SADOULET: One of the most fascinating questions
today in cosmology today is what is this
stuff, this 90 percent, I mean this 99 percent
of the stuff in the universe that we think
is around, but don't see? This problem of
Dark Matter.
DAVIS: This problem of the Dark Matter is
a fascinating development that has been known
for 50 years. Astronomers have been saying
for more than 50 years that there seems to
be Dark Matter in these clusters of galaxies.
There's Dark Matter in the outer regions of
normal galaxies. And now the idea that this
Dark Matter dominates the total mass density
of the universe, perhaps accounting for 90
or 99 percent, is what many people call the
Ultimate Copernican Revolution, that we're
not made of the dominant material which comprises
the universe. The Copernican Revolution, of
course, refers to the notion, first really
argued strongly by Copernicus 400 years ago,
that the earth is not the center of the universe.
The stars don't revolve around the earth.
The sun doesn't revolve around the earth.
It's the other way around. The earth revolves
around the sun. Now we know the sun is just
a very minuscule speck on the outskirts of
one galaxy, and there are millions and millions
of galaxies that we can see out there. So
we are clearly not the center of our universe.
But we used to think that we were made of
the most dominant stuff of the universe. Our
atoms were the dominant stuff, the hydrogen,
the helium, the oxygen, et cetera.
SADOULET: So how do we know -- because first
maybe -- there are two questions. How do we
know that there is something dark, if we cannot
see it?
DAVIS: That's the first question. How do we
know -- what is the evidence that there is
Dark Matter at all? The evidence has to come
through as gravitational effects. We use the
laws of Newtonian mechanics that have been
around for 300 years. Those are the laws that
govern the orbit of planets around our sun.
SADOULET: So we see stuff going around spiral
galaxies.
DAVIS: We see stuff going around spiral galaxies.
It turns out that we can measure the light
in the galaxies, and we have a fair estimate
of the mass of a star.
SADOULET: And we can measure the velocity
of these clouds.
DAVIS: That's right. We can see the velocity
of clouds and we know, for example, in the
solar system that we are orbiting the sun
at 30 kilometers a second, and if we were
four times further away from the sun than
we are, we would be orbiting at half that
speed, 15 kilometers a second. This is known
as Kepler's Law, just a simple relationship
of Newtonian mechanics that if you're away
from a star, your orbital velocity falls as
the square root of the distance. But in a
galaxy, it's a more complicated situation,
because the mass is not concentrated in a
central star, it's spread out over all the
light of the galaxies.
SADOULET: Where it's basically the equilibrium
between centrifugal force and centripetal
force which is --
DAVIS: And the gravitating mass. When we do
that calculation, we would say that rotation
of the outer regions of the galaxy should
be falling with distance from the center of
the galaxy, but they never fall a distance.
All the galaxies that we're observed, and
there are quite a few now, many, many dozens,
every one of these galaxies has a rotation
curve, that is to say rotational velocity
as a function of distance from the center
that is flat with distance as though there
were a Dark Matter component in that galaxy
whose density was declining with distance,
such that the total mass of the galaxy is
not converging. As you add up bigger and bigger
shells, the mass continues to grow with distance.
SADOULET: Even though we cannot see any stars.
DAVIS: We can see nothing. It is clear it's
not stars. It's clear it's not ordinary gas.
It's not ordinary. It's something that is
highly invisible. It doesn't appear to be
ordinary matter.
SADOULET: So this seems to be true for our
own galaxy. This seems to be true for many
of the spiral galaxies that we can measure.
DAVIS: That's right. On any system that the
astronomers try to measure mass on scales
larger than, say, the distance of us to the
center of a galaxy, 20,000 light years, any
system that large or larger always seems to
be dominated by Dark Matter.
SADOULET: Even the elliptical galaxies seem
to have also some Dark Matter. We can measure
global clusters, small --
DAVIS: That's right and we can measure the
confinement of their hot plasma which seems
to be confined in a way that's inexplicable
unless there's Dark Matter.
SADOULET: And this is this hot plasma because
of the x-rays that they make.
DAVIS: That's right. And we see in clusters
of galaxies, we see two evidences, two different
lines of evidence to suggest Dark Matter.
In clusters -- Zwicky, Fritz Zwicky, a famous
astronomer, 50 years ago said --
SADOULET: This was the first one to say that.
DAVIS: The first one to talk about it, well
before it's time. He said that the galaxies
-- in the Coma Cluster of galaxies a very
rich conglomeration of a thousand galaxies
--
SADOULET: Yes, there are a thousand galaxies.
DAVIS: -- a thousand galaxies or so are moving
around much too fast to be held together by
their own self-gravitation, because he could
calculate the amount of gravitational attraction
of all the material there, assuming the stars
made up the total mass --
SADOULET: Right, were similar to the sun and
--
DAVIS: And it was wrong by a factor of 50.
I mean as though 50 times more mass were needed
than appeared to be present in the stars.
I mean, this is a fairly astounding discrepancy.
And he said, "I don't understand why this
cluster exists. It must have some unseen form
of matter." And now the evidence is confirmed
in many different experiments, there's unseen
matter. Now but your question of --
SADOULET: But you spoke about a second evidence
which --
DAVIS: Oh, the second evidence, right. Well,
that's called a virial analysis of the galaxies
orbiting in the cluster. The other evidence
is the nature of the x-ray emission. Again,
this hot plasma, which is confined in the
cluster, emits radiation and by mapping its
density distribution and estimating its temperature
distribution with some simple equations, we
can show that this provides an unambiguous
estimate of the mass. Again, coming out with
a very high mass. Much higher than is given
by the stars.
SADOULET: I think there is a further piece
of evidence in clusters of galaxies.
DAVIS: Oh, I guess there's the lensing.
SADOULET: The lensing.
DAVIS: Right.
SADOULET: Yes, the arclets, the fact that
we can measure the mass of the clusters by
the amount of bending that gives on the galaxy
that shines in the background. And there again,
we obtain very similar results with it than
with the velocities on the galaxies and the
temperature of the x-ray gas.
DAVIS: Now this question though about the
nature of the Dark Matter, I guess we should
think about what is the argument that says
that the Dark Matter can't be rocks or something
that was just too small to shine, but just
ordinary material that's compressed like snowballs
or something of that sort? Why do we think
it's not the stuff that we're made of.
SADOULET: We're not sure, OK? That's really
maybe the first answer. We have a pretty good
idea of how many protons and neutrons there
are in the universe. This is the so-called
nuclear synthesis argument. One of the major
confirmations of this general picture of the
hot Big Bang is that we can compute the amount,
not only of hydrogen, but the ratio between
the hydrogen and helium, the deuterium, the lithium-7 which has been produced
in the Big Bang, and we have only one parameter
to fit a large number of data. This parameter
is just the amount of baryons and the amount
of protons and neutrons. If the best fit to
all the data gives five percent, something
like that, five percent of the critical density
--
DAVIS: Critical density.
SADOULET: -- is in baryons. The critical density
is density of the universe where it just expands
forever and just across that infinity. If
it was slightly more massive, it would re-collapse.
DAVIS: Slightly denser it would re-collapse.
Less dense it would expand forever.
SADOULET: Yes.
DAVIS: So this is the boundary line.
SADOULET: The boundary line. So about five
percent of the critical density is in protons
and neutrons, what we call baryons. And really
it depends how much Dark Matter there is.
I think the current situation of the community
is the following. I was -- in the last 10
years, five to 10 years, we have witnessed
amazing consensus building up on the existence
of Dark Matter. We are left with two problems.
One is how much there is exactly and what
is nature? How much there is exactly is this
old question in cosmology of measuring the
average density.
DAVIS: Right, whether the universe is at critical
density or below the critical density. We
don't think it's above the critical density,
but a lot of direct evidence suggests it's
below the critical density.
