Antimatter Annihilation Causes Milky Way Mystery

By staff

posted: 13 July 2009 09:20 am ET
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Patterns of a type of high-energy radiation called gamma rays throughout the Milky Way aren't the signature of mysterious dark matter as had previously been suggested, a new study shows.

Over the past five years, gamma-ray measurements from the European satellite INTEGRAL have perplexed astronomers because the distribution of the gamma rays across different parts of the Milky Way didn't match what astronomers expected to see.

Some astronomers suggested that various forms of dark matter — the intangible stuff thought to make up about 98 percent of all matter in the universe — might be the source of these gamma rays, which would be further proof the existence of the mysterious matter.

But one team of astrophysicists has shown that this explanation is unlikely, and that instead the pattern of gamma rays can be explained by the way so-called "anti-matter positrons" from the radioactive decay of elements propagate through the galaxy.

"There is no great mystery," said Richard Lingenfelter of the University of California San Diego and one of the co-authors of the new study. "The observed distribution of gamma rays is in fact quite consistent with the standard picture.

Positron annihilation

Positrons are subatomic particles that are the antimatter counterpart of everyday electrons, and our galaxy — and others — are filled with them.

When an electron and a positron encounter one another in space, the two particles are annihilated and their energy is released as gamma rays.

"These positrons are born at nearly the speed of light, and travel thousands of light-years before they slow down enough in dense clouds of gas to have a chance of joining with an electron to annihilate in a dance of death," said study team member James Higdon of Claremont Colleges in California.

The positrons are born in supernova explosions, and travel from there to all parts of the galaxy, including the galactic center, the spiral arms and even the outer reaches of the Milky Way, known as the galactic halo.

The team calculated that most of the gamma rays should be concentrated in the inner regions of the galaxy — the same pattern that showed up in the satellite data.

"The observed distribution of gamma rays is consistent with the standard picture where the source of positrons is the radioactive decay of isotopes of nickel, titanium and aluminum produced in supernova explosions of stars more massive than the sun," said Richard Rothschild, also of UCSD.

If dark matter decays or annihilations were behind the gamma rays, the positrons would instead be annihilated very close to their supernova birthplaces.

"We clearly demonstrated this was not the case, and that the distribution of the gamma rays observed by the gamma ray satellite was not a detection or indication of a 'dark matter signal,'" Lingenfelter said.