More Circumstantial Evidence for Dark Matter, But Debate Continues

By Eliza Strickland | May 6, 2009 3:55 pm

Fermi telescopeThree recent studies raised hopes that physicists had caught the first glimpses of dark matter, but the somewhat contradictory results guarantee that researchers will be puzzling over the issue  for some time to come. The latest results come from NASA’s orbiting Fermi Gamma-ray Space Telescope, which was launched last June. The evidence is a reported excess of high-energy electrons and their antimatter counterparts, positrons, which could be created as dark matter particles annihilate or decay [Nature News].

Peter Michelson, principal investigator for the instrument on Fermi that made the detection, cautions that his group is not yet claiming to have found a smoking gun for dark matter. The signal could also come from more mundane sources nearby, such as pulsars, the spinning remnants of supernovae. “But if it isn’t pulsars, it is some new physics,” says Michelson [Nature News]. The new findings are published in Physical Review Letters. Meanwhile, a satellite named PAMELA recently detected higher than expected numbers of positrons, which seems to corroborate the Fermi findings. But results from a balloon experiment conducted high over Antarctica last year add a dash of confusion to the mix.

Dark matter is the mysterious stuff that is thought to make up 85% of the universe’s matter and provides the gravity that keeps galaxies from whirling apart, but its particles have never been directly detected. One way to spot these particles might be to look to the skies. Some popular theoretical models suggest that if two lingering particles of dark matter collide, they should annihilate to create an ordinary particle and an antiparticle, such as an electron and a positron, which can be observed. Those particles should emerge with a definite energy determined by the mass of the dark energy particles, leading to a sharp peak in the energy spectrum of electrons and positrons from space [ScienceNOW Daily News].

The balloon experiment in Antarctica, called ATIC, thrilled researchers with the detection of an excess of high-energy electrons and positrons with energies ranging from 300 and 800 gigaelectronvolts (GeV), which the research team said was congruent with the hypothetical mass of dark matter particles. However, the Fermi telescope, which can detect many more particles than ATIC, did not find a sharp peak of electrons and positrons in that particular energy range–instead, its spectrum analysis showed a long, sloping hill of high-energy particles in a wider energy range. Still, ATIC astrophysicist John Wefel says he’s not ready to concede that the Fermi result rules out ATIC’s findings, and notes that while Fermi detects more particles, it shows their energies in poorer resolutions. “The difference comes down to something in the instrumentation,” Wefel says [ScienceNOW Daily News].

All three teams of researchers are looking forward to further results from Fermi that may clarify matters. “This is a real detective story, and we already have some clues,” says Michelson. “It’s possible that within a year we may know whether or not we’ve got dark matter, or at least the kind of dark matter we thought we might have” [Science News].

Related Content:
Cosmic Variance: Fermi Waffles on Dark Matter has a more technical explanation of the new findings
80beats: Does a Shower of Subatomic Positrons Mean We’ve Found Dark Matter?
80beats: Have Researchers Found Dark Matter’s Signature Over Antarctica?
80beats: Fermi Space Telescope May Follow the Gamma Rays to Find Dark Matter
DISCOVER: The Father of Dark Matter Still Gets No Respect

Image: NASA


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