COBE causes big bang in cosmology.

After searching for nearly three decades, scientiests have uncovered evidence that may solve one of cosmology's oldest riddles: How did primordial matter evolve into the stars, galaxies and galactic clusters we see today?

Instruments on NASA's Cosmic Background Explorer (COBE) satellite have picked up temperature fluctuations in the cosmic microwage background, the ubiuitous energy left over from the creation of the universe. The fluctuations represent tiny gravitational ripples -- variations in the density of matter. "This is like looking at the invisible man and seeing the footprints," says COBE scientist George F. Smoot of the University of California, Berkeley.

Cosmologists believe these ripples unbalanced the primordial universe enough to cause matter to begin lumping together and, after 15 billion years, evolve into the cosmic structures found today. Smoot's team announced its findings last week at an American Physical Society meeting in Washington, D.C.

"It's the missing link," says cosmologist Joseph Silk of the University of California, Berkeley. "The lack of fluctuations has been a major obstacle in having many people accept not just [theories of] galaxy formation but the basic premises of the Big Bang."

In its simplest form, the Big Bang theory predicts that the cosmic microwave background will have a perfectly uniform temperature. Soon after COBE's launch in 1989, the satellite's preliminary measurements indicated that the microwave background was a perfectly smooth 2.73 kelvins (SN: 1/20/90, p.36), which fit with the basic Big Bang theory. Still, this finding puzzled cosmologists because a smooth primordial universe, they believed, couldn't have evolved so quickly into the galaxies and galactic clusters visible today.

Now, after analyzing hundreds of millions of measurements taken during COBE's first year in orbit, Smoot's team has found hot and cold spots in the cosmic microwave background. These spots differ barely thirty-millionths of a kelven from the 2.73-kelvin background. The new data support some Big Bang add-on theories, such as the inflationary model, in which galaxy formation springs from small gravitational disturbances.

But theorists still have their work cut out for them. The COBE measurements suggest that the gravitational ripples probably acted in concert with other, yet unknown mechanisms. "We've seen how strong the gravitational forces in the early universe are, and they're not strong enough to cause ordinary matter to collect in clusters of galaxies," says CODE scientist Edward L. Wright of the University of California, Los Angeles.

Some cosmologists have speculated that cold dark matter, an invisible substance that hypothetically makes up a significant chunk of the universe, provided the extra gravitational push. "The nature of dark matter is still mysterious, but it seems to be required in order to make the structures that we see today," says Wright.

While the COBE findings support the cold dark matter theory, they don't rule out other possibilities. Cosmologists have suggested more exotic models -- involving cosmic strings, for example -- that both fit with the new findings and seem to explain the lumpy cosmos.

COBE is still gathering data, so more concrete answers may arrive soon. "What I think we're going to see is really a breakthrough and a revolution in our understanding of the early universe," says Smooth, "because we're going to have hard facts."