Original subtitles

I'm on tour in Australia...

Some of them might recollapse in a fraction of a second.

Others might be blown apart by loads of dark energy.

...talking about the life of our universe

to scientists and to space-loving audiences.

I just like having my brain stretched.

I'll probably just stop and have a look at the stars on the way home.

On our journey so far, we've explored how our universe began.

So, this is the oldest light in the universe.

But what about our future?

Is there an end of the universe? Is our universe eternal?

The universe literally tears itself apart.

There is some true vacuum somewhere.

We could tunnel into it at any moment.

Does it make any sense to talk about this universe

in 100 billion, 200 billion, a trillion years' time?

Will there be stars and galaxies?

And people, and questions,

and astronomers, and televisions?

What IS the fate of the universe? How will it all end?

I suppose we all get used to the idea that we live finite lives,

and perhaps that's 'cause we imagine we can leave a legacy -

our children and grandchildren, great works.

But will that legacy be eternal?

Can we imagine taking the great achievements of our civilisation

and chiselling them into granite and leaving them

as a record into the far future?

Or do the laws of nature conspire to make that impossible?

Will all trace of our civilisation one day vanish?

What will be our fate?

And what is the ultimate fate of the universe?

I'm going to investigate the theories that cosmologists have put forward

for the end of days.

There is a law of physics

called the second law of thermodynamics,

and to paraphrase it, it means that on a global scale -

so, across the universe -

things can only get worse.

So, things tend to get more disordered,

things tend to decay away.

I was once involved in putting forward the opposite proposition -

things can only get better -

but that is a gross violation of the second law of thermodynamics.

It really shouldn't have been allowed even in a song lyric.

♪ Things

♪ Can only get better... ♪

I think, actually,

songwriters should pay more attention to fundamental physics.

The first stars in our universe began to shine

around 100 million years after the big bang.

But it was another 9 billion years before our star, the sun, formed.

But our sun won't live forever,

which will obviously have a direct impact on all life on Earth.

The sun is about halfway through the phase where it's burning hydrogen.

It's making helium.

It's got what we call the 'solar luminosity' -

it's putting out that much heat and light.

But that is going to be slowly increasing.

Presumably at some point we would find it extremely uncomfortable here.

Yeah.

In about 1.1 billion years,

it will have got to 10% brighter than it is now.

That is enough to create what's called a moist greenhouse.

So life on Earth will become very problematic at that stage.

3.5 billion years from now,

it'll be 1.4 times its current luminosity,

and that's enough to evaporate the oceans.

Right.

So that's getting fairly severe at that stage.

It's really got another 5 billion years to go

that it can keep on burning hydrogen,

but it will become uncomfortable for us well before that.

The destiny of stars like our sun is to run out of fuel and collapse.

The core compresses, but the rest of the star expands enormously

and we have a red giant.

So we enter this red giant phase.

When our sun becomes a red giant

and it begins to swell and engulf the orbit of Mercury,

it's gonna start getting very hot on Earth.

We're gonna need ways to terraform Mars

and then ship billions of people to Mars,

because Mars will be cooler than Earth,

it being one and a half times farther away.

The Earth will basically get fried at that point.

We won't be able to exist.

Hopefully, if humans are still around in some form or another,

we've moved off to better locales.

So, the ultimate fate of our star, the sun, is to be this cooling ember?

That's the ultimate fate of the sun.

So, in just over a billion years, this place will become uninhabitable.

In 6 to 7 billion years, the sun will be gone.

But that, of course, isn't the end for the universe,

and matter that floats out from our dying sun

will recollapse to other stars, other planets

and perhaps other living things.

But that process can't go on forever.

There comes a point in the life of the universe

when the constant recycling, the births of new stars, has to stop.

So, what IS the fate of the universe?

How will it all end?

Cosmologists have thought about this for a long time,

and many have come to the conclusion that the end will come

in more of a whimper than a bang.

It's known as the heat death,

and it begins with a decrease in the rate of star formation.

Looking up into the southern sky

allows us to see a hint of this process.

As soon as you look through a telescope,

you do get a sense of the amount of gas and stuff out there, don't you?

You always hear this wonderful phrase 'stellar nurseries'.

So, is that really accurate,

that you really do have these places where all the action is concentrated?

Absolutely.

In order to form stars, you have to put together a lot of material,

and so you naturally pop off a lot of stars in that particular area.

I can't... I can't stop looking, actually.

Fortunately, we have very powerful telescopes

to see all the way across the galaxy

so that we can pick out stars

in all kinds of stages of their formation process,

from when they just start collapsing

to when they really kick on and turn into full-blown stars.

Stars form from collapsing clouds of gas and dust.

And the leftover material forms into planets and moons

and asteroids and comets.

But in our part of the Milky Way,

this construction process is slowing down.

Is this a glimpse of the beginning of the end?

We see that around the, sort of, solar neighbourhood,

around where the sun lives,

that there's about half as much star formation

as there was 10 billion years ago.

Why is that?

Once you form a star, it dies

and it holds some of its matter together

in a black hole or a white dwarf or a neutron star,

and you've locked that gas up forever

and it never goes back to being a star again.

So in order to keep forming stars,

you've got to keep bringing gas in from somewhere else.

So the fact that the rate is dropping...

...you said a factor of two from, what, 10 billion years ago to now,

implies, of course, that some time in the future, that's gonna stop.

So, when is that?

It's a long time. Not in our lifetimes.

I think if you do the extrapolation,

you go out 100 billion years or something, more than that.

It's a long time away.

Eventually, we will run out of gas to form new stars

in every single galaxy,

and gradually, gradually, gradually, the universe will... die away.

So, what will our descendants actually see

when they look up into the night sky in a few billion years' time?

Well, the centre of the galaxy is here, isn't it?

Yeah. It's kind of the empty bit of sky up here - Sagittarius.

Peer into the centre of the Milky Way

and, like virtually all other galaxies,

you'll find a supermassive black hole.

Ours is around 4 million times the mass of the sun.

We don't know how these supermassive black holes form,

but we do know about smaller, stellar-mass black holes.

And when the centre of those stars are more massive

than about two or three times the mass of our sun,

they actually collapse into a singularity,

is the theory,

and the space-time around them bends completely on itself,

so nothing...

The gravitational pull is so strong, nothing, even light, can't escape.

So, then we're left, I suppose, with a dark sky,

but a sky full of black holes...

Yeah. ..and, what, fading stellar remnants?

Yeah, fading stellar remnants.

Things like faintly glowing cores of long dead stars,

billions or even trillions of years in the future,

and these are glowing so faintly, you can't even see them.

Now, black holes do radiate very, very faintly,

but we won't actually be able to see that,

because they'll all be too far apart.

You get the sense, I suppose, when you talk about that,

this starry sky, we take for granted.

Although, perhaps we don't.

You know, perhaps it's the most beautiful thing

but it's a temporary phenomenon.

It's actually a very short-term phenomenon, isn't it?

We're only nearly 14 billion years into the universe's history.

So in the far, far future, hundreds of billions of years' time,

if we could still stand somewhere and look out into the night sky,

we would see just blackness,

and there would be no hints at all to tell us

about the past glorious history of our universe.

But is this the whole picture?

Science has a habit of opening up new possibilities -

often when you least expect them.

We've known our universe is expanding since the 1920s,

and we thought the expansion rate was slowing down.

Then, in the late 1990s, it was discovered

that the expansion rate of our universe is actually speeding up.

Nobel Prize winner Brian Schmidt

was as surprised as anyone by his discovery.

1998, we went through and we measured

that the universe wasn't slowing down at all - it was speeding up.

And that was... well, it looked to me like a big mistake

and that I had somehow wasted three and a half years of my life

getting some nonsensical answer.

We knew that Einstein had invented something -

called 'dark energy' or the 'cosmological constant',

which makes gravity push rather than pull,

but that seemed pretty crazy -

that was, sort of, Einstein's 'bad idea'.

If the universe continues to expand forever

and indeed continues to accelerate in its expansion,

then in the far future,

you end up with a universe that's just a sea of photons

being stretched by the expansion of space

and cooling down to the same temperature,

so you end up with a universe where there is no structure.

There's no possibility of storing any information

or processing any information,

no possibility of consciousness or life,

no structures at all,

and that sea of cooling photons will last forever.

The idea that there is a mysterious thing called 'dark energy'

which drives the accelerating expansion of our universe forever

is the standard model of cosmology.

But it's not the only theory about how our universe could end.

The 'heat death' -

this idea the universe just gradually grinds to a halt

and carries on expanding forever -

is a bit miserable,

but the physics of the far future is not very well understood.

So there are, in fact, other, and I think more exciting, possibilities.

So, like the Robert Frost poem,

it seems like things will end in ice, not in fire.

But there's a big caution here,

which is we don't really understand dark energy very well,

and so this is really only one of a couple of possibilities.

Cosmologists have come up with an even more devastating idea

called 'phantom dark energy',

which eventually leads to the universe ripping itself apart.

If the universe has this dark energy,

this stuff causes gravity to push from itself.

This is another really great way to destroy the universe.

So as the universe gets bigger,

this stuff dominates the universe more and more,

because it's part of space itself.

You make more space, you have more dark energy,

which makes dark energy stronger over the other forms of gravity.

Over time, this phantom dark energy

would start to pull apart bound orbits.

So, first, it would pull apart the galaxy.

Stars would start to wander off, 'cause it's still increasing.

It'll start to break up all structure, all matter.

And then at some point,

the entire universe will be... torn asunder, in some sense.

It's called the 'big rip'.

You reach the... the breaking point, literally,

and the fabric of the universe starts to tear a hole,

the laws of the universe and the boundaries of the universe

just start going haywire.

So that's the big rip? So that's the big rip.

I mean, that's one of the theories, right?

There's the big rip and the big freeze.

I love the big rip, 'cause...

...who doesn't like a universe ripping, right?

The 'big rip' idea can be combined with a theory we have

describing the beginning of our universe, known as inflation,

which describes a time before the hot big bang,

when our universe was expanding extremely rapidly.

So, inflation... That's right.

...that rapid expansion before the big bang,

could be envisaged as being the end of a previous universe

that's undergoing a big rip?

Exactly. It's... you know, you have these big questions,

but when you look at it fundamentally,

if you have a universe that grows quickly through dark energy,

which rips,

that would mimic pretty similar to what we think happened in inflation.

We are pretty confident we know inflation had to happen in some form

to get the universe that we see today,

so it's not crazy to think that these two things are related.

We don't have the evidence for it yet,

but it's not the worst idea we've had.

We said that inflation stops and the big bang happens.

But what if inflation doesn't stop all at once -

it just stops in patches?

So then our big bang would just be a little event

in an ever inflating space.

And there'll be other patches that stop and you get a universe,

other patches that stop and you get a universe.

And that process could go on forever.

So, although our universe dies,

other universes are constantly being created.

This is called the inflationary multiverse.

So you get this wonderful process

of big bang after big bang after big bang,

yielding universe after universe after universe.

How are we to picture that?

Are we to picture our universe riding on the back

of some kind of... potentiality of expanding space-time?

You can.

I've got a more down-to-earth way, which is,

think of a big piece of Swiss cheese,

big block of Swiss cheese, right?

Each of the openings is like a universe,

and the cheese itself is the inflaton field

driving the ever accelerated expansion of the universe.

And as this cheese expands,

more and more pockets open up inside -

more and more universes are created.

And our universe is simply one opening

in the block of Swiss cheese -

that's all that we are.

The idea of a multiverse is tricky to get your head around.

This is where we keep... the cream of the crop.

But I'm hoping a glass of Aussie shiraz

will enhance my powers of analogy.

Cheers. Thank you.

I'm tempted to draw the analogy with inflationary cosmology...

Well, of course.

...and say that each one of these is like a different universe

with different physical laws.

Each barrel in this warehouse

is taken from a different part of the vineyard,

so each barrel is a different expression through the grapes

of the landscape.

So, in that analogy,

our universe would be just one barrel -

of shiraz viognier, in this case -

amongst a whole possible warehouse of barrels,

different expressions of possibilities.

It's a good analogy. It's not too forced.

Physicists refer to each different possibility as a different vacuum,

and this opens up the chance that we could be instantly annihilated.

My favourite way to destroy the universe is vacuum decay,

which is a really fun idea

and... and one that's gotten really popular lately

because of things that we're seeing at the Large Hadron Collider.

The Large Hadron Collider at CERN, on the French-Swiss border,

is the largest scientific machine in the world.

In 2012, the Higgs boson was discovered here.

But its properties suggest

that our universe could be in an unstable 'false vacuum' state.

There is some true vacuum somewhere,

and we could... maybe tunnel into it at any moment...

...you know, and just destroy the universe.

By any moment, you probably... I mean, it's unlikely.

You're a cosmologist.

If you're still watching this, it hasn't happened yet.

The timescale for vacuum decay is many times the age of our universe,

so it's not gonna happen tomorrow.

But it's still disconcerting to think that our universe

ISN'T the stable, eternal home we once believed it to be.

But you might think, "Well, quantum mechanics, could it save us?"

"Could there be a little fluctuation that allows something to happen"

"and reboot part or all of the universe?"

You know, you never know.

But probably not.

Whether it's vacuum decay, a big rip

or a sad, slow heat death over the next 10 to the 1,000 years,

now is the time to explore our universe,

while it's still visible to us.

We live in a special time in the universe's history -

the time when the universe is young enough

that we can look out into the night sky and see billions of galaxies.

The galaxies are not receding away from us fast enough

that they outrun the light.

So, our descendants, when they look out into deep space,

will see black, inky, dark stillness.

And they'll think that this little collection of galaxies

that are all bound together gravitationally nearby,

they'll think that's it.

And that's a wrong picture of the universe,

but nevertheless, that's the picture which our descendants

are gonna naturally be led to through their own observations.

The future astronomers in 2 and 4 and 6 billion years

will think we're crazy, 'cause they can't see the evidence that we have.

I think, actually, it was Brian Greene who said to us

that cosmology essentially becomes a religion.

That's right, yeah.

'Cause all you've got is history books telling you...

Exactly.

...about the oldest light in the universe.

That's right. You can't go and reproduce the experiment.

We would just have to believe someone else's word.

And then... what are we doing?

Who are we to think, who are we to say

that WE have a complete record of the universe to draw from?

What chapters of our book of the universe

had been removed before we even acquired the book?

The fascinating thing about the far future

in an accelerating and expanding universe

is that you get to a point where you can't do cosmology anymore -

there's no such science as cosmology.

You couldn't deduce from what you see

that you live in an expanding universe

that had an origin at some point in the past.

So cosmology becomes a subject

that is only accessible through ancient books

and not accessible through the things you can see.

So, in that sense, I think that the future of the universe is sad,

because your ability to gather knowledge

about the universe as it was in its past

disappears.

How will the universe end?

I have no idea.

I don't know.

I don't know.

You know, it's very difficult to make predictions

about the fate of the universe

when there's 95% we don't understand.

OK.

We should just make a documentary called I Don't Know.

Galaxies are forming and stars are exploding

and, you know, we're seeing evidence of things happening in the big bang

and the universe is expanding ever faster,

and we're just sitting here drinking our tea.

Yeah. You know?

I've had the good fortune

to share my enthusiasm for physics and the cosmos

with Australian audiences curious to contemplate

the nature of our universe.

Amazing. Absolutely tremendous. Yeah.

Oh, he takes us on a great journey, I think.

Did you understand it all?

Uh... not all of it, but, yeah, bits and pieces of it, yes.

I love the fact that when I finish work after a really hard day,

I can come to something like this

and he can make me feel really insignificant

compared to the rest of the universe.

And I've met scientists under a truly inspiring southern night sky

who share that curiosity and desire to know more.

We've got work to do!

I suppose there are two ways

of looking at the future of the universe.

You might feel depressed

that although the universe will almost certainly exist forever,

we certainly won't.

There will come a time in the future

when it's not possible for ANY life to exist.

Or you might feel fortunate.

You might feel fortunate that we live in a time

when we can look out into the night sky

and see the light from distant stars and galaxies

and from the very origin of time itself

and understand how our universe began

and make predictions about how it might end.

Captions by Ericsson Access Services

Copyright Australian Broadcasting Corporation