Bang! A Big Theory May Be Shot

By MICHAEL D. LEMONICK

Astronomers trying to piece together the universe's past have two major pieces of evidence with which to work. The first is that the whole thing began with a Big Bang, an explosion of unimaginable heat and power, between 10 billion and 20 billion years ago. The second is that the modern-day cosmos is made up of galaxies. Gravity presumably played a role in the process, but the details are unknown.

For the past decade or so, the best scientific guess about the evolution of the universe has been the cold-dark-matter (CDM) theory, which holds that an exotic, unseen form of matter helped create the galaxies. But a new study of the universe's structure, reported in last week's issue of Nature, puts that hypothesis in deep trouble.

Scientists have long known that some kind of dark matter exists. One clue is that many galaxies spin so fast that they should fly apart; the gravity from some unseen extra matter must be holding them together. Studies indicate this material surrounds the Milky Way galaxy in a roughly spherical halo. In regions of the universe where galaxies are clustered, dark matter seems to pervade the space within the clusters. Calculations suggest there is about 10 times as much dark as visible matter. That means that the gravitational pull of dark matter is 10 times as strong. Thus, it must have played an important role in the formation of the universe.

In recent years scientists decided that dark matter is probably made of "cold" (in astronomical jargon, that means slow-moving) subatomic particles. According to theorists, dark matter would have formed sooner after the Big Bang than ordinary matter did. The dark matter would have created pockets of high density whose gravity would then have pulled in the later- forming ordinary matter. These pockets would eventually grow into galaxies, and many of the galaxies would drift together into clusters -- just the state of the universe today.

But the Nature report may have delivered a fatal blow to the theory. British and Canadian astrophysicists, reanalyzing data taken in 1983 by the Infrared Astronomical Satellite, found that superclusters of thousands of galaxies, interrupted by voids some 200 million light-years across, are common in the visible universe. Scientists do not believe the force of cold dark matter alone could have worked fast enough to create structures so large. Even 20 billion years is not enough time for thousands of galaxies to have clumped together in the way the theory says.

For the CDM hypothesis to survive this crisis would take such complicated physics that the cosmos would have to operate like a Rube Goldberg machine. For the most part, though, nature follows simple rules. So while cold dark matter may exist, astronomers are beginning to search elsewhere to solve the mystery of how the galaxies were born.