Researchers have known for five years that the universe is expanding at an ever-increasing pace, driven outward by a force no one understands. They've labeled this repulsive thing antigravity, or dark energy.

At the 202^nd meeting of the American Astronomical Society, held in this city that values racing almost as much as country music, one scientist likened cosmic acceleration to the initial moments of the Indianapolis 500, held yesterday in another racing-crazy city.

"Right now, the universe is speeding up, with galaxies zooming away from each other like Indy 500 racers hitting the gas when the green flag drops and the pace car gets out of their way," said Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics. "But we suspect that it wasnt always this way."

When the universe was smaller and more dense, regular matter and dark matter (another mysterious thing) would have tended to apply the brakes to the universal propensity to grow. At some point, however, the brakes failed.

"Dark energy has the upper hand today," Kirshner said. "But it looks like dark matter was in fact winning the battle, this sort of cosmic tug-of-war, about 7 billion years ago."

Kirshner describes the wacky cosmology as a stop-and-go universe.

Theoretical models suggest that prior to 6.3 billion years ago, the expansion would have been decelerating -- still expanding, but at a decreasing pace for a stretch of time. (This would have followed an initial period of rapidly accelerating expansion fueled by the Big Bang.)

Kirshner's group found and examined four exploded stars, called supernovae, as they existed about 7 billion years ago. The results, which also considered data on supernovae gathered by other teams, were in line with the new theory.

"It's a hint of deceleration," Kirshner said. The study was led by John Tonry at the University of Hawaii. More research will be needed to confirm the deceleration and pin down when it began and ended.

The findings build on similar results reported by SPACE.com last month.

Tonry's group, the High-z Supernova Search Team, is one of several pursuing a particular type of exploded star called a Type 1a supernova. These objects all shine with a known intrinsic brightness, so astronomers use their observed brightness and other characteristics of their light to determine how far away they are and what the universe was doing when the light was unleashed.

As the light travels to Earth, the wavelengths are stretched by an amount that reflects the stretching of the universe.

Also at the meeting, Vanderbilt University's Robert Knop presented new results on 11 distant supernova from the acceleration era, studied by the Supernova Cosmology Project (SCP). Detailed examination with the Hubble Space Telescope confirms the initial five-year-old results that cosmic expansion is indeed accelerating.

As supernova hunters find more of these objects even farther back in time, they expect to pin down the behavior of dark energy. Saul Perlmutter, a Lawrence Berkeley National Laboratory research who leads the SCP, said it is not clear whether the changing density of the universe is the only factor that allowed dark energy to take over. Perhaps, he said, the basic properties of dark energy have changed over time.

If that's the case, everyone agreed, then an unknown force just gets more puzzling.

Figuring out the past will help cosmologists theorize better about the fate of the universe.

Any racing fan knows that the acceleration spurred by a green flag can't last forever. Many astronomers now doubt the universe has such limits, however.

The acceleration is likely to continue, most of them believe, until galaxies recede from one another at the speed of light and can no longer be seen. One new theory goes further, suggesting that every speck of matter in the cosmos will ultimately be torn apart in a universe-ending Big Rip.

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