Like doubting disciples, certain astronomers spend much of their time looking for proof of an unseen hand at work in the cosmos. In 1933, a Swiss astronomer named Fritz Zwicky proposed that some otherworldly presence, massive but invisible, holds the visible world together; without it, he said, galaxy clusters would fly apart, their edges spinning off into space like peanuts tossed from a carousel. Unlikely as it seems, Zwicky's hypothesis has grown more convincing with time. Astrophysicists now know that Zwicky's missing matter is fundamentally different from the ordinary matter that makes up planets and people. It doesn't interact with light, for example, either by radiating or blocking it. And it couldn't have much interaction with everyday matter either, or its existence would be far more obvious. It betrays its presence only by the gravitational pull it exerts on the shapes and movements of galaxies.
Today the missing matter is known as cold dark matter: "cold" because by subatomic standards it is sluggish, "dark" because it cannot be detected with even the most sophisticated telescopes, and "matter" because it isn't energy, which is the only other option. The latest studies of galaxy clusters and theories of astrophysics predict that cold dark matter makes up more than 90 percent of all the matter there is. Experts have come to believe that illuminating its properties could help explain galaxy formation, unify the fundamental forces of nature, and maybe even determine the fate of the universe. And just as the earliest fossils hint at the origin of life, primitive dark-matter particles, born at the dawn of time, might reveal clues about the origin of the cosmos.
"If they exist," says Stanford physicist Blas Cabrera, "they would be the oldest stable particles in the universe."
The search for dark matter has Nobel prize written all over it. So it's little wonder things got a little testy in February at an international dark-matter symposium in Marina del Rey, California. There two teams of researchers presented results from pioneering attempts to nab the theoretical particles that are thought to constitute the bulk of dark matter. The experiments shared somewhat similar strategies for particle detection; whether or not the missing matter was found depends on whom you ask.
Dark matter by definition interacts only weakly with ordinary matter, including the instruments designed to apprehend it. Because of that, the detectors must be shielded from cosmic rays and other incoming particles that would obscure the fainter signals from dark matter. As it happens, dirt and bedrock make good shields: A detector located more than half a mile underground in a zinc mine in Japan collared another type of dark-matter particle two years ago. But the Japanese experiments showed that those particles, called neutrinos, don't have nearly enough mass to account for all the dark matter predicted either by the theories of physicists or the observations of astronomers. So dark-matter enthusiasts trained their hopes on finding more-massive particles. According to their theories, the postulated WIMPs— for "weakly interacting massive particles"— might be detected during rare collisions with atoms, which would produce vibrations and bursts of light and heat as the nucleus recoils.
Both groups reporting at the California conference looked for evidence of nuclear recoils by using underground detectors. But they differed in how they screened out extraneous signals that can penetrate even to subterranean depths. One team, working at the Gran Sasso National Laboratory in L'Aquila, Italy, studied seasonal variations in the number of recoils observed in sodium iodide crystals to distinguish possible WIMP collisions. Unlike other particle streams, earthbound WIMP flows are expected to vary between winter and summer because as Earth circles the sun each year, it changes direction relative to the solar system's rotation around the center of the galaxy. If, as physicists believe, the entire Milky Way itself is shrouded in a fixed and imperturbable cloud of WIMPs, then more WIMPs should hit Earth when it is moving through the WIMP cloud in the same direction as the rest of the solar system— just as more bugs hit the windshield when you step on the gas. But when Earth's orbit runs counter to the solar system's, as it does in winter, its speed through the WIMP cloud decreases, and so should the number of WIMP collisions.
Rita Bernabei and her colleagues from the University of Rome and the Chinese Academy in Beijing presented evidence that they had indeed found seasonal modulations in recoil events. The tens of thousands of recoils they documented over four years peaked in the summer and fell off in the winter by a small percentage, they say, and the modulation satisfied several other criteria for identifying WIMP-induced events. "This definitely shrinks [the probability of] spurious effects," the group claims on its Web site.
But new results from the detector at Stanford University challenged this conclusion. One of the most significant "spurious effects" that might crop up in dark-matter detection comes from neutrons— ordinary subatomic particles that are generated by cosmic rays. The Stanford detector, which consists of silicon and germanium disks, monitors the background of neutron-induced nuclear recoils so they can be subtracted from the total signal. "If there were WIMPs at the rate suggested by the [Italian team's] signal, then in our detector we would have seen about 20 events over the period of time we ran," says Cabrera, one of the helmsmen of the Cryogenic Dark Matter Search, a collaboration that includes 10 institutions. Instead, the Stanford detector recorded only 13 recoils, he says, and all of them could be attributed to neutrons.
The discrepancy provoked a lively debate at the California symposium but no claims to have cornered truth. The properties of cold dark matter are so poorly understood that any alleged WIMP sightings are best considered exploratory rather than definitive. Both data sets could be correct, for example, if WIMPs have unforeseen interactions that differ among germanium, silicon, and sodium iodide. And even though the Stanford experiment failed to support the Italian findings, Cabrera notes, those findings are within the range predicted by theory. "It's certainly quite possible that the rate of events was real," he concedes.
The jury will stay out until any one lab's findings are confirmed independently and repeatedly by others. But cosmologists are greeting even these initial, contradictory results with eager anticipation.
"The important story is not that these two experiments disagree but that a threshold has been crossed: We now have apparatuses with enough sensitivity to detect [WIMPs]," says Michael Turner, a cosmologist at the University of Chicago. That, he says, is a major accomplishment in itself. Both the Gran Sasso and the Stanford groups have plans to stretch the limits of detection: The Italian team is more than doubling the size of its crystal detector, and next year the Stanford group will bury its detector 2,600 feet deeper, in a defunct iron-ore mine in Minnesota. Meanwhile, at least half a dozen additional dark-matter detectors are ramping up around the globe. With so much activity, Turner maintains, physicists are sure to capture some elusive, elementary particle, whether predicted or not.
And when they do, they may discover the secrets of time and space alike. Turner shares with many of his colleagues the hope that the elucidation of dark matter will complement recent developments in astronomers' efforts to chart the shape and rate of expansion of the universe. If dark matter was in fact spawned by a budding universe, then it holds clues to the forces present at the moment of time's conception. The properties and distribution of dark matter may help explain the curious clumping of galaxies that grew from a relatively smooth field of infant matter. And the density of dark matter may decide whether and how fast the universe goes on expanding into eternity. Whatever it is, cold dark matter is likely to have profound implications for the origin, evolution, and eventual fate of the cosmos.
"We've been struggling for 70 years and now we're closing in," says Turner. "We may soon learn what holds the universe together." And we may soon learn more about what sets us apart— the world we've known until now, with its quaint arrangements of atoms and molecules, people and planets, gluons and galaxies— all bit players in a vast theater of strange, inscrutable otherness. The biggest surprise of the dark-matter saga may be that "ordinary" matter is quite extraordinary, after all.
Learn more about the search for dark matter athttp://cdms.physics.ucsb.edu