Original subtitles

Under the cover of darkness,

the world lies hidden from view.

Without light, I've no idea what lies beyond my immediate surroundings.

I'm closed in, enveloped on all sides

by the unknown.

For much of human history,

when the sun went down and the dark set in,

we were at the mercy of the night.

But over the centuries,

we've developed our own sources of illumination.

We've lit our homes, our streets, our cities,

and doing so, we've banished the darkness into the shadows.

And just as we've used light to illuminate our world,

the more we've discovered about light's properties,

the more of the Universe it's shown us.

We've seen into the depths of space...

..and back to the beginning of time.

But as we've looked deeper,

we've come to realise how little we've seen

and that the cosmos's greatest mysteries

remain hidden in the dark.

Light and dark is essentially the story

of everything we know

and everything we don't know about our Universe.

And it all begins with light.

It's such an integral part of the way we perceive the world,

it's easy to take it for granted.

But for centuries, understanding what light really is

has been one of science's most enduring questions.

The first steps toward understanding the properties of light

were made in the third century BC

by the renowned Greek mathematician Euclid.

He did it by thinking about something so obvious,

most of us don't give it any thought at all.

Placing the tiny chair very close to the camera

produces a large image on the retina,

and because we're not used to seeing tiny chairs in everyday life,

our brains are tricked into thinking

it's a normal-sized chair in the middle of the room.

The reason this illusion works at all

is because, to judge distances,

our brains rely on a simple fact -

the further away things are, the smaller they appear to the eye.

And it was by focusing on exactly why

distant objects could appear the same size

as much smaller ones closer up...

..that led Euclid to discover of one of light's most fundamental properties.

Obviously the London Eye is much bigger than my fingers, I know that,

and yet to me they look the same size.

So, how do we explain this?

Well, Euclid came up with an elegant solution.

For my finger to appear at the top of the wheel,

my eye, my finger and the top of the wheel

must all lie on the same line.

But Euclid's insight didn't just explain the tricks of perspective,

it revealed a basic truth about light itself.

Euclid had discovered that light travels in straight lines.

Realising how it travels

marks the beginning of our scientific understanding of light.

And it also meant that if we could divert it from its straight-line path,

we could change the way we see the world.

But that leap wouldn't happen for another 2,000 years.

It was eventually made in Renaissance Italy

by one of the founding fathers of modern science.

In the summer of 1609,

Galileo Galilei made the short but fateful journey from his home in Padua

to Venice, capital of the Venetian Republic.

Galileo had flame-red hair, a full beard,

and was well-known for his love of fine wines and generous hospitality,

and also for his anti-establishment views.

By this time, he'd also built up a reputation as a natural philosopher and mathematician

and he was regarded as a valuable asset to the Venetian Republic.

But although, as a professor, he had a regular income,

Galileo was never far from financial troubles.

When his father died in 1591,

Galileo, the eldest of four surviving siblings,

became the head of the household

and, effectively, took on responsibility for supporting his brother,

a poor itinerant musician,

and for paying his sisters' dowries.

By the time he came to Venice,

he still owed a significant amount of money to his two brothers in law

and so was always on the lookout for a money-making scheme.

That summer,

Venice was abuzz with rumours of a device

that appeared to do the impossible...

..a Dutch spyglass

that could bring distant objects closer.

It was just opportunity Galileo was looking for.

Back in the 17th Century,

the spyglass was cutting-edge technology

and the details of how it worked were a closely-guarded secret.

All Galileo knew was that it consisted of two lenses arranged in a tube,

and so when he developed his own, he kept it very secret, as well.

But we do know from a shopping list

that he got his glass from the small island of Murano, out in the lagoon,

and because no tools existed, he had to improvise,

for instance, buying an artillery ball

to grind the curved surfaces of the lenses.

It had been known since the first spectacles were produced,

in the middle of the 13th century,

that glass had the strange property of bending light.

But unlike spectacles,

the spyglass, an early telescope,

required a combination of lenses

in a very specific arrangement.

This is how Galileo's telescope works.

Rays of light come in from a distant object

so they're almost parallel where they meet his first lens.

This is the objective lens, and it's plano-convex,

which means it's flat on one side and curved on the other.

It's the sort of lens used to treat long-sightedness.

What it does is bend the rays of light towards each other

so that they would meet at a point.

But before this focal point,

Galileo places his second lens, the ocular lens, which is plano-concave,

and this bends the rays of light back out again

so they emerge parallel, where they enter the eye,

and then the eye's lens focuses them on the retina.

Now the magnification of a telescope depends on the ratio

of the focal lengths of the two lenses -

the distances F1 and F2.

The difficulty for Galileo

was grinding down the convex surface of his objective lens

to make it as shallow as possible

in order to maximise the length F1,

because the longer he could make that,

the greater the magnification of his telescope.

Produced in just a few weeks,

Galileo's telescope had a magnification of eight times

and was far more powerful than the original spyglass.

All he needed to do now

was cash in on his new invention.

Ever the showman, on the 21st August, 1609,

Galileo climbed one of the city's bell towers.

BELLS CHIME

LIFT MUZAK: "The Girl from Ipanema"

Obviously, he would've used the stairs!

At the top, in front of an assembled group of Venetian noblemen and senators,

Galileo demonstrated his telescope.

It was a sensation.

Using it, the Venetians would be able to see approaching ships

two hours earlier than with naked eye.

The military and economic advantage of knowing who was sailing over the horizon

was lost on no-one watching that day.

Three days later, as a grand gesture,

Galileo presented his telescope to the duke as a gift.

In return, he was guaranteed his job for life,

at double his salary.

With his finances now secure,

Galileo went on to develop a more powerful telescope,

and with it, use the ability to bend light

to change our perspective on the cosmos.

This is the book Galileo published in 1610.

It's called "Sidereus Nuncius",

which in Latin means "The Starry Messenger".

In it, he recorded his first observations of the night sky

the first anyone had ever made

using anything other than the naked eye.

Today, it's hard to imagine

how anything contained in this little book was controversial,

but you have to remember that when it was written,

the nature of heavens was thought to be knowable only to God

and the Earth was considered to be at the centre of the Universe.

These are his drawings of the moon.

Since ancient times, all heavenly bodies were thought to be perfect spheres,

but with his telescope, Galileo saw texture in the surface of the moon,

deep craters and mountains

that, from the shadows they cast across the lunar surface,

he estimated to be some six kilometres tall.

As well as showing the heavens to be imperfect...

..his telescope began to uncover their true extent,

revealing ten-times more stars

than are visible to the naked eye.

And in the final chapters,

Galileo reports the discovery of four stars

that appeared to form a straight line

near the planet Jupiter.

His drawings show how their positions change from night to night.

Although they moved, they always did so along the same straight line,

and from that, Galileo deduced that they had to be orbiting Jupiter.

They weren't stars at all, they were moons.

Through his telescope,

Galileo had seen evidence

that overturned the accepted dogma

that the Earth was the fulcrum

about which everything in the Universe revolved.

Seeing moons in orbit around Jupiter

meant that not everything went round the Earth.

So, far from being the centre of the Universe,

the Earth was just another planet.

The telescope had allowed Galileo

to glimpse the true nature of the cosmos

and our place within it.

But this way of manipulating light

had another powerful application,

one that would allow us to see into another world.

BELLS CHIME

In 17th-century London,

one of the most prominent scientists of the age

was using lenses in a very different way.

Robert Hooke had taken the basic principle of the telescope

and used it to build a microscope.

Galileo uses the telescope to discover a new world in the heavens,

and Hooke uses the microscope to discover a new world

in the very, very small.

But there's a difference, because what Galileo had presented

was a world that was bigger and more plentiful,

but it was a world that people were at least vaguely familiar with

because you can look up in the sky and see the stars,

whereas the world that Hooke presented

was really something spectacular and new.

It was a world inside the tiniest particles of matter

that no-one had ever imagined to be there before.

People didn't even realise

that there was a microscopic world there to reveal.

Hooke trained his microscope

on a huge range of materials and living things.

But it was his drawings of the exquisite detail he saw in the bodies of insects

that would become famous.

Up here, you can see a human flea, Pulex irritans, a very tiny creature,

and here we've got the plate from "Micrographia",

which is a huge image of the flea that Hooke produced,

and it's really something spectacular.

This would've folded out in the book, so it was really very large.

Some people said it was as big as a cat.

It's a work of art, really. I mean, there's so much intricate detail in there.

It is, and there was nothing like it before Hooke.

They really were unprecedented

and the shading and the quality of the images is just superb.

And it's accurate. I mean, it's... It is, it's absolutely accurate.

I was looking yesterday at images of, er, photographs of the flea and, er, there's really -

made with an electron microscope -

and there's really nothing to chose between Hooke and, er,

the current images.

This is an image of the compound eye of a fly,

which Hooke shows in amazing detail for the first time.

This is an image of the foot of a fly.

Hooke shows you the foot has little spikes in it

that allow it to clasp into the pores on a surface.

This image looks less interesting, less intricate than the others.

It doesn't look terribly interesting

but, actually, it's really quite a profound picture,

because what Hooke is looking at here is a very thin slice of cork,

which he cut with a penknife,

and he's looking at the little individual components that make it up.

And he calls them pores, and then he calls them caverns,

he calls them boxes and then he calls them cells,

and cell, of course, is the term that stuck.

These are the little constituent parts, not just of cork, but of all living things,

and so it's a profoundly important discovery

and a name that has become standard in biology.

Using glass to bend light

revealed our true place in the Universe...

..and the intricate architecture of the microscopic world.

The more we looked,

the more we saw.

With each new insight into the nature of light

came a fresh understanding of the cosmos.

And the next discovery

would take us far further...

..and enable us to read the story of the stars.

And it began with something Hooke had glimpsed through his microscope.

This is Robert Hooke's book The Micrographia,

published in 1664,

350 years ago.

It's full of...

..his famous diagrams.

Here's his picture of the flea.

It's incredible seeing it in its original form.

It really is the size of a cat!

These images really captured the public imagination and they made the book a sensation,

but for me, The Micrographia is about much more than that.

The chapter that interests me as a physicist

is one that contains hardly any images at all.

And it's this one here -

"Of the Colours observable

"in Muscovy Glass, and other thin Bodies".

Here Hooke describes the iridescent patterns of rainbow colours

he sees through his microscope

as light passes through thin materials,

like soap bubbles and Muscovy-glass,

a silicate mineral that's made up of lots of thin layers.

At the time, it was thought that white light, like sunlight, was pure,

that it came directly from God,

and so Hooke concluded that the colours he was seeing

must have somehow been added to the light,

that they were effectively created

as the light passed through the materials.

But Hooke's theory about coloured light

was about to be challenged by his greatest rival.

Isaac Newton is one of the world's most revered scientists...

..best known for his theory of universal gravitation.

And just like his laws of gravity,

Newton's discoveries about the nature of light

are among his most celebrated achievements.

But the story of how that work began is much less familiar,

And this time, there was no fruit involved.

This is Stourbridge Common,

a sleepy riverside meadow on the banks of the River Cam.

But when Newton's visited in 1664, it would've been very different.

For over 700 years, every September this place would be transformed

into what was, at its height, the largest fair in Europe.

For several weeks each year, people would descend on the common

for an annual festival of commerce and debauchery.

DOGS BARK & SHOUTING

SWORDS CLASH & APPLAUSE

This whole common would've been packed with make-shift stalls -

farming produce, brandy houses, goldsmiths, silk merchants.

There'd have been slack-rope dancing, puppet shows, music,

temptations of every kind,

packed into row upon row of wooden booths and tents.

Stourbridge Fair was a place you could buy anything you could imagine,

but when Newton came here, it's said he bought just one thing -

a prism.

He bought it because it performed the same magic

Hooke had seen with his microscope.

Newton would later write that, using his new purchase,

he would "try the celebrated phenomena of colours"...

..a rather understated introduction

to work that would produce one of the most profound insights into the nature of light.

Newton devised an ingenious experiment

to discover precisely how these rainbow colours were produced

and to put Hooke's theory -

that they were created by the prism itself - to the test.

This is Newton's own drawing of what he called his "Crucial Experiment".

In it, he arranged a prism so that sunlight -

coming in from a small hole he'd made in the shutters of his bedroom window -

passed through it and projected coloured light onto a screen.

Well, here's my light source

and here's my prism

which, if I arrange carefully,

I can get projected onto the back pillar.

Of course, none of this was new.

People knew that prisms produced coloured light,

but what Newton did next had never been done before.

He first isolated one of the colours using a slit,

so in this case,

the orange light.

He then passed that orange light through a second prism.

Now, if Hooke was right,

then this prism should add the other colours to the orange

and reproduce the rainbow.

But all Newton saw was orange light.

The prism wasn't adding any extra colour.

He concluded that the colours must be contained in the white light in the first place,

that white light wasn't pure

and prisms don't add anything to it.

Instead, they split it up into its constituent parts.

Newton named the colours that make up white light

"the spectrum",

and when this discovery was combined with the telescope

it would show us something remarkable.

The spectrum would reveal

precisely what it was we were looking at out in space.

This is a spectroscope.

As sunlight comes in, it's broken up into its constituent colours

and spread out much more finely than you'd get with a simple prism.

Now, with this camera,

I should be able to show you what I can see.

I'll just check that it's working.

Yes. OK.

When scientists first did this in the middle of the 19th century...

I'm placing the spectroscope on top.

..they saw something completely unexpected.

You can see the colours of the spectrum as Newton would've seen them,

but if you look more closely, you can see something else.

It's not continuous,

it's broken up by lots of thin black lines.

These are gaps in the spectrum.

It was soon realised that these gaps

were due to atoms in the outer atmosphere of the sun

absorbing certain wavelengths of light coming from its interior,

and that they could be used

to work out the chemical composition of the sun.

Every element absorbs a unique pattern of wavelengths -

an optical fingerprint

that can be used to determine the chemicals

that make up any bright object you can see in the sky.

And in Rome,

one man was using this technique to study light

whose origins lay far beyond the sun.

Father Angelo Secchi was no ordinary priest.

He was charismatic and viewed as something of a heretic by his fellow Jesuits.

That's because he was also a professor of physics,

with a evangelical passion for astronomy.

In 1852,

Secchi was appointed Director of the Vatican Observatory.

Within a year, he'd built a new observatory

on the roof of St Ignatius Church, in the heart of the city.

At the time, most astronomers were interested in mapping the positions of the stars

and charting their motions across the heavens.

But Secchi was different.

He wanted to know what they actually were.

So from his vantage point, high above the streets of the Eternal City,

he began to meticulously analyse their light.

Fitting a spectroscope to the observatory's telescope,

Father Secchi laboriously recorded the spectra

of more than 4,000 stars.

This is Secchi's book "Le Stelle", The Stars,

which he published in 1877.

And flicking through it,

you can see many of the observations that he made.

This one in particular is interesting.

It shows some of the spectra he recorded.

The top one here is from the sun,

but the second one is starlight.

It's from Sirius A, the Dog Star,

which is the brightest star in the night sky.

It's 8.6 light years from Earth

and over 20 times as luminous as the sun.

You can see from its spectrum this clear sequence of bands,

which is the signature of hydrogen,

because it's a relatively young star.

The Universe's hottest, brightest stars

have spectra rich in the two lightest elements -

hydrogen and helium.

But as they age, they cool,

and their spectra reveal the presence of many heavier elements.

This third one is from the star Betelgeuse,

which is a red supergiant.

It's near the end of its life

and so you can see from the many bands here

that it's composed of lots of different elements.

What's remarkable about this image is that,

I mean, it really is one of the key moments in the history of astronomy,

that we can learn so much about what distant stars are made of

just by examining their light.

But because Secchi had catalogued the spectra of so many stars of different ages,

his observations led to something even more profound -

that by analysing starlight,

we can determine the stars' life cycles...

..when they were born...

..and when they'll die.

Understanding the spectrum

had allowed us to read the story of the stars.

It's quite incredible to think

that what began as a simple experiment in a darkened room

could reveal so much about the Universe,

that the scant light from those tiny points in the night sky

could contain within it the epic drama of the heavens.

But that wasn't all the spectrum could tell us.

We know that it's made up of light of many different wavelengths,

and that those wavelengths extend way beyond the range we can see.

The spectrum, from the longest wavelengths used in radio communications,

to the very shortest wavelength, gamma rays,

covers a range of 30 orders of magnitude.

The longest are 1-followed-by-30-zeros

bigger than the shortest.

That's the same as a spread in range of weights

from that of a single grain of sand

to the weight of all the water in all the oceans on the planet.

And within that vast spread,

visible light - the frequencies we can see -

covers a factor of just two.

That's the same as the difference in weight

between this pebble and one twice its size.

Are we all set, Doctor? Yes, I think so.

And throughout the 20th century,

opening our eyes to the full spectrum

revealed even more of the Universe.

If you had infrared eyes, here's how the sky would look.

Infrared allowed us to see the Universe's coolest stars,

while radio telescopes,

sensitive to the longest wavelengths,

revealed a cosmos in turmoil...

It's the violent events that are picked up,

exploded stars and galaxies.

..and satellites scoured the heavens

for short-wavelength ultra violet.

The OAO picks the ultra-violet light from hot stars,

which the atmosphere cuts off from ground telescopes.

And here's the very latest window - gamma rays -

which are like very energetic x-rays.

Seeing beyond the visible

has allowed us to peer deep into the cosmos.

I was cock-a-hoop about this.

I, too, was wildly excited when I heard of this discovery.

But the very fact that light had proved such a useful tool

for exploring the Universe

depended on one of its most mysterious properties.

Light behaves like a wave,

but if it is a wave, what is it a wave in?

Waves are carried across the ocean by the water.

The sound you can hear now is due to waves in the air.

In the vacuum of space, there is no air

so there is no sound.

But the reason you can see me is because I'm lit by sunlight

that has travelled 150 million kilometres

through empty space.

So, what is light,

and how can you have a wave in nothing?

Answering that question would not only reveal what light is,

it would ultimately allow us to glimpse

the beginning of the Universe.

And the first part of the solution

was a discovery that challenged our most basic assumptions

about how we see the world.

To our eyes, light appears to be everywhere,

instantaneously.

When I look out at the view, there seems to be no time lag,

no delay, while I wait for the light to reach me.

But towards the end of the 17th century,

it was discovered that our senses are mistaken.

In 1672, the Danish astronomer Ole Romer arrived in Paris

to begin work at the city's observatory

and to continue his observations of the moons of Jupiter.

For more than a decade

Giovanni Cassini, the observatory's director,

had been documenting their orbits in minute detail.

Jupiter's innermost moon Io

is known to make a complete circuit around the gas giant

once every 1.77 Earth days -

that's every 42.5 hours.

Now, from Earth, we can see it disappear behind Jupiter

and then re-emerge round the other side

as it travels around in its orbit.

But here in Paris in the 1660s,

Giovanni Cassini had noticed

that the timing of these eclipses seemed to vary,

sometimes sooner, sometimes later than expected.

Soon after he arrived in Paris,

Romer noticed that these fluctuations weren't happening at random.

When the Earth was closer to Jupiter,

Io would be seen to disappear and re-emerge earlier.

But as the year went by

and the Earth moved in its orbit around the sun

so that it was further away from Jupiter,

then the eclipses appeared to happen later than expected.

Romer knew the moon always took the same time to travel around Jupiter.

His great insight was to realise that the variations were due to the fact

that light itself takes time to travel through space.

Here's how it works...

The eclipses of Io appear later than expected

when the Earth is further from Jupiter,

because light takes a longer time to cover the greater distance,

but they appear earlier when the Earth is closer

because light needs less time to reach the Earth.

Light isn't instantaneous.

It travels at a finite speed.

Today, we've not only measured light's speed with incredible accuracy,

we've seen it in motion.

This is a video made by scientists at MIT,

using a camera designed to monitor extremely fast, chemical reactions.

It has a shutter speed of around a picosecond.

That's a millionth of a millionth of a second -

the time it takes light to travel just a third of a millimetre.

Now, look what happens when I press play.

What you can see here is a pulse of laser light

moving through a water-filled bottle.

To us, this would appear as the briefest of flashes,

but the camera reveals how the pulse travels through the bottle,

scattering and bouncing around as it hits the water molecules.

Light travels so fast -

300,000 kilometres per second -

that slowed down by the same amount,

a bullet would take an entire year

to travel the length of the bottle.

It's one thing to know that light travels at a finite speed,

quite another to actually see it move.

The discovery of the speed of light was hugely significant.

Not least because it proved crucial

to uncovering what light actually is.

Born in the summer of 1831,

James Clerk Maxwell would become one of the leading lights

of 19th-century physics.

GASPS & APPLAUSE

His work on electricity and magnetism

was one of the greatest achievements of the age.

This is Glenlair in south-west Scotland,

Maxwell's family home.

While he was growing up here, he developed an insatiable curiosity about the world around him,

a desire to understand nature

that he would never lose.

The young Maxwell seems to have taken great delight

in tormenting his parents and his nanny

by constantly asking them how things worked.

"What's the go o'that?" he'd say.

If anyone ventured an answer,

the young Maxwell would only be satisfied for a moment

before asking them how they knew.

Of course, none of this is particularly unusual for a child,

but what sets Maxwell apart

is that he was just 14 years old

when he wrote his first scientific paper.

So young, that a friend of the family

had to present it to the Royal Society of Edinburgh on his behalf.

Maxwell was one of the greatest scientists who ever lived

and it was here that he carried out his most important work.

During the 1860s,

Maxwell produced a virtuoso piece of mathematics

that showed electricity and magnetism

were different aspects of the same thing.

But his calculations would show something else.

Quite by accident, they would reveal the true nature of light.

These are Maxwell's four famous equations

that describe the relationship between electric and magnetic fields.

Curl of E

is minus DB by DT.

E is the electric field, B is the magnetic field.

Curl of B over mu nought,

div of E equals zero,

equals epsilon nought equals nought.

With a bit of algebra and manipulation,

these four equations can be combined to give one single equation.

So the way it's done is like this...

We take the curl of curl of E...

Hidden deep within his mathematics

was something that even Maxwell didn't expect.

..epsilon nought... Grad E 2 div...

This second term is zero

and I'm left with Del squared of E...

..minus mu nought, epsilon nought

D 2 E...

..by DT squared.

This is the wave equation.

It tells us how an electromagnetic field

travels through space.

Now, the important bit is this here -

mu nought, epsilon nought -

because it's related to the speed that the wave is travelling.

In fact, the speed is given...

..by one over the square root of mu nought epsilon nought.

And if you work that out, you arrive at...

..3 times 10 to the power 8 metres per second,

or 300,000 kilometres per second -

the speed of light.

If electromagnetic waves moved at the speed of light,

it could only mean one thing.

Maxwell knew this had to be more than just a coincidence.

It meant that light itself had to be an electromagnetic wave.

The discovery that light is an electromagnetic wave

explains one of its most puzzling properties.

What Maxwell's equations show

is that light consists of electric and magnetic waves travelling through space.

So light is simply electric and magnetic vibrations

feeding off one another as they move.

And we now know that these electromagnetic waves have a remarkable property -

they don't need to be waves in anything,

they can travel through empty space.

I remember first learning about this when I was in my second year at university.

I was in lecture hall 33AC21 of the physics department at the University of Surrey,

the lecturer was Dr Chivers,

and I remember turning to my friend next to me

and remarking on how incredible I thought this was.

I could tell by his reaction that he thought I was a bit of a geek.

But, actually, it is incredible that in just a few lines of algebra,

you can tell what light really is.

And the fact that light travels at a finite speed

has enabled us to do something else.

It allows us to look into the past.

Looking at a mirror one metre away,

you see yourself as you were six nanoseconds ago.

From Earth, the moon appears as it was one second ago

and the sun eight minutes in the past.

The further you look out in space, the further you look back in time.

Light from the cosmos's most distant objects

has taken billions of years to reach the Earth.

But there's one source that has taken us so far back in time,

we've reached the very limit of what can be seen with light.

In 1964, while converting a strange-looking horn antenna

designed for early satellite communications

to make astronomical observations...

..Arno Penzias and Robert Wilson

began to pick up a mysterious signal they couldn't explain.

Here, we had purposely picked a portion of the spectrum,

a wavelength of seven centimetres,

where we expected nothing or almost nothing,

no radiation at all from the sky.

Instead, what happened is that we found radiation

coming into our antenna from all directions.

It's just flooding in at us and, um,

clearly was orders of magnitude more than we expected from the galaxy.

At first, they dismissed it as noise,

something unwanted, generated by the antenna itself.

Now, we had some suspicion

because the throat of the antenna came into the cab and was a little bit warmer,

and that was an attractive place for pigeons,

at least a pair of pigeons who liked to stay there, especially in the cold winter.

We didn't mind that because they flew away when we came,

except that they had coated the surface with a white sticky material

which might not only absorb radio waves but emit radio waves,

which could be part or maybe all of our result.

With the antenna cleaned, and the pigeons -

well, it didn't end well for the pigeons -

Penzias and Wilson began searching for an astronomical explanation.

But the signal wasn't coming from anything in our own galaxy.

Nor did it appear to be coming from any other galaxy either.

It seemed to be coming from everywhere.

No matter when we looked, day or night, winter or summer,

this background of radiation appeared everywhere in the sky.

It was not tied to our galaxy or any other known source of radio waves.

It was rather as if the whole Universe had been warmed up

to a temperature about three degrees above absolute zero.

And so we were left with the astonishing result

that this radiation was coming from somewhere

in really deep cosmic space...

..beyond any radio sources

that any of us knew about or even dreamed existed.

What they'd discovered was light so ancient,

it had been stretched out into microwaves

and cooled to just a few scant degrees above absolute zero,

light that had been travelling to Earth

for almost the entire age of the Universe.

It hadn't come from a distant galaxy

and it was far older than any star.

Penzias and Wilson had discovered that the entire Universe was awash with light

from the embers of the Big Bang itself.

Called the Cosmic Microwave Background,

it was released when the Universe was just 370,000 years old

and it gives us a snapshot of the cosmos in its infancy.

And here it is,

the latest image of the Cosmic Microwave Background,

taken by the Planck satellite and published in early 2013.

The different colours are fluctuations in temperature in the early Universe

and the information they contain

has proved priceless to cosmologists.

The tiny variations in temperature are caused by matter clumping together

into what will eventually become stars and galaxies.

But what's truly remarkable about this image

is that it's not just light from the early Universe,

it's the very first light there ever was.

During the first era of its life,

the Universe was a fireball of hot dense plasma

that trapped light, preventing it from moving.

Then, as the cosmos cooled, the plasma condensed,

forming the first atoms...

..and the first light,

light that would become the Cosmic Microwave Background,

was released into the Universe.

It's sort of hard to express what an astonishing achievement this is,

that from our small planet, orbiting an unremarkable star,

we've reached out into the Universe

and seen as far as it's possible to see with light.

The discovery of the Cosmic Microwave Background

appeared to complete our picture of the Universe,

the final chapter in our use of light

to explore the cosmos.

Understanding the nature of light has allowed us to illuminate our world.

We've captured it from the depths of space and the beginning of time.

At the smallest scales, light has uncovered

the microscopic structure of living things,

and at the largest, it's shown us our place in the cosmos

and told us the story of the stars.

Virtually everything we know about the Universe,

we know because it's been revealed by light.

But just as it seemed light would lead us

to a complete understanding of everything...

..in the last 30 years,

it's shown us something disturbing.

The vast majority of the cosmos can't be seen at all.

Far from being a Universe of light,

much of it is hidden in the dark.

Next time,

how scientists came to the realisation

that more than 99 percent of the Universe

lies concealed in the shadows,

and the extraordinary quest

to uncover what's out there in the dark.

Whether you want to step into the light

or explore the mysteries of the dark,

let the Open University inspire you. Go to...

and follow links to The Open University.

As the sun dips below the horizon, its light begins to fade.

Night falls and our world descends into darkness.

Today, in our street-lit towns and cities,

we rarely experience true darkness.

But without our eyes to guide us,

the world becomes a much more mysterious place.

I can't see anything now,

but strangely I can still sense the presence of the trees

enveloping me in the gloom.

I can't see them, but I know there's something out there.

And in the same way, as we've explored the cosmos,

we've come to realise we can only see

the merest hint of what's out there.

Our best estimate is that more than 99% of the universe

lies hidden in the dark, invisible to our telescopes

and beyond our comprehension.

This film is the story of how we went from thinking

we were close to a complete understanding of the universe,

to realising we'd seen almost none of it,

and the extraordinary quest to uncover what's really out there

in the dark.

It's perhaps the most important undertaking in science,

because our universe was forged in darkness.

And darkness will one day tear it apart.

For centuries, scientists have used light to build up a seemingly

comprehensive picture of the universe.

We'd discovered that the Earth was just one planet

in orbit around the sun.

And that the sun was itself a star,

made of the same stuff as the billions upon billions

of stars that light up a vast - perhaps endless - cosmos.

But there was one niggling problem that had remained unsolved

for over 400 years, and it was this -

with so many stars out there, why was there any darkness at all?

The story of the dark begins with this simple question.

And at its heart lies a deep paradox.

In the forest, no matter what direction I point my torch,

the beam will always hit the trunk of a tree.

And just as everywhere I look I see a tree,

if the universe is sufficiently large,

then every line of sight from Earth should end in a star.

The night sky shouldn't be black at all,

it should be ablaze with starlight.

First posed in the 1570s,

this question would become known as Olbers' Paradox.

One possible solution was that the Earth was surrounded

by dark stuff that obscured our view of the stars behind.

But it was soon realised that these dark clouds would absorb

the light from the stars, heat up and eventually glow

with the same brightness as the stars they obscured.

The paradox was only satisfactorily explained in the 20th century.

The answer - the reason it gets dark at night

is because the universe had a beginning.

It began with the big bang 13.8 billion years ago, and so

we only see those stars whose light has had time to reach us since then.

The sky is dark because light from the most distant stars

hasn't got here yet.

No mysterious stuff was needed to block out the light.

The dark spaces that starlight had yet to reach were empty,

and cosmologists could sleep easy at night.

But before long, we began to see hints that there might be

more out there than meets the eye,

that the shadowy recesses of empty space

might not be so empty after all.

The first clues had in fact begun to emerge from the gloom

some 200 years ago,

not in the depths of the universe, but in our own back yard.

The invention of the telescope in the 17th century had allowed us

to see the dimmest light from the deepest reaches of the solar system.

And in 1781, it had revealed a seventh planet, Uranus,

the first to be found since ancient times.

But there was something odd about this new planet.

Astronomers found that as time passed, Uranus's actual position

was drifting further and further away from the position

the laws of gravity predicted it should be at.

One explanation was that the laws themselves were wrong,

but working at the Paris Observatory,

one man came up with a different solution.

There was something else out there,

something we couldn't see that was interfering with Uranus's orbit.

In 1846, the mathematician Urbain Le Verrier

was employed at the observatory to calculate the orbits of comets

as they wandered through the solar system...

..and predict when they would light up the night sky.

Le Verrier has been described as having an almost pathological

need to impose order on everything and everyone around him,

and to have made no allowances for human error or frailty.

When asked what he was like, a colleague remarked,

"I do not know whether Monsieur Le Verrier is actually

"the most detestable man in France,

"but I am quite certain that he is the most detested."

But he was undoubtedly a mathematical genius,

and he was as harsh on himself as he was on others.

And because he was a mathematician, he set about finding the object

he thought was influencing Uranus not by scouring the skies

with a telescope, but by determining its position through calculation.

These are Le Verrier's original hand-written notes from 1846.

This one is called

"Searches of the disturbing body. Second approximation."

It contains page after page

of complicated mathematical calculations.

What Le Verrier was attempting was quite different

to what was normally done in astronomy,

where you know where an object is -

say a star or planet or comet - you then use the laws of gravity

to explain its effects on nearby objects.

Here, he didn't know where his disturbing body was.

All he had to go by was the effect it had on the orbit of Uranus.

So he made some starting assumptions about its position, and then

carried out a calculation to predict the effect it would have on Uranus.

He then compared that with what had been observed.

When the two didn't match, he went back and adjusted

his starting assumptions and repeated the calculation.

He did this again and again

until his prediction matched the observation.

On the 31st of August, 1846,

after three months of painstaking work,

Le Verrier presented his results to the French Academy.

He announced that his calculations had revealed

what he believed was a new planet,

and, crucially, that he had the co-ordinates in the night sky

that showed where it could be found.

And yet, despite this, he was unable to persuade

any French astronomers to search for his planet.

Eventually, Le Verrier sent his calculations

to Johann Galle at the Berlin Observatory.

His letter arrived on the 23rd of September,

and the new planet was found the same evening

within one degree of Le Verrier's predicted location.

His calculations were so precise,

it took Galle less than an hour to find it.

Le Verrier and Galle had discovered the planet Neptune.

A vast ice giant, 17 times heavier than the Earth

and nearly 60 times its volume,

lurking in the shadows some 4 billion kilometres from the sun.

Neptune had been hard to find

not because there was anything inherently mysterious about it.

It's dark simply because it's so far from the sun,

there's precious little light to illuminate it.

And outside our solar system,

this lack of illumination is an even bigger problem.

And it means even more stuff is hidden in the dark.

Stars are thought to contain just 11% of the atoms in the universe.

The rest - clouds of gas and dust, planets, dead stars -

we can't see, because they give off hardly any light.

The dark spaces between the stars aren't empty at all.

In fact, they contain the vast majority

of the stuff that's out there.

Up until the middle of the 20th century, most astronomers

believed that, although they couldn't see nearly 90% of it,

the universe was still, theoretically at least,

entirely visible.

But that was about to change.

Welcome to White Sands Missile Range.

In 1964, NASA scientists fitted an Aerobee rocket

with an X-ray detector...

'..two, one...'

..and blasted it to the edge of space.

High above the X-ray-absorbing layers of the atmosphere,

the detector spotted something extremely bright

in the constellation of Cygnus.

The young British astronomer Paul Murdin was fascinated by this

mysterious X-ray source, known as Cygnus X-1.

And when he joined the Royal Greenwich Observatory

in the summer of 1971,

he was given with the perfect opportunity to discover what it was.

It was known that X-rays were produced

when gas was heated to temperatures upwards of a million degrees.

DOORBELL CHIMES

Hello, Paul!

'But no-one knew for sure what could produce such extreme

'conditions out in space.'

What was it about X-ray sources that interested you?

Celestial X-ray sources had just been discovered.

They were places in the sky where X-rays came from.

It's a very energetic radiation,

it means something really powerful is happening there.

I mean, the X-rays are a flag which the star is waving at you,

saying, "Look at me, look at me, look at me - I'm really interesting."

But when Paul trained his optical telescope on the source,

all he saw was an ordinary, everyday star,

nowhere near hot enough to produce X-rays.

Most stars are in systems where there's two stars,

three stars, even five stars or many more.

It's really unusual to have a star like our sun that's on its own.

I decided therefore that I'd try

and look for evidence on the star that I could see, that there

was another star nearby and that they were circling one another.

By recording its motion night after night, Paul discovered

the star was orbiting an invisible partner, once every 5.6 days.

What you can calculate, once you know the period of a binary star,

is the mass of the system and the mass of the component parts of it.

And so, that was the thing to do next.

And then, maybe within an hour,

I knew that the star which I couldn't see

was four solar masses or more.

Something that heavy so close to the star he could see

would strip material from its outer layers, the immense

frictional forces heating the gas to such an extent it produced X-rays.

But physicists only knew of one object that could be that massive

and yet remain completely invisible.

It was something that had only ever existed in theory.

Paul Murdin had discovered the first black hole.

I was just... I was just elated.

And I had to get up from my desk and walk about a bit to calm down.

My pulse raced.

I knew it was big, but I was also a little bit frightened of it,

so I knew I had to check it very carefully

and go through it all again and check what I was doing.

But it was... It was a great hour

and I couldn't really do any serious work for the rest of the day.

And I felt... I felt really happy with myself, actually.

Thanks to Paul Murdin,

the universe now had a new and profoundly dark inhabitant.

Black holes are so incredibly dense,

their gravity warps the fabric of space and time around them

to such an extent that nothing, not even light, can escape.

As you approach a black hole,

an observer watching you from a distance

will see the light coming from you getting redder and redder.

And you will appear to be moving in slow motion

as the immense gravitational field of the black hole

stretches both space and time.

And then, as you pass through the event horizon,

the point of no return that marks the edge of a black hole,

you simply disappear, lost from the universe for ever.

Black holes are objects that would remain dark

no matter how much light you shone on them.

Through their effects on other things,

we've now discovered dozens of black holes in our own galaxy,

and estimate there must be billions upon billions

of them throughout the universe.

Including huge, supermassive black holes

millions of times the mass of the sun

at the heart of nearly every galaxy.

As strange as black holes are,

they were at least something we'd expected to find.

We had theories that predicted their existence

and described their properties.

But since the 1930s,

astronomers had seen disturbing hints of something much stranger.

Stuff that was both completely invisible and completely unexpected.

FAINT WHISPERING

As a child, Vera Rubin spent hours awake at night

staring out of the window above her bed,

gazing at the stars as they moved across the sky.

Then, in her 30s and a mother herself,

she decided to realise her childhood dream

and embark on a career as an astronomer.

FAINT WHISPERING

In the mid 1960s, the hottest topic in astronomy was quasars.

But the field was extremely crowded

and because the biggest telescopes that were needed to study them

were often in the remotest parts of the world,

working on quasars meant a lot of time spent away from home.

So Vera needed to find a research topic

that was more compatible with being a working mum,

and a smaller field where she could really make her mark.

So she began a project measuring the way stars move within galaxies

like our own Milky Way.

Whoa! HE LAUGHS

Everything in a galaxy is on the move and rotating.

In one minute, the Earth travels

nearly 2,000 kilometres around the sun.

But in that same time, the sun and the entire solar system

travel 12,000 kilometres around the centre of the Milky Way galaxy.

Ah!

I'm not liking this!

If you think this is spinning fast, think about this.

The Earth is travelling around the sun at 108,000 kilometres an hour.

Ha! And the sun and the entire solar system

are travelling at 720,000 kilometres an hour

around the centre of the galaxy.

HE LAUGHS

Can we stop it now?

That's done me in, that really has.

Thanks very much.

But when Vera Rubin measured the speed of stars

orbiting the centre of the Andromeda Galaxy,

she found something deeply puzzling.

If I plot a graph of the speed

at which planets in our solar system orbit the sun

against their distance from the sun,

I find that the closest planet, Mercury, orbits the fastest.

It's then followed by Venus, Earth, Mars and so on.

The further out you go...

the slower the orbit.

In fact, Neptune moves so slowly relative to the other planets

and has so far to go in orbit around the sun,

that it's only completed one full circuit

since it was discovered 167 years ago.

Now, if I plot the same graph again of speed against distance,

but this time, the speed

at which the stars orbit the centre of a galaxy

against their distance from the centre,

I'd expect to see for the outer stars,

that the speed drops off with distance, as it did for the planets.

But when Vera Rubin plotted her data,

she found that the further out you went,

the speed of the stars didn't drop off,

it remained roughly the same.

The planets move more slowly the further out they are

because the further you go,

the weaker the sun's gravitational field becomes.

So anything moving too fast would simply fly off into outer space.

But Vera Rubin's result for galaxies

suggested there must be an extra source of gravity

holding all those fast-moving stars in their orbits.

This extra gravity was needed

because when astronomers added up the gravitational pull

of all the dark things they thought might be lurking in the galaxy,

planets, clouds of dust, even black holes,

it always came out about ten times less

than that needed to account for

the stellar speeds Vera Rubin had measured.

There were two possible explanations.

Either Einstein's theory of gravity was wrong,

or galaxies were full of a completely new kind of stuff.

Something that wasn't made of atoms,

was completely invisible and very heavy.

A new form of dark matter.

Something astronomers named... dark matter.

Unsurprisingly, rather than accept

that galaxies were full of some mysterious unseen stuff,

some physicists once again thought tweaking the laws of gravity

might be the simplest solution.

That was until astronomers captured an astonishing image.

For me, this is one of the most

amazing pictures in modern astronomy.

It's an image of a cluster of galaxies called the Bullet Cluster.

It gets its name from this bullet-shaped cloud of gas,

which is actually a shockwave caused by the collision

not of just clouds of gas or stars or even whole galaxies,

but clusters of galaxies coming together

and passing through each other at 10-million kilometres an hour.

It almost gives me vertigo trying to imagine the immensity of the scale.

But it's not the magnitude of the collision

that makes this image so important.

It's what it did to the clusters' constituent parts.

As the clusters came together,

the stars and planets in the galaxies

pretty much passed through each other

because although they're big, the distances between them are so vast

that the chances of any two stars colliding is actually very small.

But that doesn't apply to the dust and gas

that makes up 90% by mass of all the stuff we can see in a galaxy.

When these collide, they create a huge, hot cloud -

these two pink regions in the centre of the image.

But if most of the mass is trapped here in the clouds,

then you'd expect most of the gravity to be centred there, too.

But that's not what you see.

These outer blue regions show where light has been bent round

as gravity warps the fabric of space itself.

That means most of the gravity is centred out here,

rather than in the middle.

The simplest way to explain this

is that it wasn't just stars and planets

that passed through as the clusters collided,

something else did, too.

Something massive, yet invisible.

This image is the best evidence we have yet

for the existence of dark matter.

It's now generally accepted that dark matter is real,

which means there's far more stuff in the universe than we'd thought.

In fact, there's four times

as much dark matter as there is normal matter.

And so vast swathes of the universe are not just unseen,

they're fundamentally unseeable.

The reason dark matter is so elusive

is because it doesn't reflect light and it doesn't emit light.

So we can't see it.

And worse than that, what gives normal matter its solidity

is the electromagnetic force.

And dark matter particles don't feel that force,

so they just pass straight through matter.

The only hope we have is if they hit an atomic nucleus head-on.

And even if they do, that's really hard to detect.

And so the hunt for dark matter

has turned from the incredibly large to the unimaginably small.

From scouring the skies with telescopes

to detectors buried deep underground.

When it comes to the search for dark matter,

the place I'm going to is pretty much the centre of the universe.

The Gran Sasso National Laboratory

lies beneath almost a kilometre and a half of solid rock.

And can only be reached through a tunnel

cut deep into the Italian Apennines.

The reason you'd build a laboratory underneath a mountain

is because our planet is constantly being bombarded by cosmic rays.

These collide with the upper atmosphere,

creating a cascade of particles

that shower down onto the surface of the Earth.

The rock above me effectively forms a 1400-metre-thick roof

that absorbs most of these particles,

shielding and protecting the equipment below.

But crucially for dark-matter hunters,

it passes straight through normal matter,

straight through the rock,

and the hope is, into their detectors.

Oh!

It looks like a Bond villain's evil lair.

Gran Sasso is the world's largest underground laboratory.

And for the last ten years,

it's been home to dark matter scientists like Dr Chamkaur Ghag,

who works on DarkSide-50,

one of five dark matter experiments based here.

So hairnet. Hairnet.

Or head net, in my case.

Milligram levels of dust can destroy the experiment. Right.

That looks very impressive.

Yep. Very sci-fi.

So tell me, how does the experiment work?

Well, the entire experiment is configured like a Russian doll,

where the first outer layer is the mountain itself,

protecting the experiment from radiation from space.

Then we have this tank that we're standing in.

And this tank is going to be flooded full of water.

What, the whole cylinder?

Absolutely. This is all completely filled to the brim.

About 750 cubic metres of water will fill this thing

to stop radiation coming from the laboratory and the rock around us.

That's protecting this huge metal sphere right here,

which is the final layer of protection

before we get to DarkSide itself, which is inside there right now.

That's the detector, that's the heart of the experiment.

That's the thing that will be detecting dark matter.

You haven't got a light switch up there. No.

I'm going to get up there and have a look.

DarkSide-50 is designed to detect

a new class of fundamental particles

called weakly interacting massive particles.

Predicted by theory, it's thought that these WIMPs

might be the stuff of which dark matter is made.

So that metal sphere in the centre, that's DarkSide?

That's right. That's a detector full of 150kg of liquid argon.

Dark matter particles should be

streaming through the detector all the time,

but most of them just go straight through

because they're very weakly interacting particles.

If we're lucky, one will collide with the nucleus of an argon atom,

producing flashes of light that the detector will pick up.

DarkSide is yet to begin its search,

but elsewhere in the laboratory's labyrinth of tunnels,

they're already seeing tantalising hints.

This is the XENON100 experiment that's already running

and taking data and has been for a while.

It's the most sensitive dark matter detector in the world right now.

And this is a live feed of dark matter data coming in right now.

So, what exactly... What sort of signal or shape are you looking for?

Well, what we're looking for is an initial flash of light

which will be a very sharp peak like this,

followed by a much larger peak like that one,

which is light being generated in a gas layer

on top of the liquid xenon.

Oh. That could be a good one as well, actually.

There you go.

So any one of those events, those spikes,

could be a dark matter particle?

That's right. Any one of these events

could be the signature of dark matter

interacting in XENON100.

It's just we won't know for sure until the data's been analysed.

Because it's so sensitive,

the overwhelming majority of the spikes

are due to radiation emitted by the metal

that makes up the detector itself.

But the hope is experiments like this

will definitively detect dark matter particles

within the next ten years.

Today, we think that dark matter not only exists,

but that it is a vital part of our universe,

because without it, the world that we can see wouldn't exist

and that's because dark matter not only holds galaxies together,

it's dark matter that brought the clouds of gas together

to form the galaxies in which stars could ignite in the first place.

Dark matter has gone from being a curious quirk

of the way stars move around the fringes of galaxies

to the reason there are stars and galaxies at all.

But in the late 1990s, scientists attempting

to measure exactly how much dark matter there was

made an astonishing discovery.

There was something even more mysterious

and even more elusive out there.

And to understand what that is,

you have to go back to the very beginning of everything.

The universe began with a gigantic fireball.

13.8 billion years ago, the universe was born.

In the so-called big bang,

everything was created simultaneously.

See that great flash of light?

That's all the pieces of the atoms joining together to make a gas.

And now the gas is getting lumpy.

It's making the giant galaxies of stars.

The expansion of the universe that we now see

is just a remnant of the initial violent explosion.

The big bang means that in the past,

the universe was much smaller than it is today.

And it's been getting bigger ever since.

According to the big bang theory,

the universe has been expanding for the past 13.8 billion years.

And for most of that time,

you'd expect the expansion to be slowing down

due to the combined gravitational attraction

of all the mass in the universe

trying to pull it back together again.

Now, here's the clever bit,

Cosmologists realised that by measuring

how much the expansion was slowing,

they could calculate how much stuff was out there.

In a sense, it would allow them to weigh the entire universe.

But in order to measure how the universe is expanding,

you need a reliable way to measure distances in space.

Something of known brightness, astronomers call a standard candle.

The flame in this lantern produces a fixed amount of light.

It has a specific brightness that I can measure here on the ground.

But if I let the lantern go, it'll drift away

and the light will appear to get dimmer and dimmer

the further away it gets.

Because I know how bright it really is,

by comparing that with how bright it appears,

I can calculate how far away it is.

And because every lantern's the same,

I can use the brightness to calculate the distance

to any lantern I see in the sky.

The astronomical equivalent of a Chinese lantern

is a particular species of exploding star called a Type 1a supernova.

These stars always explode when they reach the same critical mass

and so always explode with the same brightness.

So by measuring how bright they appear,

we can tell how far they are from the Earth.

As well as telling us how far away they are,

the light reaching us from distant supernovae tells us something else.

As it travels across the cosmos, light gets stretched

because the space it's travelling through is expanding.

And as its wavelength increases, the light gets redder and redder.

And this red shift tells us how fast the universe was expanding

when the light left its source, when the star exploded.

But when scientists analysed light from the more distant supernovae

they found something strange.

It was less stretched than expected.

It meant that, in the past,

the universe was expanding more slowly than it is today.

In other words, the expansion of the universe wasn't slowing down at all,

it was speeding up.

The only way the universe's expansion could be accelerating...

..was if there was a mysterious new force pushing it apart.

And just as with dark matter, physicists thought the key

to understanding this new force

might lie at the smallest possible scales...

..because quantum physics appeared to provide a ready-made explanation.

According to quantum field theory, empty space is anything but empty.

Particles are constantly appearing and disappearing,

created out of energy borrowed from the vacuum itself.

The hope was that this theoretical vacuum energy

might be the very thing that was pushing the universe apart.

And the theory allows me to calculate the energy density

of the vacuum, that's the amount of energy you'd expect to find

in a given volume.

And so if I take the energy of the vacuum

to be a sum over J of half h-bar omega J,

and if I take the cut-off energy

to be of the order of 10 tera electronvolts

which is just above the known physics

at the Large Hadron Collider, then the formula for the vacuum...

'All they needed to do was check the energy density the theory predicted

'matched that needed to drive the universe's acceleration

'and the mysterious force would be explained.'

HE MUTTERS EQUATIONS

So that would give me a value for the energy density

of the vacuum of 10 to the 35 kilograms per cubic metre.

The trouble is, the value observed by astronomers

is 10 to the minus 27 kilograms per cubic metre.

That means the theoretical number and the experimental number

are out by a factor of 10 to the power 62.

That's one followed by 62 zeros.

To give you a sense of the scale of the error,

there've been only 10 to the 17 seconds

since the big bang and the diameter of the entire visible universe

is 10 to the 27 metres...

So it's a pretty big error.

And that meant that whatever was actually pushing the universe apart,

it was something completely new.

The truth is, we know very little about what's causing

the expansion of the universe to accelerate,

but we do have a name for it - dark energy.

And we know that for it to have the effect that it does,

there must be an awful lot of it about.

Einstein's famous equation E=mc2

says that energy and matter are different forms of the same thing.

And the equivalent mass of dark energy dwarfs that

of everything else in the universe.

And it means that, today,

normal matter makes up just 4% of the cosmos.

23% of it is elusive dark matter.

And a colossal 73% of the universe

consists of mysterious dark energy.

Just think about it for a moment.

100 billion galaxies,

each one containing more than 100 billion stars,

home in turn to billions upon billions of planets and moons.

All of that is mere flotsam adrift on a vast and unfathomable ocean.

Dark matter we can't see and dark energy we can barely comprehend.

And the very nature of dark energy means the universe is getting

more unknowable all the time.

As space expands and distances become bigger,

most forces get weaker, because you have the same amount of mass

or electric charge, only now everything's further apart.

But dark energy behaves completely differently.

As the universe has expanded, the stronger it's become.

The more space there is, the more dark energy there is

and so the faster the universe expands,

creating ever more space and ever more dark energy.

And that has a profound consequence.

Just as dark matter pulled the galaxies together

to create the cosmos as we know it...

..so dark energy will tear the universe apart.

In the future, as space gets bigger,

dark energy will become ever more dominant.

And so it will ultimately shape the universe's destiny.

And if it continues to increase as it appears to be doing today,

then it will push the galaxies further and further apart

until, eventually, they slip out of view,

creating a cosmos that will become ever more dark

and ever more desolate.

The ultimate goal of modern cosmology is to understand

dark energy and the fate of the universe,

and to witness how dark matter brought everything together

in the first place.

And so to shed light on both the beginning and end of the universe,

cosmologists have embarked on a quest of epic proportions -

to map everywhere in space over the entire lifespan of the cosmos...

..starting with the darkest period in its past,

an era that began as the afterglow of the big bang faded away.

We talk about the ages of the universe in the same way

that we talk about the stages in our own lives, from its birth,

through childhood, adolescence, adulthood and even death.

So mapping the universe is really about

filling in the photo album of its life.

Here's a picture of me from 20 years ago with my children.

I know it because I have a lot more hair there.

And here's a picture of me in my early 20s on graduation.

And here's one of me as a teenager.

In the same way, by looking out into space,

we have good images of the universe

all the way back to its teenage years,

when large galaxies first formed.

But before that, we have nothing but a single image -

a picture of the universe when it was just 400,000 years old,

the cosmic microwave background - the afterglow of the big bang.

It's as though, in the photo album of my life,

I have nothing before this picture of me aged 16,

apart from this one of me and my parents in Iraq

when I was just a few months old.

This gap in the childhood of the universe,

the period between its earliest moments, through the birth

of the first stars to the formation of large galaxies

is a time known as the dark ages of the universe.

The universe's dark ages lasted for around a billion years

and they get their name because there were precious few stars

to illuminate them.

So to fill in those pages in the cosmic photo album, we'd need

something capable of seeing where there was next to no light.

During the Second World War, Bernard Lovell had developed a machine

that could see in the dark.

He'd worked on airborne radar

that mapped bombers' targets on the ground.

But his real ambition was to build

something capable of mapping the heavens.

The giant dish at Jodrell Bank was Bernard Lovell's baby.

It was designed to be the world's largest fully manoeuvrable

radio telescope, capable of scouring the entire sky

and picking up the longest-wavelength radio signals

coming from the deepest recesses of space.

The Lovell Telescope has a collecting area

of 4,560 square metres,

made up of more than 2,400 galvanised steel plates.

In the original designs, this bowl of the telescope

wasn't meant to be solid like this.

The plan was for it to be built of much lighter wire mesh.

The dish was redesigned

because astronomers had discovered a new way of seeing in the dark,

something that might ultimately allow them

to map the universe's dark ages.

Hydrogen permeates every galaxy.

It was produced in the big bang

and is the basic constituent of all normal matter, including us.

And like most normal matter,

it wasn't thought to give off any light.

But then astronomers discovered something remarkable.

As it floats around in space,

neutral hydrogen gas is constantly producing radio waves

and, crucially, those waves are always the same wavelength - 21cm.

And this meant that hydrogen could be used to map

the galaxies that it fills.

By detecting the 21cm signal, the Lovell Telescope helped reveal

the spiral structure of the Milky Way

and produced detailed maps of distant galaxies.

But galaxies aren't the only place in the cosmos you find hydrogen gas.

During the dark ages of the universe,

there were no galaxies, but there was plenty of hydrogen.

So by detecting the 21cm signal from these primordial gas clouds,

you could see the universe in its infancy

and peer into the dark ages themselves.

And by doing so, we'll be able to watch dark matter

pull the cosmos together...

..and light up the heavens.

It was during the dark ages that the hydrogen gas created

in the big bang was compressed into stars and moulded into galaxies.

It was in this era that the cosmos as we know it was born,

sculpted by the gravitational pull of dark matter.

But the machine scientists are building to map the dark ages

will see far more.

With an effective collecting area of more than 200 times that

of the Lovell Telescope, the square kilometre array

will be capable of mapping a billion galaxies,

tracking the expansion and evolution of the entire universe

more accurately than ever before.

And the hope is, that by doing so,

it will provide clues to the nature of dark energy

and the universe's ultimate fate.

Using hydrogen to map the cosmos might just represent the final

chapter of humankind's exploration of the universe using light,

a journey that began in earnest some 400 years ago.

In December 1609, Galileo Galilei began making observations

of the night sky.

Before then, what was thought to be out there was essentially

a matter of faith.

The universe at large lay unseen and unseeable.

But now, for the first time,

the nature of the heavens was something knowable -

you simply had to look up and see it.

The light captured in Galileo's simple telescope

began a chain of discoveries that would reveal

the true nature of the cosmos.

We've seen galaxies

billions of light years' distance from the Earth.

And as we've come to understand light's properties,

we've discovered the stuff of which stars are made...

..and glimpsed the beginning of the universe itself.

But the realisation that most normal matter can't be seen

and the discovery of dark matter and dark energy

mean that more than 99% of the universe lies hidden in the shadows.

And as dark energy pushes the galaxies ever further apart,

what few lights there are will begin to go out.

As the universe expands ever faster,

one by one the galaxies will disappear from view.

All that will remain visible will be the stars in our own galaxy.

It would be almost as if we'd never invented the telescope at all.

For the vast majority of the universe's life,

there'll be no way of discovering all the things we have about it.

So I don't feel disheartened that so much of the cosmos

is hidden in the shadows.

The real miracle is

that when we first looked out into the depths of space

there was any light to see at all.

Whether you want to step into the light

or explore the mysteries of the dark,

let the Open University inspire you.

Go to...

..and follow links to The Open University.