All language subtitles for How the Universe Works S06E10 Mystery of Spacetime 1080p

Narrator:
What is our universe made of?

It's the biggest
unsolved mystery in science.

Despite the name, space is not
an empty space at all.

Space itself is something.

Narrator:
There's a hidden structure

and a force that exists
within space itself...

Carroll: Space-time is something
absolutely real.

It's absolutely fundamental.

It's really part of
the fundamental architecture.

Narrator: A force that connects
everything in our universe.

It's an active player
in the game of life.

Narrator:
It underpins our reality,

tying together
all of space and time

since the very beginning.

We call it space-time.

It's everything.

Space-time is what
the universe really is.

Narrator: Space-time is how
the universe works,

but what is it?

How does it control
our past, present and future?

[ crackles ]

[ rumbling ]

[ explosion ]

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

captions paid for by
discovery communications

♪

narrator: We can't see it.

We can't touch it,

but without space-time,
we wouldn't be here.

Space-time is the fabric
of our reality.

It shapes and governs our lives.

If we want to understand
the story of the universe,

it's absolutely crucial
we understand

how space-time behaves.

Narrator:
Space-time has been active

since the beginning
of everything

and is the key to
the evolution of everything.

We have to understand space-time

in order to understand
the history of the universe,

to understand how
the universe began,

how it evolved, and what's
going to happen in the future.

Narrator:
The story of space-time

is the story of our universe.

To know how the story plays out,
how it will end,

we need to go back
to the very beginning...

...To a time when there was
nothing, no stars, no space,

a time before there was time.

Then, all of a sudden...

[ rumbling ]

[ explosion ]

our entire universe was born
in the big bang.

It started
in a instantaneous moment

where, from nothing,
our universe was created.

The very definition of
the moment of the big bang

is that space and time
were created at that instant.

It is, as far
as we currently know,

the coming into existence
of space and time itself.

Narrator:
The infant universe,

a tiny speck of energy
and space-time,

materializes from nowhere.

Then...
The universe suddenly expands.

The idea of inflation is that
a very tiny region,

in an incredibly short
amount of time,

far shorter than a second,

grew by many, many,
many orders of magnitude,

so imagine myself suddenly
becoming the size of a galaxy.

Narrator:
In a fraction of a second,

the universe grew from smaller
than the size of an atom

to the size of a baseball.

In cosmic terms,
that's like a grain of sand

growing almost to the size
of the observable universe.

The universe, at the instant
of inflation,

actually expanded faster
than the speed of light.

It seems to be a violation

of everything
you've heard in physics.

You may be thinking, "hey, hey,
hey, mr. Astronomy guy.

Nothing can move faster
than the speed of light,"

and it turns out,
that's kind of true.

But the rule is, nothing can
move through the universe

faster than the speed of light.

In inflation, it's space itself
that is expanding,

so there is no violation.

There is no paradox.

♪

narrator:
Inflating fast, the universe

went through
a phenomenal growth spurt.

At the moment of the big bang,
space-time was this entity

that was flying out
in all directions.

It was space itself
that was expanding.

Narrator: But the universe
didn't expand evenly.

One spot in the universe
was ever so slightly more dense

than a spot right next to it,

and we're talking about a tiny,
tiny fraction of a percent,

one part in 100,000,
but that was enough.

Narrator: Fluctuations
in expanding space-time

created areas
with higher density.

Inflation made these
high-density regions larger...

♪

...And this allowed
our universe to take shape.

When parts of the universe
didn't inflate

quite the same way as others,

all of a sudden, things could
start to come together.

Narrator:
As the universe cooled,

energy turned into matter...

♪

...And in the denser regions,

that matter started
to clump together.

Crucially, these regions
had more mass than others.

Mass bends space-time,

so anything that is made of
matter bends space-time.

And the more matter you have in
one place, the more you bend it.

In fact, I am bending
space-time right now.

When I flex, I bend it even more

because of my incredibly high
muscle density.

I don't bend at the maximum.

I don't want to destroy
the earth and the solar system,

but, you know, it's an effect.

It's a real thing.

Space isn't constant.

It's not something that
is always the same everywhere.

It actually bends, curves.

It warps depending on the matter
inside of it.

Narrator: We'd see a curving
grid of space-time moving

and reacting to objects
within it,

and we'd feel the curving
of space-time

as the force we call gravity.

Freese: Gravity is different
from all the other forces.

It is intimately connected with
the curvature of space-time.

Something that can bend space
and time has gravity.

That's what gravity is,

the bending of space
and time itself.

♪

narrator:
It's hard to visualize this,

but a good analogy
is a trapeze artist

and their safety net.

You can imagine a trapeze artist
falling into a net on purpose.

That net is flat

and looks like a nice,
orderly, evenly spaced grid,

but when they fall into it,
they distort that grid.

Well, that's a lot like space.

If you have matter in space,
it warps the framework.

When the trapeze artist
is resting in the net,

they're bending that
space-time grid a little bit.

If you had two trapeze artists
in there,

double the mass
in roughly the same volume,

you would get a bigger dip.

You have a bigger distortion,
and that's how space-time works.

Narrator: More mass equals
a bigger curve in space-time

equals more gravity,

but understanding the nature
of gravity and space-time

is no easy thing.

♪

it's an idea developed by
one of the greatest minds ever.

Einstein had the idea that
space itself is something,

something that can be bent,
something that can be stretched,

that we are all bound together
by space-time.

Einstein says that space
and time have a geometry.

They have a life of their own.
They have dynamics.

Narrator: Those dynamics
are what we call gravity.

The more dense
the region of matter,

the greater the gravity,
the deeper the curve.

This connection
is the foundation

of our physical reality.

It's the interaction between
matter and energy and space-time

that created the universe
that we see around us today.

Narrator: But that doesn't mean
we fully understand it.

Oluseyi: There's much more
that we don't know,

and that's frustrating.

With the laws of physics,
I can talk about

how space-time behaves,

but it does appear to be
something that stretches,

that contracts, and that gravity
is the embodiment of space-time.

Narrator: Born in the big bang,
space, time, and energy

combined to create
our infant universe.

These basic materials
were the foundations,

but how did we get
to the incredible,

complex structures we see today?

How did space-time
build our majestic cosmos?

[ explosion ]

♪

narrator: Our entire universe
was created in the big bang

13.8 billion years ago.

Everything came from nothing,

but our modern universe
is a complex mosaic of matter.

When we marvel through
our telescopes

at the fantastic structure
of our universe

and its galaxies,

you got to ask,
"where'd that come from?"

bullock: Matter in the universe
arranges itself

on a vast cosmic web.

Galaxies and galaxies' clusters

are strung out on sheets
and filaments.

Narrator: It seems this
intricate web is organized

by a cosmic architect,
space-time.

It shaped everything
from planets to galaxies,

atoms to cities.

The universe is made
of space-time.

Whatever the substance is,
time and space bound together,

that's expanding and creating
the universe we see around us.

It's everything.

Space-time is what
the universe really is.

Narrator:
It's a hard concept to grasp

and even harder to visualize.

Scientists observe the universe

in different
wavelengths of light.

This is the sun, invisible
light, x-ray, and ultraviolet.

Now imagine if we could see it
in the space-time spectrum.

We would see space-time

distorting as objects
move through it.

Space-time can warp
and push things around.

It can expand
and pull things apart.

Narrator:
But it's the shape of space-time

that dictates
how we experience it.

Imagine you're in your car.

You go up hills
and you go down hills,

so the shape of earth's surface
determines

how you travel
across earth's surface.

In the same way, the geometry
of space-time determines

how light and matter
move through space-time.

Narrator:
The rules are simple.

Matter -- in fact, any object --
tells space-time how to curve.

The curvature of space-time
tells matter how to move.

♪

because the shape of space-time
tells matter how to move,

what we call gravity,

this means that gravity
and the shape of space-time

tells matter
how to clump together

and form larger
and larger structures.

Narrator: But at the beginning,
the space-time landscape

was very different from today's.

And the very first matter

started to change
the shape of space-time.

So this space right here
has a tiny bit more matter in it

than this over here.

Wherever there was a little bit
of extra mass,

that would bend space
a little bit more.

Well, if you're bending space
a little bit more,

then more mass
would collect there.

Narrator: In the early universe,
the denser regions of matter

created deeper curves
in space-time.

And as the mass gets bigger,
as stuff falls into that well,

it gets deeper and deeper and
deeper and attracts more stuff,

and it's just a runaway process.

Narrator: Gravity increased,
pulling in more and more matter.

Hughes: It got more dense,
got more dense, got more dense,

and then, before you know it,
you've got a star,

and you've got a bunch of stars,
and you start to make a galaxy,

and these stars evolved and
began forming large structures.

They sort of burned through all
their nuclear fuel and exploded,

and they made
all the heavier elements,

and, with time, we got down
to having things like planets,

atmospheres, people, all the
things that we care about today.

Narrator: All of this started
out as energy fluctuations

in expanding space-time.

Plait: These, at first,
very tiny fluctuations

became these gigantic structures
that we actually see today.

And over billions of years,
that material began to coalesce

into individual galaxies,
stars, planets, and you.

Narrator: Fluctuations
in the expansion of space-time

laid out the pattern
of the universe.

The curvature of space-time
controlled the evolution

of everything we see today.

If space-time didn't
have that property

of bringing mass together,

then all we would be is
a thin haze of hydrogen gas,

not a very interesting
universe at all.

If space-time didn't curve
because of matter inside of it,

the universe would be
a really weird place.

I mean, there'd be no gravity.

There'd be nothing to make
things stick together.

No force of gravity means
no stars, no planets,

and no people.

♪

narrator: We owe our existence
to space-time.

But even scientists
struggle to understand it.

Oluseyi: I wish I knew
what space-time is.

We know things about space-time,

but at the same time,
we feel like we know

almost nothing about space-time.

♪

narrator: Then, in 2015,
we caught a break,

and for the first time,

we heard ripples
in space-time generated

by one of the most
violent events

in the history of the universe.

♪

[ explosion ]

♪

narrator: The universe is filled
with space-time.

♪

we think it's been around from
the beginning of everything,

quietly pulling
the strings of the cosmos...

♪

...And sometimes,
we get a glimpse

of this elusive puppet master
in action.

♪

2016 -- astronomers witness
a strange optical phenomenon,

a weird circle of light
like a cosmic halo.

This is actually
what you would see in the sky

if your eyes were as sensitive
as a telescope.

They're real.

This is not some artifact
of how we adjust the images.

Something is actually
bending space

and time itself into a lens.

Narrator:
That something is a red galaxy

which is over 7 billion
light-years away from earth.

It's bending the light
from a blue galaxy

which should be
hidden behind it.

It's called
gravitational lensing.

Gravitational lenses are caused
by objects with huge masses,

say clusters of galaxies,

that distort space and time
so much

that when light comes from
farther objects

and has to pass around
the galaxy clusters,

the light bends.

It really is a true warp
in space-time.

Narrator:
Mass from the foreground galaxy

creates curves in space-time,
which we know as gravity.

Light follows those curves
and is warped,

so it bends around the galaxy.

Massive objects
like clusters of galaxies

can bend the path of light
through space-time

a lot like a piece of glass
can bend the path of light.

So when we look
at a distant galaxy,

as the light passes
through a galaxy cluster,

we see multiple images
of the same galaxy.

We see arcs and circles

as if that galaxy cluster
were made of glass.

We are seeing the warping of
space-time literally played out

in front of our very eyes.

Narrator: Gravitational lensing
gives us a way of seeing

the effects
of space-time on light,

but it's only an indirect
observation of space-time.

Could there be another way
of experiencing space-time

right here on earth?

Not everything that happens
in space can be seen.

Sometimes you have to listen
for it, as well.

[ crackling ]

believe it or not,
space is a material

much like this iron sheet,

and like this iron,
space can distort.

If I put a very heavy weight
on this sheet of metal,

its shape is going to change,
and it's going to distort.

Amazingly,
space can carry waves,

and so can this iron sheet,
but to get this sheet wavy,

you need something
really powerful --

something like me
and my hammer.

Did you see those waves travel
through that iron sheet?

Well, waves pass through space
in exactly the same way.

We call these
gravitational waves.

Narrator:
Gravitational waves

are vibrations
from cosmic events

transmitted through
the material of space-time.

To set off waves in space,
you need the biggest, baddest,

most powerful events
in the universe,

something like the collision
of two black holes.

Narrator:
When two black holes collide,

the energy released
sends shock waves

through space-time
across the universe.

By the time they reach earth,
they're so small,

they're immeasurable...Almost.

In 2015, scientists
at the ligo observatory

made a groundbreaking
observation.

♪

they detected ripples in
space-time, gravitational waves.

Rumors began flying, but it
became clear after a while

that this was, indeed,

the first direct detection
of gravitational waves

seen by man-made instruments
on the earth.

Adhikar: When we discovered
gravitational waves,

it had been so long that
we'd been waiting for signals,

not only did most of us
not believe it,

I went so far as to be
so skeptical

as to look into all kinds
of conspiracy theories

for ways it could be fake.

When I saw this data,
I still think back on it now

and the emotional impact
it has on me,

the only thing comparable is

when I saw my daughter's face
for the first time

after she had been born.

It was that kind of
an emotional impact,

just having all of this thing
that we had worked for

coming to fruition
in one moment.

It's mind-blowing.

♪

narrator:
We can actually hear

these gravitational waves
on earth.

Part of what makes
this so amazing --

it's a bit of a coincidence,

but it's a really
cool coincidence --

is that the signals
that ligo actually measures

are in the same frequency band

as the sounds that
the human ear is sensitive to.

Narrator: We can hear the waves
change frequency

as the two black holes
get closer and collide.

[ explosion ]

it's a swoop-up in frequency
that sounds like "woop."

[ electronic woop ]

what we are hearing in that woop
were two black holes

that are orbiting
around one another...

[ woop ]
...And then coming together.

That was it.

Narrator: Listening to ripples
in space-time

has given us a powerful new tool
to investigate the universe.

We are now hearing things
in gravity for the first time.

It's a sense that we have
never been able

to apply to the universe,

and we're beginning to learn
what is out there.

The observation of gravitational
waves from black holes

is one of the most significant
findings in astronomy

by anyone in
the recent hundred years.

Bullock: It's hard to overstate
the importance

of gravitational-wave astronomy.

Much like when galileo
first pointed his telescope

at the stars
to see something new,

we now have an entirely
new window into the universe.

♪

narrator: Gravitational lensing
and gravitational waves offer us

an insight into
the complex relationship

between gravity and space.

♪

but what about the other half
of the equation -- time?

But it turns out, you know,
it's space-time.

Gravity not only distorts space,
it actually distorts time, too.

We think of time as something
that can't be changed.

It simply flows ahead
at a constant rate,

but that's not the universe
we find ourselves in.

In some crazy circumstances,
my time might even,

according to you, stop.

♪

narrator: It might even be
possible to travel through time

and go back to the future.

♪

[ explosion ]

♪

narrator: For sci-fi fans,
space is the final frontier.

For scientists, understanding
time is a much bigger challenge.

Originally, we thought of time
as the same thing

as the sun rising and setting,

but now we've come to realize
that time

is a more fundamental concept
than that.

Narrator: Time isn't just
something that passes.

Time is an essential part
of our universe.

It's part of the fabric
of space-time.

The big bang was the beginning
of space and time.

♪

since then,
space has been expanding,

and time has been
ticking forward.

[ ticking slows ]

it's been doing this
for 13.8 billion years,

creating the universe
we see today.

They're sort of two sides
of the same coin.

You can't have time without
space or space without time.

So when matter influences
space-time,

it's not just creating
the formations in space.

It's also affecting
the flow of time.

This is where space-time
becomes really cool.

Narrator:
Just as gravity bends space,

it also distorts
the flow of time.

This isn't how we perceive time.

This is actually the rate
at which time flows.

Very massive objects can warp
and twist space-time itself,

so not only is space distorted,

but time itself can
slow down or even stop.

Narrator:
The stronger the gravity,

the greater the distortion.

What has the most gravity?
A black hole.

♪

bullock: Out in space, near
a strong gravitational tug

from a black hole,
clocks can do funny things,

and this is where things start
getting really interesting.

♪

narrator:
Around a black hole,

space-time warps
and twists, slowing time down.

Scientists dream
of sending a probe there

to test their hypothesis.

♪

there's a famous way
of thinking about this

called the twin paradox

where two twins are born
on exactly the same time, right,

so they're the same age,

but one of them zips very,
very close to a black hole,

hangs out a while,
and then comes back.

If I had an identical twin
who stayed back on earth

while I flew near a black hole,

when we had our daily video
phone calls, he would see me go,

[ slowly ] "hi, there."

[ normal voice ]
and I would see him say,

[ rapidly ] "oh, my goodness.
[ babbles ]

because
you're talking too funny."

we would literally notice
that time is running

at a different pace
for the other one.

Narrator:
The closer to the black hole,

the slower time passes.

If instead of coming back home,

I accidentally fell backwards
into the black hole,

my twin back on earth would
see me slow down even more.

I'd go, "oh!"

and completely grind to halt

and seem frozen
on the event horizon.

Narrator:
Time appears to stand still.

And I would just have
a sinking feeling

that I would never be able
to come home again.

Narrator:
But if the twin could escape

from the black hole,

he would be returning
to the future.

Maybe it's only been a few days
or weeks experienced

by the one that traveled to the
black hole, while the other is,

you know, gray-haired
and has grandkids by now,

has lived decades here on earth.

Narrator: The black hole
warps space-time so much

that the ultimate
science-fiction fantasy

becomes reality.

Time travel is a staple
of science fiction,

and we know that time travel
into the past

appears to be ruled out
in our universe,

but time travel into the future
is totally acceptable.

♪

narrator: Time travel
isn't possible just yet,

but space-time has a very real
effect on our daily lives.

It controls how we age.

♪

the key to different rates
of flows of time is gravity.

If you experience a different
gravitational environment,

you will have a different
flow of time.

As I climb up these stairs

and I put myself further away
from the mass of the earth,

my own clock runs
a little bit faster.

If you go down closer
to the surface,

the more your clock slows down.

We have sensitive enough clocks

that we can measure
this different flow of time.

Narrator:
Exaggerate this effect,

and we would see the flow
of time change in front of us.

Those closer to the earth
would look slowed down.

Those higher up, the opposite,
which means the wealthy,

in their penthouses,

actually would age faster
than people on the ground.

This is a mind-blowing concept,
but it's reality.

Narrator: Earth's gravity
even controls time

high above the planet.

12,500 miles up,

global positioning satellites

crucial to navigation systems
orbit the earth.

Hughes: We here on earth use
the global positioning system

as a way of getting around,
and most people, these days,

would be lost
if they have to go

more than about a kilometer
from their house

unless they have their gps app
on their phone

to tell them where to go.

Narrator: The gps receiver
in your cellphone

bounces signals
over four satellites

to figure out
exactly where you are.

It's an exercise
in precision timing.

Onboard each satellite
is an atomic clock.

The weaker gravity in orbit
means the satellite clocks tick

fractionally faster
than those on the ground.

If we didn't know to correct
for the fact that the clocks

on our satellites
move at different rates,

the gps system here on earth
would not work.

It would actually lose accuracy
at such a rate

that the entire
global positioning system

would become useless
in less than an hour.

We correct for that
every moment of every day.

♪

narrator: Space-time
has controlled every phase

of the universe's evolution
since its birth.

Now we're discovering space-time
will also dictate

the universe's death.

[ explosion ]

♪

narrator:
Our universe started

with a bang
13.8 billion years ago.

It's been expanding ever since.

Will this expansion
last forever

or will our universe
come to a violent end?

♪

for almost a hundred years,

we've now known that
the universe is expanding.

Everything in the universe
is expanding away

from everything else.

Narrator:
We can test this by measuring

light from exploding stars.

Type 1a supernovas all explode
with the same brightness,

so scientists can accurately

work out their distance
from earth.

For decades, astronomers have
measured this light

being stretched
by expanding space-time.

♪

the universe is expanding,
and there's matter in it.

That matter has gravity,

and that is distorting
the curvature of space-time.

So it made sense to us that
as the universe expanded,

all of the matter
in the universe

would hold onto each other
gravitationally.

Plait: If there's enough matter
in the universe,

it can actually
pull on itself enough

that the expansion gets slower.

Narrator: But in 1998,

astronomers took
new measurements

and made
a sensational discovery.

People had it expected it to be
slowing down, to decelerate,

but instead,
they found the opposite.

The expansion is accelerating.

Narrator:
If expansion was slowing,

then these distant lights
should appear brighter.

Instead, they were dimmer.

They were getting farther away
much faster than expected.

It could only mean one thing.

That expansion
is getting faster.

It's accelerating every day.

Narrator: This discovery turned
our understanding

of the universe upside down.

For the first 7 billion years
of the universe,

the rate at which
the universe was expanding

was going slower and slower,

but then,
something crazy happened.

It was as if gravity
had become the opposite.

Instead of attracting
the galaxies,

it was almost as if it was
pushing them apart.

That's a very surprising result.

We're still struggling
to understand it.

♪

narrator:
Was gravity losing its power,

or was there something else
pushing space apart?

There is another ingredient
in our universe --

an ingredient
that behaves very oddly.

The mysterious quantity
called dark energy.

Narrator: Like space-time,
dark energy is all around us.

We can't see it, but it makes up
70% of the stuff

in our universe,
but what exactly is it?

Dark simply means that we have
no idea what it is.

We don't know
what form it is in.

Something is pouring energy
into the universe,

causing it to accelerate.

-We don't know what it is.
-No clue whatsoever.

We don't understand it.

That's the greatest mystery
out there today.

♪

narrator: Dark energy behaves
in mysterious ways.

Ordinary matter is attractive.

Dark energy is repulsive.

That's why it's causing
an acceleration.

Ordinary matter feels gravity.

It comes together,
but this stuff doesn't.

Narrator: Is dark energy
a new force in the universe,

or, like gravity, could it come
from space-time itself?

Dark energy may very well be
a property of space-time.

It may be that space itself
has an energy,

and it's this energy
that's driving it

to accelerate in its expansion.

Dark energy is a thing.

We don't really know
exactly what it is,

but it will have a huge effect

on the future changes
in the universe.

Narrator: So what will happen if
dark energy keeps accelerating

the expansion
of our space-time universe?

Oluseyi: Because of the presence
of dark energy,

it'll expand faster
and faster and faster,

which means the universe
is going to become a lonelier

and lonelier place to be.

All the galaxies
are accelerating away

from each other.

The universe gets dimmer and
dimmer and colder and colder.

Everything gets darker
and more desolate,

and right now that is
the leading candidate

for what's going to happen
in our future.

Narrator: Our space-time
universe eventually freezes.

Sutter: The big freeze is the
ultimate endgame of the universe

as we know it.

It is an ugly fate.

It's a depressing fate,
but luckily for us,

it's not until an unimaginably
long time from now.

♪

narrator:
Trillions of years from now,

the universe could end
in a big freeze.

♪

but a 2017 study hints

at an even more
frightening possibility...

♪

...Dark energy might be
getting stronger.

One horrible scenario

for the ultimate fate
of the universe

is if dark energy eventually
grows so strong enough

that it can overwhelm
the gravitational attraction

of a galaxy itself.

It's even able to rip
black holes apart.

♪

bullock: The very fabric
that holds everything together

could be ripped apart --
this idea is called the big rip.

Narrator:
First clusters, then galaxies

like our own milky way
will be torn apart.

Then our solar system
will break up,

and in the final half-hour
of the universe,

the earth will explode.

[ explosion ]

in the final second,
atoms will vaporize.

Everything in the universe
would individually be torn apart

by the expansion of space.

Sutter: We don't understand
dark energy. Is it constant?

Is it getting stronger?
Is it getting weaker?

At this stage,
we simply don't know.

[ whooshing ]

narrator: The future
of our space-time universe

hangs in the balance.

Will it end in a big rip,
a big freeze,

or is the end really...

Just the beginning?

[ explosion ]

narrator: Space-time controlled
our universe's birth,

and it will dictate
our universe's death.

♪

now we're discovering these
two events may be linked --

but they reveal a flaw

in our understanding
of the big bang.

All the galaxies,
all the planets and stars,

all of this matter was
compressed into a tiny volume,

shrunk down to
an infinitely small size.

Narrator: We call this tiny,
infinitely dense point

a singularity.

♪

singularities are predicted

by the general theory
of relativity...

But the universe
is also governed

by another set of rules...

♪

...Quantum mechanics.

Quantum mechanics
is our description

of the subatomic realm,
of fundamental particles

and fields and forces
and how they interact.

♪

narrator: Quantum mechanics says
that nothing

can be infinitely small
or dense,

so singularities can't exist.

A singularity is a bit where
everything kind of goes to hell

because the density
has become infinite.

Gravitational force
has become infinite.

Things just sort of
break down there,

and the equations sort of stop
making sense.

Narrator:
No singularity would mean

no big bang as we understand it,

so then, how did the universe
spark into existence?

♪

scientists now think
they have an answer,

a solution that works
with both general relativity

and quantum mechanics --
quantum space-time.

Tegmark: The successful theory
of quantum space-time

should answer the question
of what really happened

in the earliest moments
of our universe.

Hopefully, the correct
quantum theory of gravity

won't have any singularities.

It will replace the big bang
with something else.

If we only knew what
that something else was,

we might have a clue as to how
and why the universe began.

Narrator: In a quantum
space-time big bang,

there was no singularity
bursting from nothing.

The universe formed
from the remnants

of another dying universe.

It's possible that
before the big bang,

there was still a universe.

There was still space and time,
but rather than expanding,

the universe was contracting.

♪

narrator: Perhaps universes
don't end in rips or freezes.

Perhaps they collapse.

♪

an ancient universe expands

but then begins to collapse
under its own gravity,

crunching space-time
down to a speck.

♪

but instead of forming
a singularity,

space-time once again explodes.

Oluseyi:
As matter gets more dense,

then as the matter
crunches down,

this force will push
everything back out.

The universe would bounce
and reignite

in a new round of expansion,
a new big bang.

What we perceive as the big bang
is the aftermath of that bounce.

♪

narrator: This suggests
that the space-time

that dictates our lives today

comes from the collapse
of an old universe

and we live in an infinite
space-time cycle of birth,

death, and rebirth,
a bouncing space-time universe.

If we live
in a bouncing universe,

it's very plausible
the universe is infinitely old,

that it's gone through
an infinite series of bounces,

and there is
no absolute beginning.

It could be that we are
just one iteration

of an infinite number of cycles

in the lifetime
of some meta-universe.

Narrator:
We barely understand space-time.

Perhaps we will never
understand it completely,

but it's is clear.

Without space-time,
we would not be here.

Carroll: Space-time is something
absolutely real.

It's really part of
the fundamental architecture,

the furniture of reality.

♪

to really understand
the ultimate fate of our cosmos,

it's not enough just to look
more with our telescopes.

We also have to understand
the basic nature of space-time.

♪

I think the biggest thing
to take out of all of this

is that the universe is weird.

♪

oluseyi: You hear about
these very weird things

that we talk about
when we study the universe

and cosmology and relativity.

It sounds like it could be all
made up, but trust it's not.

♪

you really are, right now,
living in a far more complex

and beautiful universe

than the human mind
can comprehend.

♪