There may be a large cluster of dark matter in our galactic neighborhood
There appears to be a huge cloud of dark matter next to our
solar system. We’ve never found any of these clouds in the Milky Way before,
but precise cosmic clocks called pulsars finally made it possible.
Our best cosmological models suggest that galaxies are
embedded in thin clouds of dark matter called haloes, with smaller sub-halo
effects. But dark matter does not emit, absorb or reflect light, making haloes
and especially sub-halos extremely difficult to find.
To notice this, Sukanya Chakrabarti at the University of
Alabama in Huntsville and her colleagues used pairs of rapidly rotating neutron
stars called pulsars. Pulsars rotate at an amazingly regular rate, scattering
beams of light across the sky, making them key cosmic clocks. With a pair of
them, we can use the changes in their orbits around each other to measure the
acceleration that occurs when a massive object is nearby.
Dark matter interacts with normal matter through gravity, so
if a dark matter subhalo is near a pair of pulsars, it should stretch their
orbits slightly. That’s exactly what Chakrabarti and her colleagues found a
little more than 3,000 light-years from our solar system. “There is one pair of
pulsars and [individual] pulsars around it – there’s something in this part of
the sky that’s pulling all these pulsars in this strange direction that we
didn’t expect,” says a member of the team Philip Chang at the University of
Wisconsin-Milwaukee.
The researchers analyzed the extent of the pull and
determined that it must come from an object with a mass of about 60 million
solar masses, measuring several hundred light-years on average. They compared
the position of the mysterious, massive object with maps of stars, gas and
other objects made of ordinary matter, finding no matches. If the object is
really there, which scientists say has yet to be definitively confirmed, it
must be dark matter.
If so, it could be the only sub-halo of its size in our
corner of the galaxy. “There may be only one or two locally, but that depends
on the dark matter model,” he says Alice Quillen at the University of Rochester
in New York. “Different models predict different distributions of these
clusters.”
This is what sets Chakrabarti on the sub-halo hunting path
in the first place. “Our goal is to map as many of these sub-haloes as possible
throughout the galaxy, and we’ve just started doing that. Then the ultimate
goal is to understand the nature of dark matter,” he says.
However, pulsar binaries are relatively rare: there are only
27 for which we have observations precise enough to measure their acceleration
due to gravity, which is part of why this sub-halo was not found before. “The
amount of pulsars is finite, so we’re trying to come up with other ways to
track it with objects that are more numerous,” Chang says. If they can, we may
finally have a crucial tool for investigating what dark matter really is.