The search for dark matter
An early Christmas present?
Dec 17th 2009
From The Economist print edition
Wild rumours are circulating of the discovery of one of physics’s great unknowns: dark matter
AS The Economist went to press this week, physicists were aflutter about an expected announcement from one of the world’s most important experiments searching for dark matter—the as-yet-undetected material that, if models of the universe are correct, is about six times as abundant as the familiar, visible stuff. Physicists working on the Cryogenic Dark Matter Search (CDMS), a large collaboration whose experimental apparatus is located in Minnesota, will be making presentations on December 17th at Fermilab and the SLAC National Accelerator Laboratory in America, and on December 18th at CERN, the European particle-physics laboratory near Geneva. The speculation is that they will announce the detection of the hitherto unknown particles that make up dark matter. The researchers plan to post their results to the arXiv, an online repository of physics papers, on December 17th, with submissions to a peer-reviewed journal following shortly thereafter.
Around a quarter of the universe is thought to be made up of dark matter, which, as the name suggests, neither gives off nor reflects light. (The balance, once the small amount of visible matter is subtracted, is made of even more mysterious stuff known as “dark energy”.) However, dark matter does make itself known through its gravity. This, indeed, is why astronomers believe it must be there. Some galaxies rotate so fast that they should be throwing off their outermost stars. Only the gravitational pull of these galaxies’ unseen halos of dark matter holds those stars in. Observations of the bending of light around clusters of galaxies, as well as the way that galactic structures formed in the early universe, also suggest that there is much more to reality than meets the eye.
Although the astrophysical case for dark matter is compelling, physicists would like to be able to study it in the laboratory to tease out its true nature. One way to do this would be to produce it in a particle accelerator. When it is running at full strength, the Large Hadron Collider at CERN will certainly try to do this. Another way is to build a detector on Earth that can capture particles of dark matter as they wander in from space. This is what CDMS and other experiments like it have been trying to do for several decades.
The CDMS detector, which is located 700 metres underground in an old iron-ore mine in northern Minnesota, to shield it from quotidian sources of radiation such as cosmic rays that might confuse the detectors, makes use of an array of germanium and silicon crystals that have been cooled to within a whisker of absolute zero. As the Earth makes its way through the Milky Way, it passes through the galaxy’s halo of dark-matter particles. Every so often, one of these particles would be expected to enter one of the crystals and bump into the nucleus of an atom of germanium or silicon. This minuscule nudge will set the crystal vibrating. By listening for these telltale vibrations in the quiet of a cold crystal, the researchers running the experiment should be able to detect the presence of the passing dark matter. At least, that is the theory. However, theory also predicts that such collisions will be exceedingly rare, and observing them requires very large, very sensitive detectors.
Besides locating CDMS deep underground, other precautions have also been taken. Most intriguingly, the shielding around the apparatus is made from lead recovered from an ancient sunken ship. This, because of its age, has already lost most of its radioactivity.
The experiment last announced results almost a year ago. Through careful calibration and diligent work, the researchers running it were able to eliminate most of the residual background sources of radiation that could mimic dark matter. When they examined their data then, nothing was seen in the region where a dark-matter signal would have been expected to appear. The new results, however, are based on twice as much data, so might yield a different outcome.
If the rumours are true, a solution to one of the great problems of physics may now be within reach. If not, CDMS will at least be able to place the most stringent constraints to date on theories that attempt to explain dark matter—proving some of them as baseless as the tales that eager and hopeful physicists tell.