(gentle chiming music) - [Narrator] A lot has
changed in the last 50 years, especially in our
understanding of the cosmos. - The last 50 years, I think, have been the most
exciting ever. Our entire understanding
of the universe just turned upside down. - [Narrator] Welcome
to the dark universe, where powerful, yet invisible
dark matter holds cosmic sway, shaping even the very
galaxy we call home. - Dark matter is four times
or five times more abundant than the stuff we can see. It is fundamentally disturbing. (dramatic music) - [Narrator] But
we're on the hunt. - We're entering
discovery territory. - [Narrator] And the
darkness doesn't end there. A ghostly form of dark energy
dominates the universe, driving its expansion, maybe
even choosing its fate. - I don't think we
possibly could have grasped just how profound it was. - This is a huge discovery. - [Narrator] It
remains a total enigma. - We have absolutely no
idea what dark energy is. - [Narrator] The more we've
learned about our universe, the stranger it becomes. - This is something that
can't just be wished away. - That's where to look
for the next breakthrough. - It's potentially
Nobel Prize winning. - [Narrator] "Decoding the
Universe," right now on NOVA. (dramatic music) (dramatic music continues) ANNOUNCER:
As an American-based supplier to the construction industry, Carlisle is committed to
developing a diverse workplace that supports
our employees' advancement into the next generation
of leaders, from the manufacturing floor
to the front office. Learn more at Carlisle.com. ♪ ♪ NARRATOR:
September 5, 1977. ARCHIVAL:
Three, two, one... ...and we have lift-off! NARRATOR:
A Titan-Centaur rocket carrying the space probe,
Voyager 1, launches from Cape Canaveral. REPORTER:
The Voyager spacecraft
to extend man's senses farther into the
solar system than ever before. NARRATOR:
The Voyager program would become NASA's longest mission to date. KAISER:
I absolutely remember
the coverage. I remember watching the launch--
it was really exciting. I had no sense of the scale of where that thing was headed, to where no human-built things
had ever gone before. NARRATOR:
In 1979,
Voyager 1 flies past Jupiter. (applause) ♪ ♪ About a year-and-a-half later,
Saturn. ♪ ♪ Though it will
continue on its journey, in 1990,
it sends its last images. Looking back
at the solar system... ...Earth is just a tiny speck. In Carl Sagan's words,
"a pale, blue dot." KÖNIG:
The Pale Blue dot--
it's telling us, we're incredibly small. At the same time,
it also tells us that we've gone incredibly far. I mean in cosmology,
we're looking, we're looking at supernovae. We're looking at other galaxies. We're looking at
the beginning of the universe. But there's so much more
that needs to be done. NARRATOR:
During Voyager 1's
nearly 50-year-long mission, our scientific understanding
of the universe has grown immensely... ...and even radically changed. The last 50 years, I think, have been
the most exciting ever. Our entire understanding
of the universe has changed,
it's just turned upside down. Over the past 50 years, there's really been several revolutions in our understanding of
the universe. We can both be proud
of how far we've come, but also be excited
about how much we don't know. ♪ ♪ NARRATOR:
Looking back, the discoveries of the last
50 years are remarkable. In the 1970s,
Luke Skywalker's home, Tatooine, was as close as we got
to seeing an alien planet. In the 1970s,
we knew of precisely zero planets outside of our solar system. And in the decades since then, we've moved that
number up into the thousands. SUSAN MULLALLY:
We've discovered planets are
incredibly common in our galaxy. There are more planets out there than there are stars. So go out
and look at the night sky. For every one of those stars, there are probably several
planets. NARRATOR:
And in recent decades,
we've also learned... ...our Milky Way galaxy
isn't even one in a million. A fact made evident by this
astonishing series of images taken by the Hubble Space
telescope in 1995. Just about every single
bright point is a galaxy. Scientists now think
galaxies in the universe may number in the trillions. KÖNIG:
The Hubble pictures tell us that the universe
is the same everywhere. It's also a reminder
that we are definitely not the center of the universe. NARRATOR:
One of the most shocking
discoveries of the last 50 years, was made
in our own cosmic backyard. In 1998, astrophysicist
Andrea Ghez and her team
surprised the world when they revealed evidence
of a super massive black hole at the center of our own
Milky Way. GHEZ:
From watching stars orbit
the center of the galaxy, the mass that we infer is four million times
the mass of the sun. That is the proof
of a black hole. NARRATOR:
Astronomers now believe that just about every galaxy
has one. CLIFFORD JOHNSON:
Black holes are crucial to how
we understand galaxies. That is a huge
new role for black holes and our understanding of them. NARRATOR:
But perhaps the biggest
discovery of the last 50 years is just how much we don't know. Somehow we've missed much,
much more than we've discovered. There is some new form
of matter out in the universe, and it is four times or five times more abundant than the stuff we can see. And so that is the
shocking truth of dark matter. NARRATOR:
Dark matter-- only accepted as "real" during
the last 50 years, and now,
one of the biggest mysteries in the history of science. So far, its effects
have only been detected on the cosmic stage. But understanding it, may hold the key to the
very structure of the universe. My mum is always like, you know, "Why do we have
to care about dark matter?" And, and the truth is that without dark matter,
we simply wouldn't exist. KAISER:
Dark matter is, in part,
the story of why and how we're here. NARRATOR:
And yet, in recent decades,
scientists have uncovered evidence of something that,
today, is even more
powerful than dark matter. I just, I don't think
we possibly could have grasped just how profound and how much it would change our view of the universe. It would be like as though you had only ever experienced
land, and then one day,
you discover the oceans. NARRATOR:
It's called "dark energy." Scientists now believe it's the most powerful force
in the universe, expanding its very fabric, pushing galaxies apart, and it may even be driving
the universe's ultimate fate. But it remains an enigma. We have absolutely no idea
what dark energy is to this day. NARRATOR:
How did our vision of
the universe get completely overturned
in just a few decades? And what new surprises might lie just over the horizon? JOHN JOHNSON:
The past 50 years
of astrophysics has shown that the universe is
extraordinarily creative in what is out there. And it's very
determined to consistently subvert our expectations. BAHCALL:
We are so struggling
to figure out the nature of our cosmos. And that's very humbling,
very humbling, yet very empowering. It's a,
it's a strange combination of the two things together. ♪ ♪ NARRATOR:
Since Voyager 1 left Earth, astrophysicists and astronomers have overtaken
the intrepid probe... ...reaching farther
and farther out into space to gather data written
across vast expanses of time, and on colossal scales. In stars, nebulas, supernovas, distant galaxies and galaxy clusters. VERA RUBIN:
There's the galaxy.
We're at the object. NARRATOR:
And in the 1970s,
it was just such data, meticulous observations gathered
by an astronomer, that forced scientists to confront the idea
of dark matter. The astronomer's
name was Vera Rubin. You'll get a guide star, I'll set up for the observation. ♪ ♪ NARRATOR:
Rubin was born
in 1928 in Philadelphia. From a young age,
she was hooked on the stars. RUBIN:
By about age 12,
I would prefer to stay up and watch the stars
than going to sleep. There was just nothing
as interesting in my life as watching stars every night. I knew I wanted to be
an astronomer. NARRATOR:
In 1963, Vera Rubin traveled here, to the Kitt Peak National
Observatory in the Schuk Toak District
on the Tohono O'odham Nation, 56 miles southwest
of Tucson, Arizona. Despite having advanced
degrees for over a decade, Rubin had never been
able to collect her own data. Astronomy had few women, and many observatories
weren't welcoming. Some officially
did not allow women. But the National Observatory
at Kitt Peak had just recently opened and accepted her application. She would return
to Kitt Peak several times over her career as she began
to focus more on galaxies. So Vera Rubin really was what I would call a true
observational astronomer. She loved
to go to the telescope, do the observation, take it to her office
and analyze them, and try to see what it told her
about the galaxies. NARRATOR:
Today, in more ways than one, Stanford Cosmologist
Risa Wechsler follows in Rubin's footsteps. So this instrument behind me is the 84-inch
telescope at Kitt Peak. Vera Rubin started using
it in 1968 when she started making
measurements of the Andromeda Galaxy, and looking at how different
regions in Andromeda were moving. NARRATOR:
Rubin wanted to check
a common assumption among astronomers about
galaxies. The presumption was that stars near the center of a galaxy
would be orbiting very rapidly, and stars at the outside
would be going very slowly. NARRATOR:
That idea came from the way the planets in our solar
system orbit our massive sun. Because the attractive
force of the sun's gravity falls off with distance, the farther away from the sun
a planet orbits, the slower its orbital speed. Astronomers assumed
the stars in a galaxy would behave the same way. Like the sun in our
solar system, the bright, star-packed center
of a galaxy appeared to hold most
of the galaxy's mass. So a galaxy's orbiting stars
should act like the planets, with orbits slowing
toward the galaxy's outer edges. But no one had done
the work to know for sure. Partly, it was a technical
issue, which Rubin solved by teaming up with
instrument maker Kent Ford. He had developed a device that enhanced a telescope's
light sensitivity, making it possible
to finally see the faint stars on
the far edges of galaxies. And what they saw was
surprising. WECHSLER:
They found that the regions of Andromeda that
were quite far out were still rotating quite fast, faster than you would have
expected by the amount
of light that was there. NARRATOR:
It was a strange observation. What was keeping
those fast-moving outer stars from flying off? During the 1970s, Rubin and Ford,
along with other astronomers, gathered more and more data
from more and more galaxies. Almost none showed
the speed of orbiting stars dropping as had been expected. Still, it would take years
for the astronomy community to accept the astonishing
explanation that Rubin and others proposed; that there was a vast amount
of hidden matter surrounding each galaxy, gravitationally
holding it together. Aside from that effect,
it was undetectable. AMON:
If you didn't have some invisible mass in the galaxy, the stars would not be bound
in this, in this orbit. They would fly out. RUBIN:
We now know that
in every galaxy we study, the stars at
very large distances are orbiting with
very high velocities. And that tells us that
there is a lot of matter at very large
distances from the center. So we see a lot of matter where
we don't see very much light. And that has led
to the concept of dark matter. NARRATOR:
Dark matter. Astronomer Fritz Zwicky
had proposed the name in the 1930s to describe an "unseen mass" to explain some puzzling
observations of a nearby galaxy cluster. But the idea had been
largely ignored until Rubin and Ford came along. Today, scientists estimate there is five times more
dark matter than ordinary matter
in the universe. It's arranged in
a vast web-like structure of filaments and nodes. In the early universe,
those nodes, through gravity, attracted regular matter which eventually
evolved into galaxies. But what is dark matter? PEREZ:
Over the last 50 years, this question has become
a guiding question for huge swaths
of the physics community. We've had some of
the smartest people in the world banging heads
against the wall for decades. This is a really hard problem. NARRATOR:
There aren't many clues. Aside from
its gravitational effect, dark matter appears to interact
very little with normal matter, and can pass right through it. It also emits no electromagnetic
radiation, no light. There are forms of matter that
simply don't glow like stars do, but actually they're also not
responding to light. So they are invisible, except through
their gravitational force. It probably
is something quite exotic. It isn't any
of the ordinary stuff. NARRATOR:
So who are the suspects
for dark matter? There are the MACHOs; the
Massive Compact Halo Objects, like primordial black holes. CLIFFORD JOHNSON:
Black holes that go back to the very earliest eras
of the universe could be a major
component of dark matter. NARRATOR:
For a primordial black hole
to fit the bill as dark matter, it would have about
the mass of an asteroid, and be about the size
of an atom. And there are axions-- minuscule particles
theorized by physicists. An axion, if it exists, interacts only very
infrequently with light, and it does have some mass,
so it could be dark matter. NARRATOR:
But the suspects who garnered
early fans were the WIMPs; Weakly Interacting
Massive Particles. The search for WIMPs leads here, to the Black Hills
of South Dakota. Native Americans have long
considered the area sacred, and the Fort Laramie Treaty
of 1868 included the Black Hills
in the Great Sioux Reservation. But just a few years later, the discovery of gold
and an influx of miners, led to the Great Sioux War
of 1876. The U.S. government
seized the area, and forcibly relocated
its Sioux inhabitants. The dispute over the broken
treaty remains unresolved. ♪ ♪ Today, in the town of Lead, a retired gold mine houses the Sanford Underground
Research Facility. (elevator rumbling) (rattling) Chamkaur Ghag, a professor at University
College London, is a founding member of a team of researchers drawn
from universities around the world that's on
the hunt for dark matter, in the form of WIMPs. To get down a mile,
um, takes a bit of time. NARRATOR:
Why build a dark matter WIMP
detector so far underground? GHAG:
So a mile of rock
above us here in the Black Hills of
South Dakota that shield us from
cosmic radiation, that is bombarding us
all the time. And being underground, we're able to reduce that by
factors of millions. So this experiment
up on the on the surface just wouldn't be able to run at all. It's just far too sensitive. WORKER:
And then go ahead,
hop on the train. Ready? Yep. NARRATOR:
After the ten minute ride down, it's on to
a battery-powered locomotive, followed by a brief hike to get to the cavern that holds the lab of the LUX-ZEPLIN
or LZ detector experiment. GHAG:
The core of the experiment is xenon, it's liquid xenon. It's xenon gas
that's been condensed. And we've got to keep
that clean, and we've got to keep it cold, and so much of what
we'll see around here is all for that, really. NARRATOR:
Here is the main experiment. At its center is a
container of seven metric tons of very pure,
cooled liquid xenon. The concept is straightforward; the cooled xenon
is extremely sensitive. Even just a single collision between one of
the theoretical WIMPs and the nucleus of a xenon atom would cause the atom
to collide with others, emitting a flash of ultraviolet light,
which would be picked up by the detectors at
the top and bottom of the tank. The interaction would also
liberate electrons. They'd drift up to the top,
and emit an even bigger flash. GHAG:
So we get two flashes
of light here, and based on how these two
signals look relative to one another, we can tell whether
this was background radiation, or if this was a dark matter
particle that came in and hit
the nucleus of a xenon atom. NARRATOR:
LZ isn't the only experiment
using cooled xenon, but it now leads the pack in
sensitivity. It's the frontrunner now. And so, we're
entering discovery territory. NARRATOR:
Direct dark matter detection
experiments go back to the 1980s. Xenon-based experiments, similar in design to LZ, to the 2000s. So far,
all the experiments combined have detected nothing. But the process constantly narrows down what
dark matter could possibly be. And currently,
LZ has time on its side. The plan is to accrue a total
of three years' worth of data. GHAG:
Hopefully, there'll be a
direct detection and we'll start
to understand the nature of it. It could be that dark matter
isn't a simple one-size-fits-all WIMP. It could be that there's
multiple different types of dark matter,
different species of the stuff, and we start to understand
the dark sector as more of a zoo. ♪ ♪ I'm deeply interested in trying to make some headway
into the unknown. So the bigger the unknown,
the better for me. So yeah, dark matter being
85% of the mass of the universe, that we have no real
clue about what this stuff is. But it is profoundly important. ♪ ♪ NARRATOR:
It's been a striking
transformation. In the past 50 years,
thanks to Vera Rubin and others, dark matter
has become an essential scientific building block at the foundation of our
understanding of the universe. There is no way out
of dark matter. If you believe in general
relativity and Newton's law, if you believe in that, no way out of dark matter;
you have to have dark matter. NARRATOR:
In 1998, as Voyager 1 traveled to the far reaches of our
solar system, it surpassed the record
of a previous space probe, Pioneer 10, as the most distant
human-made object. It was 6.5 billion miles
from Earth. 1998 was also the year of one of
the greatest discoveries in the history of science. What seemed to be a force that literally creates
new space out of nothing. Today's issue of the journal
"Science" reports new information about
the evolution of the universe. A lead author
of one of the studies was cosmologist Adam Riess. Thanks for being with us. Thank you. Why did some scientists
react with what one called "amazement and horror"
to these conclusions? Why was it such a shock to them? So we were hoping we'd find
a more simple explanation, something mundane, but... But instead, you found a new force
in the universe? Well, it would appear that way. ♪ ♪ NARRATOR:
One of Baltimore's hidden gems
is the George Peabody Library at Johns Hopkins University. It has been called
"one of the most beautiful library spaces in the world." These five tiers
hold 300,000 volumes, including astronomical works
written over the centuries, that try to answer a question
that has troubled humankind perhaps always... ...when we look up at
the night sky, what is it we are seeing? And has it been there forever? ♪ ♪ Adam Riess, an astrophysicist
from Johns Hopkins, played a key role in
formulating the current scientific answer
to that question. RIESS:
Wow. This is really the original
earth-centric model. NARRATOR:
Like many in astronomy, he has a deep appreciation
that he stands upon the shoulders of giants. This is really
my favorite here. Copernicus puts the sun
in the right place. This is progress in science. NARRATOR:
But Riess has also added to our understanding of
cosmology... (applause) ...sharing a Nobel Prize in 2011 with Saul Perlmutter
and Brian P. Schmidt, for a discovery
that profoundly changed our view of the universe. RIESS:
Here's somebody
after my own heart taking the observations, Tycho Brahe. NARRATOR:
Each of these influential
thinkers from prior to the
20th century have contributed to our
understanding of "the heavens." ♪ ♪ And here comes Isaac Newton, who really develops
the mathematics. And he really lays out
how gravity works. NARRATOR:
But they all shared
something in common-- whatever the heavens were,
they seemed eternal. Earth may be at the center
surrounded by celestial spheres, or the sun
may be at the center with the planets orbiting it. Comets may come into view
and disappear, but all of these took place
on a gigantic but static stage. ♪ ♪ Even Albert Einstein
initially agreed. By the early 20th century, he had already revolutionized the Newtonian view of the world, by connecting space and time into one concept
he called space-time, and then theorizing
that gravity was the warping of that space-time
fabric by mass and energy. In 1917, he applied his new
ideas to the entire universe. But he already had
a final result in mind, the one generally accepted
by astronomers. Einstein's universe would be a largely static
and unchanging one, though gravity posed a problem. Einstein had kind of a
puzzle in his mind, because if the universe
was static, and yet all the matter in it
was attractive, gravity would
pull things together. How did it stay static? What kept it static? And he made
a remarkable discovery. In his theory
of general relativity, the gravity of matter
can be attractive, but that the gravity
of empty space could be repulsive. He called this
the cosmological constant, and he thought
it was a possibility that these
two kinds of gravities-- the attractive
and the repulsive kind from two different
kinds of aspects of space-- were causing
this kind of stalemate. ♪ ♪ NARRATOR:
Einstein's solution
didn't stand for long. And here,
at the Mount Wilson Observatory outside of Los Angeles, is where astronomers
gathered some of the data that led to its fall. ♪ ♪ These days,
you may be lucky enough to hear a
woodwind quintet playing in one of its storied domes-- the acoustics are exceptional. (quintet performing) And it is an inspiring place for theoretical physicist and graphic nonfiction author
Clifford Johnson to let his mind explore. In the 1920s,
this observatory produced two of the
most important discoveries about
the nature of the universe... (quintet performing) ...both by astronomer
Edwin Hubble-- the same Hubble the famous
space telescope is named for. Hubble changes
our entire view of what the universe is
and how vast it is. NARRATOR:
At the time,
a debate raged in astronomy: was the Milky Way
the entire universe? Or was there more to
the story? In 1925, Hubble settled the issue by proving that
the Andromeda Nebula existed outside of
the Milky Way. It, along with
other distant nebulas, were renamed "galaxies." ♪ ♪ DE SWART:
The whole notion of a "galaxy" started to become a thing
only in the mid-1920s. People started immediately
get interested in what these things are. NARRATOR:
Especially Edwin Hubble. He began measuring
the distance from Earth to various galaxies, and when he combined
his work with that of
other astronomers, he discovered something
deeply mysterious. It had to do with
the Doppler effect. (distorted pitch shifting) We typically think of
the Doppler effect in terms of sound. A siren coming toward you
has a higher pitch, because the sound waves
catch up to each other and become compressed. A siren heading away from you
sounds lower, because the sound waves
stretch out. ♪ ♪ A similar thing happens
with light waves-- if the source of light
is headed toward the observer, the light it emits will be
shifted toward blue. If the source is
moving away, the light is
shifted toward red. (film reel ticking) In fact, astronomers
working parallel to Hubble encountered exactly that. The light coming from almost every galaxy
they observed was shifted toward red, indicating the galaxies were moving away from Earth. And when Hubble checked
the redshifts against his own
distance measurements, he discovered not only
were the galaxies racing away from us, but the ones farther away
were racing away faster! What did that mean? CLIFFORD JOHNSON:
Given that we're not in a special place in the universe, we're not at the center of
the universe, the conclusion is, is that the whole universe
is expanding. Everything is moving away
from everything else. This is
another huge discovery about the nature of our
universe. ♪ ♪ NARRATOR:
Some astronomers, like Belgian cosmologist and Catholic priest
Georges Lemaître, were already
exploring the implications of an expanding universe. If you rewind the expansion,
like a film, where does the universe
"start"? Does it have a beginning? ♪ ♪ The idea
is that there was some earlier phase
in the universe where everything
was much closer together. Some very dense early phase
of the universe. And something happened
to begin to push things apart. (explosion booming) That line of thinking
led to the theory labeled by its detractors
as "The Big Bang." ♪ ♪ (explosion echoing) It wasn't until the mid-1960s that observational evidence
quieted the Big Bang critics, when two astronomers, Robert Wilson
and Arno Penzias, using this antenna
in Holmdel, New Jersey, stumbled across one of its
predicted artifacts. After the Big Bang, it took hundreds
of thousands of years for the universe to cool enough to transmit light. That initial burst has left
a faint residual glow. Today, we call it the
Cosmic Microwave Background, or CMB. The Cosmic Microwave
Background is actually a remnant
of the Big Bang. It's the radiation from that Big Bang
that we can observe from when the universe
was about 380,000 years old. NARRATOR:
Later, ground- and space-based
experiments would study this
"fossil radiation" in ever-finer detail, because its extremely
slight variations, shown here
in different colors, can reveal insights
into the structure of the early universe. By the 1970s,
thanks to the CMB, most astronomers had accepted
the Big Bang theory and that the universe
was expanding, but they also thought the
expansion was likely slowing because of the attractive force
of gravity on matter. RIESS:
Matter decelerates the
expansion. It decelerates it either
a little bit if there's only
a little matter and the universe
expands forever, or it decelerates it
a great deal, halting the expansion at some point in the future and causing
the universe to collapse. (echoing boom) NARRATOR:
Measuring how fast the expansion
was decelerating would reveal the fate
of the universe. (echoing boom) And the key to doing that
was this. ♪ ♪ One of the brightest
explosions observed in space; when a star
becomes a supernova. There are
two types of supernovas and distinctions within those, but a Type Ia supernova
is fairly common. About a quarter of all
supernovas, incredibly bright-- sometimes brighter than
an entire galaxy... (echoing boom) ...and remarkably consistent. That makes it
an excellent candidate to be what astronomers call
"a standard candle." A kind of
cosmic measuring stick... ...if enough of them
could be found. So, that was the goal of the newly formed
High-Z Supernova Search Team. Adam Riess was a member. All the way from HP... NARRATOR:
They planned to discover distant Type Ia supernovas, compare them to nearby ones, and definitively answer how much the expansion of the
universe was slowing down. But they weren't alone. There was another team,
the Supernova Cosmology Project, which had started
a number of years before us. They were more particle
physicists, and we were more, I would say,
supernova astronomers. ♪ ♪ But both competing,
realistically, for the same
telescope facilities, which were very precious
commodities to get to do
this research. NARRATOR:
By 1997, the High-Z team
had collected a large enough sample of
supernovas to get a "first read"
on the universe. Hey, look at that! NARRATOR:
But initially,
the results made no sense. So we went from saying,
you know, "this has gotta be wrong,"
to like, "this looks like
what the data says, We have to report that." NARRATOR:
In early 1998, both teams announced
the same shocking conclusion: the universe was not
slowing in its expansion, it was speeding up. Like an invisible hand, some undiscovered force was
at war with gravity and pushing the universe apart faster and faster. ♪ ♪ RIESS:
From what we can see, there's really not too much left beside the possibility of
this repulsive force. Does that mean that
the universe could just keep on
expanding forever? If you take this result
at face value, if this is really true,
the implication is yes, that the universe
would expand forever. ♪ ♪ NARRATOR:
The mysterious repulsive force came to be known as "dark energy." "Dark energy" is really the name
we give to our ignorance of what's causing the accelerating expansion
of the universe. It's a pushing out
that it does. It's a pressure,
an outward pressure, that the gravitational force
is pushing against. ♪ ♪ Overall, the universe is accelerating in its expansion because of this
dark energy effect. ♪ ♪ NARRATOR:
Today, scientists estimate
it is overwhelmingly the most prevalent form of
energy in the universe. RHODES:
We thought we knew the constituents of the universe and how it was evolving
over time. Al of a sudden,
we found out that, no,
we didn't know, because the biggest
component of the universe wasn't dark matter,
it was dark energy. NARRATOR:
So, what exactly is
dark energy? One of the simplest ideas
is that it's actually a property of
space and time itself. NARRATOR:
Scientists had always assumed the energy level of
a perfect vacuum was zero. But what if it wasn't? What if, as the universe
expanded and created more space, a repulsive energy
inherent to that space grew as well? On massive scales,
it would oppose and maybe even overcome the gravitational
attraction of matter. CLIFFORD JOHNSON:
And ironically, that's exactly
the kind of thing that Einstein had
come up with long ago, when he was trying to make
the universe static. NARRATOR:
In his formula to describe
the universe, Einstein had added
a "cosmological constant" to perfectly balance
the attractive effect of gravity and create his
"static" universe. (booming) When astronomers
in the 1920s and '30s concluded that the universe
wasn't static at all, but expanding, he dropped
his cosmological constant, and is said to have called it
his "biggest blunder." But with the discovery
of an accelerating expansion, cosmologists
revived the term. In fact, it was an idea Adam Riess turned to
early in his analysis. After convincing myself
I hadn't made a mistake, I introduced that possibility
into the analysis that Einstein's
cosmological constant existed, and the fit grabbed onto it
and said, "Yes, this--" you know, with
pretty high confidence-- "this is indeed part of
the recipe of the universe." NARRATOR:
So how did astronomers miss this most consequential
of phenomena? CLIFFORD JOHNSON:
Perhaps the reason we hadn't
noticed it before is because the way you measure
it is in terms of how much is it per unit
of space time. Perhaps a little chunk
of space right here. So you have to divide
the entire effect by the volume of
the observable universe. So that makes it
a very small number. NARRATOR:
Imagine the energy released by a match head burning. (hissing, flame roaring) The estimated equivalent
in dark energy is spread across
a cube of empty space with an edge about
seven and a half miles long-- or the amount of space
contained by about one and half million
Astrodomes. ♪ ♪ So it's small-- and maybe in the early history
of the universe, when it contained
a lot less empty space, not even that important. KIESSLING:
If we were in a period of time much,
much earlier in the history of the universe,
dark energy was a very, very small
component of the universe, and it wouldn't have
necessarily been noticeable. (booming) NARRATOR:
But that changed about
six billion years ago. By then, the universe
had grown big enough for dark energy to overcome
the attractive force of gravity and start speeding up
the expansion. Today, dark energy
dominates the universe, and it may even determine
its ultimate fate. KÖNIG:
Now we are in the
dark energy era of the universe, which means
that the universe is expanding at
an accelerated rate. If the universe would
keep on expanding and expanding and expanding... Then we seem to be looking at a far distant future in which the universe
is basically empty. It's been diluted of all the other stuff
that we otherwise can see lighting up
and dancing around us. Galaxies will just continue moving further and further
apart from each other. ♪ ♪ DE SWART:
Our nearest galaxies will
go beyond our visible horizon, beyond what we can see
in the universe. KAISER:
Space would stretch
even faster than light could catch up to tell us
there's a galaxy over there. KÖNIG
Eventually, we will not be
able to see the light coming from
another galaxy. DE SWART:
We can't see any other
galaxy anymore because dark energy and
the expansion of the universe has driven this all away. That would be kind of sad. We might not be
sort of torn apart, we just become extremely,
extremely lonely. The end of the universe
will be very cold and very dark, and... and we won't see the nearby galaxies and so on. That's the future expansion
of the universe. ♪ ♪ NARRATOR:
If it makes anyone feel better, there is still a lot of
uncertainty about dark energy, especially whether it has been
consistent over time. KIESSLING:
At the moment, we think it's been consistent, but it's potentially
Nobel Prize-winning if it's been
changing over time, and so secretly--
not-so secretly-- (laughing):
cosmologists are really hoping to find something different, because that'll be
really exciting. ♪ ♪ NARRATOR:
A "cosmological constant"
that isn't really constant could be the solution to
another vexing mystery, which some have called
a "cosmological crisis." This is a real problem. There's some tensions
in what we are seeing. This has been a great challenge
for us in the last ten years. This is something that
can't just be wished away. (booming) NARRATOR:
It has to do with how fast
the universe is currently expanding; that's known as
the "Hubble constant." To calculate it, scientists have mainly used
two different approaches. One is based on the
"baby picture" of the universe-- the CMB-- which itself
has been measured in ever-increasing detail. KAISER:
Three different generations of specially built
satellites in the sky to just do this
one thing: measure the CMB, to my mind,
mind-boggling precision. AMON:
The measurements that we have from the cosmic microwave
background right now, they are the gold standard
in our field, so high-quality that when
you make measurements from it, they are extremely
high precision. NARRATOR:
Meanwhile, other groups, including one led by
Adam Riess, have calculated
the Hubble Constant using measurements
of distant supernovas. (booming) KAISER:
Distant things from us,
but not nearly so distant as the cosmic microwave
background, phenomena that are old
in cosmic history, but not quite so old. We call them "late universe." NARRATOR:
Two different techniques-- one based on the early universe,
the CMB-- and the other
on the "late universe," using supernovas. We have the ability to
bookend the universe. To essentially see
how fast the universe was expanding
at the beginning, and how fast
it's expanding now. We're measuring the same
universe, whether we're measuring
the cosmic microwave background or a population of supernovae. To see if you can go from
one to the other. If you can predict
how fast the universe ought to be expanding. And there's all kinds of reasons
to think that either one of these kinds
of physical systems should give the same answer. It essentially lets us test
our standard model over cosmic time. And that sets up a very
beautiful robustness test for "does this model work?" NARRATOR:
But as the accuracy of
each approach has grown, the estimates for
the Hubble constant-- the speed the universe is
expanding-- have diverged, a problem known as
the "Hubble Tension." RIESS:
Many of us are quite fascinated
by the implication that we could be
missing something still in our understanding
of the universe. Or this might be
another clue about the nature of some
of these unknown parts of the universe,
the dark matter, the dark energy,
things like that. When we get mismatches, we get pretty excited
about it in our field, because these
"tensions" tell us that maybe something is
not quite right in the model. Maybe that's the hint
of where to look for the next breakthrough. This is where we find
new physics. This is where we find discovery. Is it just measurements
being made wrong? Is it modeling
being made wrong? Or are we fundamentally
not understanding something about our universe? We don't know yet. But it's telling us
that cosmology is still very exciting. MAN (on radio):
Four, three, two, one, lift off. (rocket engine roaring) NARRATOR:
So, the jury is still out, but more data is on the way. The European Space Agency's
Euclid space telescope launched on July 1, 2023. It's designed to look
ten billion years into the past with unprecedented accuracy. ♪ ♪ And Euclid is not alone
in its pursuit of answers. It will soon be joined by NASA's Nancy Grace
Roman Space Telescope, which will measure
distances and positions for millions of galaxies. But perhaps most fitting
will be the work done here, in the soon-to-be completed Vera C. Rubin Observatory,
in Chile. It will house the
Simonyi Survey Telescope that will photograph
the entire night sky every few nights using the largest digital camera
ever constructed. WECHSLER:
I think it's wonderful that the Vera C. Rubin
Observatory has been named after
Vera Rubin. She was fearless
and undaunted. She just kept going. My number one belief is that
the universe is for everyone. We all have this right
to understand our place
in the universe and how the universe works. And I think that's really
a fitting part of her legacy. ♪ ♪ NARRATOR:
In 2012,
Voyager 1 left our solar system and the sun's
protective heliosphere, sending home the
first direct observations of interstellar space. Today, the space probe
continues on its lonely journey, as it likely will long after we're gone. (clicking) Since the days of
the Voyager launch, about 50 years ago, much has changed
about our fundamental understanding
of the universe. But what will happen in
the next 50 years? ♪ ♪ RIESS:
The last 50 years was about posing some of these
very big questions. I think the next 50 years is going to be
about answering them. Now we're left with a very, very hard problem
to solve, trying to understand what
dark matter is, trying to understand
what dark energy is. Those are not going to be
easy to solve. It's a hugely exciting time to be involved in physics, astronomy, cosmology-- all the things that are
now coming together to help us understand
our universe at large. RHODES:
We're doing the experiments now that might allow us
in 50 years to say, "Wow, there was
another revolution "in the mid-2020s or
around the time 2030... that gave us a whole new way
to look at the universe." DE SWART:
Dark matter is going to be
the most exciting thing that's going to happen
in the next 50 years. Because either
they're going to find it, or it's going to
be incredibly exciting because they're
going to not find it, and then people are going to
tear their hairs out, because what
are we going to do now? ♪ ♪ It really
is truly hard to imagine what our model,
and what our thinking will be. We will find new things,
it's undeniable. What I would love to learn
more about, if I had a time capsule
to zoom forward right now, is to ask, what are the questions we
didn't even think to ask today? What's really
going to surprise us that we didn't even wonder,
to wonder about? ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪
