So, we've explained matter.
We've explained the universe out there.
Well, not quite.
We've explained between about 1 and 10% of
it.
There's a little bit, you know, the majority
of it,
is some mysterious stuff called dark matter.
Now this is very odd.
There's stars or stuff out there that we cannot
see yet we know that it's there.
Now how on earth do we know that?
Let me show you a picture.
This is a picture of a spiral galaxy.
It's just like a Catherine wheel going around
on firework night.
You can get the feeling of the thing whirling
around like this.
It's not just a feeling of it whirling around,
it actually is.
How do we know that?
Well we go and look at the lights being emitted
from the stars.
Remember, when we were whirling around,
and you heard the pitch going up and down
as it came toward you
and went away from you?
Same thing with this.
You look at the light from the stars on the
edge of this spiral galaxy.
And you find some have been shifted towards
the blue,
as if they're coming towards you, and some
have gone to the red,
as they go away.
So from this you get the feeling that this
whole galaxy is swirling around.
And you can measure the speed that it's swirling
around.
We can also weigh it.
And what we find is that when you weight the
galaxy
and compare it with the speed it's whirling
around, the sums don't add up.
Let me show you what I mean.
I asked Bryson to make a galaxy for us and
he's obliged.
And this is what happens with a spiral galaxy.
The thing is rotating around and there's some
stars sitting there quite happily.
But what's supposed to happen according to
our sums, is the following thing -
If the sums are right, the stars should just
fly off because it's going around so fast.
Yet they don't.
Let's just look at this picture again.
So we see these stars whirling around and
staying in the galaxy.
Yet when we look at all the matter in the
middle,
the amount of gravitational tug that it can
provide to hold them on the trail, isn't enough.
Those stars have no right to stay there, they
should just fly off the wheel
like in Bryson's demonstration.
How do they stay in place?
There must be a much bigger tug going on to
hold them there.
Provided by stars that we can't see.
Dark matter.
Now, it's not just this one galaxy that's
got this problem,
there are many, many like this.
And not just individual galaxies. We see galaxies
and galaxies rotating around each other.
And we begin to find that the sums don't quite
balance.
It really does look as if there's this dark
matter in the universe, maybe 90% of it.
Now how on Earth do we go and look for dark
matter?
Well one way is by analogy with H.G. Wells
story about the invisible man.
You didn't see him directly, but when he bumped
in to things,
you were aware of his presence indirectly.
That's the sort of thing we're doing here.
Now what is really exciting is that as I was
preparing these lectures,
the first news came out that perhaps the first
sightings of dark matter have been made.
And this is shown in these pictures that were
published just a month or two ago.
Some astronomers were looking at some stars.
And it's been processed through the computer.
You can see these little pixels to show the
stars.
I want you to focus on this star in the middle,
and we're going to play in a moment
we'll show what happened over a period of
weeks.
So this was at the beginning of a period of
time.
Now a few weeks later this had happened.
Suddenly it was brighter in the middle.
And then a bit later still it had dimmed away
again.
So there you are on Earth, looking out at
a star in the distant heavens.
And suddenly it gets brighter and goes away.
Now what's going on?
The idea is that a dark star has become between
you and it.
Sounds very odd. Let's just look at this again,
fast,
to convince ourselves that it is brightening
and going away again.
So that's what was seen.
How on Earth can it be that a dark star has
come
between you eyes and that distant star and
made it brighter.
You'd think that it would block it out.
No!
Another of Bryson's amazing replications of
the universe here
will show us how this really works.
Now this might look to you like just a sheet
of latex rubber.
In actual fact this is the universe.
And at this end, we have got the astronomer
who's going to be looking out at a distant
star.
And at this end we have the star.
Now can I have a volunteer to send me some
light beams from the star?
Would you like to have a go?
Right, what's your name?
Hello, Paul.
Now could you just roll a couple of balls
from here.
First of all just roll one straight so we
can get the idea across the flat plane.
So you see this would just come straight in
to the telescope waiting to receive it.
Now let's see what happens when I put the
dark star in the middle.
Fire one straight at it again. See what happens.
It hits, you see. You don't see that one.
But let's see what happens to a light beam
that goes slightly to the side.
You want to move that slightly to the side,
that's right.
Now let's see what happens with this.
You pick it up, you detect it. It's got bent.
Let's try with a light beam on the other side.
Fire it a light beam.
That also got bent.
So suddenly you are seeing more than you would
have done before.
Thanks very much.
