All language subtitles for How the Universe Works S10E05 Hunt for the Universes Origin 1080p

There's a mystery at
the very heart of the universe.

We don't know how old
the cosmos is.

Understanding
the age of the universe

is fundamental to understanding
the universe at all.

It's at the heart of everything.

It's more than just
celebrating a birthday.

We want to know
how much mass is in it,

how much energy is in it,
how it behaves.

We have to have this number
nailed down.

The age of the universe

enables us to not only
understand where we came from,

but potentially,
the fate of the universe,

what will happen millions and
billions of years from now.

But our quest to discover
the age of the universe is

starting a war.

Usually Nature
just whispers to us.

Now Nature is screaming
in our ear

that we're doing something
wrong, and that's exciting.

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We think the universe
started with a bang.

Everything that has ever
existed is squashed up

in this space smaller
than a pinhead,

and all of a sudden,
space just starts expanding

everywhere at once.

The idea that
the universe grew from

a ball smaller than a pinhead
is hard to understand,

but figuring out
when it happened

sounds like it should be
more straightforward.

It seems like a simple
question right?

But it turns out, getting

the age of the universe is
pretty tricky.

Scientists have
just a single fact

as their starting point...

The universe is expanding.

When people realized
the universe was expanding,

they thought they finally had

a way to estimate the age of
the universe.

Take the universe now
and run it backwards in time.

Things get closer and closer
until they come

to a single point.

That time to that point is
the age of the universe.

The expansion rate is
so important,

it's been given its own name...

The Hubble constant.

The Hubble constant
is the present day

expansion rate of the universe.

It is a key ingredient
to understanding

the entire expansion history
of our universe and its age.

Scientists
discovered a strange

radio signal
permeating the cosmos.

It's the remnants of ancient
light from the early universe.

We call it the cosmic
microwave background, or CMB

for short.

The cosmic microwave
background radiation is

simply the afterglow
of our Big Bang,

the way the universe looked
when it was 400,000 years old.

The European Space Agency
launched the Planck satellite.

Using sensitive radio receivers,

the orbiter studied the sky in
every direction,

measuring tiny changes in
the temperature and polarization

of the radiation signal.

The CMB has all these variations

in temperature, and they're not
randomly generated.

They are there because of
physical processes

that occurred when the universe

was in its primordial
fireball phase.

The red blobs are
where matter was hottest,

and the blue areas are
where matter was cooler.

The smallest red blobs are where

hot material was packed
tightly together.

That's where material in
the universe would have

been denser, and that's
where galaxies would

preferentially form.

It's so cool to get to look at
those blueprints and study them

and see how that baby universe

later grew up into the universe
we see around us today.

Although it doesn't
look like much,

hidden within this picture
is almost everything

we can know about the universe.

In a complex process using

different mathematical models,

cosmologists figured out how
the ancient cosmos

captured in the CMB

became the universe
we see today.

They worked out how
the universe got from

small to big and how fast
that expansion happened.

The data from
the cosmic microwave background

is absolutely the gold
standard for cosmology.

It's beautifully clean, we can
understand it really well,

and we have a lot of confidence

that what we learn from it
is pretty robust.

By running
the expansion backwards,

we get an age...

13.82 billion years.

Job finished!

But it's not quite a slam dunk.

The figure must be verified.

We don't make
a single measurement

using a single technique.

We make multiple measurements
via multiple techniques.

Another group
of scientists use

a totally different method

to calculate
the age of the cosmos,

measuring objects
that we can see

in our universe to determine
how far away they are and how

fast they're moving away from
us as the universe expands.

The most direct and most
accurate measurements

are using what is known
as parallax.

Parallax is the apparent shift
in an object relative

to the background
when it's viewed

from two different locations.

So if I look at my thumb with
one eye, and then I close it

and look at the other eye,
it looks like my thumb moves.

If I move my thumb
closer to my face,

then the distance it moves
back and forth changes.

It appears to move back
and forth more.

That parallax difference
as we move the thumb closer

and farther from the face

is the way we measure
distances to distant objects.

Using parallax,
we can measure

the distance to bright stars
called cepheids

in the Milky Way.

Cepheids
are stars that burn 100,000

times brighter than our sun,

so they're extremely bright,
and they pulsate, meaning they

get brighter and dimmer over
a regular time period.

Cepheids that pulsate at

the same rate have
the same brightness.

They're known
as a standard candle.

A standard candle is something
that is a standard, meaning

we know how intrinsically
bright it is.

So all we have to do is measure

the brightness that we appear
to perceive on Earth,

and then you solve
for the distance.

So imagine that
you're on the street.

By looking down the street,

you'll see that the street
lights get dimmer and dimmer

the farther away they are,

but that's not
their intrinsic brightness.

Their intrinsic brightness
is the same.

So by seeing how faint
the farthest away ones are,

you can understand how far
away they are from you.

We can use
standard candles to measure

the distance to stars
farther away.

But there's a big problem...
Throughout the universe,

there's a competition between
the expansion pushing things

apart and gravity pulling
things together.

In the Milky Way,

there's so much matter
that gravity wins.

Even looking at galaxies in
our neighborhood,

the expansion is tiny,

but at cosmic scales of very
different galaxies,

matter is more spread out,
and expansion wins,

so we can only measure expansion
over massive distances.

The way we start to measure
distances to things that

are farther and farther away

is to use something we call
the distance ladder.

Each category of object
that we observe

is on a separate rung
of this ladder.

Measuring the distance to one
will then inform us how far

away the second rung is
and then the third rung.

So each rung depends on
the previous rung, and from

stacking these together, we can
start to measure things very,

very far away from us.

Using parallax
to measure cepheid stars in

the Milky Way
gives us a benchmark.

We can then use
their standard brightness

to measure cepheids
in other galaxies.

The next rung is a brighter
standard candle called

Type 1A supernovas.

They can be seen
in galaxies farther away.

Finally, we can measure light
from distant elliptical

galaxies, and by looking
at how red the light is,

we can work out how fast
they're moving away from us.

So those three things
give us the nearby universe,

the somewhat far away universe,
and the very distant universe,

rung by rung.

March 2021.

Scientists measure
the light from 63

giant elliptical galaxies,

the farthest rung of
the distance ladder.

They hope to get the most
accurate measurement of

the Hubble constant to date
and a precise age

for the universe.

Their calculations make
the universe

13.3 billion years old,

not too far away from
the figure of

13.82 billion years

given by the cosmic
microwave background,

a difference of around 6%.

That sounds trivial, but that
equates to hundreds of millions

of years of cosmic history
that either happened

or didn't happen.

50 years ago,
when we weren't quite as good

at measuring everything about
the universe,

we would have been thrilled to

have our numbers
agreeing to this level.

But nowadays, having
a difference like this,

it's unacceptable.

Clearly, the two
techniques do not agree.

Cosmologists split into
two camps.

We had hoped that these two
methods were like building

a bridge from either side
and then meeting in the middle.

But they're not.

Now we know that something is
going on

we don't understand.

Even though
these measurements

are roughly the same,

it's really dangerous to just
accept them and assume that

everything's fine,
because in science,

usually, the initial really big
discoveries start off

as small differences,
but then you pull

on that thread,
and something wonderful emerges.

So does a simple question,

how old is the universe,
unravel everything?

The universe
is expanding outwards.

The rate it's growing is
called the Hubble constant,

and it's the key to working out
the age of the universe.

So the Hubble constant might
just seem

like some academic number that
doesn't mean anything,

but that number contains
information about

the composition,

the evolution, and the fate
of the universe.

It's an important number,

but there's a problem.

Our best measurement methods
don't match.

It's incredibly frustrating
to not

know how old the universe is.

It's even more frustrating
to know that

there's two experiments,
which are excellent experiments

that we firmly believe in,

that completely disagree
with each other.

My hair fell out a long time
ago over this kind of stuff.

This has been
the number-one question

for over half a decade.

There must be something
wrong with one of the methods.

There's a definite
sense in the community

that whichever camp
you happen to fall into,

the problems lie on the other
side of the fence.

So if you're mainly working
with the cosmic

microwave background,
you probably think

something is up
with the distance ladder.

If there's a problem
with the distance ladder,

there's a prime suspect.

The ladder relies on stars
that have a predictable

brightness called
standard candles.

But there's evidence that
these stars are not always

the same brightness.

So if you expect an object to
have a particular brightness,

and it has
a different brightness,

then whatever conclusion
you draw that relies on

the brightness of that object is

gonna be off somewhat.

Think of the stars
like streetlights.

If one light is broken
and dimmer than the others,

you might think
it's farther away.

The concern with
the distance ladder is that

if any of the single rungs
is not perfect,

then the entire ladder might
be out of whack

by the time you get to the top.

What we need is
a fresh approach

to measuring the age of
the universe.

We're hoping we could
bring in a tie breaker,

a referee, a brand new method

that didn't care about
any of this or any

of that, and tell us what is
the Hubble constant.

We may have just found one.

This observatory
doesn't have a telescope.

It's hunting
for an invisible wave,

a disturbance in spacetime
itself, caused

by massive objects
accelerating or colliding.

It's known as LIGO.

LIGO stands for the Laser

Interferometer Gravitational
Wave Observatory,

and it is a ground-based
gravitational wave detector.

A perfectly stabilized
beam of laser light bounces

in a five-mile-long,
L-shaped tunnel.

As a gravitational wave passes
through the detector,

space stretches,

forcing the light to travel
a tiny bit farther.

You're bouncing a laser
over an incredible distance

and trying to measure as
spacetime itself

gets stretched and deformed

whether that lazar had to
travel a tiny bit further

or a tiny bit shorter,

and a tiny bit here is
the width of a single atom

over miles
and miles of distance.

LIGO has already detected
colliding black holes,

but it's also received
a signal from something

less massive.

Neutron stars are
the densest thing in

the universe other than
black holes.

They're the last stopping point
before you would collapse

all the way to form
a black hole.

They're the size of
Washington, D.C.,

but they can have
the mass of two suns.

A collision between neutron
stars is incredibly powerful.

It's one of the most energetic
events in the universe, and it

distorts the fabric of
spacetime very strongly,

because their gravity
is so strong.

But unlike
black hole mergers,

neutron star collisions can
also send out light.

In 2017, LIGO sent out
an alert... more than

70 telescopes on Earth and in
space swung into action.

This binary neutron star
merger was the first time

we had witnessed
gravitational waves

and light waves coming from
the same event.

It was groundbreaking.

This event is ideal
for Hubble constant hunters.

The light tells us how fast

the colliding stars
are moving away from us.

Gravitational waves
give us the distance.

If we know how far away it is
and how fast it's moving,

that's the Hubble constant.

Having neutron star
mergers added to your arsenal

of ways of measuring

the universe's expansion
is great, because it's

completely independent.

It uses physics that's not
related to either

of the two competing methods
we have so far.

Sounds perfect.

The result?

So this brand-new
measurement that

were hoping would be
a tie breaker...

ended up coming right in
between these two extremes.

Thanks for the help.

But it might not be
as bad as it sounds.

The number of neutron star
collisions where

we have detected gravitational
waves and light... one.

We shouldn't be at all
disheartened by the fact

that this hasn't
actually decided

the problem, because there's
a huge margin for error

when you have just one object.

We would like
something like 100 events

like this neutron star merger.

That might seem like
a huge improvement we need,

but actually,
it's very feasible that

in the next decade,
we'll get there.

Gravitational waves
may give us a precise age of

the universe,
but there is a chance

they'll tell us the problem
isn't with our measurements,

but with our understanding
of the cosmos.

If we keep getting different
answers for the Hubble constant,

especially depending on
the method we use,

that's a big clue that we don't

understand something
fundamental about

the universe's evolution,

its makeup, something important.

Our search for the age of
the universe just might

destroy our model of how
we think the cosmos works,

plunging physics into chaos.

We don't know
the age of the universe.

We had hoped that the results
from our experiments would be

like building a bridge,
starting at opposite ends

and meeting in the middle.

As time goes on,

as the evidence accumulates,

these two sides of the bridge
are not gonna meet.

Something has to give.

Some believe the problem
lies in the way

we've interpreted the picture
of the early universe,

the pattern hidden in
the cosmic microwave background.

We're really confident in
the data that we have from

the CMB, but it's actually
an indirect

measurement of
the universe's age.

It depends on our model of
the universe being right.

It could be, it could very well
be that our fundamental

cosmological model
that we've used

to successfully describe
the universe is coming up short,

that there's something
wrong in there,

that that engine is broken.

That engine is
the standard cosmological model.

Based on our knowledge of
particle physics

and general relativity,

it's like an instruction manual

for how the universe works.

Rewriting it is
a radical suggestion.

For the most part,
it matches what we see,

but it does struggle
with one thing.

As the universe expands
away from the Big Bang,

the intuitive thing you would
expect is for gravity to start

pulling it back together again.

So over time, gravity
would just reverse that

and pull everything back in,
back to a single point.

But what we see in
the data is completely opposite.

What we see is that
the universe is not only

continuing to expand,

but it's speeding up faster
and faster all the time.

To explain
this weird phenomenon,

the cosmological model relies on

the existence of
a strange, unknown force...

Dark energy.

Dark energy is
the most perplexing

and mysterious thing I've
encountered in my research.

Dark energy is a term
that we slap

on this idea that
the universal expansion

is accelerating.

That's about all
we know about it.

We don't know what's causing it.

We don't know how it behaves.

We don't know what
it was like in

the past or what it's like
in the future.

So we just call it dark energy.

It's invisible...
It fills the whole universe

and pushes galaxies apart.

In some sense,
it's like a spring,

a contracted spring, and you
let it go, and it wants to push

everything away.

And things
get stranger.

Dark energy doesn't dilute as
the universe expands.

As empty space gets created

or expands, the dark energy
associated with that

stays the same.

It basically populates
all this empty space.

Imagine I'm draining
a bucket of water,

and water just magically
appears out of nowhere.

That's like how
dark energy behaves

as the universe is expanding.

Dark energy
plays an important

role in the standard
cosmological model.

If our understanding of it
is wrong,

then so too is the model,

which means the age of
the universe we get from the CMB

is wrong, too.

Since nobody has a clue
what dark energy is,

there are a lot of
different theories.

But the biggest question of
all is simply, is it constant?

Our standard assumption
about dark energy is that

it's pushing apart the universe

with the same strength
throughout the history

of the universe.

Now physicists are
wondering if that idea is wrong.

Maybe, in the early universe,
dark energy acted differently.

Hey, you know that whole dark
energy thing that's messing

with the universe today?

Maybe it messed with
the universe back then.

It could be that dark
energy really has affected

the rate of expansion a lot
more than we thought.

This is gonna throw a big
monkey wrench into our idea of

how old the universe is

and what it was like
at different eras.

The theory is called
new early dark energy.

So the idea behind
new early dark energy is that

dark energy was present during
the very early periods

of the universe,
but in a very different state.

Just like you can think of
water being present in

two states,

it can be liquid water if
the environment is quite hot,

or it can be frozen water

if the environment is colder.

We call that a phase change.

Maybe in the early universe,

dark energy underwent
a phase change, as well.

It was different before then
and acts differently now.

According to the theory,
this more energetic state of

early dark energy pushed apart

the early universe
much faster than we thought.

So that speeds things
up in the opening moments of

our universe,

which starts to actually bring
things back into agreement

when you look at
interpreting both

the cosmic microwave
background and the distance

ladder measurements.

One of the things
that we see in the universe

is that things change with time,
density changes,

matter changes, energy changes.

Why not dark energy?

Adding new early dark energy

to the early universe
changes the standard model.

The CMB gives a higher figure
for the expansion of

the universe, and finally,

an age that matches
the one given by

the distance ladder method.

If you think about that bridge
analogy, where the two parts

just don't meet,
the early dark energy adjusts

the angle of the early
universe part of the bridge,

and it just gets them to
actually meet in the middle.

It's still controversial,
but new dark energy may be

detected in detailed
measurements of

the cosmic microwave background.

I mean,
in one sense, like,

do we really need
to overcomplicate

the universe here?
But you know what?

The universe is under
no obligation to be simple.

But there's one thing
physicists can agree on.

Dark energy truly is
a can of worms we've just

opened, and there may be some
big changes coming up.

There is a more
radical possibility.

Maybe we need to ditch dark
energy altogether and question

one of the most famous theories
of all, general relativity.

Is it possible?

Did Einstein make
a colossal mistake?

In trying to work out
the age of the universe,

physicists have started
a revolution,

a revolution that could
overturn everything we thought

we knew about how
the universe works,

including the bedrock
of modern physics,

Einstein's theory of gravity,

general relativity.

Underlying everything,

all of cosmology,
is general relativity,

but maybe we need a completely
new understanding

of gravity.

Gravity is a strange force.

It's always attractive.

The Earth pulling on us
gives us our weight.

The force of gravity
acts over huge distances.

The sun tugs on objects
throughout the solar system.

The Milky Way pulls on
other galaxies.

On the one hand,
gravity is incredibly familiar

to us, you know, the apple
falling from the tree

and all of that stuff,

and we also know that gravity
behaves in a very

predictable way
throughout our solar system

from all the spacecraft
and things we've sent out.

But when it comes to how it
behaves on incredibly tiny

scales and also on incredibly

large scales,
covering the whole universe,

it's possible that we just
don't yet have the right

picture of what's going on.

Einstein's model
of gravity has remained

largely the same for 100 years.

So much of modern physics
is really standing on

Einstein's shoulders,

but at the same time,

we can't ever take
anything for granted.

Claudia de Rham works on
a theory called massive gravity.

It's based on a key part of

Einstein's theory that says
gravity doesn't have mass.

Once you understand that
general relativity is the theory

of a massless particle,

the immediate response
should be,

well, what if it was massive?

The theoretical particle
that carries

gravity is called the graviton.

If gravitons
don't have any weight,

then there's nothing to slow
them down as they speed

through the universe.

They can act over
infinite distances,

just like photons of light.

So one galaxy on this side of
the universe can actually pull

on a galaxy that's right on
the other side of the universe.

But if gravity
has weight, things change.

In some sense,

if we attach a little backpack
to our graviton particle,

its effect is to slowly slow
it down just enough so as to

make its effect
on very large distances

being a tiny little bit
weaker, and that's our way to

switch off the effect of

gravity on huge
cosmological distances.

If gravity is a little bit
weaker, a galaxy on this side

of the universe can't pull on

one on the other side of
the cosmos.

It has a huge effect on
the expansion of the universe.

If the force of gravity
actually just switches off at

large distances,

then you no longer have to
counter the fact that

everything is pulling

everything else together,
because it isn't anymore.

So that would quite naturally
explain why

the expansion of our universe
would be speeding up.

This acceleration is what
we see in the universe today.

Currently, we use
dark energy to explain it.

So,
if the graviton has mass,

that means that we can
get out of the universe

what we see without the need
for dark energy.

What if actually
what we were observing

is simply
the first sign of gravity

switching off
at very large distances.

Maybe we're just observing
the first effect of

the graviton having a mass.

Without dark energy
to deal with,

the universe is a lot easier
to explain.

Maybe we don't need
these complicated physics.

Maybe it's just all the normal
ingredients of the universe,

but operating under
a different set of rules.

Claudia hopes her theory
will soon be put to the test.

Around 2037,

we'll have a new
gravitational wave detector,

the Laser Interferometer
Space Antenna, or LISA.

It'll be bigger than LIGO
and will orbit the Earth.

When LISA get out there
in space,

we'll even have a bigger
handle on

gravitational waves evolving
throughout the whole universe,

and so it will allow us to go
very deep in our understanding

of gravity.

LISA is a system of three
satellites arranged in a giant

triangular formation,
1.5 million miles apart.

It should pick up very low

frequency gravitational waves
from more ancient events,

perhaps even shockwaves from
the birth of the universe.

If the graviton has mass,

then the waves will arrive
more slowly than predicted,

but until we receive those
signals, all bets are off.

It's a big deal to propose
a difference in gravity,

but then again, we don't know.

I'm making no bets.

The universe has proven itself
to be so deceptive.

So I'm gonna wait until it
tells me what it is.

The question of
the age of the universe

opens Pandora's box,

and the expansion rate
of the universe

holds another secret,

our ultimate fate...
How the universe will end.

We know exactly how
the Earth will end.

In around 5.4 billion years,

the sun will turn
into a red giant,

expanding to 1,000 times
its current size.

The Earth will be destroyed.

Humans, if we still exist,

will have long deserted
our home planet.

But how will the universe end?

The age of
the universe enables us to

not only understand
where we came from,

but potentially,
the fate of the universe.

What will happen millions and
billions of years from now?

If scientists
confirm the value

of the Hubble constant,

the elusive figure that tells
us just how fast the universe

is expanding,
it will tell us the age of

the universe, and it will help
us predict its end.

Measuring
the Hubble constant is

measuring the expansion
rate today,

right now, it's like checking
your speedometer at one moment.

But just because
it's your speed now,

it doesn't mean it was
the same speed when you left

your home or the same speed
when you'll be on the freeway.

How the expansion
changes over time

will control the fate of
the cosmos.

So depending on
the Hubble constant,

the universe could
continue to expand.

It could accelerate
its expansion rate,

or it could be decelerating.

At the moment,
galaxies are racing apart.

A continually expanding
universe will

cool down as it spreads out.

Another name for this eternal
expansion is the Big Freeze,

because as everything
gets spread out,

the density is lower, and
there's no more opportunities

for temperature differences.

Everything just gets colder and
colder and colder and colder,

slowly, eternally approaching
absolute zero.

The more matter is spread out,

the less chance there is for
star formation.

And so the universe's
continued expansion means our

night sky and every night sky
in the universe will inevitably

continue to get darker
and darker and darker as things

move further away
and as stars die off.

Eventually,
all the stars will go out,

and there'll just be
the leftovers,

which we call the degenerates,
black holes,

white dwarfs, rogue planets.

It's gonna be
a very, very sad place.

The last refuge
of any matter at all

will be black holes.

You've got a big black hole in
the middle of each galaxy,

over trillions of years,
everything in galaxies fall in,

so finally, you're left with
big black holes over vast

distances, separated

almost universes away.

So getting towards
the Big Freeze,

black holes themselves
start to evaporate.

There won't even be black
holes at the end of this

accelerating universe.

All that's left is very,

very low energy photons and
a little bit of matter dispersed

throughout the universe,
and there's nothing left.

That's it.

We call that
the heat death of the universe.

There's no longer any place
that has more energy

or more heat... it's all just
thin, barely there photons.

It's fascinating scientifically,

but from a human standpoint,

not a lot of fun to think about.

But if
the Hubble constant,

the expansion rate of
the universe, keeps increasing,

then the end of the universe
could be a lot scarier

and come a lot sooner.

One possibility
is that the expansion

of the universe will accelerate

and continue
to accelerate forever,

faster and faster and faster.

And if that happens, we face
a scenario that we call

the Big Rip,
where actually the whole

of space essentially just
gets ripped to shreds.

So the solar system
is gonna get ripped apart,

then the sun and the planets
themselves will start to get

ripped apart.

And finally, it works its way
down to atoms, and atoms get

ripped apart, and we're
starting to see effects on

space and time.

Space is ripped apart.

Time comes to a stop.

So in this scenario,
time and space have no meaning.

If everything is
infinitely far apart,

then space doesn't really exist.

It's sort of beyond
our comprehension.

Working out
the expansion rate will

tell us which scenario we face,

but for now, the lifespan of
the universe is unknown.

Maybe we need to investigate
the other end of the timeline.

But how can we get a fix on

the age of the universe
without understanding

its origin?

As you go back in time
towards the Big Bang,

our knowledge of physics
really goes out the window.

Temperatures off the
scale, pressure off the scale...

The way everything behaved is
just so different that

the rules we have now do
not apply.

The biggest problem of all...

What came just
before the Big Bang?

Einstein's general relativity
predicts that all the matter

and energy in the universe

was concentrated down
to a single point,

the Singularity.

The Singularity is like the part

of those old maps
that says, "Here be dragons."

Singularities are a problem.
We don't like them.

This is where basically you
have a finite amount of matter

in the universe,
but it's squeezed down

into zero volume, so it would
be infinitely dense.

Infinite densities don't
actually happen in nature.

This is a sign that
our math is breaking down.

This is a sign that we need

to replace that with
a new understanding.

Many now believe
Einstein was wrong.

There was no Singularity
begging the question,

could the age of the universe
be infinite?

Scientists
investigating the age of

the universe are struggling
to understand its origins.

Could that be because
there was no beginning?

Could the universe be infinite?

Because we think
we live and we die,

we project that onto
the universe.

But that may not be the case.

The idea
of an infinite universe is

no more strange than the idea
of a singularity.

And in fact, throughout
most of history,

astronomers thought that
the universe was

probably infinite.

The foundation
of our mathematical

understanding of the universe,

Einstein's general relativity,
has a problem.

It doesn't translate to
the world of the very tiny,

which is why its laws break
down close to the Big Bang.

General relativity does
a great job at describing things

on scales that you and I are

familiar with and things like
how planets move

and how galaxies evolve,
all the big stuff.

Quantum mechanics, on the other
hand, describes the world of

the very small,
the world of the atoms.

The problem is that
these two theories

don't fit well together at all.

A new theory known as
loop quantum gravity,

brings quantum theory
and relativity together,

and it makes a stunning
prediction.

So one possibility
is that the end of the universe

could kind of match onto
the beginning of a new universe

and create a cycle of universes,

one after the other.

Nicknamed the Big Bounce,

it predicts a universe that

stops expanding and switches
into reverse.

And the idea here is
that the universe can expand for

a time, stop expanding,

and then begin
to contract again.

And some have suggested
that perhaps

there's a cycle of expanding
and compressing.

It bounces back over again.

One of the appeals of
the bouncing model is

that it allows us to get
beyond the Singularity.

A bit like
recycling on Earth.

All the components get crushed
down and then reused,

giving the cosmos
no beginning and no end.

If the universe is cyclic,

does the age even have
a meaning?

Age is a construct of humanity,

because we need to count time.

But if the universe is infinite,

maybe it doesn't matter in
the big scheme of things.

A contracting
and expanding universe

messes with the concept of age.

But the very idea of

an expanding universe provides
another cosmic curveball.

It might not be alone...
It might

be just one ageless universe
among many.

It's an idea embedded in
the math of the Big Bang.

The most popular theory
we have in astrophysics,

what put the bang into
our Big Bang, is inflation.

This idea that there was
a kind of dark energy

on steroids that made our
universe double over and over

not every seven billion years,

but every split second,

creating out of
almost nothing, a big bang.

When the universe was just
a hundredth of a billionth

of a trillionth of
a trillionth of a second old,

it underwent a period of rapid
expansion called inflation.

It doubled in size at least
90 times, going from

the size of a subatomic
particle to that of

a golf ball.

The problem with this
inflation is that

it doesn't really stop.

It just makes this ever bigger
space and says that,

yeah, well, okay, there was
one region of space where

this crazy doubling stopped
and galaxies formed,

and that's us.

But there's this vast realm
out there where inflation is

still happening.

In the spots
where inflation stops,

parallel universes form.

This eternal inflation
means that

new universes are popping into
existence all the time,

but they're completely separated
one from the other.

Many of my colleagues
hate parallel universes.

They just don't like the idea
that our universe is

so big and most of it is
off limits for us.

If you are willing to be a bit
more humble and accept

that the reality might be
much, much bigger

than we will ever see,

then parallel universes
feel pretty natural.

It's really interesting
how everything

in the universe is
tied together.

We can start with a simple
question like how old is

the universe, and here we are,
questioning virtually

everything about the universe.

Cosmology's

century-long search for
the age of the universe

forces us to question
our cosmological model,

the nature of gravity,
and even time itself.

The age of the universe
does bring up sort of

profound philosophical
questions about how

a universe can even start,

how can you create something
from nothing?

The vast majority of whatever

the universe is,
is eternally hidden to us.

So we answered the questions
how big, how old,

and those very answers
show us that we don't

even know if we've asked the
right questions to begin with.