Physicists may not know what dark matter is, but they're getting a better idea of what it's not. Data from NASA's orbiting Fermi Gamma-ray Space Telescope puts a crimp in particle theorists' favored explanation of the mysterious stuff whose gravity holds the galaxies together, ruling out a hefty range of masses for the hypothesized particles, a team announced this week. Curiously, the result comes not from the Fermi team itself, but from other particle astrophysicists who analyzed the Fermi data.
For decades astronomers and astrophysicists have know that the whirling galaxies do not seem to contain enough stars and gas to hold themselves together. Instead, some mysterious and otherwise undetected mass must provide the gravity that keeps the fast-circulating stars from flying out into space. This missing stuff is known as dark matter because it doesn't emit or reflect light of any wavelength.
Although dark matter is invisible, it may have other signatures. A top candidate for dark matter is the hypothesized "weakly interacting massive particle" (WIMP), so called because it interacts strongly with gravity but weakly or not at all with other forces. In many models, a WIMP is its own anti-particle, meaning that when two dark matter particles meet, they annihilate to produce more-familiar particles, including gamma rays detectable by Earth- and space-based telescopes. Physicists expect WIMPs to have a mass somewhere between 1 and 200 giga-electron volts (GeV)--roughly between 1 and 200 times the mass of a proton--although in principle the mass could be higher.
The latest analysis of the universe's uniform backdrop of gamma radiation now carves a 10 GeV hole in the low end of that range, Kevork Abazajian, a particle cosmologist at the University of Maryland, College Park, reported here this Sunday at the oddly timed April Meeting of the American Physical Society. Abazajian and his team looked at 18 different ways WIMPs could annihilate and calculated the expected spectrum of photons for a range of possible WIMP masses and rates of annihilation. They then compared these expected spectra with the actual unpublished spectrum from Fermi, the most sensitive gamma-ray detector ever built. If a given mass and rate of annihilation results in more gamma rays than Fermi has actually seen, the physicists could rule out that combination of parameters. By the end, they'd ruled out masses from 5 GeV to 15 GeV.
The team has also put a snag in the general WIMP scenario. To explain the current abundance of dark matter in the universe (as inferred from galactic observations and other data), the rate of annihilation, which governs how often WIMPs were produced in the early universe as well as how often they annihilate now, must fall within a narrow range. That state of affairs is known as the "WIMP miracle." For some masses and some of the possible annihilation mechanisms, that annihilation rate would produce more photons than Fermi has actually seen, Abazajian says. So at least in those mass ranges, the miracle is truly impossible.
"In a lot of these different scenarios he's considered of how dark matter can annihilate ... he's started to rule out well-motivated theoretical models," said Jennifer Siegal-Gaskins, an astrophysicist at Ohio State University and a member of the Fermi Large Area Telescope (LAT) collaboration who attended Abazajian's talk. The LAT collaboration, which presented the unpublished spectrum at conferences, is racing to publish its own dark-matter analysis, she says. "I'm just really curious to see the actual paper and see how the analyses differ."
As Fermi collects more data, "more and more chunks will be taken out [of the landscape of possible WIMP masses and annihilation rates] or the discovery is going to be made," Abazajian said. If the last chunk disappears, "we'll have to go back to the drawing board" to explain dark matter.