The Dark Side of the Cosmos

WHEN CHARLES ALCOCK PEERS UP at the nighttime sky, he wonders not at the luminous stars but at the blackness that enfolds them. The Milky Way, Alcock knows, is like a sprinkling of bright sequins on an invisible cloak spread across the vastness of space. This cloak is woven out of mysterious stuff called dark matter because it emits no discernible light. A sort of shadow with substance, dark matter dominates the universe, accounting for more than 90% of its total mass. Yet scientists, struggling to interpret just a few sparse clues, know virtually nothing about it. The dark matter could be made up of giant planets, failed stars, black holes, clouds of unknown particles, or even, so far as the laws of physics are concerned, bowling balls. “After all this time and all this effort,” sighs Alcock, head of astrophysics at Lawrence Livermore National Laboratory, “we still don’t know what most of the universe is made of.”

Over the coming decade, Alcock and others believe, this collective ignorance may at last be dispelled. Small bands of determined researchers are embarking on elaborate hunts for the hidden side of the cosmos. Some, using telescopes, are taking aim at the dark halo that rings our galaxy, searching for large, dim objects like burned-out stars. Others are positioning electronic detectors in underground tunnels, hoping to entrap phantom particles that may be so prevalent that they drench the universe like invisible drops of rain. “Someday soon,” predicts University of Chicago astrophysicist David Schramm, “one of these groups is going to strike gold — Swedish gold,” the kind that bears the likeness of Alfred Bernhard Nobel.

A new finding announced last week can only encourage such searches, for it supports the growing conviction that dark matter exists in astonishing abundance. At a meeting of the American Astronomical Society held in Phoenix, Arizona, a team of scientists reported that the dark equivalent of 20 trillion suns lies hidden in a small group of galaxies located millions of light-years from earth. They based their calculation on the recent detection by the Rosat X-ray satellite of a cloud of hot gas that suffuses a seemingly empty region between two of the galaxies. The gas molecules are moving at such high velocities, explains Richard Mushotzky of NASA’S Goddard Space Flight Center, that a “cloud like this would have dissipated into space long ago, leaving nothing for us to detect, unless it was held together by the gravity of an immense mass.” The unseen mass needed to perform this function may outweigh the amount of visible material by an astounding 30 to 1.

If such a ratio prevails throughout the universe, the implications are vast. First, it would mean that there might be so much matter in the universe that the outward expansion ignited by the Big Bang would eventually be counteracted by the force of gravity. The universe would ultimately cease its expansion and begin to collapse under its own weight, imploding in a catastrophic finale that theorists have dubbed the Big Crunch. But the presence of so much dark matter also has implications for the question Alcock ponders: What is all this stuff made of? The more dark matter there is, the less likely it is to resemble ordinary matter.

Dark matter was first postulated in the 1930s by the astrophysicist Fritz Zwicky, who observed that galaxies in the far-off Coma cluster were whirling around one another faster than the laws of physics would allow. They should by rights have been flung out into deep space, unless, as Zwicky contended, the gravity from some massive, invisible substance was holding them in. For decades the idea was rejected as too bizarre. “It smacked of angels dancing on the head of a pin,” recalls theoretical physcist Joel Primack of the University of California at Santa Cruz.

That view has gradually changed over the past 20 years as astronomers became convinced that dark matter not only exists but exists in great quantity. Much of the evidence comes from the kinds of motions Zwicky noted and also from the mysteriously rapid rotation rates of individual star systems, particularly those known as spiral galaxies. Another clue, uncovered largely by AT&T Bell Laboratories astrophysicist J. Anthony Tyson, is the bending of light from distant galaxies. The light is presumably distorted by the gravitational pull of invisible matter.

Just what this mystery matter is made of has been the subject of some truly wild speculation. “The list of candidates,” says Rocky Kolb, a theoretical astrophysicist at Fermi National Accelerator Laboratory near Chicago, “depends on whether or not you believe in a WYSIWYG universe.” WYSIWYG stands for “what you see is what you get” (dark-matter aficionados are inordinately fond of acronyms). WYSIWYG types like to assume that dark matter is most likely made up of the same basic building blocks as ordinary, visible matter: protons, neutrons and electrons. One possibility is that dark matter is nothing more exotic than planet-like objects that are bigger than Jupiter but too small to shine like the sun. Such objects, known as MACHOs (massive compact halo objects), may be orbiting our own Milky Way like swarms of giant bees.

Over the next four years, a 1.3-m telescope on Mount Stromlo, in Australia, mounted with sophisticated digital cameras, will methodically search for MACHOs by peering at stars in the nearby dwarf galaxy known as the Large Magellanic Cloud. If MACHOs exist, explains physicist Christopher Stubbs of the University of California at Santa Barbara, who helped design the experiment, they should occasionally pass between the earth and these background stars. Because gravity bends light, the MACHOs would act as lenses, causing the stars temporarily to brighten enough for the cameras to detect.

But even if MACHOs are found, they are unlikely to resolve the dark-matter conundrum. Physicists have calculated that there is an absolute limit to the amount of ordinary matter in the universe. If dark matter adds up to more than that — as last week’s announcement and other new findings suggest it might — then at least some of the dark matter must be made of something different from the matter we know.

The most obvious candidate is the neutrino, a fast-moving brand of particle that whizzes through the cosmos in great abundance. Though physicists initially flocked to this explanation, there are two considerable drawbacks. First, no one knows if neutrinos have any mass at all, although some recent experiments have hinted that they might. Second, and more important, computer models of a cosmos built largely of neutrinos fail to match up with the universe as we know it. The models imply, for example, that galaxies should have formed relatively recently, while in fact they are very ancient.

As a result, physicists have increasingly come to believe that dark matter — or at least some of it — is made of something no one has ever seen. Santa Cruz’s Primack calls this idea “the ultimate Copernican revolution.” Says he: “Not only will the earth no longer be the center of the universe, it won’t even be made of the same sort of stuff.” The unknown ingredient could be “weakly interacting massive particles,” or WIMPs — sluggish but ubiquitous bits of matter predicted by theoretical physicists. Says University of California, Berkeley, astrophysicist Joseph Silk: “The only thing that’s uncertain about WIMPS is their existence. If they exist, then they are the dark matter.”

This instant, in fact, quadzillions of WIMPs may be streaking harmlessly through our bodies. Alternatively, the mystery matter might be made of “axions,” equally speculative little items predicted by other theories and whimsically named after a laundry detergent. Both are known in the trade as cold dark matter (cold refers not just to their temperature but also to the fact that they move slowly, unlike hot, zippy neutrinos).

Physicists are in pursuit of each of these possibilities. At the Center for Particle Astrophysics at Berkeley, director Bernard Sadoulet and his colleagues are putting the final touches on a contraption designed to catch a WIMP. It consists of a solitary crystal of germanium immersed in a frigid bath of liquid helium; the whole apparatus will eventually be placed underground to screen out more conventional particles. Should a wayward WIMP happen to jostle one of the atoms in the crystal, the impact would create a telltale spike of heat, detectable by delicate sensors. Meanwhile, at Lawrence Livermore National Laboratory, Karl van Bibber and his colleagues are hoping to build an axion trap using a magnetic field 150,000 times as strong as the earth’s.

The success of either group would be cause for celebration, and yet such an achievement would probably solve only a piece of the dark-matter puzzle. In the past five years, astronomers peering deep into the cosmos have discovered huge structures: superclusters of galaxies with names like the Great Wall and the Great Attractor, and empty regions like the Great Void in the constellation Bootes. Mathematical models indicate that such superstructures would be unlikely to exist if all dark matter were cold. The latest thinking: maybe dark matter includes both cold particles like WIMPs or axions and hot stuff like neutrinos; the former would have husbanded ordinary matter into galaxies and clusters of galaxies, while the latter helped create the giant structures.

Such a hodgepodge is considered a cumbersome and ugly solution by many theorists. Says Princeton University’s Jeremiah Ostriker: “It’s like you’re making soup, and you add a little salt, and it doesn’t taste right, so you add a little pepper.” Still, in the confounding world of astrophysics, the simplest and most elegant theories often fail, and there is no reason to assume that the recipe for the cosmos would be bland.