All language subtitles for BBC.The.Sky.at.Night.2025.The.Expanding.Universe.1080p.HDTV.x264.AAC.MVGroup.org

In the 1920s, everything we thought
we knew about the universe changed.

Astronomers looked up into
the night sky

and realised that the galaxies
and other bodies appeared

to be moving away from us.

There seemed to be only one
reasonable explanation.

They had discovered evidence
that the universe was expanding.

This is a pretty mind-bending concept,

but there's an easy way to imagine
it.

OK, so bear with this.

The stars on this balloon
represent galaxies.

And the black balloon itself
represents the fabric of space-time.

OK. One more.

So let's say
that this star here is us,

and then we've got this galaxy nice
and close,

which is about two centimetres away.

And then we've got another
one over here which

is four centimetres away.

So what I'm going to show you now is
the universe expanding in real time.

By my measurements, this
has just doubled in size.

And in that time, this galaxy

has gone from two centimetres
away to four.

But this one's gone
from four centimetres away to eight.

So it appears, from
where we're measuring,

that the further away galaxy
has moved away faster.

Now, the analogy is not perfect.

I mean, for one, it looks like
the stars are stretching,

but the galaxies
aren't actually growing themselves.

Also, it kind of looks
like everything

is expanding from a central point,

which isn't real
in the actual universe.

However, it really shows that idea
that the fabric of space-time,

and therefore
the universe, is stretching.

And a deeper understanding of this
expanding universe could

be just around the corner.

Welcome to The Sky At Night.

When Edwin Hubble discovered
the universe

was expanding around 100 years ago,

the foundations of astrophysics
were changed forever.

It suggested the universe started
as one point

in time and space and underpinned
the theory of the Big Bang.

But understanding that it
is expanding was just the beginning.

What followed was

the challenge of getting an accurate
measurement of how fast.

Ever since then, we've been trying
to pinpoint that rate of expansion.

With each new generation,
we get closer.

But we're not there yet.

Right now, in the UK,

astrophysicists are working on two
brand-new telescopes.

And with these telescopes, we hope
to find the answers.

It's only by understanding our
expanding universe

that we can unlock some of
the biggest questions,

like the size, age,

and even ultimately
the fate of our universe.

Chris is at Radcliffe Observatory
in Oxford to look back in time.

It's still astonishing to me
that we don't just have to imagine

the start of the universe,
we can actually see it.

It's like a palaeontologist watching
a T-Rex with binoculars.

By looking at ancient light

called the cosmic microwave
background, or CMB,

and comparing it to what we see
around us

in the present day universe,

cosmologists like David Alonso

can work out just how fast
the universe is expanding today.

So, David, what is
the cosmic microwave background?

Well, so the cosmic microwave
background is essentially

the first light that reaches us

from the beginning of the universe,
right?

Cos if it's expanding today,

it must have been
a lot more contracted in the past.

So that means that all particles
that made up the universe,

they would be interacting
with each other very, very often.

The early universe was a dense

soup of particles thick enough
that light couldn't move freely.

As the cosmos expanded and cooled,
it became transparent,

letting light travel across space.

And it's light from this time that
we see as the CMB.

With tiny variations,

these blobs on the map giving us
a glimpse of the early universe.

So how can we use the CMB to measure
the expansion of the universe?

So if you imagine that this is
the size of one of these blobs,

right, given by this ruler here,

and if I put this ruler very close
to me,

I will have to look like this
in order to see the two ends of it,

right, cos it spans
a very large angular size.

If I bring it further away from me,
then it looks smaller, right?

You can grab it, yeah. So as it goes
this way... Exactly.

..it gets smaller. It gets smaller.

You know, it does like this, right?

It forms a smaller and smaller angle
as you measure it.

Exactly, and so if we know exactly
the size of the ruler

and we measure the angle,

then we can know how far away
from us it is.

And so if I do the same thing with
the blobs of the CMB,

what this tells us is what is
the distance

that the photons of the CMB have had
to travel since they were emitted.

Satellites like Planck have scanned
the skies,

capturing the ancient
light left over from the Big Bang,

producing wonderful maps of the CMB
from space.

Like this one released in 2013.

And by starting here

and winding forward
to our present day universe,

scientists are able
to estimate how fast

the universe
must be expanding right now.

But a new ground-based observatory

is about to provide
the sharpest images of the CMB yet,

revolutionising cosmology forever.

The new experiment that I'm
involved in

is something called
the Simons Observatory.

The Simons Observatory is this set
of telescopes put

in the Atacama Desert in Chile,
and so it's a new observatory

made out of seven telescopes.

There's one telescope called
the Large Aperture Telescope,

which is a huge six-metre telescope

that will measure the CMB
at really, really high resolution.

And then there's
the Small Aperture Telescopes.

There's six of them.
We call them the SATs.

And the main goal
for those telescopes is to measure

the polarisation of the CMB

at large angular scales,

cos that tells us something
about other properties

of CMB that we're interested in,

which could tell us about
the very early universe.

It's possible that these new,

sharper images of the CMB will
confirm the calculations of

the current speed of
the expansion of the universe.

Now, while the ancient light of

the CMB can be used to calculate
that speed,

other methods use
light from exploding stars

or supernovae
in the present-day universe,

and these two ways of measuring
the speed the universe is expanding,

well, they don't match,
and that's a problem.

It's interesting
that we get measurements

of this expansion rate
that are really, really similar,

but not exactly the same.

Do you think that's just
a mistake somewhere?

Do you think that's new physics?

Like, what will happen in the next,
say, ten years on that?

In terms of the CMB,

there's been measurements of
the expansion rate from experiments

that have come in between Planck
and us, and they all seem to agree.

I think it'll be great when we do
this with Simons Observatory,

we'll be able to get a completely
independent measurement of it,

so we will know actually
whether it's real or not.

I don't want to say
whether it's real or not,

cos I don't know!

If it turns out that it increases
in significance

the level to which these two numbers
are different,

it will be really exciting, cos
that is then definitely telling us

that there's some new physics,

either in the very early universe

or in the very late universe
that we don't understand,

and then we need to figure out
what it is. So how are things going?

I know Simons
is just getting started. Yeah.

But when can we come
and talk to you about results?

When will you have cosmological
results, do you think?

In principle, Simons Observatory
is going to be observing

for five years,
and we will be putting

out results every year or so,
but in between getting the data

and being able to say this is
the number four expansion rate,

for example, there's a lot of work
that needs to happen

and a lot of making sure. So
sometimes that gets delayed.

I would say two years from now,
maybe come to me,

we'll have the first results.

It's intriguing
that this measurement of how fast

the universe is expanding

made with this CMB doesn't agree
with the result you get

if you use local stars
or supernovae,

things that surround us
in the universe today.

And that difference is one of the
hottest topics in modern cosmology.

Now, it might be that we just don't
understand supernovae well enough.

It might be that there's something
we're missing about the CMB,

about the early universe,

but there's also the possibility

that something's really wrong
with our cosmology,

that this difference is telling us
that there's something

we don't understand, that there's
a novel force

or some new effect
that we've never thought about.

Is that true? We don't know,

but the Simons Observatory
may well tell us.

While we wait for the new CMB
observations of the oldest light

to see if it can help determine

the exact speed
the universe is expanding today,

it's new strides in the study
of more recent phenomena

that may help solve
another cosmological conundrum,

one that came
about from studying supernovae.

Because these supernovae
were so distant,

they gave us a glimpse
of our cosmological past

and a way to trace how rapidly

the universe
had expanded through time.

And they made
an astounding discovery.

The expansion of the universe
wasn't slowing down.

It was actually speeding up.

This was a real surprise.

If anything,

gravity should be slowing
the expansion down,

like dust and debris settling
after an explosion.

Ever since, we have made huge
strides in calculating

that rate of expansion,
and when we pin it down,

it will reveal how our universe
has changed in the past,

but more importantly,

it will show us how it will change
in the future.

Phil Wiseman specialises
in studying supernovae

and this accelerated expansion
of the universe.

Now, you use stars to understand
the expansion of the universe.

But can you tell us how that works?
Yes.

So a particular type of star
called a white dwarf -

it's actually
a small, old dead star -

if it's near another star,

it can actually gain material
from that star.

And once it reaches a certain mass,
it will explode.

Explode as a supernova.
Supernova? Lovely.

OK. And because it always

is exploding around
that same mass limit,

it's almost always
the same brightness.

And that's actually really helpful
for us.

I've got a little demonstration
that I can use to help explain this.

I love a demonstration.
So this little light bulb,

we're going to pretend is a star
about to explode. Yes.

And when it does explode,
it will light up.

But before we light it up,
if we could turn the lights down...

Magic.

Pretending we're now
in the universe.

So the star now explodes.

OK, so it's gone supernova.

And just like the light bulb,

we know how bright it is.

So all Type 1a supernovas have
more or less the same brightness.

Exactly that. Aha!

OK. So if I take this star,

pretend we now have a new star
that we explode from here...

..this should now seem fainter
for you.

It does, but I guess it's
the same intrinsic brightness,

because it's the same light bulb
or the same type of star.

Exactly. So you'd be able to measure
the distance

to this light bulb
by how faint it now looks to you.

Yes, yes. And what we'll do
is take it one step further,

explode one in the distant universe.

And so now you can
measure more distances,

you can measure distances
further out in the universe

and therefore actually further
back in time.

I'll bring you the supernova back.

Yes.

Astronomers then combine
the measured redshift

of a supernova's host galaxy
with this distance.

Redshift is what happens

when light from an object
gets stretched

into longer, redder wavelengths

as it travels through

the expanding universe.

The further a galaxy has been
retreating,

the more its light is redshifted.

This tells astronomers how fast
the host galaxy

of the supernova is moving
away from us.

This combined data gives us

a snapshot of
the speed of expansion over time.

So we've been using these.

But there's a new kid on the block,
a new telescope.

Vera Rubin.
Can you tell us how that's going

to actually improve the observations
of these Type 1a?

The Vera Rubin Observatory,

which is now already starting
to observe the sky in Chile,

is probably and arguably
the most amazing,

most technologically advanced
telescope that we've ever built.

But the real centrepiece
is the detector,

what actually detects the light
from the universe.

That's by far the largest digital
camera that has ever been built,

3.2 gigapixels.
The charge-coupled device,

the CCD that is the detector? Yes.
And we have them

in our mobile phones, but on
a much smaller scale. Exactly.

Exactly. So this CCD is actually
so sensitive

that if you were stood in London

and someone had a candle
in Honolulu,

then Rubin would be able to see
that candle.

Yow. OK. That's impressive.

So you're observing supernovae
at the moment,

but how is Rubin going to be
a game-changer?

To put this in terms of pure
numbers,

so far,
over the history of astronomy,

we've found somewhere

between 20,000, 30,000
and 100,000 supernovae.

Rubin will find a million a year.

Whoa! OK, so brace
yourself for the data.

It's coming through.
It's just...pfft.

So with the onset of Vera Rubin,

it's going to be incredibly
exciting,

all these supernovas coming through,

but how will that help us refine
the expansion rate of the universe?

By putting ten times more supernovae
on that graph

than have ever been on before,

drawing a line, which is the
expansion rate, becomes much easier.

Oh, yes.
The other great point with Rubin is

that most of these supernovae
will be further back in time,

further away than where most of
the supernovae we've got before are.

So that gives you
a better lever arm to compare

the expansion rate now to
the expansion rate back in history.

Yeah. And I suppose it's
the gradient of that curve

that really gives us
the understanding.

And so having more data points
up here will fix it better.

Wow.

Getting a better understanding of
this changing expansion rate

could give us a better
understanding of how our universe

has been changing in the past,

but it could also tell us
about our future.

If you were a gambling man,
what do you think -

is the universe
going to continue expanding,

is it going to contract?

I'm really having to sit
on the fence with this one.

We've made some tentative
breakthroughs

in the last couple of years
that would suggest

that it might not be
the expanding on forever universe

that we've thought it has been
for the last 25 years. Right.

Rubin will put this to bed. Yes.

But I'm going to sit on the fence
until it has. More data needed!

Isn't that always the way? Yeah.

If astrophysicists work ever harder

to measure the universe's expansion
over time,

what's causing that acceleration
still remains a mystery.

When it was first discovered
that the expansion of

the universe was speeding up
and not slowing down,

the term "dark energy" was coined
to represent

what might be causing
the acceleration.

Dark energy isn't something we've
seen

or even remotely understand.

It's just sort of like a nickname.

It could have just
as easily have been called

"invisible power"
or even "dark force".

We can't detect it directly.

We have to learn about it
by observing how it shapes

our universe's matter
and speeds up its expansion.

Sesh Nadathur is working on probing
the effects dark energy

has on our universe's
cosmic structure.

So you work with dark energy,

and I'm going to start you off
with a big question.

What is the evidence
for dark energy?

All the evidence that we have
for dark energy

really comes from us measuring
distances in the universe,

and how fast the universe
has been expanding with time.

And the evidence showed in 1998

that the expansion
had been getting faster with time.

So it was accelerating. Mm-hm.

And we don't know what's causing
that acceleration,

but whatever it is,
we call it dark energy.

But I guess we can't really
investigate dark energy directly.

We're looking at the effects, right?

Exactly. That's right. So how are
astronomers tackling this

right now, then? Well, one of the
things that we're doing with

a big survey collaboration called

the Dark Energy Spectroscopic
Instrument,

or DESI, that's what I work on,

is we're building a giant 3D map
of millions of galaxies.

We've got up to 30 million now,

but we're aiming
for over 50 million.

And looking at the patterns
in that map

in order to measure distances
even more precisely,

and therefore work out how
the universe has been expanding

going back over
the last 11 billion years,

and we use those distances
to determine how fast

the universe has been expanding
with time to great precision.

And that allows us

to measure what's been happening
with dark energy.

And you say you've done 30 million
of them already so far.

Yeah. Wild. OK,
so how is this map, then,

this 3D map, helping us to measure
the speed

of the expansion of the universe?

So there's a very cool feature

that we can extract once we've got
a map that's big enough,

which is actually
a leftover little pattern,

like a ripple, that comes from
the very early universe

and just got frozen in time
at this early time.

And now you can still see it in
the distribution of galaxies today.

Just after the Big Bang,
before the CMB was emitted,

the universe was a hot plasma
of particles and light.

In this plasma, pressure waves,
or sound waves,

rippled outward
through the early universe.

These ripples,
called baryon acoustic oscillations,

or BAOs, left a faint imprint
on the distribution of matter.

When the universe cooled enough
for atoms to form,

these ripples were frozen into
the large-scale structure

of the universe.

The cool thing about this pattern

is it serves
as, like, a little scale on our map.

Once we see it,
we know how far away

those individual
pairs of galaxies are,

so we can work out how far away
they must be from us today.

And we can use that
to measure distances

to galaxies at various points
in our map.

11 billion years
are then already mapped out.

That's quite a lot already.

This is impressive work,
but when combined with the data from

the ancient CMB light
and more recent supernovae,

things get even more exciting.

We can actually combine
our measurements of these ripples,

the BAO feature, with other data
to extend that reach.

So, for instance, we get data from

the cosmic microwave background,
or the CMB,

which is even earlier,
further back in time,

and also, with
the supernovae explosions,

which help us map out
more precisely

the very, very local universe
that is very close to us.

And by putting
all of these three together,

we can span an even wider range
of time and distance.

OK, so that's very impressive.

But, of course, now I need to ask,
what are the results?

Well, the results have been very
exciting because what we've found is

that the rate at which the universe
appears to have been

expanding over this time

does not match our simplest model of
what we thought dark energy was,

which is what we've been working
with for the last 25 years. OK.

And in that model, dark energy was
a cosmological constant,

which means that it was
the same over all time and space,

but now it appears
to be contradicted by this new data,

which seems to show
that it has been changing with time.

If this is confirmed, the changing
behaviour of dark energy

over time could not only give us

a better understanding
of what dark energy itself is,

but it also might solve
cosmology's other problems.

We seem to be kind of like on

the precipice of rewriting
the physics books.

Our understanding of the universe
is just fundamentally changing.

How was this for you,
kind of discovering this

and seeing this for the first time?

Well, I've spent
the last 15 years just thinking

that dark energy was going to be
a cosmological constant,

and we just measure it better
and better.

So this has been a real surprise.

In fact, at the time
when we first saw our results,

one of my colleagues sent me

a picture of our results
with no comment

and just an emoji
of an exploding head.

I like that. And that basically sums
up our reaction to it.

It's new, it's very exciting,

but we don't quite know
what to make of it yet.

I like that. It's kind of classic
academic, sort of like, you know,

just an emoji
for, like, physics is different now.

We are constantly testing
and measuring

the universe around us
because the truth is,

we only know and understand

a tiny portion of how
the universe actually works.

So DESI, along with
the Vera Rubin Observatory

and the Simons Observatory,

will all be needed together
if we're going to finally unravel

the mystery of what is accelerating
the expansion of the universe.

While we scratch our heads over

the mysteries of
the expanding universe,

it's time to stop and look up,

to see what wonders
to look out for in the coming weeks.

Pete is at South Hert's Astronomical
Society to tell us more.

This month brings us the Northern
Hemisphere's autumn equinox,

a time when the length of day
and night

are more or less the same.

That occurs on 22nd September.

Now, the nearest full moon
to this equinox

is defined as the harvest moon,
so called because it's said

its light helps farmers
gather their crops.

In astronomical terms,
the difference between moonrise

for the fuller phases of the moon
over consecutive days

is at a minimum
at this time of year.

On average, in the UK, the
difference between moonrise times

from one day to the next
is around 50 minutes,

but around the equinox
it's typically less than 15 minutes.

To locate the rising moon,
just look east northeast

in the early evening.

On 6th October,

the full moon rises at 18.14 BST.

The next day, on 7th October,

moonrise is at 18.25 BST,

and on 8th October,

moonrise is at 18.40 BST.

That's an average
difference of 13 minutes.

Earth isn't the only place
to have equinoxes.

All of the planets have them,

and they make
for some interesting viewing.

Let's take Saturn, for example.

This had an equinox
back in May of this year,

when Saturn was effectively
sideways onto the sun,

and the sun was on Saturn's
equatorial plane.

This resulted in the rings appearing
very narrow and thin,

and Saturn's largest moon, Titan,

being able to pass across
the planet's globe,

behind the planet's globe
and also into Saturn's shadow.

From Earth's perspective,

we saw Saturn go edge on
back in March of this year,

and at that time the rings
virtually disappeared from view.

However, at the time, Saturn was too
close to the sun to be seen,

and after that,
the rings began to open up again.

But the jostling
positions between Earth

and Saturn means that we get to see
the rings

go virtually edge on again between
mid-November and mid-December.

And it's after that that they start
to open up properly once again.

Saturn is currently
in a good position for observing,

reaching opposition
on 21st September.

Around this date, it can be found

due south around 1am BST,

roughly one third of the way
up the sky.

It's near two asterisms,

or unofficial patterns known
as the circlet and the water jar.

There's a special Titan transit

occurring in the early hours
of 20th September,

as dawn is brightening,

when the moon and its shadow

cross the planet's globe in sync.

Such a sight is very rare.

Titan's shadow transits will
end in October,

meaning you'll have to wait
approximately 13 years

for them to start up again.

Jupiter is also coming
back into view,

and it, too, is heading for
an equinox in December 2026.

This is currently providing
some great opportunities

to see interactions between Jupiter

and its outer Galilean moon,
Callisto.

Jupiter can be seen climbing high
above the southeast horizon

in the run-up to dawn.

It's the brightest thing
in the sky

apart from the moon when it's
around, and if you wait long enough,

Venus appears low in the east
as dawn is breaking.

Using a telescope, it's possible
to see Callisto's shadow

pass across Jupiter's disk
on 15th September

between 02.10 and O5.00 BST.

As ever, if you do take some great
images of the skies,

do share them on our Flickr account

and you can find details of this
and my full sky guide at...

Meanwhile, here are some of
our recent favourite submissions.

You sort of get used to the idea of

a Big Bang
and an expanding universe,

but it's intriguing to hear

that we don't understand
the details of this expansion,

that there might be
new physics lurking there,

waiting to be understood.

I'm excited by these new telescopes
that will help us solve

these mysteries and tell us
new things about the cosmos.

And speaking of mysteries,

remember to send
in your space mysteries

for our show coming up with the hit
Radio 4 programme Curious Cases.

If you have a question
the team could investigate,

submit it now to...

Goodnight.