Our galaxy's dark matter is clumpier than once thought,
according to a new computer simulation.
The model, created by one of the most powerful
supercomputers in the world, shows that the spherical halo of dark
matter that envelopes the Milky Way contains dense clumps and streams of
the mysterious stuff, even in the neighborhood of our solar system.
"In previous simulations, this region came out smooth,
but now we have enough detail to see clumps of dark matter," said
researcher Piero Madau, an astrophysicist at the University of California,
Santa Cruz.
Dark matter, which scientists can only detect by noting its
gravitational effect, is thought to make up about 85
percent of the matter in the universe. Its composition remains a mystery,
though some scientists think it's made up of hypothetical particles called
WIMPs (weakly interacting massive particles), which could annihilate each other
and emit gamma rays when they collide.
The new simulation, described in the Aug. 7 issue of the
journal Nature, implies that dark matter could be detected by the
recently launched Gamma-ray Large Area Space Telescope (GLAST).
"That's what makes this exciting," Madau said.
"Some of those clumps are so dense they will emit a lot of gamma rays if
there is dark matter annihilation, and it might easily be detected by
GLAST."
So far, though many teams have been looking for WIMP
particles, no one has conclusively detected them.
"There are several candidate particles for cold dark
matter, and our predictions for GLAST depend on the assumed particle type and
its properties," said Juerg Diemand, a postdoctoral fellow at UCSC who led
the new research. "For typical WIMPs, anywhere from a handful to a few
dozen clear signals should stand out from the gamma-ray background after two
years of observations. That would be a big discovery for GLAST."
The model took about one month to run on the Jaguar
supercomputer at Oak Ridge National Laboratory in Tennessee. By following the
gravitational interactions of more than a billion parcels of dark matter over
13.7 billion years, the computer could predict how the dark matter in the
universe developed over time based on leading theories of how dark matter
interacts.
"It simulates the dark matter distribution from near
the time of the Big Bang until the present epoch, so practically the entire age
of the universe, and focuses on resolving the halo around a galaxy like the
Milky Way," Diemand said.
The research was funded by the U.S. Department of Energy,
NASA and the Swiss National Science Foundation.
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