MAGAZINE OF THE SOCIE TY FOR SCIENCE & THE PUBLIC MAGAZiNe OF THe SOCie TY FOR SCieNCe & THe PUbLiC
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Dark matter nothing to
fear, if it’s there or not
For several decades now, scientists have
pondered the mystery of cosmic darkness. Most of the matter in the universe
is not of the Earthly variety, but rather
consists of some invisible (hence “dark”)
species of subatomic particles with very
little tendency to interact with ordinary
protons, neutrons and electrons.
This dark matter does fully participate in gravitational
interactions, though, which is how astronomers came to be
aware of its existence. Visible matter’s gravity cannot explain
how galaxies cluster together or the speed with which the
outer edges of galaxies spin. Explaining the architecture of the
cosmos requires vast amounts of dark matter; observations
and calculations suggest that the Milky Way galaxy, home to
the solar system, sits within a huge cloud of weakly interacting
massive particles, conveniently acronymized as WIMPs.
As the galaxy turns, the Earth and sun would plow through
this cloud, subjecting everyone going along for the ride to a
constant bombardment: Billions of WIMPs pass through your
body each second. But thanks to their reluctance to interact
with ordinary atoms, few actually collide with any of your
body’s atoms. By one recent estimate, actual WIMP collisions
with atomic nuclei in a typical person’s body range from 10 per
year to 1 per minute (about 100,000 times per year).
Such exposure to WIMPs is nothing to worry about, though,
astrophysicists Katherine Freese of the University of Michigan and Christopher Savage of Stockholm University report.
You probably get a higher radiation dose from cosmic rays and
other natural radioactive substances each second than from a
lifetime of WIMPs, the astrophysicists write in a recent paper
online at arXiv.org ( http://arxiv.org/abs/1204.1339).
And maybe the WIMP exposure is even less. A new study
of the motions of stars in the solar system’s galactic neighborhood doesn’t see the gravitational influence expected if dark
matter WIMPs are as abundant as they’re supposed to be.
By some accounts, such a lack of WIMPs suggests a crisis
in current theories of the cosmos. But as Nadia Drake reports
(Page 5), the new finding may just indicate that the dark
matter cloud isn’t as uniform and spherical as the simplest
calculations assume. And it’s not clear that the method used
to test for dark matter’s presence could actually find it.
So there seems to be no reason to fear dark matter, or to
fear that it isn’t really there, either.
—Tom Siegfried, Editor in Chief
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