By Geoff Brumfiel
Researchers are once again proposing that an orbiting telescope may have seen evidence for dark matter -- the undetected material that is believed to permeate the Universe.
The Fermi Gamma Ray Space Telescope has captured flashes of high-energy γ-ray light that might come from dark matter, according to Lisa Goodenough of New York University in New York City and Dan Hooper at the Fermi National Accelerator Laboratory in Batavia, Illinois. In a paper posted on the arXiv pre-print server1, the duo suggests that flashes seen at the Milky Way's core could be caused by the collision of dark-matter particles with their antiparticles. "We were really shocked just how well a simple dark matter model accommodated this data," says Hooper.
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But not everyone is convinced. This latest analysis, which points to a dark-matter particle weighing around 30 gigaelectronvolts (GeV), seems to be incompatible with other recent studies. And given that ordinary astronomical sources could be causing the signal, Goodenough and Hooper's proposal is "pretty shaky", according to Jan Conrad, a physicist at Stockholm University.
Measurements of the faint afterglow left by the Big Bang suggest that dark matter could make up as much as 85% of the matter in the Universe, but to date, no one has been able to observe a dark-matter particle in the laboratory. That has led many physicists to conclude that dark matter rarely interacts with ordinary matter, except through the force of gravity. However, that doesn't mean dark matter is completely undetectable: some believe that when dark-matter particles and antiparticles collide and annhilate each other in a flash of energy, ordinary, detectable particles and radiation such as γ rays could be produced.
Real rays
Goodenough and Hooper's analysis is the latest in a recent flurry of such dark-matter proposals. In August 2008, an orbiting satellite known as the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) reported an excess of electrons and their positron antiparticles near Earth. A few months afterwards, a high-altitude balloon experiment spotted a similar electron excess far above Antarctica. Both the electrons and the γ rays could be created by the annihilation of dark-matter particles with their own antiparticles, theorists have suggested.
But the similarities end there. The electron signal seen by PAMELA suggests a dark-matter particle with a mass of upwards of 100 GeV, whereas the balloon experiment suggests that the dark matter would be several times more massive than that. A paper based on data from the Fermi telescope and posted on arXiv in July 2009 claimed to have found γ-ray evidence that was consistent with PAMELA's findings (see 'Ray of hope in dark-matter hunt'), but mission scientists later said it was probably caused by noise in the telescope's wide-angle camera.
Unlike the previous studies, Goodenough and Hooper's observations suggest that "these are likely to be real γ-rays," says Julie McEnery, a project scientist for the Fermi telescope mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland. But that doesn't mean that the γ rays are coming from dark matter, she says. Many other objects, such as the remnants of exploding stars and pulsars, can create γ rays -- especially in the cluttered galactic centre. "The galactic centre is a very challenging region," she says.
Hooper agrees, but he adds that, regardless of its origins, the data fits dark matter models remarkably well. "When I look at this data, it lines up perfectly," he says. "It quacks like a duck."
