of
many around the world going underground to shield
experiments from cosmic rays. Each are racing to detect dark
matter, an invisible particle that has only been indirectly
observed but never captured. We've been at this
for now a decade, and we have yet to see
a dark matter particle. NARRATOR: The hunt
for dark matter started almost a century ago. Astronomers finally had
the tools to see deep into the night sky, and
the questions began. So it wasn't until the 1920s
that the technology developed well enough so that we could
take little, fuzzy patches that people had noticed
in their telescopes and resolve them and
figure out what they are. NARRATOR: Then Edwin Hubble
shocked the world and declared the universe was bigger
than just the Milky Way. People realized that some
of these little, fuzzy patches are separate galaxies just
like our Milky Way galaxy. NARRATOR: As astronomers
discovered new galaxies, Caltech professor
Fritz Zwicky looked up to the neighboring Coma
Cluster of galaxies and observed something strange. When he measured what the
motions were in the Coma Cluster of galaxies, he got
an estimate for how much mass there was in that cluster. Then he compared it to how
much mass you could actually see by looking at the galaxies. NARRATOR: Something
didn't add up. The galaxies were
moving too fast within the cluster for the
amount of illuminated stuff he could see. By his calculations, there
should have been 160 times more illuminated mass to account for
the random speeds of galaxies in the cluster. Something else was affecting
their motions, but what was it? He analyzed their motions
and concluded that the cluster could not be stable unless
there was a large amount of dark matter present. In 1933, he was one
of the first people to really grasp the significance
of the presence of dark matter. He called it missing matter. NARRATOR: Dark matter--
an invisible mass that was gravitationally
attractive and was able to affect the
speeds of entire galaxies in a cluster. Revolutionary thinking, yes. But the discovery
was largely ignored. I think that people
took Zwicky seriously, but they didn't jump
to any conclusions. This was a time when
the universe was just beginning to be explored. It was the 1920s when we first
realized there were galaxies outside our own. What we didn't know back then
was whether it was simply galaxies or stars or gas or
dust that we couldn't see or whether it was
something truly different. NARRATOR: Zwicky's
observations were based on measuring the mass
in the stars and galaxies. But how do you weigh
stuff in space? You can't go and put
the sun on a scale. It's a little bit hard. But what you can do is you can
measure how fast the planets are moving around the sun. And the more stuff
there is in the sun, the faster those planets have
to move to stay in their orbits. NARRATOR: Newton and Einstein
both said the more mass or stuff you have in an
object, the more gravitational pull it will have. And the further an object
is from the center, the slower it should
travel in orbit because the gravitational
pull is weaker. According to Einstein's
general theory of relativity or even according to Newtonian
gravity, all of the galaxies are pulling on each other. NARRATOR: It's like the sun's
influence on our solar system. The mass of the sun pulls
Mercury faster than Pluto because Mercury is
positioned closer to the sun. Likewise for a galaxy, you
would expect as you got further and further away, things are
moving more and more slowly to stay in their orbits. NARRATOR: But Zwicky
didn't observe that. Neither did a young
scientist named Vera Rubin 50 years later. She was observing the
rotational curves of galaxies similar to the Milky Way. Like Zwicky, her observations
also seemed strange. What Vera Rubin
actually measured was as you got further
and further away, the velocity of the orbiting
gas and dust remained constant. NARRATOR: What Rubin observed
was as if a city were a galaxy, and every car on the road
was a planet or star. And despite the
amount of traffic, every car traveled around
the city at the same speed. This same consistent
rotational speed despite the amount
of stuff or traffic was exactly what Rubin observed. The outer parts of the galaxy
were rotating fast enough that there must be
a lot more mass. Otherwise, the galaxy
would have flown apart. MICHIO KAKU: The only way
to resolve this paradox are galaxies which spin
10 times too fast is to assume that
there is a halo, a halo of invisible matter surrounding
the galaxy, keeping the galaxy whole. NARRATOR: Dark matter was
present in the galaxies. And it had enough mass to keep
the rotation speed constant. Imagine that I
am the dark matter. This ball is a star orbiting me
because my gravitational force is keeping it in place. But even if you
couldn't see me, you would know that there
must be something here. Otherwise, the star would just
zoom off in a straight line. There must be something
causing that gravity. And that's how we know that
there must be dark matter. NARRATOR: Rubin
estimated that there was 10 times more dark matter
than ordinary illuminated stuff. MICHIO KAKU: Since then, we've
analyzed hundreds of galaxies. And they all have
the same pattern. They all rotate too
fast for their own good. And they need dark matter
to hold them together. NARRATOR: This time science
paid attention and started to wonder, what is dark matter? How do you find something
that is invisible in space? They needed to see just where
dark matter was hiding out in the universe. And even if they
couldn't see it, science realized that
dark matter exposed itself by bending light that
passes through it. It's called
gravitational lensing. And it's a virtual
spotlight that uncovers any invisible
stuff in the universe. RICHARD ELLIS:
What it does do is it does what all matter
does in that it can deflect the light ray. So a light ray can be deflected
in its path by dark matter. NARRATOR: By tracing the
battered light's path, gravitational lensing detected
dark matter concentrated in the halos of galaxies. Gravitational lensing
proved to be infallible. And dark matter's presence
was suddenly revealed. RICHARD ELLIS: This technique
of gravitational lensing is the most precise because
we can actually pinpoint not just how much
dark matter there is but how it's distributed
in its position on the sky. And that's because we can
measure the distortion of the light rays passing
through the dark matter. How do you know that
your glasses are there? Because it bends light. In the same way, by looking
at Hubble space pictures of the universe and looking
at the distortion of light as it goes through
galaxies, we actually have maps of dark matter. Most of the mass of the
galaxy is from the dark matter. The ordinary matter accumulates
in the gravitational field of the dark matter. NARRATOR: But once dark
matter came on the scene, scientists wondered if it was a
new undetected particle or just invisible ordinary matter. RICHARD ELLIS: When
people found dark matter, everybody wanted to
know, well, what is it? You know. And of course, the
first answer is it's just the stuff that
makes up you and me, but it's not shining. NARRATOR: Scientists started
to investigate objects in the universe that
didn't emit light. Black holes were considered. They don't emit light, can
draw matter to themselves, and are detected with
gravitational lensing. SEAN CARROLL: It could take a
form of black holes or MACHOs, Massive Compact Halo Objects,
which are basically dark, small stars that don't
give off a lot of light. NARRATOR: MACHOs hide out
in the halo of the Milky Way and are detected by
gravitational lensing. But there weren't enough
to account for the amount of dark matter needed. Failed stars like brown
dwarfs were also suspected. They are massive enough to
make up dark matter's presence. Whatever dark matter is,
there is way more of it than the ordinary matter
of stars and planets-- 10 times more. SEAN CARROLL: All of the stuff
that you can construct from ordinary atoms, protons
and neutrons and electrons, cannot possibly be enough to
account for the total amount of matter that you see
in galaxies and clusters. NARRATOR: Scientists continued
to present new suspects as the search continued
for dark matter. Previously-discovered exotic
particles like neutrinos were reconsidered. Like dark matter, neutrinos
are passing through the Earth millions of particles at a time. But they are too light to
account for dark matter's effect on gravity. And scientists can
recreate neutrinos in particle colliders. They also come from the Sun. DAN BAUER: Axions is also
another possible dark matter candidate. They were invented to
explain a particular glitch in one of the particle
physics theories. They would be extremely light. So you search for them in
a completely different way than what they're doing. But there would be-- they
would also be very numerous. And so they could possibly
be the dark matter. NARRATOR: Axions are very
light and are believed to have been created at
the moment of the Big Bang just like dark matter. But theories suggest that
they could change to protons while dark matter is stable. After exhausting all
the usual suspects, many scientists
believe dark matter is a new, exotic particle
unlike anything on Earth. And billions are passing
through us every second. [music playing] Up until the discovery
of dark matter, scientists believed the universe
was made only of protons, neutrons, and electrons-- the stuff everything
on Earth is made of. RICHARD ELLIS: And we
know it has some mass. And we're left with something
that we have not yet detected. NARRATOR: But to be a perfect
dark matter candidate, it must have certain
physical properties. And none of the usual suspects
were fitting the crime. SEAN CARROLL: So we know
that the dark matter is some ponderous substance. We know that it's not
moving too quickly. And we know that
we can't see it. Dark matter particles
are not traveling at the speed of light. And they don't interact with you
and me or anything pretty well. And that's why it's been
so difficult to track down these particles. And it doesn't interact
with ordinary matter except through gravity. If I had some dark matter in
my hand, it would have weight. But first it would dissolve
right through my fingers. There aren't any candidates
for cold dark matter within what we call the standard
model of particle physics. NARRATOR: Like an invisible
man passing through walls, dark matter is passing through
Earth billions of particles at a time, never colliding
with ordinary matter. So the most popular idea for
what the dark matter could be is something called a WIMP. NARRATOR: WIMPs are Weakly
Interacting Massive Particles. They have not been detected,
but their characteristics match the perfect
dark matter candidate. At the Soudan laboratory,
the Fermilab team has gone underground,
braving thousands of bats to try and capture
a WIMP particle. DAN BAUER: This is called the
cryogenic dark matter search. CDMS is the acronym for it. This was an iron ore mine
until 1962 when it shut down. We're a half mile underground. NARRATOR: Fermilab has designed
a machine that at subzero temperatures can detect a
dark matter particle passing through it. And its sensor is made of
germanium, a dense metal jam packed with atoms. DAN BAUER: It's a very
pure block of germanium. It's got on the
surface of it a pattern of tiny, little
thermometers basically that are able to detect
when a particle passes through this hockey
puck-sized chunk of germanium. Dark matter is streaming
right through us right now without
doing anything. Very occasionally, it will bump
into the nucleus of an atom. And that's the signature
that we're hoping to see. NARRATOR: To get a
clean dark matter hit, Fermilab needed to filter
out junk from space. DAN BAUER: We would
get so many particles that it would be really trying
to sift a needle in a haystack. NARRATOR: Fermilab's experiment
picks up all matter that passes through this detector. The less junk in
the air, the easier it will be to detect a
dark matter particle. Because dark matter
doesn't interact easily with regular protons
and electrons, Fermilab has frozen the
germanium pucks to near subzero temperatures. DAN BAUER: If a dark matter
particle comes through and hits a nucleus, it will actually
change the temperature of the crystal very slightly. And so we're looking for that
tiny change of temperature in the crystal to signal that a
dark matter particle has passed by. NARRATOR: 16 germanium
pucks sit in a chamber inside a clean room. So we're suited up. We're about to go into
the experimental room. It's a class 10,000 clean room. That's why we're all suited up
so we don't carry in any dust because that would cause a
background for the experiment. So let's go inside. So we have ventilation
counters that are catching any cosmic ray
particles that get all the way down underground here. So right here is what
keeps our experiment cold, that tiny, little bit
above absolute zero. This is a dilution refrigerator. Way inside here are the
germanium and silicon detectors. So we're just waiting for a
WIMP, a dark matter particle, to get down to this depth and
hit one of those remaining silicon crystals that's buried
way inside all the shields. NARRATOR: Fermilab
has been visiting the mine for nearly
two years, trying to capture the dark matter. This is far harder
than it sounds. Although billions of particles
are passing through Earth at one time, it's a
one-in-a-million shot dark matter will interact
with ordinary matter. Getting a dark matter particle
to hit a germanium atom is like an archer trying to hit
a bullseye when the target is a mile away. DAN BAUER: All these green
lights indicate particles passing through the germanium
and silicon detectors that we saw downstairs. These are almost certainly
all background particles. But maybe it's buried in
there some place as a WIMP. But we won't know until
we analyze the data. NARRATOR: Hunting dark
matter is tedious. Each day, the Fermilab team
reports to the underground lab, analyzes data, and perfect
their ping pong game while waiting for the one hit
that will prove dark matter exists. But for all this
effort and waiting around, no dark matter has
been detected by Fermilab or by anyone else. DAN BAUER: Unfortunately, we've
seen precisely zero dark matter particles so far. MICHIO KAKU: Any day now,
we may have the announcement that physicists have captured
dark matter in a bottle. We have a hypothesis. It certainly seems to explain
the universe we live in. But the plain fact is
we haven't yet detected this cold dark matter particle. NARRATOR: Finding dark
matter will not only give us proof of its
existence but might also answer the other big
questions of space. SEAN CARROLL: Detecting dark
matter directly will give us a window into what was going
on 1/10,000 of a second after the Big Bang. NARRATOR: If scientists can
discover what dark matter is, they might also discover how
the universe behaved early in its life. RICHARD ELLIS: Dark matter is
not only a mysterious quantity in the universe. But also it's
fundamental to our-- you know, why
we're here in fact. It would be difficult to
form galaxies and hence the solar system and
hence life on Earth. dark matter and dark energy can't be told without going
back to the beginning of time to the moment of the Big
Bang when space didn't exist. There is no center
where you can point to. And that's exactly the
analogy for the Big Bang. There is no direction in the sky
from which all the galaxies are expanding. NARRATOR: From this moment of
nothing to a violent explosion, space was created. And the universe began
to grow from a seed. Particles formed in a nuclear
fusion of gas and energy. Ordinary matter was reacting
with other ordinary matter. The early universe was
in fact a nuclear reactor when it was one-minute old. NARRATOR: And 380,000 years
later, bits of particles began to cluster creating
the seeds from which stars and galaxies would later form. RICHARD ELLIS: Bigger lumps
grow to form yet bigger lumps. And so gravity is
slowly pulling force-- bringing objects together. NARRATOR: What
scientists now realize was that at the moment
of the Big Bang, dark matter was created. And it played a critical role
in helping ordinary matter clump together to form
stars and planets. Like steel girders used
on a building site, dark matter's
slow-moving particles acted like scaffolding upon
which ordinary matter could attach itself. We believe that
because it's cold and doesn't interact very much
that dark matter was pulled together by gravity
very slowly over time and actually formed the seeds
around which normal matter coalesced into galaxies. It's like a cosmic
web like a spider's web where there are
strands of dark matter and where these strands
intersect like a scaffolding pattern. So in a sense, the dark
matter is the framework. It's providing the scaffolding
for the shining galaxies that we can easily see. SEAN CARROLL: They are really
like the Christmas tree lights. They're not the
actual Christmas tree. They're the things that
are visible from very, very far away. But the reality of the galaxy
is a big halo, most of which you don't see. You see the shiny bits that
are stars and planets that have accumulated at the center
of that large halo, which is mostly dark matter. NARRATOR: Scientists have long
wondered why galaxies formed in seemingly random
patterns across space. Now scientists know it's
because of dark matter's gravitational pull. MICHIO KAKU: The universe is not
uniform at all but has voids. It has clumps. It seems to have
bubble-like regions. Well, we now believe
it's due to dark matter. NARRATOR: In the last
year, astronomers have been able to take their
theory one step further and create a detailed
3D map of dark matter in the universe using
gravitational lensing. SEAN CARROLL: And Einstein said
that gravity affects everything just like gravity is
caused by everything. So one of the things that
is affected by gravity is light itself. Because when light goes
through a dark matter, it bends just the
way light bends when it goes through glass. NARRATOR: And light
doesn't discriminate between ordinary
matter and dark. Both types of matter
batter light's path as it travels
through galaxies. Like plotting a course
on a map, astronomers have traced thousands
of light sources as they pass
through dark matter. It has given science the most
accurate picture yet of where dark matter hides in space. We can compare that
map of the dark matter with where the galaxies are. And lo and behold, we
find that the dark matter is acting as the skeleton. It is the backbone around
which the visible material is clumping. NARRATOR: By mapping
the universe, astronomers can also
look back in time and predict how much matter
was created at the Big Bang. SEAN CARROLL: So if you know how
lumpy the universe appears now and when it was half its
current size or its half its current size before that,
you can infer the total amount of stuff in the universe. It gives us another
very nice way of matching onto what we
believe is the total amount of dark matter. NARRATOR: It's
estimated dark matter makes up 23% of the universe
while ordinary matter makes up only 4%. You need a lot of dark matter
to account for the total amount of gravity that exists in
these clusters and galaxies. NARRATOR: But what makes up
the final 73% of the universe? Scientists were
shocked to discover a new mysterious dark
energy was dominating space. And its repulsive energy is
driving the galaxies apart. Science always assumed that
although the universe continues to grow in size,
it would eventually slow in its expansion or
perhaps even collapse on itself. Gravity would overcome
any momentum it had. But while measuring
the expansion history of the universe,
scientists were shocked to realize that the universe
wasn't slowing down. It was speeding up. And a grim fate awaited
any living thing. The universe
will disintegrate. And temperatures will become so
cold that any intelligent life will freeze to death. NARRATOR: From the moment the
Big Bang created the universe, space has been expanding
and never stopping, carrying galaxies along for the ride. SEAN CARROLL: The space
in between the galaxies is expanding. But galaxies are not expanding. The Earth is not expanding. The solar system
is not expanding. NARRATOR: Edwin Hubble first
discovered galaxies were moving away from the Milky Way in
1929 by realizing the more distant galaxies move
faster away from us than the nearby ones. He realized he could measure
their velocities by studying their wavelengths
through a prism. It's called measuring
the redshift and is still used to
measure distances in space. ALEX FILIPPENKO: And he found
that in fact, the greater the distance of a
galaxy right now, the greater the speed with
which it's moving away from us, that is, the
greater its redshift. NARRATOR: A few years
ago, astronomers decided to use redshift measurements
to measure the expansion history of the universe. But how do you measure the
entire expansion history of the universe? How do you travel back
12 billion years in time? RICHARD ELLIS: We
have the capability of going back in time
directly to observe the past. So much in the same way as
a geologist looks at layers in the Grand Canyon
and as he goes down to lower and lower layers
looking back in history. If you look at progressively
more distant galaxies, you're looking at them as they
were at progressively greater times in the past. NARRATOR: To measure
expansion history, scientists used
type Ia supernovas as their standard candle. ALEX FILIPPENKO: One
example of a standard candle might be a 100-watt light bulb. You could have a bunch of
these things sitting around in your room at different
distances from you. Then the more distant
ones will appear fainter. And the more nearby ones
will appear brighter. NARRATOR: Type Ia supernovas are
always consistently brilliant no matter where
they occur in space. ALEX FILIPPENKO: A supernova
is the colossal explosion of a star at the
end of its life. It just goes kabam. And it occurs when a dying
star known as a white dwarf goes through a nuclear runaway
and just literally blows itself to smithereens. We find the type Ia supernovae
in very distant galaxies. So they look really faint. And they-- then we compare
them with type 1a supernovae in nearby galaxies whose
distance were-- distances were measured using
Cepheid variables or some other technique. NARRATOR: Using these
type 1a supernovas, two different teams
set out in the 1990s to measure the deceleration
rate of the universe. But to capture
supernovas as they occur, astronomers had to put the
universe on surveillance. You can compare
this a little bit to perhaps surveying a casino. So all these cameras
are on all the time. And most of the time, they
don't find much of anything interesting. But occasionally, they find
what they're looking for. You have to look at
lots of galaxies. So what we did is we took large
telescopes with cameras that have wide fields of
view about as wide as say the width of the full moon. And we took many
snapshots of space using this wide-field camera. And each-- and each snapshot
contains tens of thousands of galaxies. And by comparing the apparent
brightness of the distant type Ias with those of the nearby
type Ias in nearby galaxies, we can get the distance
of the distant galaxies and hence the amount of
time that we're looking back in the history of the universe. NARRATOR: After the two teams
studied the results of 60 type 1a supernovas, scientists
were shocked at their results. The universe wasn't
slowing down. Its expansion was speeding up. We all expected that expansion
to be slowing down with time. Because after all,
all of the galaxies are pulling on one another. We were so confident
we were going to measure the rate at which the
universe was slowing down, and then we found of
course a negative answer. The universe is
not slowing down. It's speeding up. And this was just a big mystery. ALEX FILIPPENKO: That
is really, really weird. You know, we expected to
measure some amount of slowdown. And instead, it's
expanding more quickly. That's like the
wrong sign, right? We were really afraid that
we had gotten completely the wrong answer. We rechecked our measurements. We checked the analysis. A bunch of people on
each of the two teams did the measurements and
analysis independently and kept getting
the same result. MICHIO KAKU: One of the
greatest shocks in the world of cosmology just in
the last few years has been the realization that
our universe is accelerating. NARRATOR: This repulsive force
driving the universe was called dark energy, an invisible
energy that was nothing anyone expected or understood. ALEX FILIPPENKO: It suggests
that over the largest distances in the universe, there is a
repulsive effect that dominates over gravity. NARRATOR: And dark energy was
creating space, taking galaxies along for the ride. ALEX FILIPPENKO:
This energy that appears to fill the universe
and stretch the expansion of the universe faster
and faster with time is now known as--
as dark energy. So here I throw the apple. And initially,
it's decelerating, and then dark energy makes
it accelerate away from me. So you throw the apple. And it just zooms away
faster and faster with time. NARRATOR: Like the apple
forever traveling into space, galaxies are being carried
away as more space is created. [music playing] So if you could imagine,
you know, a classroom populated by chairs, and
those chairs are slowly getting further apart from one
another within a classroom. Because if all the chairs
are being stretched apart like an expanding universe, no
matter which chair you sit on, you'll find all chairs
are moving away from you. So the chairs are
not really expanding. In fact, the chairs are
the same size really. What's happening is that
the room is getting bigger. The space in between the chairs
is being stretched apart. More space is being created
in between the galaxies. So you have individual galaxies
remaining of constant size in a universe where all of space
is getting bigger and bigger. NARRATOR: Dark energy is very
different from dark matter. ALEX FILIPPENKO:
It doesn't clump up like galaxies do in clusters
or like stars do in galaxies. Instead, it appears
to be pretty uniform. And we find the same
amount of acceleration no matter which
direction we look at. RICHARD ELLIS: It's
probably smooth, although some people believe
there may be structure in its distribution
and its influence. Dark energy is the energy
of the vacuum, the energy of nothing. Even nothingness has energy. And it's pushing
the galaxies apart, creating a runaway universe. NARRATOR: It appears dark
energy and dark matter have been at war
with one another since the beginning of time. Science believes dark energy was
created along with dark matter at the moment of the Big Bang. It has always existed
in the universe. The gravitational
forces of dark matter kept it in check, slowing
down the expansion of space during the first nine
billion years of time. This changed five billion years
ago when the universe grew big enough so that dark
matter was dispersed throughout the universe, and
dark energy wasn't so affected by dark matter pull. As a result, the universe began
to expand at an accelerated rate. Dark energy is
a constant term. That was probably
very insignificant when the universe was hot
and dense in the beginning. So it didn't really matter
whether dark energy is there or not. It is there, but it just
plays no role at all. And then, as the universe gets
cooler and less dense, bigger, so gravity becomes
less important, and then dark energy takes over. It's a property of space that
we don't yet fully understand. NARRATOR: As the
universe expanded, astronomers realized dark
energy won its struggle with dark matter and started the
acceleration five billion years ago. ALEX FILIPPENKO: So there came
a time about 5 billion years ago when the dark energy
started dominating over the attractive matter
in the universe. So in a sense, if you plot
the force versus time, the gravitational attraction
is declining with time. The repulsion is
increasing with time. And about 5 billion years
ago, the two curves crossed. And that's when the
universe started accelerating in its expansion. RICHARD ELLIS: Dark energy
is fundamental to understand because it tells us where
the universe is going. What's the fate of the universe? Is it going to expand forever
and get cold and dark? Or is there some end in sight? NARRATOR: Dark energy now drives
the expansion of the universe. And it doesn't seem
to be stopping. The repulsive effect of
the dark energy increased. Because the more space
there is between galaxies, the greater is the cumulative
effect of the dark energy, the repulsive effect. NARRATOR: And
individual galaxies seem destined to a
lonely existence. MICHIO KAKU: So it looks as if
this is the end of everything. NARRATOR: Surprisingly,
the theory of dark energy was proposed and discarded
long ago from one of physic's greatest minds. He called it his
biggest blunder. His name was Einstein. And he might have been
onto the greatest discovery of the 21st century 80 years
before anyone had a clue. dg and expanding space. But in the early 20th
century, astronomers believed the universe was
only as big as the Milky Way and would never grow in size. But Einstein had just formed
his theory of relativity and decided to test it
on the static universe. But as hard as
Einstein tried, he could not balance his equation
to equal a static universe. His calculations showed a
universe that must either expand or contract. He realized that if you had
a universe that was smooth, that was uniformly
distributed with stuff, his theory unambiguously
predicted that it should either be expanding or contracting. NARRATOR: So Einstein proposed
a repulsive vacuum energy that would hold the
universe in balance with attractive gravity. He called it his
cosmological constant-- a constant energy that would
hold the universe in balance. MICHIO KAKU: He introduced the
cosmological constant or dark energy to hold the
universe static. NARRATOR: When Hubble
announced space was expanding, suddenly Einstein's cosmological
constant seemed irrelevant. And he labeled it
his biggest blunder. MICHIO KAKU: Now it turns out
that dark energy, the concept that he threw away back
to the 1920s, is in fact the dominant force blowing
the universe apart. Einstein's so-called
blunder will eventually determine whether or not the
universe dies in fire or ice. And the betting is the
universe will die in ice. NARRATOR: In trying to survey
how the universe behaves, Einstein had erroneously
predicted dark energy and what is the total makeup
of the universe. The total amount of stuff in
ordinary matter and dark matter is not enough to account
for the curvature of space that we observe. NARRATOR: Like looking
across a horizon on Earth, the size of the universe is so
great the curvature of space appears flat. SEAN CARROLL: That ordinary
matter, dark matter, and dark energy together,
that makes a prediction for the curvature of space. And that prediction
comes spot on. You get the right answer. MICHIO KAKU: Our satellite
data now has revealed the fact that 73% of the matter energy
content in the universe is dark energy. Dark energy, which was
once Einstein's blunder, is now known to be the
dominant force in the universe. His blunders are
our great discovery. NARRATOR: Scientists are at the
very beginning of understanding what effect dark
energy will have on the fate of the universe. Ideally, we'd like to measure
how dark energy is behaving as the universe ages. Eventually, when the dark
energy completely dominates over dark matter, the universe
will enter a stage known as exponential expansion. For every given unit of
time, it'll double in size. And unless the dark energy
changes sign some day and becomes
gravitationally attractive, the fate of the universe is to
expand forever more and more quickly with time. We don't understand if
the vacuum energy is driving the acceleration of the universe
why it has the amount it does. That is one of the
deepest puzzles remaining in theoretical physics today. Trillions of
years from now, it's going to be a very lonely place. We'll look up in the sky. And the skies will
be practically dark. The oceans will freeze over. And it looks as if this is the
death of all intelligent life. It looks as if dark energy
and the laws of physics are a death warrant to
all intelligent life in the universe. NARRATOR: In discovering
dark matter and dark energy, science is one step
closer to defining the theory of everything-- one equation that will
define the entire workings of the universe. MICHIO KAKU: Once we have
the theory of everything, we'll be able to answer some
of the deepest questions ever since man and women
first looked at the heavens. This could be the crowning
achievement of 2,000 years of investigation into
the laws of nature ever since the Greeks asked the
question, what are things made of? NARRATOR: For now
dark matter and dark energy continue to be the
greatest cosmological questions of the 21st century. DAN BAUER: It's certainly
frustrating, yes. I mean, it's humbling too
to know that, you know, all we know about physics is
restricted to normal matter. And yet there's all this other
dark matter and dark energy that we really understand
very little about. NARRATOR: It's the
beginning of a new era and the mysteries of the
dark side of the universe. DAN BAUER: It's the
wild west as far as particle astrophysics, which
is what we call this field. It's a property of space that
we don't yet fully understand.
