Original subtitles

In 1929, Edwin Hubble made an alarming discovery.

He found that wherever he pointed his telescope,

it revealed that everything was getting further away.

The universe seemed to be expanding, and if it was expanding -

they checked and it was - and you think about it for any

length of time, which they did,

you have to conclude that it must be

expanding from some kind of starting point.

Hubble had stumbled across what was then a revolutionary idea,

but something that is now scientific orthodoxy.

Our universe started 13.8 billion years ago in an instant.

ALL: This was the first period of the birth of the universe.

It is known as the Big Bang.

Nowadays, our understanding of the birth of the universe is

extremely detailed.

Then it underwent a dramatic expansion.

ALL: This was the second period in the birth of the universe.

It is called inflation.

Thanks to science, we think we know exactly how we got to now.

BOTH: Atomic matter condensed to form the stars

and planets that make our universe.

ALL: This is the standard model of cosmology.

And not content with painting the biggest picture of all,

science has also created a comprehensive list

of what the atoms we're made from, are made from.

There are six quarks.

ALL: Four types of gauge bosons.

ALL: Six leptons.

And the Higgs boson.

ALL: This is the standard model of particle physics.

Together, these two paradigms should explain everything.

And yet, just at the point where things seem to be coming together,

some researchers are worried that there's an increasingly

strong possibility that we might have got the science wrong.

That our current theories are looking shaky.

That we don't understand our universe

or what we're made of, or anything, really.

How does any theorist sleep at night knowing that the standard

model of particle physics is off by so many orders of magnitude?

We have no idea what 95% of the universe is.

It hardly seems that we understand everything.

This is about what the universe is made of.

This is about our existence.

What is it that they say? They say that cosmologists are always wrong but never in doubt.

There are more theories than there are theoreticians.

OK, I'm going to be honest here, but we're in the strange situation

that it seems like every other year there's a new unexplained signal.

Maybe we're just going to have to scratch our heads

and start all over again.

Nestling beneath the huge Andes Mountains that dominate

the whole of Chile lies its capital.

It was founded by the Conquistadors in 1541, who gave it its name,

Santiago, St James, after the patron saint of the motherland.

But in Spanish, Iago also means Jacob, and it was Jacob who,

according to the Bible, dreamt about climbing a ladder to heaven.

While the mountains may hint at a metaphorical stairway

to paradise, they also provide a practical route to enlightenment.

That's why British astrophysicist Bob Nichol is here.

He's en route to some of the biggest telescopes on the planet,

perched aloft on the roof of the world, where he's continuing

the work of trying to understand how the universe works.

So the amazing thing about cosmology is that it only really started

in the 1920s, so when people started looking through their telescopes,

they didn't know whether these fuzzy things out there in the universe

were actually within our own galaxy or actually separate galaxies from

our own. And then it was the great astronomers like Hubble that came

along and measured the distances to these faint nebulae that you

could see in your telescopes, and suddenly discovered that they were

much further away than we expected and therefore had to be outside

our galaxy and therefore discovered a universe of other galaxies.

The discovery of a universe that was far more complicated

than anyone could have imagined...

..and the idea that it all started in an instant...

..suddenly provided a credible creation story

that didn't rely on myths and magic.

The idea of the Big Bang and the expanding universe was

a triumph for modern astronomy.

And everyone was happy with it,

until 1974, when astronomers discovered a big problem.

So in the solar system,

we have a sun in the middle, which provides all the gravity.

And then coming further out from that, we have all the planets.

They're lined up and rotate around the sun,

and the speed by which they go round the sun decreases

as a function of the distance away from the sun.

So by the time you get to the outer planets,

they are moving a lot slower than the ones in the centre.

So, for example, Neptune takes 165 Earth years to go round the sun.

So if I was to draw a graph of that, it would look a bit like this.

So...

..you would expect the speed of the planets in the centre to be

high, and as the gravity got weaker,

the speed would get smaller

and smaller and smaller until you got out here.

Now, we have the same set-up in our galaxy.

We have a large supermassive black hole in the centre

and we have stars orbiting around the centre of the galaxy.

So you'd expect that the stars further away from the centre

of the galaxy would be moving slower than the ones on the inside.

But that's not what we see.

What we see is the speed of the stars is constant with distance,

so the stars out here are travelling at the same

speed as the stars in the centre.

Wherever the speed of stars in spiral galaxies were measured,

they produced the logic-defying flat rotation curves.

The only way they made sense was if there was more matter than

we thought, producing more gravity.

And since the extra stuff couldn't be seen, it was given

the slightly sinister title "dark matter".

Dark matter is a really interesting problem.

It sounds exotic, but it doesn't have to be.

Professor Katie Freese is a theoretical physicist.

That is to say, the physics she deals with is theoretical.

Katie herself is real.

There's a lot of dark things out there in the universe.

Until I shine my light at these bottles, I can't see them

and as soon as I take away the light, they're dark.

That's what people thought. They thought it might be gas,

it might be dust.

The dark matter could just be ordinary stuff that you can't see.

These ordinary, but dark, dark matter creatures are called MACHOs -

massive compact halo objects.

But the trouble was that even the most generous

estimates for how much the MACHOs might weigh fell pathetically

short of what would be needed to explain the strange

goings-on in spiral galaxies like ours.

Another explanation was required.

Well, there's an alternative idea for what the dark matter could be.

What we think it is, is that it's some new kind of fundamental

particle. Not neutrons, not protons, not ordinary atomic stuff

but something entirely new.

And these particles are everywhere in the universe.

They're flying around in our galaxy, they're in this room.

Actually, there would be billions going through you every second.

You don't notice, but they're there.

These theoretical dark matter candidates are called WIMPs -

weakly interacting massive particles.

But because they interact weakly with ordinary matter,

the stuff from which we and scientific instruments are made,

catching them is about as straightforward

as trapping water in a sieve.

In fact, in the early days of dark matter, these particles were

so theoretical that no-one had any idea at all about how

they might get hold of one, even in theory.

Then, in 1983, freshly minted theoretical physicist

Katie Freese had an epiphany.

I was at a winter school in Jerusalem

and that's where I got into the dark matter business.

I met a man named Andre Drukier.

He's a brilliant, eccentric person.

He's Polish,

he speaks English, French, German, Polish,

all at the same time.

And he knew where to go for the New Year's party.

And he started, believe it or not, in that evening,

over the cocktails -

cocktails have always been good for science -

started telling me about work that he'd been doing.

Drukier had hit upon a way of detecting neutrinos, real particles

that share some characteristics with the proposed WIMPs.

So what we realised is you could use exactly that same

technique for WIMPs.

WIMPs have the same kind of interactions,

they have the weak interactions, the same ones that the neutrinos do.

I, at the time, was a post-doc at Harvard and I convinced

Andre to come to Harvard for a few months. And there, we also

worked with David Spergel, and the three of us wrote down some of the

basic ideas for what you might do if you wanted to detect the WIMPs.

WIMPs, the particles that could be dark matter, are like ghosts.

They travel through ordinary matter.

But they are particles,

so every once in a while, one of them

should collide with the nucleus of an atom, in theory.

What's more, the theoretical collision should release

a photon, a tiny flash of light - dark matter detected.

Simple, in theory.

If you were to try to build one of these experiments on a table top

or in a laboratory on the surface of the Earth,

then your signal would be completely swamped by cosmic rays.

These would just ruin your attempt to do the experiment,

because the count rate from the cosmic rays would be so high

that you'd never be able to see the WIMPs.

So what you have to do is go underground.

It is because of the ideas that Katie had in the 1980s that

thousands of scientists have been scurrying underground

in search of the dark ever since.

Juan Collar is one of them.

His search for dark matter has taken him to Sudbury, a small

town in Canada, perched just above the North American Great Lakes.

To look at it now,

you wouldn't think that this place owes its existence to

one of the most catastrophic events the world has ever witnessed.

Millions of years ago, a gigantic comet crashed into what is

now Sudbury, creating, to date, the second largest crater on Earth.

The comet brought with it lots of useful metals that ended up

under what became known as the Sudbury Basin.

When humans became clever enough,

they sunk holes into the crater so they could get the metals out.

The area's nickel mines are responsible for, amongst other

things, the town of Sudbury's main tourist attraction, the Big Nickel.

What they're less well known for is the part

they play in the search for dark matter.

Juan and his colleagues regularly make the two-kilometre

descent into the darkness in pursuit of the universe's missing mass.

He's been making the journey for some time.

How long have you been doing experiments underground?

In my case, since 1986.

It's been a while.

So you haven't found anything yet? No.

Do you ever feel like giving up?

Well, after walking a mile underground like this...

This is not the right time to ask me that question, don't you think?

There's ups and downs, of course, but, yeah.

Every so often you have to wonder about the fact that we may be

looking in the wrong place, right?

But someone has to do that job.

I mean, in physics a negative result is also important.

You close a door,

and then we can get to work looking for other possibilities.

The scientists are heading for an underground

laboratory in which it is hoped that the super-shy dark matter

particle may one day show its face.

Because anything brought in from the outside world could

give off radiation that might look a bit like dark matter,

every trace must be removed before entering the lab.

No-one is allowed near the ultra-sensitive detectors

without being thoroughly cleaned

and given a special non-radiating outfit to wear.

Here in this near-clinically clean environment is a bewildering

collection of experiments, some of them several storeys tall,

all designed to catch dark matter in the act of existence.

Most of the experiments intend to record the hoped-for

flash of light, produced when WIMPs collide with atoms.

But Juan's experiment works in a totally different way.

Juan has decided to listen, rather than look, for dark matter.

So, Peter, this is the inner vessel of Pico-2-L,

what we call this project.

And it goes inside that big recompression chamber.

We have cameras that look inside

and the principle of operation of this detector is the following -

we put a liquid in there that is

a rather special liquid. It's what we call a super-heated liquid.

It makes it sensitive to radiation, so when particles like the liquid

that goes in there normally - it's now empty - they produce bubbles.

The number of bubbles tells us about the nature of the particle

that interacted.

You can see these copper things here. These are electric sensors.

They are very sophisticated microphones and through sound

we are actually able to distinguish...

differentiate between different types of particles as well.

What sound would dark matter make?

It's actually very soft. It's not the loudest.

So if you find a WIMP it'll have a wimpy noise?

Very wimpy indeed, yes.

Juan has scaled up this idea in his latest detector.

Because a bigger detector means a greater hit rate.

Assuming, of course, that there's anything doing the hitting.

So this is 260.

It's a much larger bubble chamber,

about 30 times larger in active volume than

the one we were looking at before.

We explore the same principle.

We listen to the sound of particles, etc.

It's just a much bigger version.

In some of the models they have developed for these dark matter

particles, the rate of interaction is as small as one interaction,

one bubble in our case, per tonne of material per year, or less.

Confident?

Confident? Not really.

You do your job the best you can

and then you hope for the best, but...

..nobody knows if there's WIMPs out there or not. We're trying.

But confidence is not something that

you typically find among experimentalists.

The fact is, though, that though the hunt for dark matter has

so far proved to be the world's least productive experiment,

the world's large telescopes are providing increasing evidence that

the elusive WIMPs, whatever they are, really are the dark matter.

This array forms one of the world's largest telescopes.

In fact, its name is the VLT -

the Very Large Telescope.

We're in the Atacama Desert in Chile,

at the top of a big mountain at the European Southern Observatory,

so there are four massive telescopes

that we use to stare into deep space

and they give us even more information

on the dark matter that fills our universe.

The Very Large Telescope has produced some staggering images,

but perhaps one of the most compelling is this one.

This image shows a large cluster of galaxies.

Such large objects can bend light

of the galaxies that are behind it.

We call this technique gravitational lensing.

These arcs are distant galaxies behind the cluster

that have been brightened and stretched

as the light passes through the cluster and gets bent.

And what's very interesting is this technique

allows us to measure the mass of the lens,

and when we do that using these arcs,

we find the mass of the lens is about 100 times more

than the light we see in this image.

But second of all, and more importantly,

it tells us that the dark matter that we can't see

is more distributed and acts as a dark matter cloud of particles.

So this is conclusive evidence of dark matter,

but it also is conclusive evidence that that dark matter

must be more spread out than the galaxies we see here,

and in fact it tells us it has to be a cloud of dark matter particles,

not just individual objects in the cluster.

So here's the thing. Dark matter has to have mass.

Remember, that's the reason it has to be there in the first place -

all those speeding stars. And it seems that

it's not just matter we can't see because it's not shining.

So it has to be some kind of other stuff

that we can't see by definition.

And more than that, it has to be some kind of material

that's capable of clumping together in something like a gas.

And all this adds up to one thing -

we're looking for a new particle.

And when it comes to new particles,

there's really only one place to come - Switzerland...

and France.

This place might look like a third-rate

provincial technical college,

but if the hunt for dark matter has taught us nothing else,

it has shown that a book should never be judged by its cover.

And so it is with this place,

because beneath the dismal architecture

lies the most exciting piece of scientific apparatus ever created.

This is CERN, the world's biggest physics lab,

home to the Large Hadron Collider,

the largest particle accelerator on the planet.

It's here where scientists investigate what stuff is made of...

by smashing it apart.

Protons are fired around its 27-kilometre-long circular tube

in opposite directions at nearly the speed of light,

before being smashed together.

EXPLOSION

Waiting to trawl through the debris resulting from those collisions

are two-thirds of the world's particle physicists.

One of them is Dave from Birmingham.

He is in charge of one of the huge detectors

which record each and every collision.

I have to admit, I come down here a few times a week

and pretty much every time I come in,

my jaw still drops when I see ATLAS in front of me.

I mean, it's incredible that we built this detector

and that we're able to operate it.

So the whole detector itself is about eight or nine storeys tall,

and so we're about halfway up at the moment,

so four or five storeys above the base of the detector.

The total weight of the detector is about 7,000 tonnes,

which is about the same as the weight of the Eiffel Tower.

While it might weigh the same, the ATLAS detector

shares few other characteristics with Paris's most famous flagpole.

Fitted with 100 million detectors,

it produces the equivalent of a digital photograph

40 million times a second, providing Dave and his team

with a permanent record of the precise nature

of each particle's demise.

When the protons collide,

most of the time the particles they produce... Nearly always

some new particles are created, but they tend to be

low-mass particles so they tend to be the familiar quarks,

the familiar hadrons, the protons, the neutrons, pions,

which are also light hadrons.

But sometimes, very rarely,

you produce these much more massive particles,

and that's where we're looking for. So if we are producing

Higgs particles or we're producing even more massive particles -

which would be ones we don't know about,

they would be ones beyond the standard model -

these are the guys that we're really looking for.

The LHC has been switched off for two years while it's been upgraded.

Now it's been switched on again

and will run at twice the energy it did before.

It might be that more new particles might emerge.

If they do, they could well be the elusive WIMPs,

one of which could well be the dark matter.

The idea is that we're looking for imbalances of momentum in the event

that signify that there are unobserved particles

going off with high energy carried out of the detector.

So what you're actually seeing is an absence of something?

What we're seeing is an absence of something,

an imbalance of something, yes. It's some particles that we can't observe

and we can infer that they're there by looking at the rest of the event.

So that's beautiful, isn't it? That you can find dark matter which you can't by definition see

and you discover it by not seeing it? Exactly, yes.

On the face of it, this is an extraordinary,

not to say logically contradictory idea,

that ordinary matter smashes into itself

to produce invisible matter that can't readily be detected

because it only interacts weakly

with the stuff that produced it in the first place.

And yet this is precisely what is being predicted

in another part of CERN

by theoretical physicists like John Ellis.

My job as a theoretical physicist is to try to understand

the structure of matter, what makes up everything in the universe,

the stuff that we can see, the stuff that we can't see.

It's the stuff we can't see

that is currently occupying most of John's time.

So the astronomers tell us that there are these dark matter particles

flying around us all the time,

between us as we speak.

But they've never detected these things.

Now, we were going to try to produce them at the LHC.

It sounds like a bold statement

but it's based on a very conventional idea -

namely, that everything we can see and can't see

has its origins at the point of the Big Bang

when things were as hot as it's possible to be.

And it's only in the LHC that, at least in theory, energy levels

approaching those not seen since the moment of creation

can be reproduced.

EXPLOSION

Now, at those very early epochs,

we think that there were other particles

besides the ones that are described by the standard model,

particles that we can't see.

Now, we believe that this dark matter must exist,

because if we look at galaxies,

if we look at the universe around us today,

there has to be some sort of unseen dark stuff,

and we think that stuff must have been liberated from the particles

that we can see very early in the history of the universe.

If John and Dave can make a suitable WIMP at CERN,

the picture will become much clearer

for Juan and the deep mine fraternity.

Suddenly there'll be something to shoot at.

If the astronomers find a dark matter particle, you know,

hitting something in the laboratory,

they don't know what type of particle it is.

But if we put our two experiments together,

like pieces of a jigsaw puzzle,

we may be able to figure out what this dark matter actually is.

Linking a manufactured particle from CERN

to underground WIMP detections

would indeed connect two pieces of the jigsaw.

But there's a third piece -

one that provides evidence of dark matter in its native habitat.

This is Chicago, Illinois.

# You only love me for my record collection

# You say you never felt a deeper connection... #

Chicago is the home of the deep-dish pizza, Barack Obama,

and Reggies blues club at 2105 South State Street.

# Let the record spin cos you like it like that

# We're hanging on by the way it spins round

# You love me for my records and you wanna get down... #

Guitarist Charlie Wayne and his band The Congregation

are entertaining the crowd with one of their newest songs.

MUSIC CONTINUES

Charlie has been in many bands over the years, and has often been

in two minds as to whether he should become a professional musician.

CHEERING

But for the time being, he has a day job.

And a day name, too.

During the day, guitarist Charlie Wayne becomes

Associate Professor Dan Hooper, physicist.

So, I'm a professor of astronomy and astrophysics

at the University of Chicago, but I also do

research here at Fermilab, as part of the theoretical astrophysics group.

In addition to being the centre of particle physics

in the United States,

they have a strong programme in cosmology and particle astrophysics.

They study questions like, how did the universe begin?

How did it evolve? What's dark matter and dark energy?

Some of my favourite questions.

And while Charlie dreams of commercial success

and induction into the Rock and Roll Hall of Fame, Dan has his eyes

on the glittering prizes that can be won through academic study.

So, this is my office, this is where I do my work.

So what does work mean, Dan?

So, I'm a theoretical astrophysicist. Which means my research is

done on chalk boards, and pads and paper, and my computer.

I don't run any experiments. I don't build anything.

Fermilab is named for Italian-American

Nobel Prize-winning physicist, Enrico Fermi,

whose name is also given to a class of subatomic particles, fermions.

It's appropriate, then, that Dan works here,

because it's possible that he, too, has identified

a type of particle - something that could be a dark matter WIMP,

something that Dan's colleagues are already calling the Hooperon.

OK, so in many theories of dark matter,

these particles of dark matter are themselves stable.

They'll sit around and basically do nothing, throughout

the history of the universe, but in those rare instances where

they collide with each other, they can get entirely destroyed or

annihilated and leave behind in their wake these energetic

jets of ordinary material. So these jets might include

things like an electron that might fly around here and just move

through the magnetic fields of the universe, or they might

include particles called neutrinos, which are really hard to detect.

And then they could also include, and usually do, some particles

that we call gamma rays which are just really high-energy photons.

So if the Fermi telescope, which is my cartoon picture

of the Fermi telescope here, happens to be looking

in the direction that the gamma ray came from, you could record them

and maybe see evidence of this sort of process going on,

especially in the centre of the Milky Way,

where there's so much dark matter.

Liftoff of the Delta rocket carrying the gamma ray telescope,

searching for unseen physics in the stars of the galaxies.

The gamma ray-detecting Fermi telescope is also

named for Enrico Fermi, but confusingly,

it has nothing to do with Fermilab. But because the data it records

is made public, anyone, including Dan, can take a view on what it's seeing.

In 2009, I was sitting at my laptop just like this.

And I had a mathematical routine written to, you know,

plot the spectrum in the galactic centre regions. So how the different

photons came with different energy, how many of them were different energies,

and most of the backgrounds predict something pretty flat,

not exactly flat, but pretty flat, and dark matter predicts a bump.

So I plotted up, and for the first time I hit enter

and, you know, run the plotting routine and this plot comes up,

and there's this big old bump. You just couldn't miss it.

It was a giant bump in the inner galaxy.

The bump of gamma ray activity that Dan has seen

could be due to many things.

Pulsars emit gamma rays, for a start, and there are plenty of them in the Milky Way.

But the energy levels that make up Dan's bump

theoretically matches the annihilation profile of particles that could,

theoretically, be dark matter - Dan's particle, the Hooperon.

It really was the thing I did the analysis looking for.

And it just stared back at me

and said, "This is the thing you might have been looking for."

It was exciting.

Exciting it may be, but, as yet,

the data that feeds Dan's bump is currently just raw data.

The Fermi telescope collaboration has not yet confirmed it.

Until they do, the excess gamma rays could be anything,

even a problem with the gamma ray detector.

But if it is real, if this third part of the jigsaw

falls into place, it will not only be good for Dan's career, it will

also confirm what this man has been saying for more than 30 years.

He is Professor Carlos Frenk, FRS, creator of universes.

So, Carlos, what is this place?

Well, this is my institute,

the Institute for Computational Cosmology of Durham University.

This is where I work.

That's my office up there,

and it's here that we build replicas of the universe.

Back in the day, when WIMPs and MACHOs were still debated,

and Carlos was just starting out in his scientific career, he and his

friends made a compelling case for one particular type of dark matter.

"Dark matter," they announced - with all the certainty of youth -

"is not only of the WIMP variety, but, furthermore, it is also cold."

It was 1984 and the University of California in Santa Barbara

had organised a six-month workshop on the structure of the universe.

I was there with my three very close colleagues, and they were

George Efstathiou from England, Simon White and Marc Davis.

We were very young, at the time, we were only in our 20s,

and my first job was to try and figure out,

together with my colleagues, how galaxies formed. And to

our amazement we realised that a particular kind of dark matter

known as cold dark matter, was just... Would do the job just beautifully.

Now that idea, at the time, was really not accepted.

It was very unconventional. Because the idea that dark matter existed

was not generally accepted and that it should be an elementary particle,

and cold dark matter was just outrageous, but that's how we were.

We were outrageous, too. We were young, reckless.

I remember George Efstathiou used to wear a leather jacket

and drive a bike, very, very fast motorbike.

Simon and Marc were completely reckless skiers.

I was the only reasonable individual of the gang of four,

and then in the summer of 1984, we had

a conference in Santa Barbara - by the beach, sun shining,

beautiful day... I will never forget.

I gave my first ever talk on cold dark matter,

and at the end of it, I thought it had gone rather well,

but at the end of it, a very, very eminent astronomer came up

to me, whom I had met before when I was a student in Cambridge,

and he says to me, "Carlos, I've got something important to tell you."

He says, "I regard you as a very promising young scientist but

"let me tell you something, if you want to have a career in astronomy,

"the sooner you give up this cold dark matter crap, the better."

And I remember how my world crumbled. And I went up to Simon,

and I said, "Simon, this is what I've just been told."

And Simon just looked at me for what seemed a very long time,

and he said, "Just ignore him, he's an old man."

He was 42.

HE CHUCKLES

Since he was told to drop it, Carlos has shown again

and again that his ideas about cold dark matter really do seem to

hold water, at least mathematically.

And with the advent of computer visualisations,

bare numbers have been transformed into the intensely beautiful

infrastructure of our universe.

This is not a picture of the real universe,

this is the output of our latest simulation. So what

we do to simulate the universe is that we create our own Big Bang

in a computer, and then, crucially, we make an assumption about the

nature of the dark matter, and in this particular case we have assumed

that the dark matter is cold dark matter, and this is what comes out.

An artificial virtual universe, but it is essentially

indistinguishable from the real one. And it is this that validates

our key assumption that the universe is made of cold dark matter.

Of course, the obvious drawback with dark matter is that you can't

see it...

But in his universe, Carlos can simply colour it in,

mainly purple in this case.

So this is the backbone of the universe, this is

the large-scale structure of the dark matter coming to us vividly.

You can almost touch it from this realistic computer simulation.

This is cold dark matter.

When I look at these amazing structures that come

out of the computers, and the fact that

I have largely contributed to cold dark matter becoming

the standard model of cosmology, I'm just so glad I didn't listen

to my eminent colleague in the 1980s, who told me that the quicker I gave

this up, the likelier it was that I would have a successful career.

I'm just so glad I didn't listen to him.

So cold dark matter it is, then.

Carlos and his young guns were right.

Their ideas are now enshrined in the standard model of cosmology.

And the standard model of cosmology is a theory that's

accounted for everything very well.

It explains how Hubble's expanding universe originated.

Our universe started...

13.8 billion years ago...

In an instant.

It tells us how the universe got to be the size it is.

ALL: This was a second period in the birth of the universe.

It is called inflation.

It predicts precisely how much dark matter there is in our universe.

ALL: 26% dark matter.

But it's a description of a problem, rather than of a thing,

and this is where it gets frustrating, because there

should be an answer from the standard model of particle physics.

There are six quarks...

ALL: Four types of gauge bosons.

Six leptons.

And the Higgs boson.

But there isn't, because, so far, there isn't a particle

in the standard model of particle physics that provides us with

dark matter for the standard model of cosmology, cold or otherwise.

At CERN, they're hoping to put that right.

John Ellis thinks they might have found some likely dark matter

particle candidates down the back of a mathematical sofa, twice as

many particles as the standard model currently provides, to be precise.

This idea goes under the name of...

Supersymmetry.

So the particles of the standard model include the electron,

and then there's a couple of other heavier particles

very much like it - called mu and tau.

Other particles include neutrinos and quarks, up, down, charm,

strange, top and bottom quarks.

Photons, gluons and W and Z are force-carrying particles.

Now, as I've written it, these particles wouldn't have any mass,

but there is the missing link, the infamous Higgs boson,

which gives masses to these particles and completes the standard model.

Now, what supersymmetry says is that in addition to these particles,

everyone has a partner or mirror particle, if you like,

which we denote by twiddle,

so there's a selectron, there's a smuon,

there's a stau, there's a photino, there's a gluino, sneutrinos...

Supersymmetry, or SUSY if you're in the know,

is, according to its devotees, a rather beautiful notion that

not only explains an awful lot of problems in physics

and cosmology, but also provides us with a dark matter particle,

perhaps, if it's real, as opposed to just a nice idea.

And so far, it's been as elusive as, well, as dark matter itself.

We were kind of hopeful that with the first run of the LHC,

we might see some supersymmetric particles, but we didn't.

And the fact of the matter is that we can't calculate from first principles

how heavy these supersymmetric particles

might be, and so what the LHC has told us so far is that they have

to be somewhat heavier than maybe we'd hoped. But when we increase

the energy of the LHC, we'll be able to look further, produce heavier

supersymmetric particles, if they exist, so let's see what happens.

Also waiting to see what happens

and interpret the 40 million pictures per second that the

ATLAS detector will produce, will be Dave Charlton and his team,

but not all of them are convinced they'll see supersymmetry at all.

I have to say, I'm not the hugest fan of supersymmetry.

It seems slightly messy, the way you just add in, sort of, one extra

particle for every other particle that we know about.

I would prefer something a bit more elegant.

People have been looking for SUSY for decades, right,

and we've been building bigger and bigger machines

and it's always, it's always been just out of reach, like it

always just moves a little bit further away.

It's always receding over the horizon.

And it's getting to the point where, now with the LHC, it's going up in

energy and that's such a huge reach now that if we still don't find it,

then...you know,

it starts to look like it's probably not the right idea.

As an experimentalist, it's really my job to have an open mind

and really to look at all of the possibilities and try

and explore everything we might discover.

The theorists might have their own favourite theories

and say, you know, you should discover supersymmetry,

or you should discover something else.

I don't know. Nature will tell us what's there.

If you're beginning to think supersymmetric particles that

may or may not be there, and that in any case we might not be able

ever to detect, are looking less and less likely, then you're not alone.

In Seattle, at the University of Washington,

Professor Leslie Rosenberg is on his own search.

And he's not looking for SUSY.

So, Leslie, what's wrong with supersymmetry?

Well, I don't know that anything is wrong with it.

As an experimenter, I suppose I'm not spun up about it.

It's not something that I could squeeze and break like a balloon.

If I try and squeeze it, the balloon expands and evades me.

It's... Things are loosy-goosy

unless you've got something definite to look at.

So imagine that you're looking for Martians

and you have no idea what a Martian looks like and you do an

experiment where you're looking for someone that's purple, and they're

half-a-metre tall, with three antennae. And you publish a paper saying

you've excluded this particular Martian. Well, Martians could be

12 metres tall and they could have no antennas and they could be

a nice shade of puce, and you really haven't excluded Martians.

Professor Rosenberg has dug his own hole in the ground, in which

his dark matter search is about to begin.

He's looking for yet another theoretical particle that

nobody has ever seen, except in the form of mathematics.

But it's not supersymmetrical, and it has a name.

It's a type of WIMP called an axion.

This is the axion dark matter experiment, ADMX.

This piece of it is one of the major components.

It's a large, super-conducting magnet, 8-Tesla...

much, much bigger than the Earth's field.

And this is the actual insert being assembled for the next run here.

So the idea of the experiment is so straightforward.

When we insert this insert into the large magnetic field here,

nearby axions scatter off the magnetic field -

and, oh, my goodness, there are a lot of axions.

But the number of scatters is very small.

That's why it's a hard experiment.

And those few microwave photons, as a result of that scatter,

get amplified, get pushed out of the experiment

and detected by the

low-noise room-temperature electronics,

and if the axion is the dark matter, we should be able to answer

the question - does it or does it not exist as dark matter?

As ever, it's a simple enough question to ask, but unlike

certain other set-ups, Leslie is hopeful that his experiment is

straightforward enough to stand some chance of providing a simple answer.

I can really see it as being a particle in nature,

and I'm really driven, as we all are driven here, to try and find it.

And if you don't?

We will dust ourselves off and move on.

I mean...

God can be tough, and if God decides axions are not

part of nature, then that's the answer.

There's not much I can do about it.

We will have an answer, though.

I-I will be still living when we have an answer.

There are many other theories where people will be long-dead

by the time the theory is fully, fully vetted.

But it's not just axions.

There are other cold dark matter candidates

competing for God's attention.

One that glories in the name of the sterile neutrino

isn't even cold, it's warm.

Carlos and the gang of four may have been wrong all along.

In recent years,

Carlos has been flirting with the idea of warm dark matter and has

even created a computer simulation of it in our own Milky Way.

Cold on the left, warm on the right.

This is still tentative.

It's still controversial.

But here's a prediction for what the halo of the Milky Way should

look like if the universe is made of warm dark matter.

It should be much smoother with far fewer small clumps.

And the beauty of this is here we have a prediction,

cold dark matter versus warm dark matter, that's eminently testable.

It's now incumbent upon observational astronomers to

tell us, with their telescopes, whether the Milky Way is

in a halo like that or whether the Milky Way is in a halo like this.

If it turns out to be that the universe is not made of cold dark matter,

I will be rather depressed, given that I've

worked all my life on cold dark matter.

I will be disappointed, but not for very long,

because that's the way science is.

You have to accept the evidence and if it turns out that I've

wasted my life working on the wrong hypothesis, so be it.

What I really want to know is - what is the universe made of?

Let it be cold, let it be warm.

I just want to know what it is.

At Fermilab, that answer might be inching slightly closer.

CHATTER

A representative of the Fermi telescope collaboration is

preparing to make an announcement.

This is the moment Dan Hooper has been waiting for,

ever since he first identified the excess gamma rays in the centre

of the Milky Way and saw the bump they produced in his graph.

Professor Simona Murgia will shortly reveal

whether the raw data that hints at the presence of a Hooperon

is real or simply the product of a loose wire on the satellite.

OK, so here is some more information about the Fermi mission.

Professor Murgia's analysis of the Fermi telescope data

is rigorous and extensive.

So this spectrum in gamma rays of the globular class gives you

a good indication of the spectrum of population in the second pulsars,

so these...

But there's only one thing Dan wants to hear.

The signal was consistent with dark matter annihilating again.

I will have, hopefully, new interesting results to come. Thanks.

So what we find when we look at the data with our analysis,

is that there seems to be an excess which is consistent with

a dark matter interpretation, meaning that it has

a distribution that is very similar, very consistent with what we

think the dark matter distribution in our galaxy should look like.

As I see it, they see, essentially, the sort of excess we've been

talking about for years.

That's a great step.

They haven't been saying that until very recently.

So I think it's very exciting because this could be

the first time that we are seeing dark matter shining.

However, there is a lot more work that we need to do to

actually confirm that what we're seeing is dark matter.

So, we're heading in the right direction? Right direction.

Maybe not there yet, but definitely in the right direction.

So you're happy that the last few years' work

hasn't been a complete waste of time?

It doesn't seem to have been a complete waste of time.

OK, good.

It might be that, finally, science is making inroads

into the mysterious non-visible world of dark matter, perhaps.

If the Hooperon checks out,

and if all the fingers being crossed in Switzerland

and France pay off, then, at least in theory, the deep-mine scientists

will simply have the formality of looking in the right place.

Dark matter identified, standard models intact,

Nobel prizes handed out.

You would think that would be that, the end of the story.

But you'd be wrong, because there's another problem, another

dark thing that is a description of something we don't understand.

It's called dark energy.

So, 15 years ago some astronomers observing distant supernovae

saw that the distance to those supernovae was larger

than they expected, and so the only way that they could

understand that was to have a universe that started accelerating

three billion years ago, and whether that carries on accelerating

or not, we don't know, but what we do know is that there has to be

another component to the universe which we call this dark energy.

But you don't know what it is? No idea. Not at all.

No-one knows what it is?

No-one. No-one.

There are more theories than there are theoreticians.

And that's a problem, because according to the standard

model of cosmology, it makes up most of the universe.

Our universe consists of 4% baryonic matter.

26% dark matter.

And 70% dark energy.

And because dark energy seems to make sense,

at least at a theoretical level,

it's the role of experimentalists like Bob

to think of ways to explain it.

That's why he's come here to the Dark Energy Survey

at Cerro Tololo, where one of the world's largest digital cameras

scans the night sky in search of more supernovae

and an ever more accurate picture of the universe's expansion history.

You can probably see some of the stars, and in here will be

some of the supernovae that we're hunting to measure dark energy.

So are you hopeful?

I am hopeful.

I think we will be able to make at least a factor-of-ten improvement

with using this instrument, than we have today.

And then if we don't get that, we'll have to wait for LSST.

The LSST, the Large Synoptic Survey Telescope,

is being built on another Chilean mountain and is due to come

on stream in 2021, representing a significant jump in resolution.

With this instrument, we can observe about 3,000 supernovae.

With the LSST we'll be able to observe about a million supernovae,

and that should really nail it.

OK. It won't though, will it? Actually?

THEY LAUGH

See...

It'll nail it, it will nail it.

What, what will it nail?

Well, it'll nail the expansion history of the universe

and then, hopefully, some bright theorist will come up with...

So it's not going to nail dark energy.

It'll just show you how it's expanding?

It'll show us how the universe is expanding

and then, hopefully, that will give us some direction

in which to understand the true nature of dark energy.

It could be that cosmology stands on the cusp of revealing

the true nature of our universe.

Then again, it may stand on the cusp of nothing at all.

It might be that the only way to progress is not to look harder,

but to embrace a new physics that's currently,

like the dark universe, just out of reach.

HE EXHALES

HE LAUGHS