Thursday, April 02, 2009

Has dark matter’s destruction been detected?

By Clara Moskowitz
updated 9:38 p.m. CT, Wed., April. 1, 2009
When dark matter is destroyed, it leaves behind a burst of exotic particles, according to theory. Now scientists have found a possible signature of these remains. The discovery could help prove the existence of dark matter and reveal what it's made of.

No one knows what dark matter is, but scientists think it exists because there is not enough gravity from visible matter to explain how galaxies rotate.

An Italian satellite called PAMELA (Payload for Antimatter Matter Exploration and Light nuclei Astrophysics), launched in 2006 to measure radiation in space, found an overabundance of particles called positrons, which are the antimatter counterpart to electrons (matter and antimatter annihilate each other).

This positron signature could have a variety of causes, but a prime candidate is dark matter, the intangible stuff thought to make up about 98 percent of all matter in the universe. When two dark matter particles collide they can sometimes destroy each other and release a burst of energy that includes positrons.

"PAMELA found a number of positrons much higher than expected," the mission's principal investigator Piergiorgio Picozza told "Many think this could be a signal from dark matter, because for positrons this behavior fits very well with many theories of dark matter."

Potentially huge for physics
The finding, detailed in Thursday's issue of the journal Nature, is not a total surprise, but it could make a huge splash if confirmed.

"This kind of signal for dark matter has been predicted as a possible leading signature for over two decades, and [the PAMELA scientists] are seeing just the kind of things one might expect," said University of Michigan astrophysicist Gordon Kane, who was not involved in the research. "There's a very good chance that this is the most important discovery in basic physics for decades."

Positrons are often created when cosmic rays interact with atoms in the gas and dust between stars. But this source cannot produce enough positrons to account for PAMELA's findings. Another possibility is that the positrons PAMELA found were produced by dense spinning stars called pulsars. To distinguish between this option and dark matter, more data will be necessary, either from PAMELA or from the Fermi Gamma-ray Space Telescope, launched last year.

"We hope to have detected dark matter, but now we need other verification coming from other experiments," Picozza said.

Decades-old mystery
Even so, some scientists are excited to have come so close to possibly discovering the presence of dark matter, which has eluded researchers since it was first conceived in the 1930s.

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