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Runaway Universe Still Around

CHICAGO--Instead of coasting to a stop under the force of gravity, the expanding universe appears to be picking up speed in its post-Big Bang voyage--a surprising finding reported by two teams of astronomers earlier this year. Now new data presented at a meeting from 29 to 30 October here at the University of Chicago strongly endorse the existence of a bizarre repulsive force puffing up the universe like a big soufflé.

The evidence for the mysterious repulsion comes from exploding stars called supernovae. When seen at great distances, corresponding to earlier times, they are unexpectedly dim, implying that the expansion of the universe has sped up since then, sweeping them unexpectedly far away. One by one, participants wrote off possible confounding factors--such as obscuration by cosmic dust or errors estimating a star's distance--that could spoil this scenario. Nothing emerged to explain away the results. If the findings continue to hold up, the universe may have a mysterious background energy permeating empty space, known as the "cosmological constant," or "lambda." "I personally very much dislike the cosmological constant," said Mario Livio, an astrophysicist at the Space Telescope Science Institute in Baltimore. "But given the existing observations, there is a lambda."

Both of the international groups that authored the original finding, the High-z Supernova Search Team and the Supernova Cosmology Project, also presented new data at the meeting to bolster their claims. The Supernova team has now fully analyzed 42 distant supernovae with more in the pipeline, while the High-z team is now adding about a dozen new events to the 16 it has already published. The new data reinforce earlier estimates that about 30% of the universe's energy is in the form of matter and 70% in the mysterious lambda. The two groups are highly competitive, but "you will see that we are in remarkably violent agreement" about the results, joked Supernova Cosmology Project head Saul Perlmutter, an astrophysicist at Lawrence Berkeley National Laboratory and the University of California, Berkeley.

Both teams will now be reaching for more distant supernovae to see whether earlier in cosmic history--when the same amount of matter was crammed into a smaller universe--gravity reined in the boost of the cosmological constant and squelched the acceleration, as theorists predict. Perlmutter's team took the lead last month, discovering what is probably the most distant supernova yet using the 10-meter Keck Telescope in Hawaii. By now the groups have found so many supernovae that Perlmutter's team has taken to naming them after composers--instead of numbering them--to keep things straight. The most distant, roughly 8 billion light-years away, has been dubbed Albinoni after the 18th-century Italian composer.