In the News
dark matter,
where art thou?
New study finds dearth of
invisible substance in the
solar system’s neighborhood
STORY ONE
By Nadia drake
Scientists using Earth-based experiments to detect parti- cles of dark matter might now know why they keep coming
up empty-handed: There may be no dark
matter in the solar neighborhood — at
least, not within 13,000 light-years,
reports a study appearing in an upcoming issue of the Astrophysical Journal.
Though a mysterious substance different from ordinary visible stuff supposedly makes up more than 80 percent of
the matter in the universe, the new work
suggests that the solar system lives in a
dark matter desert. That’s a problem,
because most astronomers think the
Milky Way galaxy is embedded in a large
dark matter halo.
If the results hold, scientists will have
to reconsider what sort of shape that
dark matter halo takes, although it’s
unlikely to cause any major cosmological upheavals.
“This doesn’t mean that dark mat-
ter does not exist,” says study coauthor
Christian Moni Bidin, an astronomer at
the Universidad de Concepcion in Chile.
“The result is only that dark matter is
not where we expected it.”
Dark matter explains many things,
including the rotation speed of the Milky
Way. Its outer reaches spin more rapidly
than expected based on the amount of
mass sequestered in visible stars. Scien-
tists have deduced that unseen matter,
probably in the form of exotic subatomic
particles, engulfs the speeding gal-
axy, exerting a gravitational force that
accounts for the rapid rotation.
Scientists believe the Milky Way is embedded in a large halo f dark matter, depicted here as a fuzzy blue cloud. But a new study failed to find dark matter in the solar neighborhood.
can’t directly observe it in space. Instead,
they infer its presence by looking for its
gravitational effect on things like stars,
gas and light from distant galaxies. For
the ne w work, a team in Chile went hunting for changes in the paths of stars
crossing the sky, sinuous indicators that
masses of the dark stuff lay nearby.
More than a year ago, astronomers
studied the motion of roughly 400 red
giant stars that hovered far above the
galactic plane in the vicinity of the solar
system. Now, the team has calculated the
amount of matter needed to explain the
stars’ movements. That total equals the
mass of stuff astronomers can see.
Ergo, no dark matter.
It’s a challenging measurement, and
one that may not have been done right,
says Chris Flynn, an astronomer at
Swinburne University of Technology
in Melbourne, Australia. When Flynn
applied the team’s method to a simulated galaxy with known quantities of
dark matter and stars, it failed to detect
