Suppose the sun went dark one day, so
that we could no longer see it - how could we tell if it was still
there? One way to check would be to keep track of the motions of
the Earth and the other planets. If the planets don't fly out of
their orbits, then there must be something generating the gravitational
pull that keeps
them in their
orbits! In fact, if you know the approximate masses of the
planets, you can do better than that. Each planet's orbit and
mass allow you to calculate the mass
of the object the planet is orbiting. By observing that
the mass you get is roughly the same for all of the planets, you
can conclude that the object the planets are orbiting must fit inside
Mercury's orbit. In other words, you can learn a lot about an
object (in this case, the invisible sun) without ever seeing it!
In cosmology, we can't quite do this, but we can do something
analogous. By studying the redshift of light from a distant
spiral galaxy, we can determine how rapidly different parts of the
galaxy are moving toward or away from us. This lets us construct
what's called a "rotation curve" - a plot giving the relationship
between orbital speed and distance from the galactic center for the
stars and gas that make up the galaxy. This plot lets us map out
the distribution of matter in the galaxy.
For a given galaxy, we can compare the rotation curve we actually measure
to the rotation curve we
to get if the
galaxy contained no dark matter at all - in other words, if the visible
matter was the only matter in the galaxy. It turns out that these
two curves don't agree! If only the stars were present, one would
expect that the galaxy's mass was concentrated at its center, and that
the material in the outer parts of the galaxy should be orbiting more
slowly than the material in the inner parts. Instead, we see that
matter throughout the galaxy orbits at an almost constant speed - the
rotation curve is said to be "flat"!
From the observed flat rotation curve, astrophysicists have concluded
that galaxies are immersed in large clouds of invisible matter.
This dark matter halo seems to have a diameter far larger than that of
the visible part of the galaxy, and to have several times the mass.
The Virial Theorem
Even in blob-like systems where astronomers can't define a rotation
curve (elliptical galaxies or galaxy clusters, for example), they can
still use velocities to estimate masses. In this case,
astronomers treat these systems statistically and compute an average s
for its members, and hence an approximate
average kinetic energy. A result of classical mechanics called
the "virial theorem" relates the average kinetic and potential energies
of a gravitational bound system, and the potential energy (since it is
caused by gravity) is related to the total mass. This analysis
indicates that both elliptical galaxies and galaxy clusters reside in
large dark matter haloes.
In large galaxy
clusters, the space between the galaxies is filled with
a very thin cloud of ultra-hot gas (more than a million degrees).
These gas atoms are moving so fast that when they collide they release
x-rays. By studying these x-rays using instruments such as the
Chandra space telescope, astronomers can estimate the velocity of this
gas. This in turn allows them to estimate the cluster mass using
the virial theorem, as described above. Again, these results
support the idea of a massive dark matter halo.
For more detail, see pages by Martin White
Jeff Filippini, UC Berkeley
Cosmology Group (August 2005)