Based on earlier experiments and a preliminary run in 2003, the CDMS team hopes to detect between 6 and 15 WIMPs over the next two years. At the same time, the workers know full well that they may find nothing at all. WIMPs are still a theoretical construct—but right now, they are physicists’ best guess about what glues the universe together.
One of the hocky-puck-size
detectors used in the CDMS
experiment, where it is shielded
from radiation and cooled to
near absolute zero.
”What we’re doing is high risk, but it’s a good place to look,” says Stanford University physicist Blas Cabrera, one of the team leaders. “Of course, it’s always possible that nature chose something else.”
The first hints of dark matter were found as far from the depths of the Soudan Mine as you can imagine: in the Coma galaxy cluster, about 320 million light-years away. In 1933 Caltech astrophysicist Fritz Zwicky observed that outlying galaxies in the cluster are moving so rapidly that the whole group should fly apart. The gravity of all the stars in the cluster is not enough to hold it together, so something else had to be binding the cluster. It didn’t show up in photographs, even in silhouette, but there had to be a lot of this mysterious dark stuff—more than 10 times the mass of all the stars—to keep the Coma cluster from spraying galaxies all over the cosmos.
Zwicky’s explanation was not very popular at the time. “Scientists don’t like things they can’t see,” says the CDMS’s Sadoulet. There was also the problem of Zwicky himself, known for contemptuous sarcasm and vituperative rages that alienated students and colleagues alike. He reputedly derided the astronomers at Mount Wilson Observatory as “spherical bastards” because they looked like bastards from any side. “Fritz Zwicky was ahead of his time,” says University of Chicago astrophysicist Michael Turner, a leading dark-matter researcher. “But he did not play well with others.”
Still, no one could disprove or resolve Zwicky’s “missing matter” problem. And when Vera Rubin, an astronomer at the Carnegie Institution of Washington, showed in the 1970s that there was a matter deficit not only in galaxy clusters but also in individual galaxies, interest perked up. “Today there’s no lack of evidence for dark matter, but not then,” Turner says. “Vera Rubin really made a strong case, and that’s when people began seriously to look at it. Individual galaxies brought the problem much closer to home.”
The next step was to figure out what dark matter was. The only thing scientists knew about it was that it neither emitted nor absorbed light. Maybe it was just large accumulations of dim but familiar objects, like extremely faint red stars or white dwarfs, some astronomers speculated. Then around 1980 a team of Soviet researchers claimed that neutrinos, evanescent particles ubiquitous throughout the universe, might have enough mass to make up the matter deficit. Perhaps this was dark matter.
Further research showed that neutrinos were not the answer, but the Soviet experiment had “changed the conversation,” Turner says. Scientists began to regard dark matter not only as a dilemma for cosmologists but also as a problem in particle physics. During the 1980s cosmologists used other techniques to compute the amounts of matter in the universe more precisely, leading to a growing realization that there were not enough atoms in any form to provide the needed gravity. Whatever dark matter was, it did not consist of stars, gas, or any kind of known object.
In the late 1990s this picture grew even more confusing, when scientists found evidence of another dark entity, called dark energy, that seems to be causing the universe to expand at an ever-accelerating rate. Fortunately, a pioneering space mission called the Wilkinson Microwave Anisotropy Probe (WMAP) clarified this muddle by delivering the first accurate account of the overall makeup of the universe. The answer is decidedly strange. Dark energy makes up 73 percent of the universe, dark matter another 23 percent. Atomic matter—everything around us and everything astronomers have ever seen—accounts for just 4 percent.
“We have an extraordinary claim—that the universe is not primarily star stuff. We want to know what it’s made of.”
Cabrera compares the makeup of the cosmos to a lit Christmas tree. “The lights are the stars,” he says, but the tree—representing dark matter and dark energy—is unseen behind the lights, and no one knows whether it’s a white pine, a spruce, or a cedar. "So here we are with a big problem,” Turner says. “We have an extraordinary claim—that the universe is not primarily star stuff. We want to know what it’s made of.”
Today cosmologists are convinced that the answer lies in physics theory, which predicts the existence of fundamental particles that have not yet been discovered, many with the right attributes for dark matter. Sadoulet became interested in the dark-matter hunt after reading a paper in the mid-1980s suggesting that the particles were massive. If so, he realized, “we could detect them by elastic scattering” of a target nucleus. “That was the first part of the revolution.” A second paper suggested that these particles would have a very slow interaction rate—weak interactions, in other words. “Then the whole thing came together,” he says: weakly interacting massive particles, or WIMPs.
