The Hunt for Dark Matter
Even with all
of the galaxies that Bothun and others expect to find, researchers
still say much of the matter in the universe is unaccounted for.
the Big Bang theory, the nuclei of simple atoms such as hydrogen
and helium would have started forming when the universe was about
one second old. These processes yielded certain well-specified abundances
of the elements deuterium (hydrogen with an extra neutron), helium,
and lithium. Extensive observations and experiments appear to confirm
the theory's predictions within specified uncertainties, provided
one of two assumptions is made: (1) the total density of the universe
is insufficient to keep it from expanding forever, or (2) the dominant
mass component of the universe is not ordinary matter. Theorists
who favor the second assumption need to find more mass in the universe,
so they must infer a mass component that is not ordinary matter.
Part of the evidence for the second theory was compiled by Vera
Rubin, an astronomer at the Carnegie Institution of Washington who
received NSF funding to study orbital speeds of gas around the centers
of galaxies. After clocking orbital speeds, Rubin used these measurements
to examine the galaxies' rotational or orbital speeds and found
that the speeds do not diminish near the edges. This was a profound
discovery, because scientists previously imagined that objects in
a galaxy would orbit the center in the same way the planets in our
galaxy orbit the Sun. In our galaxy, planets nearer the Sun orbit
much faster than do those further away (Pluto's orbital speed is
about one-tenth that of Mercury). But stars in the outer arms of
the Milky Way spiral do not orbit slowly, as expected; they move
as fast as the ones near the center.
the material in the Milky Way's outer reaches to move so fast? It
is the gravitational attraction of matter that we cannot see, at
any wavelength. Whatever this matter is, there is much of it. In
order to have such a strong gravitational pull, the invisible substance
must be five to ten times more massive than the matter we can see.
Astronomers now estimate that 90 to 99 percent of the total mass
of the universe is this dark matter-it's out there, and we can see
its gravitational effects, but no one knows what it is.
At one of NSF's
Science and Technology Centers, the Center for Particle Astrophysics
at the University of California, Berkeley, investigators are exploring
a theory that dark matter consists of subatomic particles dubbed
WIMPs, or "weakly interacting massive particles." These heavy particles
generally pass undetected through ordinary matter. Center researchers
Bernard Sadoulet and Walter Stockwell have devised an experiment
in which a large crystal is cooled to almost absolute zero. This
cooling restricts the movements of crystal atoms, permitting any
heat generated by an interaction between a WIMP and the atoms to
be recorded by monitoring instruments. A similar WIMP-detection
project is under way in Antarctica, where the NSF-supported Antarctic
Muon and Neutrino Detector Array (AMANDA) project uses the Antarctic
ice sheet as the detector.
In the spring
of 2000, NSF-supported astrophysicists made the first observations
of an effect predicted by Einstein that may prove crucial in the
measurement of dark matter. Einstein argued that gravity bends light.
The researchers studied light from 145,000 very distant galaxies
for evidence of distortion produced by the gravitational pull of
dark matter, an effect called cosmic shear. By analyzing the cosmic
shear in thousands of galaxies, the researchers were able to determine
the distribution of dark matter over large regions of the sky.
shear "measures the structure of dark matter in the universe in
a way that no other observational measurement can," says Anthony
Tyson of Bell Labs, one of the report's authors. "We now have a
powerful tool to test the foundations of cosmology."