MACHOs 1, WIMPs 0
Two types of dark
matter have been proposed to solve the missing matter problem: Normal or
baryonic matter (made up of protons, neutrons and electrons) provides the
building blocks for plants, animals, stars and anything else that we can
see. Some of these objects are easily detectable. The rest, considered
to be dark matter, were dubbed MACHOs (massive compact halo objects).
A more exotic form
of matter, called nonbaryonic, is composed of particles that are so far
only theoretical. This expected sea of massive particles, with names like
neutralinos, photinos and axions, are thought to interact weakly with normal
matter, and are thus called WIMPs
(for weakly interacting massive particles).
WIMPs have never been detected.
The new study does not change beliefs
about WIMPS, and Oppenheimer says such matter almost certainly exists and,
by most estimates, dominates the mass of the universe. Pinning down how
much matter exists is a key component in helping to answer one of the most
important questions in cosmology: Will the universe expand forever, or
will it one day reverse course and crash together under the force of its
own collective gravity?
Where is the matter?
The majority of dark matter in our
galaxy is thought to reside in the halo, a vast sphere some 300,000 light-years
in diameter that surrounds the main galactic disk, where most of the younger
stars are nestled.
The disk rotates about the galactic
center. Our Sun, some 26,000 light-years from the center, is motoring along
at 492,000 miles per hour (220 kilometers per second). But objects in the
halo do not rotate in the same manner; rather, they move on more independent
orbits at faster speeds.
The white dwarfs found in the study
are thought to be technically a part of the halo, based on their movement
and their old age. But the halo pervades the galactic disk, and these white
dwarfs are in fact moving through the disk. They are all within 450 light-years
of Earth.
It is this relative proximity that
made it possible to find them.
In the study, Nigel C. Hambly of the
University of Edinburgh in Scotland used new digital techniques to study
existing photographic plates from sky surveys of the Southern Hemisphere
dating back to the 1970s. Hambly and colleagues picked out a handful of
candidate stars and studied them further with a 157-inch (4-meter) telescope
in Chile, analyzing their faint light emissions for telltale clues of the
composition and temperature.
The group found 28 previously undetected
white dwarfs. An additional 10 stars that had been cataloged but not fully
identified were determined to be white dwarfs.
The area surveyed represents some 10
percent of the sky, but does not go deep into it, because more distant
white dwarfs would be too dim to appear. So while the results indicate
that at least 3 percent of the galactic dark matter is in the form of white
dwarfs, it is not possible to know whether that figure could be higher
-- to as much as the 35-percent limit predicted by other studies.
And Oppenheimer cautioned that it is
not possible to apply the results directly to other galaxies, though some
extrapolations might be made to younger galaxies that appear to be developing
similarly to our own.
"To find this type of dark matter farther
out in our own galaxy will be very difficult," he said. "To try to look
for it in another galaxy is nearly impossible."
Andrew P. Digby of the University of
Edinburgh in Scotland, Simon T. Hodgkin of Cambridge University in England
and Didier Saumon of Vanderbilt University in Nashville also worked on
the study.
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