One of the biggest mysteries in the universe
is dark matter.
Astrophysicists have been observing the universe
for many decades now and they tell us that
in addition to all the visible matter we can
see around us there must be some other type
of matter out there.
So far we've only see it through gravitational
forces; maybe it has other forces, maybe we
could detect it, we don't know.
One of the biggest puzzles in understanding
of astrophysics and cosmology is what is this
dark matter.
The story of dark matter started back in the
1930s when the Swiss astrophysicist Fritz
Zwicky was observing galaxies in a cluster
of galaxies called Coma.
He found something very strange.
What he discovered was that these galaxies
we're moving too fast.
What do I mean by that?
What I mean is that you can calculate from
the galaxies that you can see how much gravitational
field they generate and then you can calculate
what speed galaxies would have to have in
order to stay inside this gravitational field.
The same way, for example, you can calculate
the speed of the Earth from knowing that the
gravitational field in Solar system is generated
by the Sun in the middle.
So what he found was that those galaxies in
the Coma cluster were moving much too fast.
If there was only gravitational field that
was generated by the visible matter then they
should be flying away from each other, not
staying inside the cluster.
So he said: there must be some additional
invisible form of dark matter there.
It was the 1930s.
For many years people didn't pay a lot of
attention; probably the person who really
convinced astrophysicists and other scientists
that this dark matter has to be real was the
astronomer Vera Rubin.
She did something a little bit different.
What she did was look at the motions of stars
in a galaxy and she felt much the same effect
like Fritz Zwicky: she found out that the
stars in the galaxies were moving too fast.
The gravitational field generated by the stars
themselves was not sufficient to keep the
stars in orbit around the center of the galaxy.
That was the 1970s.
Since then there'd be many more pieces of
evidence confirming the reality of dark matter.
For example, we can measure the properties
of the background radiation that fills the
universe that was produced when atoms were
born 380,000 years after the Big Bang.
Using that light from just after the Big Bang
we can figure out what there was in the universe
when it was at that young age.
We can tell that obviously there was radiation,
obviously there was an ordinary visible matter
that we’re made of, but in addition there
had to be this dark matter.
Using these observations we can in some sense
weigh the amount of dark matter and we can
compare it with the amount of visible matter.
Dark matter is somewhere between five and
ten times as much as visible matter in the
Universe.
That's why I say that the existence of dark
matter is well-established and it's the biggest
puzzles we have in our understanding of the
universe.
Next question is, what is this dark matter
made of?
One possibility is that it is made of some
sort of miniscule particle, like the ones
that we see in our particle accelerators,
although it would have to have different properties
and the particles that we know about could
not explain the dark matter.
For one thing, the dark matter particle would
not have any electric charge or, perhaps,
have a very-very small electric charge, so
it's not the same as an electron or a proton.
It would have to have very weak interactions,
so it couldn't be made of neutral elementary
particles like neutrons, for example.
It would have to have a relatively large mass,
so it couldn't be a neutrino, for example.
So it would have to be some additional type
of particle, something that we've not met
before.
There's all sorts of different ideas about
what that dark matter particle might be.
Depending on those ideas people propose different
ways of trying to detect it and pin down exactly
what it is.
When you're doing these experiments to look
for dark matter you can look for it directly.
According to the theory, here we are sitting
in this room and there are dark matter particles
coming through it all the time.
And you think: well, if there’re dark matter
particles coming through all the time it must
be simple to detect them?
Well, not so simple, because their interactions
must be very-very weak and if you try it to
measure their properties here on the surface
of the Earth you would have enormous backgrounds
from things like the cosmic rays that come
from the outer space.
So what people do is to go to into deep underground
laboratories where they don't get these cosmic
rays, they’re shielded by the Earth.
There they look for very rare, very weak collisions
between these possible dark matter particles
and ordinary matter.
So there's a lot of experiments that they’re
doing that at the moment but so far they haven't
found any signs of the dark matter particles.
What else could you do?
Well, if they really are particles and if
they weigh less than, let's say, a thousand
times as much as a proton then collisions
of protons at high-energy particle accelerators
might produce particles of dark matter.
That's been one of the big experimental programs
at the CERN Large Hadron Collider and they’ve
been looking for the production directly of
these dark matter particles.
You don't actually see the dark matter particle
itself, because it doesn’t have the electric
charge, doesn't interact in the detector,
but after it’s being produced it carries
away energy and momentum that you can detect.
So what you do is you draw out a sort of balance
sheet of all the energy and momentum after
the collision and you look to see whether
it balances or not.
If it doesn't, you say: well, maybe that's
dark matter.
Those are two of the ways that people are
currently looking for dark matter particles.
Another possibility that people are studying
is: if you've got these dark matter particles
around us in the universe today, every once
in a while they may collide with each other.
And when they collide they might produce particles
that we can actually detect, for example,
amongst the cosmic rays.
That's another thing that astronomers and
astrophysicists are doing: they are looking
for unexpected particles in the cosmic rays.
But again, so far no luck.
In view of this lack of success so far, people
are considering other possibilities for this
dark matter.
For example, a few years ago gravitational
waves were discovered from the merges of black
holes.
So people revived an old suggestion there
maybe the dark matter isn't in the form of
small particles at all but instead it's in
the form of massive black holes.
Well, there's all sorts of problems with that
idea: how'd you make that many black holes?
There’s all sorts of the astrophysical restraints
limiting the possible density of such black
holes.
So I don’t think that idea works, but still:
maybe some fraction of the dark matter might
be made up out of black holes.
Another possibility is that the dark matter
doesn't consist of individual objects, individual
particles, but instead is in the form of some
sort of wave through the universe and depending
on how deep or how dense this wave is then
you make it some dark matter.
There are people who are looking for that
possibility.
Anyway, that's where we are at the moment.
Dark matter is a big puzzle, it's a puzzle
that's been with us for more than 80 years.
Astrophysicists, cosmologists, particle physicists
have been studying it in great detail for
at least 40 years.
We've come up with lots of ideas for what
that dark matter might be, the ideas range
from very heavy black holes all the way down
to waves of invisible particles.
There's all sorts of experimental efforts
to look to see what that dark matter might
be.
At the moment we have no direct evidence but
for me it’s one of the most exciting outstanding
questions, I'd say, not just in astrophysics
and cosmology, but also a particle physics
because it suggests that there's a whole world
of particles out there which we don't know
anything about yet.
