New supernova goes the cosmic distance.

Astronomers have discovered a supernova more distant than any previously known. This exploded star lies about 9.5 billion light-years from Earth. Dubbed Dudley Doright by its discoverers, the supernova could be one of the shining lights that will test the astounding finding, reported last year, that the expansion of the universe is accelerating.

A team including John L. Tonry of the University of Hawaii in Honolulu and Nicholas B. Suntzeff of the Cerro Tololo Inter-American Observatory in La Serena, Chile, announced the discovery of the supernova--officially designated SN 1999fv--in a Nov. 19 circular of the International Astronomical Union.

The discovery team, as well as another group, has used this type of supernova to probe the fundamental nature of the universe. Members of this supernova group, classified as la by their composition, are known as standard candles because they have the same intrinsic brightness in/ both nearby and distant galaxies.

Because light from a faraway galaxy takes several billion years to reach Earth, astronomers observe such a galaxy as it appeared when the universe was younger. If cosmic expansion had recently slowed, there would be less distance between Earth and a remote galaxy than if the expansion had proceeded at a constant speed. A supernova in such a galaxy would therefore look brighter than if the expansion had been constant.

Early last year, the two teams announced that they had found exactly the opposite. Distant supernovas looked about 20 percent dimmer than expected, indicating that the universe has over the past few billion years revved up its rate of expansion (SN: 12/19&26/98, p. 392).

It's possible, though, that the two teams were fooled. Intervening dust could have made the supernovas look dimmer, or the more distant ones might have a slightly different composition than nearby supernovas, causing them to appear fainter.

Extremely distant supernovas provide a means to settle these conundrums, Suntzeff notes. That's because these supernovas can reveal whether the expansion of the universe had decelerated when it was very young. The youthful cosmos was so dense that the gravitational tug of matter would have dwarfed any antigravity term that astronomers have invoked to explain why cosmic expansion later sped up.

Dust or compositional differences could not mimic both deceleration at early times in the universe and acceleration at more recent times, astronomers say. By finding a large sample of supernovas that lie more than 10 billion light-years from Earth--no small feat--researchers in as few as 2 years might test whether cosmic acceleration is genuine, Suntzeff notes.

Studying extremely distant supernovas "is one of our best near-term bets," agrees Philip A. Pinto of the University of Arizona in Tucson.