By Peering Back into the Beginning of Time, a Satellite Finds

By MICHAEL D. LEMONICK WASHINGTON

THEY WERE, BY FAR, THE LARGEST and most distant objects that scientists had ever detected: a swath of gargantuan cosmic clouds some 15 billion light-years from earth. But even more important, it was the farthest that scientists had ever been able to peer into the past, for what they were seeing were the patterns and structures that existed 15 billion years ago. That was just about the moment -- or more precisely, an infinitesimal 300,000 years after the moment -- that the universe was born. What the researchers found was at once both amazing and expected: NASA'S Cosmic Background Explorer satellite -- COBE -- had discovered landmark evidence that the universe did in fact begin with the primeval explosion that has become known as the Big Bang.

In anticipation of the announcement, an overflow crowd had crammed into the meeting of the American Physical Society in Washington last week, and they were not disappointed. "If you're religious, it's like looking at God," proclaimed the leader of the research team, George Smoot, an astrophysicist at the University of California, Berkeley. Princeton astrophysicist David Spergel, who had recently co-authored a theory that was demolished by the COBE results, cheerily admitted, "We're dead. But this is great stuff . . . It's the most important discovery in cosmology in the past 20 years."

The existence of the giant clouds was virtually required for the Big Bang, first postulated in the 1920s, to maintain its reign as the dominant explanation of the cosmos. According to the theory, the universe burst into being as a submicroscopic, unimaginably dense knot of pure energy that flew outward in all directions, spewing radiation as it went, congealing into particles and then into atoms of gas. Over billions of years, the gas was compressed by gravity into galaxies, stars, planets and, eventually, even humans.

The first evidence of this scenario was established in 1964, when astronomers discovered the cosmic microwave background, the original radiation from the Big Bang. The second part, though, was much trickier. In order for gravity to make galaxies out of atoms, it needed something to work with -- some chunks of space in which the atoms were closer together, a region of greater than average density, so that they could draw surrounding matter in. The excess densities need not have been very large, but they had to be there if matter was to congeal. And if they were present, they should be visible to a sensitive enough probe in the form of warm and cool spots mottling the microwave background.

The COBE satellite was designed to be sensitive enough, but the first maps of the microwave sky it beamed down showed nothing. That was not a big problem. The research team knew that the cosmic microwaves are polluted with local microwaves from the Milky Way galaxy and that it would take months of computer analysis to weed out the unwanted signals.

In the end, it took more than a year. What finally appeared on the computer screens at the Goddard Space Flight Center in Maryland was a map with blotches of all sizes indicating regions of the sky where the microwaves are a minuscule 30 millionths of a degree warmer or cooler than average -- almost imperceptible, but enough to save the Big Bang theory. Says University of California, Berkeley, astronomer Joseph Silk: "They've found the missing link. This removes the biggest remaining objection to the Big Bang."

Because the microwaves have been traveling for 15 billion years to get to the COBE sensors, the warm patches have long since evolved into groups of galaxies. Even the smallest patch observed by COBE is by far the largest area ever surveyed. The structures dwarf the "great wall" of galaxies discovered in 1990. The largest spans one-third of the known universe, or 10 billion light-years, which is 60 billion trillion (60 followed by 21 zeros) miles.

COBE is designed to see just the biggest structures, but astronomers would like to see much smaller hot spots as well, the seeds of local objects like clusters and superclusters of galaxies. They shouldn't have long to wait. Astrophysicists working with ground-based detectors at the South Pole and balloon-borne instruments in the stratosphere are closing in on such structures, and may report their findings soon.

If the small hot spots look as expected, that will be a triumph for yet another scientific idea, a refinement of the Big Bang called the inflationary universe theory. Inflation says that very early on, the universe expanded in size by more than a trillion trillion trillion trillionfold in much less than a second, propelled by a sort of antigravity. Bizarre though it sounds, cosmic inflation is a scientifically plausible consequence of some respected ideas in elementary-particle physics, and many astrophysicists have been convinced for the better part of a decade that it is true.

One prediction that comes out of the theory of inflation is that the mix of big and small hot spots in the early universe should follow a characteristic pattern. The spots COBE found conform to that pattern, and scientists like Smoot expect that the smaller hot spots will too. Another prediction of inflation is the surprising notion that everything astronomers can see, including all the stars and galaxies, constitutes just 1% of existing matter. The other 99% of the universe is dark and invisible.

There is already strong evidence that at least some dark matter must exist. The Milky Way and virtually all other galaxies rotate so fast that they should literally fly apart -- unless the gravity from invisible halos of dark matter is holding them together. The halos still fall short of what inflation requires, but bolstered by the latest results, theorists are sure the rest will be found. Says Paul Steinhardt, a University of Pennsylvania physicist who helped develop inflation: "We were confident in our theory, of course, but it's always nice to know that Nature is cooperating."