Findings Offer Clearer View of How To Detect Unseen Matter in the Universe

Dark Matter "Highway" (artist's conception)

Troy, N.Y. — Astrophysicist Heidi Newberg at Rensselaer Polytechnic Institute and her colleagues suggest that a "highway" of dark matter from another galaxy may be showering down on Earth. The findings may change the way astronomers look for mysterious cosmic particles, long suspected to outweigh known atomic matter.

The findings of Newberg and researchers at the University of Michigan and the University of Utah have been published in the March 19 issue of Physical Review Letters.

Scientists believe that about 90 percent of the mass in the universe is made up of particles called "dark matter." This belief is based on an unseen gravitational pull on the stars, but observations to directly detect dark matter have been sketchy. One Italy-based research group, called DAMA (for DArk MAtter), has made steady claims to have detected particles of dark matter, but so far the results have not been confirmed.

But, the disruption of a dwarf galaxy called Sagittarius, which is being torn apart and consumed by the much larger gravitational pull of the Milky Way, may be the key to reconciling the results of dark matter experiments of DAMA and other research groups.

The dwarf galaxy's entrails of stars and dust, like a long piece of ribbon, are entangled around and within our galaxy. The so-called "trailing tidal tail" can be seen to extend from Sagittarius' center and arcs across and below the plane of the Milky Way. The leading part of the tail extends northward above our galaxy where it then turns and appears to be showering shredded galaxy debris down directly on our solar system, Newberg and colleagues say.

"As the Milky Way consumes Sagittarius, it not only rips the stars from the smaller galaxy, but also tears away some of the dark-matter particles from that galaxy. We may be able to directly observe that in the form of a dark-matter highway streaming in one direction through the Earth," says Newberg, who has recently identified stars near the sun that could be part of this leading tidal tail.

WIMPs, or Weakly Interacting Massive Particles, are the most likely form of dark matter. Astrophysicists measure the possibility of WIMP detection based on calculations that the particles are coming from the Milky Way's galactic halo.

As Earth orbits around the center of the galaxy, the planet flies through this cloud of dark matter. As that happens, millions of these weakly interacting (and therefore difficult to detect) particles could be passing through each of our bodies every second.

As a result of the new findings, scientists now have another source in which to look for these dark-matter particles, says Katherine Freese, University of Michigan researcher and co-author of the Physical Review Letters paper. Freese, her graduate student Matthew Lewis, and Paolo Gondolo from the University of Utah have calculated the effects that the tidal stream would have on dark-matter detection experiments.

"If you expect to see only halo WIMPs, there will be an extra set of particles streaming through the Earth that were not accounted for," Freese says. "Scientists will need to adjust their calculations to look for this. Finding this stream would represent a smoking gun for dark-matter detection."

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