All language subtitles for Secrets.of.Size.Atoms.to.Supergalaxies.S01E02.WEBRip.x264-ION10

On scales far beyond
human perception,

there are strange beasts,

exquisite palaces,

wondrous landscapes.

Some just a few thousandths
of a millimetre long.

Others dominate the vast
expanses of the cosmos.

Thanks to ground-breaking
new technologies,

I'm setting out to explore
these hidden worlds.

Mind-blowing to look at that.

Each one of these dots is a galaxy.

It's sending you a daily email
from space.

At the tiniest
and the largest scales,

I'll see how the laws of physics
have bizarre consequences,

overturning everything
we thought we knew.

My journey will take me to
the frontiers of modern science,

revealing our latest discoveries

and our biggest
unanswered questions.

This is the story
of how the universe works

at scales we can't normally see,

from nanoparticles
to galactic superclusters.

When we move to scales
beyond our imagination

the universe behaves
in extraordinary ways.

But about three centuries ago,

when objects like this
were first created,

we had no real understanding at all

of the true size of our universe.

You might recognise this.
It's called an orrery

and it's a beautiful little
mechanical model

of the solar system.

Now, it's not really a toy.
It's been used for centuries

to demonstrate the relative orbits
of the planets around the sun.

But you shouldn't take this
model literally

because it's not to scale,
both in terms of the relative sizes

of the planets
and their distances from the sun.

Imagine the Earth were about
the size of a football.

Now, in reality, our planet is
12,700km in diameter,

so this has been shrunk down
roughly 60 million times.

So what if we were to shrink
the sun down by 60 million times

so it's the correct size relative
to this Earth. How big would it be?

We can now see just how wrong
the orrery is in terms of scale

because the sun would
in fact be this size.

The sun is over 100 times
the diameter of the Earth.

You could fit one million
Earth-size planets inside it.

The sun's mass is the source of
its immense gravitational power

over the whole solar system.

The next question is, on this scale,

how far apart should
the Earth and sun be?

Again, this is very different
from the orrery

because if the Earth is here
next to me, the sun would be...

..2.5km away.

The distance between the Earth
and the sun

is actually a very significant
figure in astronomy.

You see, in space,
distances are so vast

that using kilometres soon
becomes very impractical

and so astronomers have come up with
their own units of measurements.

And, as it happens, the distance
between the Earth and the sun

is one of them.

The sun is 150 million kilometres
from the Earth,

a distance known as
one astronomical unit.

So, our neighbouring planet, Mars,
orbits the sun at an average

of 1.5 astronomical units.

The asteroid belt, containing
millions of floating rocks,

orbits at between two and three AU.

Jupiter orbits at an average
of five AU.

Saturn at ten AU.

And Uranus at 20 AU.

And Neptune orbits at an
average distance of 30 AU.

Many of us think this is
the end of the solar system...

..but if you travel
beyond the planets

to about 120 AU from the sun,
you'll encounter

a giant, mysterious structure which
helps protect us here on Earth.

It's a bubble called
the heliosphere.

The heliosphere was only discovered
in the middle of the 20th century

but astronomers of earlier times
did glimpse hints of its existence.

The clues were in the behaviour
of comets -

icy bodies of rock and dust

which orbit our sun
at different distances.

Comets have two tails which develop
as they approach the sun

and they point in slightly
different directions.

One of them is made of dust and
it often trails behind the comet

in a kind of arc, but the other
straight tail, called the ion tail,

is made of gas and that's
the one we're interested in.

Here's the puzzle that baffled
the astronomers of the past -

the ion tails of comets
always point away from the sun.

It's as though there's a wind
being produced by the sun

but how can this be possible?

I mean, space is empty, isn't it?

How can there be a wind
blowing through empty space?

A clue to this cosmic mystery
can be seen

when there's a total eclipse
of the sun.

The sun has its own
fiery atmosphere

which shoots away
from its surface.

It's called the corona.

It consists of colossal ejections,
loops and flares

of a kind of superheated gas
called plasma.

But the plasma extends far beyond
the corona, out among the planets.

So the sun does have a wind,
it's called the solar wind,

and that's what pushes
the comets' tails away from them.

The force driving the solar wind

comes from the sun's intensely
powerful magnetic field.

I guess a lot of people
don't realise that the sun

even has a magnetic field.
Yes, it does, that's right,

just like the Earth. The sun's is a
bit more complicated and it varies.

It's a bit more dynamic, but the
basic principles are the same.

At its most basic level,
the sun's magnetic field

is quite similar to that
of this bar magnet.

And you can see quite clearly that
there are regions near the equator

where the magnetic field lines
are basically closed,

they're closed loops, whereas over

the poles, the magnetic field lines

actually point out further
into space.

At the sun's surface, the magnetic

field lines twist and turn,

raising the temperature
to millions of degrees.

This heat makes the charged
particles move so fast

that they break free
of the sun's gravity.

The particles then flow out

along the magnetic field lines

to become the solar wind.

Here they're trapped, here they can
get away far from the sun

and that's what forms
the solar wind.

Tim has another demonstration
to show how the solar winds

create the enigmatic heliosphere.

Water coming out of the hosepipe,
that's like the solar wind

going away from the sun
in all directions,

and then it forms a boundary
all the way around.

The water flowing outwards
from the hosepipe

collides with the water
already in the pan,

creating a turbulent boundary.

That's exactly what happens
in the solar wind

and that boundary forms actually
beyond all of the planets,

something like 100 times further
away from the sun than the Earth.

The water's forming a barrier here
because the water that's already

in the pan is stopping it from
spreading out any further.

It provides a natural barrier.

What's the equivalent to that
when it comes to the solar wind?

What's stopping that from
spreading out into space?

Right, the equivalent boundary there
for the solar wind is actually

the tiny amounts of material
that live between all of the stars.

So we think of space as totally
empty but actually it isn't.

There are tiny amounts of dust, gas
and even plasma-charged particles,

and collectively we call that
the interstellar medium

and that's the material
that the solar wind hits

that causes this boundary.

The interstellar medium -
the stuff which the solar wind

is flowing into,
is mostly composed of gases,

especially hydrogen and helium.

Gases can move and flow,

so in that sense they act
a bit like water.

If we do think of space as a fluid,

then the heliosphere
is a bit like a boat,

carrying us through
the interstellar medium.

Like the hull of a boat, our
heliosphere helps protect us

against the interstellar elements...

..and the storms of space.

Space doesn't just contain
gas and dust,

it's also contaminated
with radiation

produced by cataclysmic
events like supernovae.

Fortunately for us, the magnetic
bubble of the heliosphere

also acts as a shield,
scattering and deflecting

much of this radiation
and preventing it from reaching us.

So, in a sense, our heliosphere
is like a life raft

carrying us through the vast
and dangerous waters of space.

As it pushes its way through
the interstellar medium,

the heliosphere even creates
a bow wave,

just like the bow wave
at the front of a boat.

It's thought that most if not
all stars have heliospheres.

Our telescopes have taken
photographs of other stars

whose heliospheres
are clearly visible.

Sometimes we can even make out
the bow wave.

In 2020, the European Space Agency
launched a mission to the sun,

a probe called Solar Orbiter.

And among the scientists
who built it

was a team from Imperial College
London, headed by Tim Horbury.

Half past seven, and we're due to
launch in about three and a half

hours' time. It's really exciting,
the tension's really building.

It's looking really good.
One and a half hours to go.

Go, Solar Orbiter.

You see it launch
and then we get this feeling.

It's like a vibration of the launch
because it's such a powerful event.

It just makes you tingle because you
know that it's your baby, almost.

Off it goes.

I'd already been working on the
project for 13 years at that point.

Wow. And to have such
a personal connection

with what was going on
was incredible.

For Solar Orbiter's instruments
to work at all,

the engineers have
had to protect them

from the extremes of temperature
near the sun.

The heat shield is always
pointed at the sun,

so wherever we move around,
we orientate the spacecraft

so that that is what's
pointing at the sun.

The instrument that Tim and Helen
built is a magnetometer.

It's mounted on a boom at the back.

The heat shield can get to
temperatures of up to 500 degrees

when it goes in close,
but the electronics,

which lives in the main body
of the spacecraft,

we have to keep that below
50 degrees. So from that,

front to the back, which is about
half a metre, we have to lose

all that heat, and it's much better
for us actually to be out here.

Because although it's cold,
it's a stable temperature,

and that really helps us to
keep the measurements stable.

The Orbiter takes photographs
of the sun's surface

and the magnetometer measures
its magnetic field.

The results are then sent
straight back to Earth,

to the delight of Tim and the team.

Do you want to have a look? Yes,
please. So, this is the kind of data

that we get, and every day we can
see what the spacecraft is doing

and what the instrument is doing.
It is quite remarkable, isn't it,

that it's sending you a daily email
from space every day,

updating you about the spacecraft.
It really is.

The orbiter has just started
taking photos like this,

a gigantic eruption of plasma
from the sun's surface.

And the magnetic readings
will help us understand

why all this activity is happening.

The aim is for Solar Orbiter
to help unlock

the mysterious workings
of the heliosphere,

the bubble which helps
protect us on Earth.

Every object in the solar system
is held in place

by the sun's enormous
gravitational power.

And as we move beyond
the heliosphere

to around 100,000 AU from the sun,

it's thought there's one more
massive structure out there.

A sphere of icy objects
called the Oort cloud.

At these scales we start using
a new unit of measurement -

the distance light can travel
in one year.

So, the Oort cloud
is believed to extend

up to three light years in diameter.

Now, travelling outwards,

I'm going to leave our
solar system behind.

We now know that the beautiful
ribbon of stars

we can sometimes see stretching
across the night sky

is our own Milky Way galaxy,

but around 100 years ago, most
cosmologists didn't even believe

there were such things
as galaxies

and the universe seemed
a much, much smaller place.

In the first decades
of the 20th century,

one of the biggest debates
in cosmology revolved around

a fundamental question -

just how big is the universe?

Many astronomers believed
that everything we could see

in the skies, in fact every
object in the universe,

was part of one continuous
giant structure

of stars, dust and gas
all clumped together.

So the argument went, what we refer
to today as our Milky Way galaxy

was in fact the entirety
of the universe.

But some astronomers
thought differently.

They argued that our universe
is composed of

an unknown number galaxies.

Colossal independent islands
of stars,

located at vast distances
from each other.

So our Milky Way would just be
one galaxy among many.

Now, if that idea was right,

then the universe would
have to be far, far larger

than had ever been suspected.

The debate revolved around some
mysterious objects in the sky,

fuzzy patches of light
called nebulae.

Although they were very faint,

improving technology
meant astronomers

could start to get
to grips with them.

Telescopes of the time
were even able to make out structure

in some of these cloudy objects,

like this large nebula in
the constellation of Andromeda,

photographed around 1901, 1902.

Some astronomers argued
that nebulae like this

were relatively close to us.

Large clouds of dust perhaps,
where new stars were being born.

But others argued that nebulae

were independent galaxies
in their own right,

made up of billions of stars

at mind-boggling distances away
from us.

The debate was solved
to a great extent,

thanks to the astronomer
Henrietta Swan Leavitt.

For many years her work
was largely ignored,

but it's finally being acknowledged
by a new generation.

Henrietta Swan Leavitt
worked at Harvard Observatory

about 125 years ago
as a research assistant.

We actually refer
to them as human computers.

But back then, computers
were people, mostly women,

who crunch the data and do things
like measure the positions of stars,

their brightnesses,
make all these catalogues.

She was absolutely dedicated
to her work.

She catalogued so many stars,

looked at so many photographic
plates over and over.

So here are some
of Leavitt's photographic plates.

Tell me what we're seeing here.

Yes, these are photographs
of the night sky.

They're negative images.

Leavitt dedicated herself to
studying a very weird type of star

known as a Cepheid variable.

These grow and shrink
at regular intervals,

so they pulse in brightness
at predictable periods

which vary from days to months.

Leavitt concentrated on
looking for variable stars

in two mysterious nebulae,

the Small and Large
Magellanic Clouds.

I find it hard to believe,
looking at a picture like this,

she managed to discover
and catalogue 1,777 variable stars.

Mind-blowing to look at that
and think about how kind of

painstaking that work
must have been.

And looking at these stars,
she noticed something

that I think really just
changed our understanding

of the universe forever.

Leavitt realised that there
was a strict relationship

between how intrinsically bright
these stars are

and how quickly they pulse.

So if we look at these
four pairs of lights,

the pair at this end
are clearly the dimmest,

but they also flash the quickest.

Exactly, yeah.

The brightness increases
but the flashing slows down.

And at this end, these
are the brightest pair,

but they're also
flashing the slowest.

And that's exactly
what Leavitt noticed,

that the brighter stars
flash more slowly

and the dimmer stars
flash more quickly,

and it's really what her main
contribution to astronomy has been.

Here's the reason
for its importance.

If we see two Cepheid variable stars

and they're pulsing
at the same rate,

then we know they must also
have the same brightness.

So, if one seems dimmer to us,

then we know that it must
be further away.

Leavitt had found a way we can
measure huge distances in space.

In 1923, Edwin Hubble
pointed a telescope

at the mysterious Andromeda Nebula

and he found that it contained
a Cepheid variable.

He measured its pulse rate
to be very slow - 31 days.

So, using Leavitt's discovery,

he could calculate that it must be
very, very bright.

But because it looked
so faint when seen from Earth,

it must be very, very far away.

In fact, the Andromeda Galaxy is two
and a half million light years away.

So the mystery of these faint,
wispy nebulae was solved.

As suspected, some of them are
indeed collections of gas and dust,

relatively close to Earth.

But others, like Andromeda,
are their own independent galaxies

floating at unimaginable
distances from us.

So now that we can identify

which celestial objects
are part of the Milky Way,

we can create a map of it,
starting with our solar system.

Michelle, I can see the Oort cloud
and the sun in the middle.

Yeah, so the Oort cloud is sort of
the edge of our solar system almost,

and we don't know exactly
how large it is,

but it's roughly two to three
light years in diameter.

OK, let's see what else is in
our local neighbourhood

if we zoom out a bit more.

So what is starting
to come into shot now?

Yeah, so Alpha Centauri is our

nearest neighbour star system.

So it's actually three stars,

two large stars and one
smaller, fainter star as well.

But we can also see Procyon there,

that's about 11 light years
away from us.

And then we also have 61 Cygni
over here.

Compared to our solar system, we're
talking about quite large scales now

but to people like me who study
other galaxies in the wider cosmos,

this is still a really
tiny distance.

For me, it's not even a real unit
of measure until we get to,

you know, 100,000 light years or so.

So all these stars
we're looking at here,

they're just sort of in the way.

You want to be able to see way
beyond that. Yeah, exactly.

Normally I don't worry about
anything that's less than

a few 100,000 light years across.

So let's zoom out
much, much further

from the stars
of our local neighbourhood...

..to encounter an important
structure in our Milky Way,

which is over 10,000
light years long.

So what we're seeing here now
is the Orion Arm,

which is our local or
our home spiral arm where we live.

And spiral arms are kind of
the largest visible structures

we can see in spiral galaxies,

and that's because they're full of,
like, the youngest, brightest stars.

The Orion Arm is home to one of
the most famous astronomical objects

visible from Earth -
the Orion Nebula.

A dense molecular cloud
lit up by the stars it contains.

It's well over 1,000
light years away from us.

Moving outwards from the Orion Arm,

we start to see
the whole structure of our galaxy.

OK, wow. So there it is,
the Milky Way galaxy.

Where are we now in the Milky Way?

So if we start with the centre
of the galaxy here,

where we have this
bright elliptical bulge,

we're about 25,000 light years
away from that.

So up here.

And so how far does
the entire galaxy stretch?

So we don't know exactly,

but we think roughly,
from end to end,

it's about 100,000
light years across.

It's the force of gravity
which creates all this structure.

Gravity also draws in
hot, young, massive stars

to create the bright central bulge.

In some regions of our galaxy,

the interstellar medium is drawn
together into areas of high density

where turbulence sculpts the gas

and dust into astonishing shapes.

These are stellar nurseries,

where pressure from the force
of gravity heats up the hydrogen,

making it hot enough
to ignite fusion.

These are the places
where new stars are born.

But by studying the operation
of gravity at a galactic scale,

we've encountered a mystery.

As our galaxy revolves
around the centre,

what keeps it all together
is the mass of all those stars

and gas and dust acting as
a kind of gravitational glue.

During the second half
of the 20th century,

astronomer Vera Rubin pioneered
the study of galactic rotation.

She worked with instruments
that could measure it

with ever-greater accuracy.

As she did the maths, she realised
something extraordinary.

OK, so we've shrunk the Milky Way

galaxy down to manageable
proportions

and we're looking
at how it's rotating.

Now, Vera Rubin was one
of the leading figures

who realised there was a problem

with the way galaxies
like this rotate.

Exactly, and so what we've got here
is a simulation.

We've set it up so we've mapped in

all of the visible matter
we can see.

So the stars, the dust,
the gas and the galaxy.

And now if we just set it rotating

at the speed we know it's rotating
at, we can see what happens.

As the galaxy revolves,

it drifts apart in all directions,
losing its shape.

OK, so it's not holding together.

There's clearly a problem.

Precisely and this is what
Vera Rubin realised.

And the problem is, if we just
take all the mass that we have here

and add it up, there's not enough.

It can't provide enough gravity
to stop the galaxy

from flying apart
when it's rotating.

Rubin came to an
astonishing conclusion.

Either our theories of gravity don't
apply universally at large scales,

or there's a huge amount of matter
there which we can't see.

It's now thought
that perhaps around 85%

of all the matter in the universe

is composed of what's known
as dark matter.

We can't detect it directly,

but it must be there
to hold our galaxy together.

Most cosmologists now think
our galaxy is surrounded

by a halo of dark matter,

a kind of scaffolding which
keeps it from flying apart.

Some estimates say it could be up
to two million light years across.

The light we can see
with our own eyes

reveals just a small fraction
of what's really out there.

That's because many astronomical
objects give off radiation

right across what we call
the electromagnetic spectrum.

This is the
electromagnetic spectrum.

On this end you have gamma rays,

which have the shortest wavelength

and the highest energy.

Then X-rays and then ultraviolet.

On this side you have
infrared radiation

and then microwaves
and finally radio waves,

which have the longest wavelength

and the lowest energy.

And this slice in the middle,
this is visible light.

It's what our eyes

have evolved to see.

The rest of the spectrum is,

to our eyes at least, invisible.

In the past few decades, we've
launched telescopes into space

that can photograph the universe
across the spectrum.

And as we look beyond visible light,
it's like pulling back a veil...

..giving us astonishing new insights
into the very larger structures.

Things we never suspected
were there.

Astronomers take photographs
of the same object

with instruments sensitive
to different wavelengths.

They then combine these
to produce images

that our eyes alone
would never see.

These incredibly sophisticated
imaging techniques

have given us privileged glimpses
of astonishing events,

frozen moments from the birth, life
and death of stars and galaxies.

Our ability to penetrate
the mysteries of the cosmos

has also led to some
bizarre surprises.

ORGAN PLAYS

Astronomers have found
massive objects in space

that effectively
give off musical notes.

The obvious question
that people would ask is,

you know, how can you hear sound
in empty space?

In space, no-one
can hear you scream.

Absolutely, but there are
still particles in space,

very diffused gas throughout
all of space

and it's possible for sound
to propagate in that

as ripples through the gas.

Using an X-ray telescope
called Chandra,

Andrew and his team took these
photos of a region of space

250 million light years away.

They show massive energy bursts,

shock waves rippling out
through the interstellar medium.

Each ripple is effectively
part of a sound

with a wavelength
of cosmic proportions.

Andrew has calculated
that these sound waves

make one of the lowest notes
ever detected in the universe.

You've calculated that
the sound is a B flat?

Indeed. Which is why
we're here - because George,

who's the senior organ scholar
here at St John's Chapel,

is going to play us some notes.

First of all,
George plays us a B flat

in the mid range of the organ.

Every time George
goes an octave lower,

he has to double the size
of the organ pipe he's using.

Three octaves down and he's using
the longest pipe at his disposal,

and the note is getting to be
as low as we humans can hear.

You can almost hear
the individual oscillations.

It's like a pneumatic drill.

So we've gone down three octaves?
Yes. But to get to the sound...

We have to go down
another 54 octaves,

so we're going to have to keep
doubling the size 54 times

and it would then be galaxy-size.

The organ would have to be
the size of a galaxy? Yes.

THEY LAUGH

The note is so low
because the object only gives off

one sound wave
every ten million years.

The same kind of massive ejections
have been detected again and again,

all coming from
the centre of galaxies.

This extraordinary image shows
that millions of years ago,

the centre of our own galaxy gave
off gigantic bubbles of energy,

spanning about 50,000 light years.

So what can possibly lie
at the centre of galaxies

which give off
these colossal outbursts?

This is a panorama photo
of our own Milky Way,

but seeing what lies at the centre
is a scientific challenge.

In order to see
what's at the centre,

why isn't it simply a case
of pointing our telescopes

at the core of the galaxy
and just looking?

It's because of this black stuff.

It's dust particles
that lie between the stars.

They're produced by the stars,

but they're all over
this interstellar space.

Wherever you look,
there is going to be dust.

It's material that's
not dissimilar from smoke,

and you know how smoke blocks light.

Essentially, it becomes impenetrable

when you're looking along the plane
of the galaxy.

Fortunately, dust is only a problem

in the visible part
of the electromagnetic spectrum.

Andrew shows me
the same view of our galaxy

using a telescope that detects
infrared radiation,

not visible light.

And in the infrared,
we can see through the dust.

Looking through,
we can see many more stars.

There's 100 billion stars
in the galaxy.

Most of the stars in our galaxy

rotate around the centre
at roughly the same speed,

about 220km a second.

But that's not true
for the innermost stars.

All of these stars
will still be going round

at 220km per second,

but then as we get right
towards the centre,

we find there's a group of stars

which are actually moving
in a strange way.

They're moving much faster,
and you can see that this one

is swinging round in an orbit
around something around there.

It goes up to 5,000 kilometres
per second, much faster,

and something must be doing that.

It's by studying strange mysteries
like the orbits of these stars

that astronomers have come
to an extraordinary conclusion.

The only possible solution for this,

the only thing known to physics
that we can fit inside that orbit

and have that gravitational pull
is a black hole.

A black hole with a mass
of four million suns

and therefore there's
a supermassive black hole

at the centre of our galaxy.

A black hole is formed when matter,
such as a massive star, implodes,

becoming so dense
that not even light

can escape its gravitational pull.

It's now thought that there may be
a supermassive black hole

at the centre of every large galaxy
in the universe.

In 2019, a team of astronomers

managed to take the first-ever
photograph of a black hole

in a galaxy called M87.

A key issue here is that the black
hole itself cannot emit anything.

Light only falls into it, it doesn't
come out of the black hole.

And what we can see is emission

from material swirling round
the black hole

and it's in what we call
an accretion disk.

An analogy for the accretion disk
of a black hole

can be found in a kitchen sink.

As water goes down the plughole,
much of it swirls around the rim.

In supermassive black holes,
some matter is swallowed,

some swirls around
forming the accretion disk.

The result is an astonishing
build-up of heat caused by friction.

It gets to hundreds of thousands
of degrees.

All of the emissions from that
accretion disk, which include jets,

it releases an enormous
amount of energy.

We're now pretty sure
this is the answer

to the puzzle
of the giant outbursts.

They're coming from
the supermassive black holes

at the centres of galaxies.

Andrew believes these black holes
have had a fundamental role

in shaping the galaxies around them.

The jets of high energy particles
and radiation

are believed to have triggered the
formation of generations of stars.

Andrew has also found
that these explosions of energy

can blow the interstellar medium out
of a galaxy, halting star formation.

Galaxies come in all shapes
and sizes.

They range from dwarf galaxies,

perhaps just a tenth the size
of the Milky Way,

to giant ones, many times its size.

It's now thought that there may
be up to two trillion galaxies

in the observable universe.

But here's the question...

Do all these galaxies just
float around by themselves,

moving through space serenely
as independent objects,

as lone travellers?

Or do they come together as part
of even larger structures?

And how can we even
answer that question?

Fortunately, to help
tackle this puzzle,

cosmologists can take advantage
of a very peculiar phenomenon

we're all familiar with
here on Earth.

TRUMPET SOUND LOWERS
AS CAR PASSES

What you heard there was
the famous Doppler effect.

As the car approaches,
the pitch of the trumpet is high.

But as it passes me,
that pitch drops.

The effect happens
because, as the car approaches me,

the sound waves bunch together,
shortening the wavelength

and raising the pitch.

As the car drives away from me,

the waves are spaced out,
lowering the pitch.

Light waves do the same thing.

When a source of light is moving
towards us, its waves shorten,

which moves them towards
the blue end of the spectrum.

When it's moving away,

the waves are shifted
towards the red.

So, by measuring the spectrum of
light given off by a cosmic object,

astronomers can tell if it's moving
towards us or away from us.

It's one of the uses of
a technique called spectroscopy.

I'd like to give you an example of
how spectroscopy works in practice.

A few days ago, I asked
my local astronomy club -

the Hampshire Astronomical Group -

to point one of their telescopes
at the Andromeda galaxy.

Now, this is the nearest
large galaxy to us

and the one that Hubble used
to make his big discovery.

Using an instrument
called a spectroscope,

they were able to record
its spectrum.

And here it is.

Now, there's a lot of science here,
so I'll take you through gently.

Here's the spectrum,

this black and white image
that was actually taken.

And here it is again in all
the colours of the spectrum.

Now, this wiggly line here
is the intensity of the light

at different wavelengths,

different colours
that's coming from Andromeda.

These dips here are signatures
of particular elements in Andromeda.

And this big one here
is the important one,

this is the signature of sodium.

So you see this dark line here,

that corresponds
to the wavelength of sodium.

Now, if Andromeda wasn't moving
towards us or away from us,

there would be no Doppler shift
and we would expect to see this dip

at a particular wavelength.

We should see that dip
between these two vertical lines.

But, in fact, you can see
it shifted slightly to the left

to shorter wavelengths
or slightly towards the blue end.

That means Andromeda's light
is blueshifted,

which means it's moving towards us.

And from the amount of shift,

we can work out how fast
it's approaching us.

You might well ask,
"So what, who cares?"

But the remarkable thing is

that by measuring a tiny shift
in a wiggly line,

we're able to calculate
that the Andromeda galaxy

is heading straight towards us
at 300km per second.

So, through spectroscopy we've
discovered our galactic destiny.

It will take about
four billion years,

but Andromeda is predicted to hit
our galaxy in a cataclysmic merging

which may even eject the solar
system from the Milky Way entirely.

Using spectroscopy
and other observations,

we now know that galaxies move
through space in complex ways.

Under the influence of
the vast power of gravity,

many of them are drawn together into
what are known as galactic groups

of up to about 50 galaxies.

These groups can be drawn
into larger structures

called galaxy clusters of perhaps
1,000 or more galaxies.

And these clusters
can group together

forming the largest known structures
in the universe,

galactic superclusters.

These consist of millions
of galaxies

and can stretch across distances

greater than 100 million
light years.

In recent years,
cosmologists have discovered

the galactic supercluster
which we are part of.

Over the past decade,

Professor Helene Courtois

of the University of Lyon

has been working with an
international team of astronomers

on the epic task of mapping it.

Hello, Helene.
Hi, Jim. It's nice to see you.

Helene worked day and night
using video conference calls

to colleagues at radio telescopes
across the globe.

These telescopes are in different
time zones, so when do you sleep?

Yes. Oh, we sleep when we can!

But I love observing, so I always
try to be on all the shifts.

I've seen you describe yourself
as a cosmographer.

Yes. What does that mean?

So it's someone who is making maps.

I try to find out
where are the other galaxies

compared to our galaxy,
and then I measure distances

and co-ordinates in the sky
of those galaxies.

My speciality is not only
to make maps,

but to map the motions
of the galaxies in the universe.

So I am a dynamic cosmographer.

I'm going to do galaxies all my life
because what I want to see

when I open my computer,
I want to see beauty.

So, every morning, I look
at what the telescope

has been running in the night
and I see galaxies all the time.

I don't want to do any other job.

First, Helene and colleagues
plotted the positions

of many thousands of galaxies,

creating intricate 3D maps
like this.

Each galaxy is just a little dot.

Then they measured
the spectrum of every single galaxy

to see if it's blueshifted
or redshifted,

and that meant they could
work out the direction

in which each galaxy
is moving and at what speed.

They found tens of thousands
of galaxies

all flowing in the same direction,
forming a giant supercluster.

And they fly like this,
like a dance,

so the motions of galaxies are
correlated, they travel together.

They are not like
with random motions.

In some parts of space,
they travel together

and this is how
we make this discovery.

In 2014, they made an
astonishing announcement.

They'd mapped
the giant supercluster

in which our own galaxy resides.

They gave it a Hawaiian name -
Laniakea.

Its true size is incomprehensible
to our minds,

confined as we are to our earthbound
scales, shapes and sizes.

But there are landscapes
here on our planet

which can give us an inkling
of what Laniakea must be like.

To help us understand what these
galactic superclusters look like,

cosmographers like Helene often
use the analogy of a river system,

with smaller streams flowing
into larger streams

which then flow into rivers,
all heading towards the sea.

At the larger scales
of the universe,

galaxies move together through space

along pathways
which resemble rivers.

The power of gravity makes raindrops
fall into streams

and streams to flow
downhill into rivers.

In the same way, galaxies are pulled

by the immense power
of gravitational attraction,

heading to an enormous
concentration of mass.

So let me give you a tour
of Laniakea.

This is our home galaxy,
the Milky Way.

And we live within
what's called the local group

which contain some of the galaxies
we've met before.

For example, here's the Small
and Large Magellanic Clouds.

These are dwarf galaxies that were
studied by Henrietta Swan Leavitt.

They're about 200,000
light years away.

Over here is the giant
Andromeda galaxy,

which of course is on
a collision course with us.

It's over two million
light years away.

Our local group stretches up
to ten million light years across,

but it only forms a small part
of the structure

which Helene and her colleagues
have discovered.

So let's see what the whole
of Laniakea looks like.

Each one of these dots is a galaxy.

And each one of these lines
is a pathway that they follow.

Our Milky Way and all the other
galaxies in our local group

sit in one of the streams
over there.

All of the galaxies are being pulled
by incredible gravitational forces

along these pathways.

They're all moving towards
a central mass

called the Great Attractor.

The Great Attractor
is still a mystery,

but it's thought to have
the mass of trillions of suns,

attracting galaxies across hundreds
of thousands of light years.

And there we have it,
our home in the universe.

Laniakea contains about
100,000 galaxies like ours

and 100 trillion stars.

It stretches across
half a billion light years.

Looking at all of this, you can see
why they called it Laniakea.

It's a Hawaiian term
meaning 'immense heaven'.

These giant superclusters
are only just being discovered,

so we're only now starting to get
to grips with the way they work.

From earthly scales to
the rotation of galaxies,

the force of gravity
is well understood by physicists,

but it doesn't necessarily mean
we understand how gravity holds

these giant superclusters together.

So cosmologists like
Helene use superclusters

to investigate the workings of
gravity at the very largest scales.

Our question, our physics question,
is gravitation.

How does it work on large scales?

Does it work just like on Earth?

And was it always the same in time?

So we use the universe as a lab
with huge masses - the galaxies.

From the infinite small
to the infinite large,

it's the main question of physics
of the 21st century.

We still miss the full
understanding of it.

By studying superclusters,

a few cosmologists
are beginning to question

some of our most cherished
scientific principles.

According to our current model
of cosmology,

the laws of physics suggests

that structures much bigger
than Laniakea cannot exist.

If I look at this sheet
of sandpaper,

at this scale it looks
completely uniform,

there's no pattern to it.

But if I zoom in,

using this portable microscope
connected to my laptop,

suddenly I see structure.

Mottled colours of black
and brown and yellow,

shades and speckles,
which are completely invisible

when I'm zooming out
looking at the whole sheet.

According to our current theory,

a similar principle
applies to the universe.

After the Big Bang,

matter was scattered pretty evenly
throughout the cosmos

because the same forces
acted equally on everything.

The power of gravity
brought together galaxies

and then galactic clusters.

But over a certain size,

gravity is too weak
to bring structures together.

Like my sandpaper, the universe
should appear featureless.

This is what's called
the cosmological principle

and it's one of the foundational
pillars of modern cosmology.

But recent findings
have cast some doubt

on this extremely important idea.

Remarkably, one of those discoveries
was made by a student.

Alexia teaches violin to students

to help fund her way through
her PhD,

and while doing her research she
came across something remarkable.

It was really exciting,
but completely accidental.

My supervisor had this idea
of using this new technique

to map what's out there
in the universe

and I happened to stumble across
this giant, great structure.

It was completely serendipitous.

The ingenious method
which Alexia used

involved very distant objects
called quasars.

These are immensely bright
centres of galaxies,

billions of light years away,

thought to be powered
by supermassive black holes.

For Alexia, they were
extremely useful

because you could use them
as if they were spotlights,

shining a light on the dark corners
of the universe.

We're going to use these two torches
to represent quasars.

These very distant powerful
sources of light.

Now, you have a simple demo here,
this plate of glass.

Tell me what these blobs are.

These little blobs here

are representing faint galaxy
and galaxy clusters.

Now, without the quasars, we
wouldn't be able to see them.

They're too faint and too distant.

But with the quasars, they
essentially act like a torchlight

that illuminates this faint matter.

As the light
passes through a galaxy,

some of it is in effect blocked.

We can see that some
of the light was absorbed

and therefore something
must have absorbed that,

and therefore there
is matter lying there.

Using this method, Alexia has
been able to create a 3D map

of a huge chunk of the universe,

and she seems to have found
a pattern.

These red blobs are all galaxies
or galactic clusters,

and together they appear
to form a giant structure

in the shape of an arc.

We've used three different
statistical tests

and all three of the tests
show that the giant arc

is actually more than
just a random fluke.

That it's really there. Why is
it so unexpected and exciting?

So we have this thing in cosmology
known as the cosmological principle

and it says to us that
on the larger scales

there should be no structure
or pattern in the universe.

The cosmological principle
has a specific cut-off limit

and that's estimated to be
about 1.2 billion light years.

But the giant arc here is over
three billion light years wide,

so it begs the question
how something like that can form

in our current understanding
of cosmology.

The giant arc is so big
that, if we could see it from Earth,

it would look 35 times bigger
than our moon,

even though it's nine billion
light years away.

The reason it's so mysterious

is because it's too big
to be held together by gravity.

And it's not the only one.

Even bigger superstructures
have been found,

such as the Hercules-Corona
Borealis Great Wall,

thought to be three times bigger
than the giant arc.

It's early days in the story, but
this debate could be a game-changer

because all our theories
of how the universe was formed

after the Big Bang are based
on the cosmological principle.

Time will tell, I guess.
It's still too early to be sure.

It's still too early,
but everything is built on top

of this assumption
of the cosmological principle, so

it's kind of like taking the bottom
piece of a Jenga puzzle out...

And everything collapses.
..and the whole thing collapses!

Carrying out investigations
at these mind-boggling scales

continues to challenge
our most cherished beliefs,

transforming everything
we thought we knew

about how the universe behaves
and how we came to be here,

and I can't wait to see
what we discover next.