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Dark Matter, or Murky Data?

29 February 2000 7:00 pm
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A group of physicists claims to have identified dark matter, the shadowy stuff thought to account for 90% of the universe's mass. Announced Friday at the Fourth International Symposium on the Sources and Detection of Dark Matter in Marina Del Ray, California, the discovery would solve the decades old mystery of what hold the galaxies together--if the finding holds up.

There must be more mass in the universe than meets the eye, otherwise the galaxies themselves would fly apart. Like riders on a fast-spinning carousel, the stars whirling around a galaxy are flung outward by their own inertia. Indeed, in the outer reaches of a spiral galaxy like the Milky Way, the stars travel so fast that the gravity from all the other visible matter in the galaxy can not hold them. So some other type of matter, invisible and elusive, must provide the missing mass and the extra gravity. One theory holds that clouds of weakly interacting massive particles--or WIMPs--engulf the galaxies, giving them the heft they need.

To search for WIMPs, researchers from the Dark Matter (DAMA) experiment at the Gran Sasso National Laboratory in Italy looked for a slight seasonal variation in the number of particles entering their underground detector. The detector picks up muons, gamma rays, neutrons and other particles as flashes of light in an array of sodium iodide crystals. Presumably, occasional WIMPS would leave a similar flash--but a with telltale seasonal variation.

If our galaxy spins in the middle of a huge stationary WIMP cloud, then Earth should plow through the cloud at a pace averaging 220 kilometers per hour. But the speed of that wind should vary slightly as the Earth zips around the sun. The DAMA detector should count more particles in summer, when Earth pushes directly into the WIMP wind, and fewer in winter, when it swings away from the wind. A year ago, the DARMA team announced tentative evidence of a seasonal variation (Science, 1 January 1999, p. 13). Last week, they reported a seasonal signal that has only one chance in 10,000 of being a statistical fluke.

The DAMA researchers still have to rule out all possible sources of contamination that might vary throughout the year, says Michael Turner, a cosmologist at the University of Chicago. And researchers from the competing Cold Dark Matter Search housed at Stanford University reported that all of the particles seen in their much smaller detector were probably neutrons. "With respect to the DAMA results," says Stanford physicist Blas Cabrera, "we're ruling out the signal that they've seen."

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