So on behalf of the Royal Society, I’d like to
welcome you all here tonight.
As you probably realise, this has been an
exceedingly exciting year for physics and of course,
locally in Edinburgh with Peter Higgs getting
his Nobel Prize, I think almost everyone we
know now has some feeling for what fundamental
physics is about.
But even very recently, as recently as three
weeks ago there have been new discoveries
announced about how the universe is evolving.
And tonight, Clare will be telling us about some
of these things and how to interpret them.
Now, Clare did her PhD at University of Cambridge
and then she worked at DESY in Hamburg,
also the University of Geneva and now is one
of our Royal Society university research fellows
at the University of Nottingham.
But you're not here to listen to me.
You're here to listen to Clare, so Clare...
[Applause].
Thank you, and thank you so much for coming tonight.
It's really lovely to be here with you.
And yeah, what I want to talk to you about
is the dark side of the universe.
So I think probably few of us in this country
are lucky enough to see the beautiful skies
that they get in the middle of the desert.
This is a picture from Death Valley in Nevada
and you can see the arc of the Milky Way
across the centre of the picture there.
And we know that there are things outside,
things away from the Earth, things out there
in the universe because we see the light from them.
You can see all these stars making up the
Milky Way by the light that they give off.
But this talk is not going to be about that.
This talk is going to be about dark things
in the universe.
So what do we mean by that?
So 'Dark', dark has a lot of meanings in the
English language.
There are a couple of them here from the OED,
and these are the meanings that we're going
to take for this talk.
So dark things are things that don't give off light,
an absence of light or things that are hidden
from view.
Things that are obscured, that you can't see.
What this talk isn't about is dark in the moral sense.
So unless, just to make this completely clear
from the beginning, this is not a talk
about Star Wars.
And we are not going to be passing any moral
judgements on any of the components of the universe.
So how do we know that the universe has a
dark side?
As we just said, we see things in the universe,
we see things in this room because they give
off light or they reflect light.
I can see all of you because the lights from
the ceiling are reflecting off you and I see that light,
the same way you see me.
And okay, we have few other senses that tell us
that things are going on around us but especially
for things far away, light is really the way that
we know things are there; the way that we
observe objects.
So if there are things that don't give off light,
how on earth are we supposed to try and find them?
How do we even know that they're there?
So to answer this question, we're going to start
with a few slightly easier ones that we maybe
stand a bit of a better chance of answering.
So we're going to for the first part of the talk
we're going to go through these questions.
How far away is a galaxy?
How do we know how far away objects are in space?
We can't travel to other galaxies and measure
the distance as we go, so how do we get a sense
of how far away things are?
How fast are they moving?
Are they moving at all?
Are they moving towards us or are they moving
away from us?
Is there a way that we can find that out?
And also we're going to ask what are galaxies
made of?
Are they just stars?
Are they made of the same things that we are
or are there other things there as well?
And so by answering these three more straightforward
questions, we'll come back to how we know that
there are things in the universe that are dark.
So the first question we said, how far away
is a galaxy?
And for this, I’m going to need a little bit of help
from the audience because what I want to show
you is the principle that we use to tell how
far away things are.
So just give me a second.
Could I; would you be able to hold one of these for me?
Thank you very much.
And then I need one... Ooh, it's gone out.
One person in the middle of the room and one
person at the back.
Would you be able to hold that for me?
Can I give that to you?
Okay, so hopefully everybody in the room can
see the candles.
Then maybe we could dim the lights just a little bit?
Now, I promise you that the candles have all come
from the same pack; they should all have been
exactly the same.
So they should be burning with the same brightness,
and maybe you can convince yourselves that the
ones that are furthest away from you look dimmer.
You see less light than the ones that are closer to you.
I'm sure that's probably what you expect, but
hopefully you can actually see that in the room
this evening as well.
Okay, so maybe then we can bring the lights up,
make sure nobody burns themselves and shall I
take that?
Do you want to just hold onto them?
Okay, save me running up and down stairs again.
So what we've just seen is probably as we said,
a principle that you know very well, which is that
if you have things that are shining with the same
brightness, if they're further away from you you'll
see less light than if they're closer to you.
Okay, and that's all that this equation here...
Okay, there is not going to be very much maths
in this talk but there are going to be a couple
of equations.
That's what this equation is telling you.
It's telling you if you have a source that's giving
off a certain amount of light, the amount of light
you see decreases the further away from you
the object is.
Okay, that’s completely what you would expect
and it's what we have just seen.
So the problem with doing...
So this is all very well in the room because we took
the candles from the same pack, we knew they were
all going to be the same so when we saw them dimmer,
we knew they were further away.
The problem we have in the universe is that
we can't go out and sprinkle the universe with
candles that we've made.
We're just observing objects from here on Earth
and then if something looks dimmer, how do you
know if it's just dimmer or if it's further away.
If one object is dimmer than the other, is it
because one is burning more brightly than the other
or because one is further away from the other one?
So what we have to do is find something that burns
in the same way; that burns in the same way
wherever it is in the universe, just like we did
with the candles that were all the same.
We're very lucky in fact, to have such an object.
They're called Type 1A supernova.
They're dying stars; stars that die in a really
spectacular way.
They collapse to form black holes and in the process,
they give off an enormous amount of light.
Because black holes are very special objects,
the way they do that, they always give off;
a star always gives off the same amount of light
when it dies and forms a black hole.
So wherever that happens in the universe, we know
the same amount of light is being given off.
And so we can use these supernovae to tell us
how far away things are, relative to one another
because if the supernova looks dimmer, it's
further away from us.
The second question we wanted to ask was
how fast is the galaxy moving?
In fact, is it moving at all relative to us?
And the way that we can answer this question
is by looking at the colour of the light from the
galaxy.
Now, hopefully maybe you remember from school that
the colour of light is related to its wavelength.
Light with longer wavelength looks redder;
light with a shorter wavelength looks bluer.
And so if an object is moving relative to us, that
changes the way that we see the light
from that object.
Now, this is really the completely same thing as
the Doppler Effect.
The change in frequency of the sound that you
hear as a police car goes past.
You hear that change in pitch as the siren goes
past you and this is exactly the same effect
only for light instead of sound.
And if an object is moving away from you,
that stretches out the wavelength for light
a little bit.
If the object is moving towards you, the waves get
compressed into a slightly smaller space and so
the wavelength gets smaller.
And so in principle, we should be able to say,
well, the light looks a bit redder or the light looks a
bit bluer so that object is moving towards us or
away from us.
But again, we have the same problem that we
don't intrinsically know what colour the light is
that a star is giving off.
You don't have any stars at home that you can test
and see the colour that it's giving off.
So all we see; we only have what we see.
Okay, we're just observing what the universe
is giving us.
But we can be a little bit cleverer than that.
Because stars emit light in a whole range of colours.
Okay, this is the light coming from the Sun,
all the different colours that make up the light
that we see from the Sun.
Okay, remember that we said red light has a
longer wavelength, blue and even purple light
has a shorter wavelength.
Now, the star or our Sun emits all this light but
some of that light collides with stuff before
it gets to us.
And in particular, some of it collides with atoms
inside the Sun.
And when that happens, for various particular
frequencies of light, the atoms can absorb
that light.
So most of the light from the Sun just comes
flying straight from the Sun to us but some of it,
very specific frequencies, very specific wavelengths
of the light get absorbed by the atoms in the Sun.
And that's why you see these little black lines
across the spectrum, so these are spectral lines.
And so different atoms absorb different wavelengths
of light and so you get these black bands where
we don’t see any of the light from the Sun
because the atoms have all absorbed it.
So we have this reference from the Sun and now
we can go out and measure the light from
distant stars and we can see what their spectrum
looks like.
So this is on the right hand side now, we have
the spectrum from a distant super cluster.
And you can see that the pattern looks very
much the same on the right hand side here.
The gaps between the lines look very similar
to what we had for the Sun and yet they've all
consistently moved upwards, moved towards
the red.
And the fact that we see the same spectrum,
so the same elements are absorbing the light from
this super cluster but we don't see them where
we expect to see them; we see them all shifted
across a little bit.
That tells us that the light, the frequency of the
wavelength of the light has been shifted compared
to what we were expecting.
Just to go back to the previous slide, we said
that things, light got redder if an object was
moving away from us and so the fact that all
of these lines, all of the light has got shifted
towards the red end of the spectrum tells us that
this super cluster has to be moving away from us.
Okay, so now we have two things that we
think we can find out about the galaxies in
our universe.
We think we know and can find out how far
away they are using the standard candles of
these supernovae, these exploding stars and
we think we know how fast; we can work out
how fast they're moving by looking at the
colours of the light that we see.
Now, they don't necessarily have to have any
particular relationship to one another.
It's not obvious, just thinking about it,
that they have to be related but in fact,
general relativity which is our current best theory
of understanding how gravity behaves in the
universe and how things move tells us that those
two things should be related to one another.
Okay, so there are some beautiful, complicated
mathematics to do with general relativity but I
think for our purposes, it's really nicely summed up
by this quote from John Wheeler, which tells you
that space-time, so space and time which in the
theory of general relativity are allowed to deform
and to move around, and to stretch and contract,
'Space-time tells matter how to move;
matter tells space-time how to curve.'
So the two things talk to each other.
The space that you're living in tells you and I
that we should be falling towards the ground
when we jump up.
And the fact that the Earth is here, it's bending
the space-time around it and that's then what
we feel in turn.
So this principle underlying the theory of relativity
tells us how the universe evolves depends on
what it's made of.
So this is the important point.
What there is in the universe determines how
it evolves.
So that tells us that distance and velocity are
potentially related, so if the universe is expanding,
that expansion is controlled by what is in the
universe through this theory of general relativity.
So if the universe is expanding, things are moving
away from us and that's the velocity that we see.
So this is a plot of observations of supernova.
So we talked about the fact that supernova are
standard candles and therefore, we can determine
how far away they are so the distance to these
supernovae are plotted on there.
Vertical axis; okay, some slightly more complicated
function of that but this is just the distance on
the vertical axis.
And on the horizontal axis, we call this the red shift
but so this is telling us how the light is moved
into the red and this is what we talked about by
the fact that the galaxies that these supernova
are living in are moving away from us.
So the larger the number on the horizontal axis,
the more they're moving away from us.
The larger the number on the vertical axis,
the further away from us an object is.
And so we have all these wonderful observations
of these supernova.
They live at different distances; they are travelling
at different speeds.
And you can see that there is a very strong
relationship between these two properties.
There is very much a clear curve going through here,
telling us that distance and velocities are related.
And we have our theory of general relativity and
we think we can predict how these two things should
be related, and how they're related depends on the
matter that we put in the universe.
Now, we've got three lines here that we've drawn
and these are different predictions from our
theory of general relativity.
And you can see that the green line goes right
the way through the middle of the data.
The red and the blue lines are a bit less well,
a bit less of a good fit to the data that we have.
They're for different stuff that we've put in
the universe.
So the blue line is a universe that is entirely
made of matter like stuff, like you and me,
particles that interact with each other, that
collapse under gravity; very familiar things.
Okay, so the blue line is a universe made of the
stuff that we know.
Unfortunately, that does not fit this distribution
of points very well.
What fits the points better is to think of a
universe that is partly made of matter,
partly made of something else and we're going
to come to this something else in a moment.
But before we do that, I want to ask the final
question, the third question that we had on
the slide at the beginning, which is what are
galaxies made of?
Now, this again is another fantastic picture of
a galaxy.
This is the Pinwheel Galaxy, and you can see
that most of the stars, most of the stuff is
concentrated in the centre of the galaxy.
We have these lovely spiral arms but they're a lot
dimmer; they're a lot fainter.
There seems to be less stuff there.
And one thing that we can then ask is how fast
is this rotating?
Obviously we can't do that from one picture,
but we can observe a galaxy and we can see
how the stars are rotating around the centre.
And again, we think we have a good theory of
how things should orbit under gravity.
We understand the motion of the planets very well.
So now just to try to demonstrate to you
orbital motion, you know that if we try and spin
things around, the more force we put in, the
faster it spins.
Okay, more force, faster motion, less force,
slower motion as it goes around.
Okay, so if things are moving under gravity,
it's the amount of mass there is that is the force
that's pulling things around in their orbits.
So it shouldn’t surprise us that the speed at
which things move is related to the Gravitational
in understanding dark matter, is to understand
constant which is capital G, and the mass around
which you are orbiting.
Okay, so the mass here is the force that's pulling
you around in your orbit.
Now, what we can't do with the yoyo is show
you the other way in which the speed at which
stars are rotating around the centre of the
galaxy varies.
That's that it should drop off with distance.
I think that probably makes sense.
We're used to the idea that forces weaken as
you go further away and so the further away you
are from the centre of the galaxy, the weaker
the gravitational force should be and the slower
your motion as you rotate around the centre of
the galaxy.
So our expectation is that the stars on the edges
should be moving slower than the stars
in the middle.
Now, what I have for you here is a simulation
of what we expect, what we just talked about
versus what we actually see when we look at
galaxies.
So our expectation is on the left and what we
see is on the right.
So let me play you the video.
I hope that it's clear to everybody that there's
quite a difference between what's happening
in these two images.
So let's play that again.
So what we see on the left is what we just
discussed.
Stars in the middle are moving quite quickly.
Stars on the edge are moving quite slowly.
What we have on the right, and remember this
is what we actually observe when we look at
galaxies, is that everything is going around fairly
quickly.
And what these two little graphs on the bottom
are showing you is where the mass has to be
in the galaxy to cause that motion.
So on the left hand side, we have lots of mass
in the centre and then it tails off as we go to the edge.
That's kind of what we were expecting when we
looked at this galaxy.
Most of the stuff seems to be in the middle,
and then there are obviously stars in the spiral
arms but the density of those light points decreases.
What we seem to actually need to match the data
is the distributional matter that's essentially
constant as we move out from the middle to the
edges of the galaxy.
Okay, and that really doesn't seem to match up
with the light that we see coming from the galaxy.
That seems to tell us that there is a lot of mass,
a lot of matter in this galaxy that we don't see.
Right, so we've talked about the fact now that
there are two reasons to believe that there are
things in the universe that we can't see.
One, because the way that distance and velocity
are related in the universe, the way that distant
objects are moving away from us doesn't seem to
be described by a universe that only contains matter.
And the fact that the way that galaxies are rotating
doesn't seem to be described by just the mass
that we see inside the galaxy.
So to give you an idea of how much this missing
stuff makes up of our universe, this is a picture
of the Trent building at the University of Nottingham.
We have a very lovely campus, very nice if
you want to come down and visit.
Yes, this is a picture that was taken earlier
in the Spring.
And now the amount of the picture that I’ve
left up, that I haven't blacked out, the area of
this picture; this is how much of the universe is
made up of stuff we can see, stuff we can understand.
95% of this picture has gone.
95% of the universe is made of things that
we don't understand.
And that's really tough for us as cosmologists,
as astronomers because all we can see is this bit.
And you can study what you can see here;
there's lots of detail in the clock tower and the
building.
From that bit of the picture, it's really difficult
to infer that there was supposed to be a flower bed
here, and a lake.
This is our job.
This is what we're trying to do as cosmologists,
is to try and work out what makes up this
black part of the picture.
Okay, so to give you the same story in a different,
in pie chart form, this is the 5% of the universe,
this is what we think our universe is made up with.
5% of it is made of ordinary matter, things we
understand and then the stuff that we don't know,
we break up into two pieces.
One we call dark matter.
That's the stuff that was missing from the galaxies.
Okay, and we call it dark matter because it seems to
clump together in the way that matter does.
It seems to live in the same places that matter does.
So that makes up, well, 26-27% of the universe.
The remaining 68% is even weirder.
And this is dark energy.
This is what we call dark energy, but these names
'dark energy' and 'dark matter', they're really
just labels for things we don't know.
And dark matter and dark energy, those are
just guesses at what might be underlying the problem.
So to try and explain to you what dark matter
and dark energy are, what we do know about them,
the very little that we know about them,
we're going to do a little analogy and just in case
anybody is unfamiliar with bread, I’ve brought
some along with us.
So I want to ask you, this is a fairly standard
loaf of bread:
What is it made of?
[Inaudible].
Okay, I can't... can somebody shout a bit louder?
[Matter].
Yes, I suppose there probably is matter we can't
see in there, but the things we can see?
Wheat flour, somebody said.
Very good.
Yeast.
Raising agent, yeah.
Some water.
Beer.
Maybe not in this loaf of bread.
Okay... Yeah.
So lots of different things can go into bread,
depending on how exotic and how interesting you
want to make it.
For a simple loaf like this, there are two very
important ingredients there that people talked about.
One is the flour that makes the structure of the bread.
That's kind of, the flour is what we see essentially here.
To make a loaf like this, the other really important
thing that we have to put in is the yeast.
We need something to make it rise.
And that, those two important things for the loaf
of bread are our two missing components of
the universe.
So dark matter is the flour, so the dark matter
is the thing that's giving us the structures.
It's what clumps together to form lumps and
strands that make up the bread.
The dark energy, we don’t necessarily see...
Okay, we don't see either of these components
but the dark energy, we see indirectly by the fact
that it's made everything expand.
So this is what dark energy does.
It's the yeast in the universe.
It's the thing that's making everything fly apart
from everything else.
Now, this is a nice little video of bread rising.
Obviously time lapse, speeded up of bread dough
rising and what I like about this video is not only
do you see the expansion but you can see the
strands of gluten in the flour trying to stick
together and they’re forming long strands and lumps
in the bread as the yeast is trying to push
everything apart, trying to make everything expand.
So let's just watch that again.
Okay, so there is a general expansion but the flour
and the yeast and the gluten are trying to
stick together.
Trying to hold onto one another and we're
getting voids and we're getting, sort of voids,
air bubbles I guess.
We're getting strands of flour in our dough which
then hopefully we'll bake and make something tasty.
Now, that is exactly what we see happens
in the universe.
There are these two things going on:
one is that the dark matter wants to stick together,
and the thing that's making it stick together
is gravity.
So gravity is pulling, trying to pull everything
together and everything is attracted to everything
else and that tends to make the dark matter
quite lumpy.
But because we have this dark energy, we have
this yeast, this raising agent in the universe,
that's trying to make everything expand and
that in particular, makes the spaces in between
things expand.
So what I’m going to show you now is a video
simulation of how we think these structures
grew in the universe.
We start from a quite uniform universe where no
place is particularly special and then we'll
see what happens.
Okay, yes. And this is from the Deus simulation.
And the colours that you see are the density
of stuff in any particular place.
So we start off with a fairly boring, fairly
uniform universe and then we see things
start to change.
Okay, now if I play that for you again,
I’ll play that for you again.
As the simulation goes on, hopefully you can
see two things happening.
One is the matter clumping together to form
these lumps and strands and the other is the
spaces in between those lumps and strands expanding.
That expansion probably only becomes obvious
towards the end of the video, but see if you can
see it.
So we're getting umps and strands just as in
our loaf of bread.
And the spaces in between these lumps and strands
are expanding.
Now, we said that there were...
The two important things that went into making
our loaf of bread were the dark matter and the
dark energy.
The dark matter was the flour.
The dark energy was the yeast that made everything
expand.
Obviously, we have to live in that universe.
Somewhere there is a sprinkling 5% of visible matter
of things we understand and that essentially
just goes along for the ride.
It tends to fall into the lumps of dark matter.
It tends to be attracted to them by gravity and so
what we have here; so this is from a slightly
different simulation.
This is the Millennium Simulation but this also matches
up very well to what we see in the universe.
This is the distribution of dark matter that they
find in their simulations, and again, we see
the same story.
We get lumps, we get strands and we get voids.
Then this is the visible matter, so this is the
stars and the galaxies, and the planets and the
life forms living in the universe.
You can see that the structure we have over here
gets imprinted on the visible matter as well.
So it falls into the same places.
So by looking at the visible matter, you can
kind of work out where the dark matter
was supposed to be or where it is in your universe,
even though you can't see it directly.
And from the way that these voids, the spaces
in the galaxies have evolved, we can work out
where the dark energy is or what the dark
energy is doing.
So this is from the same simulation.
This is a zoom in on the big lump in the middle.
And again, this is where we think the dark matter
should be, again, even as we zoom in, it still
has quite a lumpy structure.
Then this is where the visible matter lives and
each one of these dots in this simulation is a
galaxy, so this would be a really big cluster
of galaxies in the universe.
Okay.
So we've said that 95% of the universe is made
of things we don't understand.
That's probably not the point where we want
to stop.
How do we find out more, given that the defining
property of these two substances is the fact
that they don't give off light, they don't
reflect light and therefore all of our normal
ways of observing things in the universe don't work?
Now, dark matter, we have a couple of good ideas
and this is because dark matter seems to be
quite similar to the things that we're made of,
the particles that we're used to seeing.
It doesn't interact with light.
It doesn't give off light which is a bit strange,
but we saw that it formed structures in the
universe in the same way that we expected
normal matter to do.
So we can apply our normal techniques, the things
that particle physicists have spent so long working
out to try and learn about the protons and the
electrons, and the Higgs Bosons that we're made of.
That is to smash things together and find out
what you get.
So one way we can look for dark matter is in
a particle collider, in just the same way that we
looked for the Higgs boson.
We take two...
So the standard model is the model of particle
physics that describes all of the particles
that we know.
The Higgs boson was the last piece of this puzzle
to go in, and we take two of those particles and
we throw them at one another and then we
look at what we get.
Okay, typically when you smash things together,
you get all kinds of mess and all kinds of bits
and that's typically what happens in a particle collider.
And if we're really lucky, maybe when that happens
we'll produce some particles that don't look
like any of the ones that we're used to.
Maybe when we smash two protons together in
the large Hadron collider, we get a couple of
dark matter particles produced.
And if we see strange particles being produced
in a particle collider then we can start to try
and put together a story, try to see if they can
explain the dark matter in our galaxies or not.
This is a picture of a simulated event from the
CMS at the LHC.
This is the kind of thing that you get when you
smash particles together.
So the different coloured tracks here are different
kinds of particles that you're seeing and each track
or each point here is a different particle
that's been emitted.
So this is really tough to try and see a couple
of dark matter particles that are probably
not interacting with anything else very much.
In the middle of all of this standard stuff that's
going on, it was hard enough to find the Higgs boson
in all of this; finding dark matter, while it's still
possible is going to be even harder.
So is there another way that we can throw,
smash things together and see what we get?
So the colour coding in these pictures is still
the same.
Blue particles are standard particles,
ordinary particles, the things that we're made of.
Red particles are the dark matter.
And we should think of these, this process
happening from left to right so what we have here
is a standard model particle and a dark
matter particle colliding and giving us back
the same thing.
So how does that happen and why is it useful?
Well, we said that dark matter makes up most
of our galaxies and so at our place inside
the Milky Way, we should be swimming through a
bath of these particles.
There is dark matter potentially all around us
and so we could hope that occasionally some of
these dark matter particles kind of bounce off
ordinary particles.
Now, that's again, quite a difficult thing
to try and see, but if you build...
If you're very careful, you screen out all of
the kind of noisy world around us, maybe you
could hope to see that.
Maybe you could hope to see something;
you can see something you can observe,
some atom in a crystal suddenly get nudged
by something that you weren’t aware of,
that you didn't see.
And so that's what's happening here.
Actually, this is a better picture.
So this is a picture from the CDMS detector which
is trying to do exactly this using crystals made
of germanium.
So they have a crystal lattice of all of these nuclei,
so this is the nucleus of an atom, these are
the electrons going around in the orbitals.
And maybe what happens is if you’re observing
this crystal lattice very closely, maybe you notice
that one of these atoms gets slightly displaced
for no reason that you could see.
And if you could see that then you’d have a
very good idea that maybe a dark matter
particle just came in and hit your crystal.
The final way of smashing things together to
try and learn about dark matter is what we call
indirect detection because we don’t actually see the
effects of the dark matter, but you might think that
if we're living in this bath of dark matter in our
galaxy and that's made of particles,
maybe some of those dark matter particles
hit each other occasionally.
And it's possible that when that happens,
you make the dark matter particles disintegrate
and make ordinary particles; protons, electrons,
photons.
And then maybe you can see those.
These things we know how to look for.
So you're not observing the dark matter directly
but you’re seeing some consequence of there
being a lot of dark matter around.
Then there are actually some hints now that
maybe we are starting to observe this.
So what we have here are some pictures of our
galaxy and a number of different wavelengths of
light (that's what the label on the top of these
pictures is telling you),
and this is a picture of the whole sky with the
galaxy along the middle.
So the bit that's blacked out is where the
galaxy has been taken away from these pictures.
So you should imagine this picture, that each
of these images is the whole sky wrapped
around you and the line of the galaxy then is the
line across the centre of the picture.
And the bright points are where we're observing
light once we've taken away all of the stars,
all of the standard astrophysical processes
that we know of.
So we've subtracted everything we think should
be happening and we still get some light left.
Now, it's still all a little bit uncertain but there's
a kind of growing evidence and people are slowly
starting to be convinced that maybe this is
light that's being produced from the collisions
of dark matter particles.
So I think that's something to definitely
look out for in the future.
So our other mysterious substance in the
universe was dark energy.
It was the yeast in our bread, the thing that
was making our universe expand.
And how do we find out about that?
Well, this is a lot harder.
Because dark energy is essentially, the only thing
we know about it really is that it's not like
normal matter.
So all of our normal techniques, all of the
smashing together that we do to study physics
normally isn't necessarily going to work to try
and study dark energy.
We just don’t know because it's so different to
the things that we're used to working with.
So one of the simplest explanations for what
dark energy is, is it's just a constant.
It's just another constant of nature like the
speed of light, like Newton's Constant.
So this equation, this is the Einstein equation
of General Relativity.
We said that general relativity was to do with
how space-time and matter talk to one another.
That's all that's encoded in this equation here.
The left hand side is telling you what space-time
is doing, what gravity is doing.
The right hand side is telling you what
matter is doing.
They talk to each other through this equation.
The capital Lambda here is the term which,
if you give that constant the right value,
explains what we see.
If you pick a very particular value for that
lambda, you can match our observations of
the expansion of the universe.
Which sounds good, right?
That sounds like we solved that problem.
We don’t have to worry about it anymore.
Unfortunately, that’s not the case because we
can try and predict what the value of that
constant should be using...
The predictions come from our theories of
quantum mechanics, and the problem is that
our predictions don't agree with observations.
They don’t agree, not just by a little bit.
They don’t agree by a huge amount,
by up to 120 orders of magnitude.
That tells you that you’ve got your calculation wrong.
The universe is telling you you've got your calculation
wrong by a factor of one followed by 120 zeros.
Which is not great.
So if that's not the answer, if it's not as simple
as just putting in one number and everything
being fine, what can we do?
Well, maybe this wasn't the right equation to
start with.
We have lots of evidence that tells us that
general relativity works very well describing
the universe within a solar system.
But maybe it doesn't work.
Maybe it's not the right theory to describe
the whole of the universe.
That's one possibility.
The other possibility is that there is just some
new, weird kind of stuff, something completely
different to anything we've seen before that's
making up 68% of our universe and is causing
it to expand.
How do we go about working out if either one
of these is the right answer or if both of these
guesses are completely wrong?
Well, our best attempt at the moment is
the Euclid satellite which is currently being built.
It's supposed to be launched in 2020.
And it's going to go out, it's going to live at
a point on the opposite side of the Earth from
the Sun and it's going to take better measurements
of the things we talked about at the beginning.
It's going to measure how far away things are,
how fast they’re moving, what galaxies are made of.
Answer all of these questions that we talked about
at the beginning of the talk but much, much
better than we've ever been able to do before.
It's going to study the way that structures
are formed.
Does that match up to the simulations that we
looked at, that are our best current understanding
of the universe?
So even though we don’t have a good idea of
really specific ways to look for dark energy at
the moment, if we can measure all of these
things much better, maybe we'll start to see
where exactly is it that our theory doesn't
match up with observations?
Where is the dark energy hiding?
Is it doing other things other than just
causing the expansion of the universe to accelerate?
And if we have more information then maybe we
can start coming up with a better explanation
for what dark energy is.
So to bring you back to this picture of the
University of Nottingham, or 5% of the
University of Nottingham, a huge amount of our
universe, 95% of our universe is stuff that
we can't see and that we don’t understand.
So this is the concluding thought for my talk.
It's quite, it's maybe an uncomfortable
statement or a slightly scary statement maybe,
that there is so much out there that we just
have no idea about but I think it's also really
exciting statement about our universe.
It tells us that there is so much more to learn,
that we are not done understanding the world,
the universe we live in and that there are going
to be more and more questions to ask
as we go forward.
So I think there are some younger people in the
audience, some younger than me anyway,
and this is a great thing to look forward to
doing in the future, understanding more of the 95%.
Thank you.
[Applause].
So I think you should sit down here.
So Clare, this really was a fascinating talk and an
absolutely fascinating area.
Could we start talking a little bit about dark matter
and what the particles might actually be?
In that so far, the LHC has not come up with
anything that is beyond the standard model.
Right.
And yet, the [unclear] experiment, looking at the
X and gamma rays from towards the galactic
centre might suggest the kind of particles.
What would your interpretation of that be?
What sort of particles do you think there might be?
So I think nobody knows for sure at the moment.
There are lots of ideas going around.
One of the most popular, at least until recently
has been the idea of super symmetry which is the
idea that every particle that we know of has
a companion particle that is just a lot heavier,
with kind of a mirror particle except it's a
lot heavier.
And there were a lot of people for a long time who
thought that these mirror particles, these
super symmetric particles could be the dark
matter that's living in the galaxy.
The problem is that the natural expectation for how
heavy these particles would be would mean that
you kind of would have started to see them
at the LHC by now, or maybe not quite yet
but maybe soon.
So when the LHC comes back on at increased
energy and luminosity, maybe there will be a
real chance of seeing something?
Yeah. I think there are a lot of people
hoping for that.
And if you don't see anything then those models
of trying to explain dark matter are in big trouble.
Right.
Some people have set up experiments looking for
axioms which people also think might be dark matter
particles.
Could you tell us what axioms are?
Yes.
Axions were actually invented to solve a completely
different problem, which is that in our theory
of protons; how protons and neutrons are made
of quarks and what the quarks do and how they
interact, there is a number in that theory that has
to be really, really small and nobody really
understands why that particular number has
to be so small.
So the axion is a particle that was first invented
to try and explain, to give you a natural way of
saying why this number is small.
You introduce a new particle and that forces the
number to be small.
But axioms, once they're in your theory, you can
ask yourself what else they do.
And if they're heavy enough and you make enough
of them, as the universe is evolving then they
also clump together and form in galaxies in the way
that we expect dark matter to.
But there's been no experimental evidence  for
axioms, I guess?
No.
People are looking for them.
There are lots of really nice experiments looking
for axioms.
One of my favourite experiments is light
shining through walls experiment.
Yes.
So there's a group of people actually at the old
lab that I worked at in DESY in Hamburg who are
trying to shine lasers through walls and the way
that you think this might work is that photons,
so the light and your axioms can interact with
each other in this theory and light can turn
into axioms in particular circumstances.
And so you shine your light at your wall but
before the light gets there, it turns into the axion
and the axion quite happily goes through the wall.
Then it turns back into light on the other side,
and so you could maybe try and hope to see
that, but so far they haven't seen anything.
Some of my colleagues in Germany are building a
laser for this, in fact.
It's a very interesting experiment.
It really is. Yeah.
So maybe we should move onto dark energy.
Now, I’m puzzled why it's called energy because
since e=mc^2 according to Einstein,
you would think if it was energy, it would have
mass and that mass would self gravitate.
And it would behave almost like matter and not
cause expansion but actually cause contraction.
Why is it called energy?
I don't, honestly I don't know where the name
comes from: it had that name before my time.
And I’m not sure if it's a terribly helpful label.
As you say, it leads to the wrong expectations
of what this stuff should do.
I think it would be great if somebody could...
Find another name for it.
To find another name for it.
Now, about two weeks ago there was evidence
from the BICEP2 experiment in the South Pole
that at a very early stage, the universe underwent
remarkable expansion, very fast expansion
before matter was formed, etc.
Due to something called, supposedly an
inflation field
Is it possible that that inflation field is still what
is driving the universe to expand and that
inflation field is dark energy?
If it is, what is an inflation field?
Okay, so that was lots of questions but,
so what we think happened at the beginning
of the universe is there was another period of
very rapid expansion and you’re right,
it is in many ways very similar to what dark
energy is doing.
It's the same kind of process of expanding the
spaces in between particles in a very rapid way.
And so again, you need something to cause that.
It can't just be caused by normal matter and so
one of the things you can introduce is an inflation
which is very similar to some of our theories of
dark energy and it causes the universe to expand.
It's not obvious that it can be the same thing
causing the expansion at the beginning of the universe
compared to what we're seeing now,
because everything was much smaller and much
denser at the beginning of the universe and
energies were higher.
Things were hotter and everything was much
more dense.
So to have the same effect, you need something
that works at higher energies at the beginning
of the universe compared to what we're seeing now.
So you need something that happens at quite
a specific; happens to cause the expansion at a
particular moment in time you need to have something
that works with those energies and those
temperatures and the beginning of the universe was
a very different environment to what we're seeing now.
So whilst the same kinds of processes are going on,
the energy scales are very different and that
seems to make you think it's not just the same thing.
It might be related but it's not the same thing
causing expansion today.
Do we have any understanding of that earlier field
to cause the initial expansion in the universe?
So the BICEP results give us a much better idea of
what might be going on.
So the way that we learn about the early universe
is we can see light that has travelled to us
from the very early universe, so it was produced in
this very hot era but it travels towards us
and essentially, it cools down.
It becomes redder in the same way that we
talked about before in the talk,
as the universe expands.
And that light carries with it the imprints of
what happened in the very early universe.
So by studying the light, you can learn about
what happened at the very beginning.
And what BICEP did was look at how that light
is polarised and that again, gives you more
information so we're starting to learn what
that field maybe did a little bit more.
At the moment, it seems a little bit conflicted.
BICEP seems to be telling you one kind of thing
happened whereas our previous best measurements
which were the Planck satellite, tell us something
slightly different happened.
So I think there is still something to be worked
out there; everything's not quite agreeing.
Right, and Planck will announce another set of
results in the Autumn; is that correct?
Yeah. So Planck is a satellite experiment
studying this light from the early universe whereas
BICEP, as you said, was just a telescope or is
a telescope at the South Pole.
So what Planck is able to do that BICEP can't is
study the whole sky.
So BICEP only sees a small patch.
Planck can see the whole sky, and they're
studying the polarisation of the light.
At the moment, it's not quite clear when the data
is going to come.
Hopefully before the end of the year, and that,
yeah, by studying more of the sky and different
frequencies of the light which BICEP also isn't
able to do at the moment, it gives you again,
even more information and you can see if this
tension is still there or if it resolved itself.
Yeah.
I noticed also fairly recently that up until now,
people have thought that a cosmological
constant that stays a constant could explain the
observations of dark energy.
Yeah.
But I noticed that Pan-STARRS are one of the
experiments looking at supernova.
It seems to be suggesting that in fact, it's not
a constant and that the acceleration is in fact
increasing: is it increasing or decreasing;
I can't remember?
I think it's increasing.
Increasing, yeah.
Do you have any thoughts on that?
I think that would be really, really exciting if
it were true.
For me personally, so I mostly work on dark energy
at the moment and for me, that would be the
most exciting thing somebody could see, is that
real evidence that it's not a constant.
There is something new out there that's
changing and evolving.
I think it's still too early to say whether that's
really what they've seen.
There are a lot of these tensions at the moment
in cosmology between different kinds of
observations giving you a slightly different story
about how the universe has evolved and I think
we need to understand a little bit more about...
Because observing things in cosmology or in
astronomy is very difficult, you have to be sure
you understand how the stars are burning,
could your observations be being contaminated
from something else?
And just making sure that we're really seeing
something truly cosmological and not something
coming from some other source.
I think we still have to pin all that down.
And there's something very strange about
gravity and general relativity in a sense, that
general relativity is a pure field theory.
I mean, as yet there is no proven way you can
quantise gravity and every other field that we
know about; whether that's magnetic field,
weak interaction, strong interaction, etc,
they're all quantifiable and relatable to each other.
But gravity stands absolutely alone.
Do you think that what we're seeing is that
maybe once we have a sort of theory of quantum
gravity that we might understand better the
observations that are being made, or do you think
these are independent things?
No, I think both of these things are telling you
that you don't understand gravity as well as
you might hope you did.
So from the observational side, obviously
our expectations are not matching up with what
we're seeing and as you say, on the theoretical
side it seems very strange that we can quantise
everything else.
We can talk about quantum theories of electrons
and photons but not of gravity.
It's not obvious that the two things have to be
connected in that the problems with quantising
gravity come in at a very high energy.
That's where you really have problems.
Whereas problems of dark energy and to a latter
extent, dark matter, happen at much lower energies.
So it's not obvious that those two things are connected
but it could be that if you have a better theory
of gravity, you start to understand how these
things can be happening.
And I think that actually studying dark energy
and dark matter, understanding where the gravity
is breaking down, that's another way into working
out what the true, full quantum theory of
gravity might have to be.
Yes.
So I think before we open to questions,
I must put in a plug for my own field,
the search for gravitational waves,
that we should be able to check independently the
expansion of the universe and whether or not the
supernova results are in fact borne out really by
using signals of a totally different kind.
Hopefully that will be true now within a couple
of years.
Anyway, that plug over.
So I think now can we have questions for Clare,
please.
Do we have any young people in the audience?
That is people younger than Clare, to ask
questions first?
Have we no young people tonight?
I think you're about the youngest here, it looks like.
Well, we'll just open then in general to questions.
There was somebody over here.
Thank you very much.
That was very interesting.
By definition, we can only look at the effects
of the 95% that we can't see on the 5% that we can.
If we keep on looking and we can't find evidence
through the Hadron collider or whatever,
is there a chance that actually, our model of what
we can see is the problem, and that that is the
thing that needs fixed?
That's a very good question.
So there are a few things that we don't understand.
So we think; there are lots of things we understand
very well about the model of what we can see
and the discovery of the Higgs-boson is kind of
just the most recent...
the most recent results that have matched up
with what we predicted.
So there are lots of things we do understand very well
but as we were talking about with Jim before,
there are things we don't understand like this fact
that there is a number that’s very small in
the [unclear] or of how protons and neutrons
interact with one another.
And some ways of fixing that problem also lead to
an explanation of dark matter.
So there is definitely room for those things
to be connected.
Some people also try and explain the theory,
explain dark energy through connections to neutrinos.
So there are some coincidences to do with the
energies at which these things happen which might
lead you to think there is a connection between
neutrinos and dark energy.
I think that’s a lot less well developed and not
obvious that it's true but there are definitely
links, kind of hints that maybe there are connections,
that there are things we don't understand with what
we can see that could be connected to what
we can't see.
So it's a bit of both, basically.
Am I correct in fact, that what we observe about
neutrinos, that is not consistent with the standard
model of particle physics, I think?
Is that correct?
That's a very good point.
So in the standard model, it's very difficult
to explain why neutrinos have masses and why
they have the masses that they do.
And we're very, very sure that they do have
these masses now and we have very good
evidence for that, so that's another mystery...
So it tells us there is something incomplete
about the standard model of particle physics.
Yeah.
Sorry.
Next question?
I'll take one at the back here.
With the orange top.
Young lady with the orange top.
Just wait until the microphone comes.
It's a bit of a random question.
We’re not able to see the dark energy but are
we able to hear it in the same way that the
universe has you know, we can hear the universe,
we can detect that kind of like sound.
Is it something that we can detect it in other
ways, other than visual?
So the problem is that space is empty or a lot
of it is empty and sound waves can only travel
in a medium.
So the only reason that you can hear me when
I'm talking is the fact that there's air in
between us and we're making the air vibrate,
and that's what's carrying the sound.
Light really for anything very far away,
light is really the best way we have for
studying anything.
I mean, it would be great if we had another way...
Well, so in fact...
Radio, of course.
Well, radio waves are kind of really light...
Alright, there you go.
Just at longer wavelengths, but the gravitational
waves actually would be...
That is really a different medium.
There you're really seeing the ripples in space-time.
The gravitational field, yes.
You're seeing the ripples in the gravitational field
at audio type frequencies, in fact, which you
can almost...
It's almost like hearing because the sort of system
that you use to try to pick these up is like a
big microphone looking for force fluctuations.
And of course, neutrino astronomy is the other
thing independent of light that can tell us, and
will tell us about strange things happening
in the universe.
Okay?
Was there a question down here?
Microphone. Good.
Good evening.
The gentleman further back with his neutrino
question made me think of something.
Like light, neutrinos flow freely within the
universe and between...
Sorry, within the galaxy and between the galaxies
and I was wondering, dark matter;
you've shown it as we would assume it to be
in a direction with a specific discrete galaxy but
do we believe dark energy and/or dark matter
is concentrated within galaxies or does it also
exist potentially within the void between the
galaxies, which is a much greater proportion
of the universe?
So dark matter we think essentially lives in
a galaxy and dark energy is everywhere.
So dark energy is in the voids.
Dark matter is just in the galaxies.
But actually the connection with neutrinos and
the fact that they're sort of kicking around
as well...
So for a while, people wondered if neutrinos
actually explained dark matter.
So you know, you'd kind of expect them to fall
into a galaxy but because they have a lot of
energy and a lot of speed, they don't kind of
clump together in the way that heavier particles do.
And that's one of the successes
that we've had in understanding dark matter
is to understand that neutrinos can't be dark matter.
I don’t know whether...
Well, it's a funny kind of success but it's that
they're just too hot.
They’ve got too much energy and so they don’t
give you the kinds of structures that you see,
that we're seeing in the sky now.
Over here, on the left.
Good evening.
I guess theoretical understanding of science and
practical uses have always had a weird dance
between each other.
Sometimes we understand something and then we
make it useful and sometimes we find something
useful and then we try to understand it.
If we're trying really, really hard to understand
dark things, do we have any reason why we're
trying to find things about it?
Does dark matter matter?
[Laughter].
No, a very, very...
Yeah, very good question, especially because
to some extent you're all paying for all of
this research through your taxes.
It's very difficult to say at the moment what it
would be useful for because we don’t understand
anything about it, or very little about it.
But we have the historical precedent that every
time we've understood something better,
something about our universe in a new way,
it has eventually led to revolutions in our lives.
So at the turn of the last century, people were
studying quantum mechanics which was at the
time, a completely esoteric theory of things that
were happening on super small scales that didn't
seem to have any relevance to anybody.
And now our understanding, so 100 years later
our understanding of quantum mechanics is the
basis for transistors and all of the telecommunications
that we use.
It's what underlies MRI scanners, if you're
ever in hospital so it's very difficult to predict
exactly what the payoffs will be but we can hope...
Our expectation is, or a precedent tells us that
every time we've understood something better before,
it's led to benefits in the future.
And remember, your GPS wouldn’t work if people
didn't know about general relativity and know
what corrections to put in for the timing.
So I mean, Einstein did a lot of good things for us.
You know, and people like Clare, as they
understand dark energy will do equally good
things into the future.
There was a question further up at the back?
In fact, there's two up there at the back,
so I think the young lady first.
Thank you.
Hi.
You've been talking about the universe expanding
and space-time and I was wondering if you could
help clear up for me, when you say space-time
is expanding, is it stretching?
Is it generating new space-time?
Anything about that would be great.
No, I agree.
It's a difficult thing to get your head around.
So I would say that the distances between
galaxies are expanding, so galaxies themselves are
held together by their own gravity, so we are
not expanding at the moment.
But it's the spaces in between things that
are growing.
Does that answer your question?
No.
Okay, do you want to try...?
Okay, so Einstein talked about space-time as
almost a fabric.
So what I’m...
Gravity stretches it or curves it, so I’m
wondering, as the universe is getting bigger,
I mean it's not just void, is it?
There's virtual particles and if it was all
condensed at one point, how is it getting bigger?
Is it stretching or generating new stuff, or...?
Yeah, so you can think of it as the fabric
in this, in the analogy of space-time being a sheet
that we're all living in and we're bending the sheet.
But that sheet is also being stretched at the
same time.
And what is doing that stretching, it's being
caused by the fact that there is other matter there.
If there was no matter in the universe,
no dark energy, no dark matter, it wouldn’t
necessarily be expanding.
And it takes something to kick that off,
and this theory of inflation that we talked about
is what puts all that energy into the universe.
That's what starts the expansion and then it
runs on from there, kind of stretching the sheet.
Think about the sheet model.
I mean, Einstein relied a lot on these simple
mechanical models to understand, you know,
to try himself to understand what general relativity
meant and I think the sheet models are very good,
in fact, if you just think of a stretchable
rubber sheet.
So maybe in front, yes.
Gentleman in front.
Thank you.
Yes, I just wanted to pick up on the point that
you discussed just towards the end there
about quantum mechanics and general relativity
being the thing that hadn't been merged yet
with quantum mechanics.
But earlier in your talk, you mentioned that when
you were looking at the cosmological constant
in predictions not agreeing with observations,
you mentioned quantum mechanics in that context
and I just wondered...
That was to 120 orders of magnitude.
I wondered how, the two theories not being
linked, yet still a prediction was able to be made?
Right.
So the way our prediction for the cosmological
constant comes about is what you're essentially
seeing there is the energy contained in the vacuum.
So the energy...
So quantum mechanics tells you that even in
a vacuum there should be particles kind of popping
in and out of existence.
Quantum mechanics tell you that everything is
kind of probabilistic and things can come and
go in this way.
So even your vacuum is not quite empty.
There is this kind of background noise of things
coming in and out of existence.
There is some energy stored in that, and that
energy behaves in the way that the cosmological
constant should in the Einstein equations.
It should be; it's the same at all points of space.
You don't think vacuum changes from one place
to another and it's the same in all directions.
Depending, no matter which way you look at the vacuum,
it's still the vacuum.
So there is some energy stored in that and we can
try and predict from our theory of quantum
mechanics because these are ordinary particles
popping in and out of existence, we can try and
predict what that energy should be and that's
the number that goes horribly, horribly wrong
when you try and calculate it.
I think it's also interesting, you can link
quantum mechanics and gravity actually through
the Heisenberg Uncertainty Principle.
Now, what Heisenberg said was that if you
tried to measure the position of an object with
great precision, that you kick the object and you
make it move and you give it momentum and
fundamentally can't measure the position and momentum,
you know, both very, very accurately.
So if you now think of trying to measure a particle,
you know it's position, very, very accurately;
you give it momentum and momentum is equivalent
to giving it energy.
So the more closely you try to determine a
particle’s position, the more energy you give it
through quantum mechanics.
You can give it enough energy that the mass
associated with that energy is such that there's
gravitational collapse and you form a mini black hole.
And in this way, you can actually show that there
is a minimum distance, in fact that you could
ever measure without in fact causing this
black hole effect.
That's called the Planck distance and the time it
takes light to cross that is called the
Planck time.
So there is, I mean, it must be that gravity
is quantifiable if you believe in Heisenberg,
if you believe in quantum mechanics.
The trouble is it's hard to get much further
than that, I think.
Any more questions around the middle here, perhaps?
Clare, you mentioned that experiments were going
on to try and determine the nature of dark matter.
But you yourself are involved with dark energy.
Right.
I wonder, in what sort of experiments you can
plan for that would possibly test dark energy?
Right.
So we talked a little bit about observations from
the Euclid satellite.
You could certainly make better observations
of the universe to try and find out more.
You can; so something that I’ve actually,
that's what I’m working on at the moment are
laboratory experiments to try and study
dark energy.
These are a little bit more speculative in the
sense that they only test a sort of subset of
our theories of dark energy and yeah, so
there are some theories of dark energy that you
can try to test in the lab.
The way you test them is because dark energy
essentially gives you an extra force in the
universe, so we have four forces that we think
we understand: gravity, electromagnetism and the
strong and the weak forces in atoms.
And maybe dark energy can be a fifth force.
That's certainly an idea that a lot of people
are thinking about, but if there is this extra
force then it has to be behave in a kind of
peculiar way.
So not like the normal forces that we're used to.
It's a force that can be stronger or weaker,
depending on the environment.
So in particular, the dark energy force would
be weaker in dense environments and stronger
in diffuse ones.
And so what we're trying to do with the lab
at Imperial is trying to make quite a diffuse
environment or a laboratory vacuum where you can
try and look for extra forces between different
kinds of particles.
That's hopefully going to get up and running soon
and we're quite excited to try and see what
that tells us.
Okay.
There was another question at the back.
The gentleman with the orange T-shirt.
Thank you.
So from what you said about dark matter,
so it seems to obey the laws of gravity like
normal matter does and we can see its
influence on a galactic scale.
Wouldn't it make sense for it to also collapse on
itself like normal matter and form bodies like
planets so we could observe normal bad matter
orbiting kind of dark matter planets, or
whatever they are?
Yeah.
No, you certainly might expect that to happen.
It depends, how much that happens maybe depends
a bit on your theory but people certainly have
tried to look for the existence of dark stars which
are kind of stellar size lumps of dark matter.
And it’s a slightly larger scale but dwarf galaxies
are dominated, completely dominated by dark
matter and there is only a little bit of visible matter.
So that's kind of essentially what you said,
a lump of dark matter being orbited by some
visible matter.
So dwarf galaxies are just kind of little, small
galaxies that we can see.
That makes sense. Thank you.
Yes, question here.
Yeah, hiya.
Following on from that, does that not sort
of imply that we're an anomaly in the universe?
You mean that we're made up something
that's quite rare?
If 95% of it is not here, are we just like,
well, outside the norm and there could be a
whole universe or galaxy which we can't see?
Yeah.
I think that's completely possible.
I mean, we've been talking about dark matter,
especially as if it's just one kind of particle
whereas we know that we are made up
of lots of different kinds of particles.
There is actually no reason why dark matter
wouldn’t be like a whole new collection of
different kinds of particles and some of these
ideas that we were talking about to do with
super symmetry would predict that.
Then maybe you could expect that there is
a whole hidden universe going on that we can't see.
So potentially, there could be a lecture
somewhere where people were talking about
the 5% of the galaxy they can't see?
They're probably a lot less worried about
not understanding 5% than we're worried
about not understanding 95%, but yeah.
Have we any more questions in the audience?
Ah yes, one more here.
I hope this isn't a daft question, but from what
you say, it's likely or the possibility exists
that there is dark matter here in this lecture theatre,
on Earth, whatever.
If that's the case, why don't we see local
gravitation effects of that?
So it's fairly, on the kind of distance scales that
we experience in this room or on Earth, it's
essentially completely uniform.
So it's not kind of localising in a way that would
mean we'd feel the gravitational effects of that.
And you have to remember that gravity is
actually a really weak force.
It takes the whole of the Earth pulling down on
this glass, the whole of the mass of the Earth is
pulling down on the glass through gravity,
and yet I can hold it up quite easily.
So gravity is quite a weak force.
And if dark matter is quite diffuse at this level,
it would be quite hard to see the gravitational
effects of that.
You stand a better chance if it interacts in
a different way, in a stronger way and that's
kind of what we're looking for in the particle
collider and the other experiments that we said.
Actually, I see we do have a young gentleman,
do we, just here in the audience that I didn't
notice earlier.
If there is anything you'd like to ask...?
Go on.
Often, in fact the best questions come from those
who are quite young.
Is there anything else you'd like to ask?
Don't be nervous.
No? Okay.
Maybe it's all been answered.
So I think, oh, another question here.
Good.
Hi. I don't know anything about physics.
I was wondering, I didn't quite understand why
the gentleman here said about the energy at
the start of the universe couldn’t be the same
as the dark energy that you're looking for
because you were talking about dark energy
as being yeast in bread.
And you were saying that it couldn't be the
same because there is different mass now and
there's different like heat, temperature
and things but yeast depends how it rises
and how fast it goes, then it stops and it
depends on different things like heat and the
mass of the bread or whatever.
So why couldn’t it be the same thing?
Just getting slower because there's less heat
and less mass; I don't know?
Right.
No, that’s a very good...
Do you want to...
I think it's a very good question.
No, I’ll let you answer it.
I don't know the answer.
But it's a very good question.
Yeah, so I mean, okay...
Analogies break down at some point but I think
you can kind of see...
So if I wasn't trying to make a loaf of bread with
normal flour but I was trying to make a loaf
of bread with I don't know, something much
heavier and much denser, so...
[Inaudible].
Yeah.
Or in fact, you even know with different kinds of
bread flours that they rise differently.
Some are denser and it's harder to make them
rise and certainly if you fill them with too
much raisins and other seeds and things, then
they also don't rise very well.
And if you wanted to make that bread rise,
you'd have to put in either more yeast or maybe
something more powerful, or yeast and some
raising agent or something like that.
So it's the fact that you have something that
doesn't want to rise in the same way and so
you need more; something more to produce
the same effect.
At the back...
We know that it took  us 14.8 billion years from
the big bang to come to today and we know in
the next billion, billions of years the visible
universe will probably burn out and be so far
apart from each other at that universe.
Is there any prediction of what the dark
matter, would happen to it in such a scale
of future events?
So our predictions of what the universe will
do in the future are really quite sensitive
to how much dark matter there is, how much
dark energy there is and what those things are.
If dark energy kind of carries on at the same rate
that we're seeing it today, causing the expansion
at the same rate we're seeing it today, we
get one prediction.
If it's in fact, as we mentioned before; it's
actually a theory that's evolving and changing
with time then maybe we'll get a different set
of predictions for the future.
So yeah, it's very hard actually to make reliable
predictions for the future of the universe because
there is so much that we don't know still
about what's in the universe today.
I'd like two questions at the front here.
The left, with the lady on my left first.
I'm quite struck that you're talking about matter
and energy.
What is the universe expanding into?
Because I think that has to be a factor in
the equation as well, because in my logic,
space and time come into it but what is it and
how are you explaining this away?
Right, so it's...
It's not so much that the universe is expanding
into something; it's that as we said,
the distances between things are expanding.
So one analogy people like to make is if
we were points on the surface of a balloon, as
we blow up the balloon, those points get
further apart.
They’re stretched apart on the surface of the
balloon and that's kind of like what's happening
with the universe.
The problem is you're going to say,
well, I can see that the balloon is expanding
into the space around it.
We're limited in cosmology in how much we can
see of the universe.
We only actually have one patch of the universe
that we can see, and that’s because we think that
the universe is only as the gentleman at the
back said, 14 billion years old and light travels
at a finite speed.
So there is a furthest distance away that we
could ever hope to see and outside of that,
who knows what's happening?
Who knows?
That's right.
Okay, I think the lady in blue here next.
Thank you.
It's just really a comment to the person who said
why does it matter, or why do we find,
we want to find something that we don't know,
like dark matter or energy, dark energy?
Well I think as a kind of human being,
like why in biology long ago, you know,
when we didn't know or we couldn’t explain why
the infections transmit through water or other things,
and air.
Then we worked out there is something there but
we cannot see it, like the microbes of the bacteria, etc
before the microscope.
So I think in the same way, in cosmology if we
know there is, or we have evidence there is
something else that we don't know, of course the
human beings want to find out what it is.
Just a thought. No.
Yeah, I think that’s true and I’m sure that
both for myself and for Jim, that's the reason that
we do this on a day to day basis.
That's right.
And how exciting it is to be finding out new
things about the universe.
New things all the time, yes.
Well, I think really we need to bring this to a
close now, and I would very much like to thank
Clare for an excellent talk and to wish her all
the best.
You know, over the next sort of few years
of getting a much better understanding of
what dark energy is and of what dark matter is.
So Clare, thank you very much indeed.
[Applause].
That was very good, excellent.
