All language subtitles for Light and Dark 1080p HDTV x264 AAC MVGroup org

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.