Reports in mid-February that a team of Italian and Chinese researchers had finally detected the source of dark matter raised hopes that the identity of this elusive material had finally been discovered. Yet almost as soon as the ink was dry on those initial reports, a team of American scientists announced that its experiment, which is just as sensitive, had failed to corroborate the dark matter detection.
"There is a great deal of interest in the Italian results because it would be extremely important if they had discovered most of the material of the universe," says David Caldwell, professor emeritus of physics at the University of California at Santa Barbara and a founder of the U. S. team. "However, our results do not confirm their observation."
Almost every astronomer agrees that most of the universe is made up of dark matter. This elusive material radiates no light, or at least so little that it can't be detected. Astronomers know it exists because galaxies behave as if they had a gravitational pull some ten times stronger than their visible mass accounts for. Leading theories about the formation of the universe also suggest that visible matter makes up only a small fraction of the total matter in the cosmos. In essence, at least 90 percent of the stuff in the universe can't be seen.
The nature of this material remains a mystery. Some scientists think much of it consists of ordinary matter. Isolated black holes, for instance, would not give off any detectable radiation. Less exotic objects like white dwarfs, brown dwarfs, or Jupiter-sized planets would radiate so little light that they couldn't be detected even in they filled the halo of the Milky Way Galaxy.
Other researchers bet that exotic elementary particles make up the bulk of the dark matter. One variety, called the neutrino, has been detected -- although no one has proved that neutrinos have mass. On the other hand, some physicists think that Weakly Interacting Massive Particles (WIMPs) -- hypothetical particles that only rarely interact with ordinary matter -- could be the dominant form of dark matter. In reality, both ordinary and exotic matter undoubtedly contribute.
Both the Italian-Chinese and American teams are searching for WIMPs. If they exist, WIMPs should permeate the galaxy and Earth would constantly plow through them. As Earth revolves around the sun, however, the speed at which the WIMPs rush by will vary -- changing the detection rate. It was this seasonal variation that the Italian-Chinese group, called DAMA for Dark Matter Experiment, claimed to see.
The Cryogenic Dark Matter Search (CDMS) involves 55 scientists from 10 U. S. institutions. It attempts to find WIMPs with heavily shielded and supercold detectors located 30 feet underground at Stanford University in California. The CDMS can distinguish background events resulting from known particles interacting with the detector from those that are likely to be dark matter interactions.
CDMS scientist Harry Nelson of UC Santa Barbara says "Our experiment is smaller but better able to distinguish the dark matter from garden varieties of radioactivity." And CDMS has yet to see dark matter -- at least at anywhere near the levels reported by the Italian experiment.
Both groups plan to refine their experiments in the near future. In 2001, CDMS will expand their detectors from a mass of 0.6 kilogram to 10 kilograms and move from the Stanford tunnel to a deep mine in Minnesota. The rock in that mine will protect the experiment from cosmic rays that can cause a spurious dark matter signal. So the search will go on, and presumably one day resolve this ongoing mystery.
3/6/00
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