Seemingly in defiance of common sense, the cosmos appears to be permeated by a repulsive force that is counteracting gravity on large scales. That is the reluctant conclusion of an international team of astronomers who have used distant exploding stars called supernovae as survey markers for gauging how the expansion of the universe has changed over time. Gravity should have gradually slowed the outward rush, but as the team announced last week at a small astronomy meeting in Los Angeles, their measurements imply that the expansion has actually sped up in the billions of years since the stars exploded.
Theories of the big bang had led astronomers to expect that the universe contains a specific density of mass, just enough to slow cosmic expansion gradually to a halt. But when earlier observations did not turn up enough matter, they began to speculate about a mysterious background energy--the so-called cosmological constant--that might make up the deficit. Physicists do not have a good explanation for the energy. But astronomers thought they might be able to detect it from its effect on cosmic expansion, because it would push rather than pull on the universe, speeding up the expansion over time.
The High-z Supernova Search Team, which has members in Chile, Australia, the United States, and Europe, went looking for signs of an acceleration by studying supernovae at enormous distances, corresponding to times as much as halfway back to the big bang. All of these explosions are thought to have roughly the same intrinsic brilliance, so they can serve as approximate "standard candles," whose apparent brightness is a measure of their distance from Earth. The 14 distant supernovae in the study turned out to be, on average, 10% to 15% farther away than they would be even in a low-density universe, in which the expansion would have slowed very little. "Not only don't we see the universe slowing down; we see it speeding up," says Adam Riess of the University of California, Berkeley, lead author on the paper being prepared on the results.
"My own reaction is somewhere between amazement and horror," says Brian Schmidt of the Mount Stromlo and Siding Spring Observatory in Australia, who leads the group. "Amazement, because I just did not expect this result, and horror in knowing that [it] will likely be disbelieved by a majority of astronomers--who, like myself, are extremely skeptical of the unexpected."
Other astronomers do express caution, saying there could be still-undiscovered differences between galaxies now and billions of years ago--and hence in the brightness of the supernovae they host. But just last month, another group of astronomers presented supernova data that pointed, more tentatively, to the same effect. Says Rocky Kolb, a cosmologist at the University of Chicago, "Even the most conservative explanations for the results are quite amazing."
* For more details, see the News  story in tomorrow's issue of Science.