The two groups found, though, that nearby supernovae looked dimmer than they should, implying that the universe was growing faster than expected, speeding up, under the influence of some form of antigravity — perhaps embedded in the fabric of space-time itself.

The results were buttressed by studies of radiation left over from the big bang, which suggested that some two-thirds of the mass-energy of the universe resides in this dark energy.

"But there was always a nagging doubt," Riess told his colleagues on Friday, that dust or some other astrophysical effect was dimming the supernovae, mimicking the effects of acceleration. If that was the case, supernovae even farther away than the ones already observed should be even dimmer than the supernovae nearby.

On the other hand, if it was really an antigravity energy in space, then as space expanded, the push from this dark energy would grow along with it. In the early years of the universe, the dark energy would have been too small to counteract the gravity of the matter in the universe and the expansion would initially have been slowing. Once the universe got big enough, though, the dark energy would dominate and the universe would start to expand.

Riess described the difference between the matter (most of which is dark) and dark energy as follows: "One pulls, the other pushes."

To test which of these ideas was true, Riess and his colleagues had to find supernovae farther in the past than previous surveys had — about 7 billion light-years.

In 2001 Riess and his collaborators found Hubble observations of a supernova 10 billion years in the past. It proved to be anomalously bright, lending credence to the idea that a dark energy had taken over sometime in between.

"But a single object is just not robust enough," he said. For the last year he and his colleagues have used the Hubble in collaboration with a large galaxy survey known as GOODS to find distant supernovae.

"We found lots of weapons of mass destruction," he said, showing Hubble pictures of some of them exploding with the brilliance of small galaxies 8 to 10 billion light-years away.

More important, they were brighter than expected. When he plotted their velocities against distance, or time in the past, Riess found that the universe had to have changed direction, from slowing to speeding up, over a period of time about 5 billion years ago, the so-called cosmic jerk, using the technical term for a change in acceleration.

"It's great to see it," Riess said of the turnaround.