Personally, I am sure that the universe began with a hot big bang,
but will it go on for ever?
And if not, how will it end?
I am much less certain about that.
The expansion of the universe stretched everything out,
but gravity tries to pull it all back together again.
Our destiny depends on which force will win.
And the influence of gravity in turn depends on what the universe is made of,
and just how much of it there is.
It won't be easy to find out if,
as we suspect, most of it is dark matter, stuff we can't even see.
As a child in India, Priya Natarajan dreamed of becoming a poet.
Twenty years later, in Cambridge, she's a astrophysicist,
trying to build a perfect model of the universe.
There is this impression that everything the scientists do is very circumscribed,
whereas it is not, because we are bringing,
the ways in which we perform a model, the way the ingredients that we put in,
and the ways in which we choose to mix
the ingredients has a lot to do with our individual creativity and our feelings,
and our sense and intuition of how things ought to be.
So it's almost like writing poetry, where you pick a particular poetic form.
For instance, you could pick the sonnet, or you could pick the Japanese haiku,
which, each of these forms has a set of rules,
so you operate within a set of rules, which is
very much like the laws of physics, that you operate within in a model.
But then, inside the, inside the form or inside the model,
there's a lot of freedom, there's a lot of choices you can make.
Priya's life was changed when she was awarded
a top science scholarship at Cambridge.
Her area of cosmology is profoundly creative.
Priya's subject is the fate of galaxies in our universe.
We see the stars that shine in the galaxy,
and we also have evidence that there is some gas in the galaxy,
because we can see the light that's scattered off the gas.
But, er, as it turns out, galaxies contain a lot more than just that.
Priya's imagination was fired when She came across research done in the 1950s
by a young American scientist, Vera Rubin.
What Vera Rubin did in her work was map the speeds of stars at different distances
from the centre of a huge spiral galaxy.
Vera Rubin noticed something which made nonsense of what she'd been taught.
Stars spinning round the centre of
galaxies were supposed to behave just like the planets that orbit the sun.
But they don't.
With our solar system,
you have the sun in the centre and you have sort of the planets orbiting around,
and since the dominant gravity is that of the sun the planets
that are the outer planets, they move much slower than the planets on the inside.
So naturally what people expected to find was similarly in a galaxy,
if you measure the speed of the stars away from the centre towards the edge,
you expect it to fall off.
And what Vera Rubin found instead,
when she actually measured that for a spiral galaxy,
was that the speed stayed the same as she,
as she sort of mapped the speed of the stars from the inside out,
all the way out to the edge, they stayed, they stayed the same.
Vera Rubin saw that the galaxy center
could not be the exerting gravity on the stars.
For all the stars in a galaxy to move at the same speed as each other,
there had to be a force at work all around them.
But nothing which could produce such a force could be seen.
Vera Rubin suggested that
every galaxy must be surrounded by matter that we cant see.
But the scientific establishment at the time was not ready
what she called dark matter.
Her announcement that there was dark matter associated
with every individual galaxy, er, was received with much scepticism,
because of the far-reaching implications
it had and because also of the inferred percentage of dark matter.
From her work, she inferred that almost ninety per cent of the mass
in a spiral galaxy had to be dark.
The movement of stars in a galaxy must be controlled by stuff we can't see.
Around every galaxy, there needs to be a halo of dark matter,
invisibly locking all the stars in place with the gravity it exerts.
The stars are not stranded in the vastness of space,
it's just that we can't see at least ninety per cent of what's out there.
It could be as much as ninety-nine per cent.
For a cosmologist like myself,
it's crucial to know precisely how much dark matter
there is in order to know what will become of the universe - eventually.
The total mass of our universe is what decides the, er, the fate of our universe.
Whether we continue expanding or whether we stop and decelerate or we turn around,
back on ourselves, so the ultimate fate of
what really happens to us depends on how
well we have made an inventory of the mass in the universe,
and therefore if such a large fraction is indeed dark,
that has very important conseguences.
For example people now doubt that Vera Rubin was right.
Dark matter determines the future of the universe.
To know our ultimate fate, we need to be sure that dark matter exists,
and how much of it there is.
Searching for the invisible is not for the faint-hearted.
When Chris Stubbs first told his colleagues
he wanted to look for dark matter, they told him he was out of his mind.
That wasn't the way to a safe job.
There's an infinite amount of science to do,
and I think the trick is to choose carefully what you spend your time doing,
and my taste for a long time has run towards fundamental problems that
may be very difficulty to address experimentally,
but which have a very large impact on our understanding of the universe.
Chris Stubbs staked his career on finding something that couldn't be seen.
But nobody knew exactly what dark matter was,
they had to decide how to target their search.
We're carrying out an experiment to look for a particular kind of dark matter,
which we call machos,
which is a short-hand that stands for massive compact halo objects.
The idea is that our galaxy has a big halo
of dark matter around it that's made out of
astronomical objects that for one reason
or another don't shine like the stars that we see.
These astronomical objects, the decomposing corpses of dead stars,
are thought to litter the universe.
From the size of our earth to ten times the size of the sun,
these machos might explain what Vera Rubin had seen.
If so, they'd be found in the outskirts of galaxies.
It was the right place to search.
But finding them would be like looking for a black bat on a dark night.
The one thing that he knew was that his subjects were fairly heavy,
and if they were heavy,
they would have a gravitational effect on light passing nearby.
The macho-hunters turned to Einstein's general theory of relativity
to tell them how gravity affects light.
Einstein's general theory of relativity tells us that
light passing close to an object like the sun
or a lump of dark matter is deflected, it gets bent,
and the effect that a mass has on the light
coming from a distant star or galaxy is just like putting a lens
in front of the star, it distorts the image and it makes it appear brighter.
According to Einstsines theory
if the huge mass of a macho passed between us and a distant group of stars,
we would see the stars would progressively brighten,
and then fade back to normal.
Chris Stubb's team analysed thousands of images of stars to prove their point...
Two years after we started the experiment, we were looking through the data,
trying to understand how to analyse it, and much to our surprise,
found exactly what we thought we were looking for,
in that we saw a star get brighter and then fainter again with
exactly the signature that's predicted by general relativity.
OK, this looks like a definite, it fits it pretty well.
...draw it...
A nice microlensing curve...
Our experiment has detected a previously unknown component of this galaxy.
It's a stunning result.
The first dark matter had been detected.
It really is out there.
But did these machos solve the riddle posed by Vera Rubin?
Could there be enough of them around each galaxy
to exert the gravitational effects she had observed?
The simplest point of view is that the machos are ordinary matter made up of
the same material that exists in stars, it just didn't happen to end up in stars,
and we think we know exactly how much,
we think we know how much ordinary matter there is in the universe,
and it just isn't enough to solve the dark matter problem of the universe.
We need to find something else to account for more of the dark matter.
Machos were big, so why not try something tiny?
One candidate was a well-known particle, the neutrino.
This is produced in atomic bomb explosions,
so it would also have been produced in the big bang explosion.
If it had a tiny mass of its own, it could be the dark matter.
Or it could be one of the so-called exotic particles
whose existence was predicted by theory,
but which were very hard to detect in reality.
Perhaps they are actually out there,
silently shaping the evolution of the universe.
Carlos Frenk, Professor of Astronomy at Durham University,
is fascinated by how things grow and change.
I think there is a great parallel between the evolution of the largest system
that we know of, which is the universe, and the evolution of
the biological system, like a person or like my son.
I often wonder what are the factors that are going to influence
the development of my son, whether he will be,
er, a physicist, or a scientist, whether he will be a musician or
will whether he'll be anything else.
I don't guite understand what are the forces
that are going to drive him in one direction or the other.
The universe, however, is simpler,
because we have a much greater mastery over the laws that
govern the evolution of the universe.
Carlos is confident he can describe
how the baby universe grew into the universe we know today.
He believes that the dark matter exerted a powerful influence.
Carlos' fascination with how things evolve has found its outlet in physics.
He doesn't yet know whether it will be the same for his son.
Carlos' dream is to grow a perfect model of the universe in his computer,
from its violent birth to the complicated picture we see today.
That will only happen
if he programmes in exactly the right characteristics for the dark matter.
He starts every attempt just a second after the big bang.
Prior to that time,
the universe was made up of a cosmic soup of elemental particles and radiation.
No-, nothing else could exist in the midst of this tremendous heat.
But then, er, after about a hundred seconds,
the universe had cooled down a little bit to a mere ten billion degrees,
but this temperature is now low enough that, er,
the first thermonuclear fusion reactions can take place.
Carlos' belief is that at this critical moment,
a mysterious group of particles broke free from the pack.
Instead of forming the galaxies and stars,
they began to cluster together to become the dark matter.
These clusters of dark matter had a gravitational pull on the ordinary matter.
As the lumps of dark matter grew bigger and bigger in the expanding universe,
they pulled in more and more ordinary matter, which condensed to become stars.
Eventually, one billion years after the big bang,
the first galaxies began to form.
In the early 1980s, Carlos was ready to build a model of the universe.
The idea that was around at the time was that the dark matter
could consist of small elementary particles called neutrinos.
Carlos wondered whether the neutrinos from the big bang
could be the dark matter.
That was the very trendy, fashionable ideal, if you like,
in those days, and, it was the first concrete proposal
we had for what the dark matter could be, and, er,
this was very significant, and some people would say, a very significant idea,
so people would say it was the signal of the beginning of the revolu-,
revolution in the way in which we study the universe,
because for the first time the neutrino hypothesis
provided a concrete starting point that we could explore
in an unambiguous fashion using the tools of evolutionary cosmology.
Neutrinos travel across the universe at virtually the speed of light.
Every second about a hundred trillion neutrinos pass straight through your body.
The universe is thick with them.
But nothing seems able to stop the neutrino.
When they meet a solid object like the earth,
they simply float through it, and this makes them extremely hard to catch.
But it's important that we do,
because we must find whether the neutrinos has mass - without mass,
they wouldn't exert gravity and so couldn't be the dark matter.
In northern France, Yves Declais wants to catch neutrinos,
and see if they do have any mass.
When you prepare your bait, when you, when you have prepared your detector,
when you have prepared your trap, you have to,
to install it at the right place, and so you have to go,
well, and you think you will be able to detect neutrino,
well, you will be able to get out some fish, out from the river,
and after that, you have not only to wait, but you have to work and you,
you will see what will be the result, you will see, er, how many neutrino,
you will see how many fish you will get out of from the river.
Neutrinos are produced in nuclear reactions.
So Yves Declais has set up his project
in a decommissioned bunker beside a nuclear power station.
In this experiment, we want to see if the nature of
the neutrino change between the source of the neutrino
and the detector one kilometre away.
And if the nature of the neutrino change during this path of one kilometre,
we can demonstrate, we can prove that this is related to the existence of,
of a mass for the neutrino.
It is a little bit complicated to compute, but it, it is a very, very simple system.
Once they had set up their eguipment, the scientists must wait.
Apart from routine checks, there's nothing to do until
their highly sensitive detector begins to pick up passing neutrinos.
All the data from the detector is fed down ground lines to the chateau,
where Yves and the team live.
The want to establish if they detect less neutrinos
now then when they set up the same eguipment much nearer to a reactor.
If the numbers aren't the same, they will know that the neutrino has mass.
It may take years to find out, but it will be a result with huge implications.
This is really important, er,
for particle physics and also for cosmology and astrophysics.
One of the best candidates for the dark matter,
for the missing matter of the universe,
is the neutrino, if the neutrino has a mass.
Even if the mass of the neutrino can be very tiny, very, very, very small,
because the universe is completely filled by a lot of neutrino coming
from the whole universe, from the first traces of the universe.
Meanwhile, Carlos Frenk was hammering away at a theoretical approach.
His idea was to assume that neutrinos had mass,
and see what kind of a universes would allow his computer to build.
So we programmed our computer to follow the evolution of a universe
in which the dark matter was made up of massive neutrinos,
and the aim was to produce in the computer a synthetic universe that
we could then compare with the real thing.
So we programmed our computer up in this fashion,
and I let it churn away over Christmas, and when we came back,
we saw the first maps being generated by the computer.
Here we have a recognizable universe, a credible universe,
something that's made galaxies, something that's made galaxy clusters,
something that is competitive vis-a-vis the real universe,
there was a great sense of elation at the thought that
we might have solved what was already clearly then and sadly still is today,
the main unsolved problem in cosmology, with, one,
one of those feelings that you have once in your lifetime,
and you think you're really stumbled upon something major.
Now, that was, that was our first impression.
But his first impressions were deceptive.
Carlos was so elated to have grown a computer universe that
at first he didn't notice the truth.
On closer inspection,
his modelled universe didn't guite look like the real thing.
But Carlos was not tempted to give up his computer modelling.
He was still convinced that studying the universe would prove
easier than pursuing some other branch of science.
So it is paradoxical that we can understand the universe better than
we can understand a tiny little part,
my son is an insignificant little speck in this gigantic universe,
and yet I can understand the universe better than I can understand my son.
And so often I regard myself as being very lucky that I am a physicist,
rather than a biologist or a psychologist,
they have a much tougher time than we do,
because we deal with systems that are intrinsically simple,
biologists and psychologists deal with these, er,
much more complex and in some ways magical world of humans,
who are essentially unpredictable.
Particle physicists urged Carlos to consider a hypothetical particle,
one of the so-called exotic particles.
Why should he try only known particles? They argued.
Any more that his son should practice only one skill.
The neutrino had been to lively, or hot, to build a realistic universe.
They urged Carlos to try something more sluggish,
something he would call 'cold dark matter'.
Our next step, then, was to change our starting assumption,
and now, er, take the dark matter to be composed of cold dark matter.
I must say we were very sceptical when we started this new project,
by then we have got slightly fed up with particle physicists trying
to tell us astronomers what the universe was made of,
particle physicists are supposed to be working on something else,
and they have no right to come and tell us astronomers
what our universe is made of, er, so our approach at first was, er,
a really fairly, cynical, we, er, started off saying, right,
let's go and do this particle physicists right,
let's go and rule out, we'd ruled out neutrinos,
now let's go and rule out cold dark matter as well,
and get those guys off our backs,
so they can go and do their own thing with accelerators and
we can keep on doing our own thing, trying to understand how galaxies form.
What happened was that, er,
these cold dark matter universes turned out to be far richer and
far more interesting than we ever had any right to expect.
Carlos has blended neutrinos and cold dark matter in his latest computer model,
but can he be sure cold dark matter exists?
The, er, burden of proof is on the experimentalists
who now have to go and detect these particles,
and until that happens then, er,
we cannot be by any means certain that this is a correct theory.
But, if they do succeed, if they do succeed,
this really will be an outstanding achievement,
and I think it's not an exaggeration to say that
if the dark matter turns out to be an exotic elementary particle,
this really will now go down in history as one of
the greatest scientific discoveries ever.
The evidence suggests that most of the universe
is made up of something no-one has ever seen.
By its very nature, cold dark matter has to be hard to detect.
Finding a way to do so is one of the most difficult tasks in physics today.
Here, in the North Yorkshire Moors,
a small team of British scientists is tipped to win the race.
They are led by Neil Spooner.
It's very astounding that at the end of the twentieth century
we actually don't know what most of the universe is made of,
not ninety per cent, maybe even ninety-nine per cent, er,
and that it sort of puts one in, as, as a human being, in,
into some perspective, that, you know,
we, we, the earth is not the centre of the solar system,
etc, and, and maybe we're not even the only life now,
er, and we're not even made of particularly common matter,
in that we're not the typical matter that's around,
because most of it's dark matter, which we don't know what it is.
Like neutrinos, cold dark matter is very difficult to distinguish
from the rain of other cosmic particles that bombards the surface of the earth.
Any detector would have to be protected, deep underground.
It just happens that my father is, is a, a mining engineer by profession,
so I just asked him what was the deepest mine in Britain, naively,
thinking it would be a coal mine, which would be useless for us,
because it would be guite difficult to, to work in a coal mine,
because of the, er, the safety aspect of it.
But it turned out that, er, he looked it up and,
and the answer was Bowlby, which is a salt mine, which is ideal for us.
Bowlby mine is not only the deepest in Britain, but the deepest in Europe.
The lift travels at twelve miles an hour for five long minutes,
taking them a mile underground.
At this depth, the air is ten degrees hotter than
it is at the surface, and very dry.
Their intricate scientific eguipment must be built to
withstand these hostile conditions.
It's a technological fight, trying to work deep underground,
in the salt mine, which is an environment which, well,
we're dealing with fairly intricate electronics and,
and we're trying to be clean and trying to that in a mine is,
is sort of very, very difficult.
The Yorkshire miners seem to enjoy working next to a group of
fundamental particle physicists.
So they all say, have you found it yet?
We just go, not yet, but we're working on it, or something.
It might seem odd to the miners that
if dark matter makes up ninety-nine per cent of the universe,
it should be so hard to find.
But the particles that Neil Spooner is looking for
aren't known as weakly interacting massive particles for nothing.
These so-called wimps try to avoid contact at all costs.
These particles are neutral, they're not charged.
Their interaction is like a sort of billiard-ball effect.
If they interact, which mainly they don't, but when they do,
they just will strike an atom, which will recoil.
So what we're looking for is, is these little recoils of atoms.
We're talking about very small distances,
thousandth of a millimetre or something, and move off.
And as this atom recoils, it gives off some energy.
In our case, light, and you try and detect this light.
To shield their detector from radioactive rays from the rock,
they suspend in two hundred tons of distilled water.
We're over a kilometre underground, so we've got rid of the cosmic rays,
and then we're in the water,
so we can screen off the stuff coming from the walls,
and then we've got our detector in the middle,
sitting there waiting for a wimp.
Our detector was a simple crystal which gives off little bursts of light when,
when struck by a particle, and we have to really amplify this light,
because it's very, very low level.
We use this device called a photomultiplier, to convert, er,
light into electrons, and these are then multiplied and you get,
for every one that comes in you get about a million coming out,
and that provides a nice big signal which you can then measure and record.
In the last year or two, we have made fairly significant progress,
we've improved our detector such that
we're now about fifty times more sensitive than anyone else was previously.
But we still need to get probably another hundred times better.
If we do that, then we should see them or we should not see them.
If we see them, then obviously that's very exciting,
and maybe we've discovered what dark matter is or
what most of the dark matter is.
If we don't see them, then that's also pretty exciting,
because it's got to be something and if it's not wimps then it,
maybe it's not machos, and maybe neutrinos don't have mass, we don't know,
but, er, it's got to be something, so that would deepen the mystery.
Even when the dark matter, or I should say when the dark matter is discovered,
because it's not a guestion that dark matter is there to be discovered,
and it will be discovered, I can say that with complete certainty, well,
as complete as a scientist can ever do, but when the dark matter is discovered,
I think the whole jigsaw of our universe will fall into place,
we will understand not only why our universe looks the way it does,
we will understand why there are galaxies, how they came to be,
why there are planets, why there are stars but we would al-,
also understand what the ultimate fate of our universe will be.
There are two possibilities.
If there's only a fairly small amount of dark matter,
the universe will continue to expand for ever,
getting colder and colder, and more and more empty.
On the other hand, if there's a lot of dark matter,
gravity will slow down the expansion of the universe and stop it eventually.
Then the universe will begin to contract and will end up in a big crunch,
like the big bang in reverse.
From what we know now, it could go either way.
If I placed a bet, I think I know which fate I'd back.
But how would I collect after the big crunch?
Whichever way the universe eventually goes,
its evolution is being affected by dark matter right now.
But before we have even discovered what it's made of,
some astronomers have begun mapping its effects.
The challenge of mapping the universe is that
you have to do it in three dimensions,
and human beings are very good at making two-dim-, two-dimensional maps,
it's the challenge of getting that third dimension and putting it in your head,
being able to close your eyes and see in three-D what's around you,
it's a challenge, but it's on.
So, I think we'll lay things out with the eguator down here, or something...
When we first started this mapping business, it was very, very primitive,
and people knew that there were clusters over there
and a everywhere over there and so on,
it was a little bit like Stanley going to darkest Africa,
he knew where the Congo river was, and the Nile maybe,
but not much about anything else.
It looks like it's over there.
Sandra Faber's enthusiasm for her subject is legendary.
Through numerous collaborations,
she has done as much as anyone to further dark matter research.
Her mapping technigues were to reveal far more than
the patterns already established with conventional two-dimensional maps.
What we're going to see here is three slices of the universe,
and earth is down here in this diagram, right here at the point,
and now we see the first slice being displayed like this,
each little black dot is a galaxy.
This was taken from the southern hemisphere, and now we see the,
finally, the third slice is coming up here.
And what's interesting about these maps is that
the galaxies are not uniformly distributed in space.
Instead, what we see is that they tend to pile up along these walls;
some people have called them 'soap bubbles',
and then the insides of these spaces are called 'voids',
they're relatively empty of galaxies,
and of course this whole structure is expanding as the universe expands.
Now, the guestion is, of course,
why do the galaxies trace this beautiful large-scale structure?
Sandra's hunch was that
this structure was created by an unseen web of dark matter.
So she set out to prove it,
using the most advanced telescopes in the world.
Observing is, er, almost mystical,
it's the act that really puts me in contact with the rest of the universe.
Sitting there and accepting these photons,
er, I imagine projecting myself back along that same path and,
in some way, I know this sounds ridiculous
being in communication and communion with where they came from.
I often think if somebody's looking back at me,
I wonder if their telescope is bigger than mine.
Even through telescopes of this size,
galaxies appear as tiny specks many millions of light years away.
But to prove her theory,
Sandra Faber needed to achieve measurements of unbelievable precision.
The slightest variation in air temperature can invalidate a night's observations.
Before each run, the telescope is cooled with liguid nitrogen.
Once everything is ready they retreat to the warmth of the control room
from where they can aim their telescope
Ah! So this is our next observation. What was this galaxy?
Oh, this was, er, NGC 5813...
Fifteen years ago when Sandra and
her colleagues created a new mapping technigue
they hoped this would reveal the effects of dark matter across the whole universe.
They were completely astonished by the picture that
they finally pieced together.
Just suddenly it came to us,
that if we plotted all of these motions of galaxies,
with an enormous region of space, including us,
was moving roughly in parallel like a big river of galaxies
at the breakneck speed of six hundred kilometres per second,
and that was a definitely new thought for us,
that really struck us very strongly,
we said, what have we discovered, this is truly remarkable.
Then we began to look at our survey in more detail,
and we saw that in fact, off in the distance there,
towards which this great river was flowing,
was a very large structure,
which one of us later named 'the great attractor',
and it turned out to be a very, very big supercluster,
a supersupercluster of galaxies,
and our motion towards that is due to its gravity, it's pulling all of us in,
and it's something like fifty to a hundred billion years from now,
our galaxy will be one of several thousand on orbit
in the great supercluster called the great attractor.
The force of the dark matter around the great attractor is pulling us
across intergalactic space at six hundred kilometres a second.
An unseen web of dark matter is controlling the shape of the universe.
It's dragging all the galaxies together into clusters and superclusters,
and leaving behind huge voids in space.
The way the dark matter clusters will affect exactly
how galaxies form and how superclusters, voids, walls and so on form,
dark matter is key, it, er, it is controlling the motion of everything else,
it's making the galaxies form,
it's making the large-scale structure of form, it's in charge.
We can still only guess about where dark matter is taking us.
If Sandra Faber were pushed, she thinks she knows how she'd place her bet.
Well, currently, it looks as though there's not enough matter in the universe,
guite, to retard the expansion.
If we had to bet right now we'd probably bet
that the universe will expand for ever.
It's a fascinating idea, if the universe expands for ever,
what will happen to it as it cools off?
Stars are gradually consuming all the gas in galaxies,
over time it will all be used up,
those stars will burn and use up their fuel and die, become cold,
dead remnants, white dwarves, maybe some black holes in there.
Galaxies are ever merging to make yet larger structures,
but will become ever dimmer as the stars in them die out,
and ultimately even the very stuff of which stars are made,
the protons, neutrons, and so on, will decay,
and it may be that the ultimate state of the universe is to have no matter at all,
a sea of, er, of, er, elementary particles, dead photons, nothing else.
And the alternative of a big crunch is not much better.
A few years ago, when I was giving a lecture,
I was asked not to mention the end of the universe,
in case it depressed a stock market.
But I can reassure worried investors, either way,
the universe is good for many billions of years more.
The end may be coming, but not just yet.