Original subtitles

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.