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Supernovas Light Up Cosmic Growth Spurt

15 May 1998 6:30 pm
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Our corner of the universe may be expanding slightly faster than the universe as a whole, according to unpublished work described in a Research News story in today's Science. By analyzing the brightness and motion of distant, exploding stars, astronomers have also picked up clues to the nature of the mysterious energy that appears to be filling space and causing cosmic expansion to accelerate.

The new findings rely on exploding white dwarf stars. Because these supernovae all blow up with nearly the same brightness, their apparent brightness as seen from Earth can be translated into distances. The supernovae can be seen across most of the visible universe, at distances corresponding to earlier times in cosmic history. By plotting the distances of the supernovae against the speed at which they are rushing away from Earth--easily found from the redshift, or stretching, of their light--astronomers can see how cosmic expansion has changed over time.

Idit Zehavi and Avishai Dekel of The Hebrew University in Israel, and, independently, Adam Riess of the University of California, Berkeley, noticed a slight kink in the plot at a few hundred million light-years or so from Earth. They say the kink may indicate that our region is expanding about 6% faster than the universe at large. The researchers think the gravitational pull of large clumps of galaxies at the edges of our cosmic neighborhood might be tugging local galaxies outward, speeding up the expansion and resulting in a more tenuous region in which our own galaxy sits.

Meanwhile, Peter Garnavich of the Harvard-Smithsonian Astrophysical Observatory and his colleagues are looking much farther out along the supernova diagram. The exact shape of its subtle curve should reflect just what kind of energy is boosting the expansion over large scales. The group is comparing the observed curve to the curves expected from the unchanging push of the so-called cosmological constant and from the more varied shoves of other forms of background energy, which theorists have named quintessence or X-matter.

So far, says Garnavich, the unrelenting push of the cosmological constant fits the data best. He and his colleagues caution that the result is preliminary. But Saul Perlmutter of Lawrence Berkeley National Laboratory and the University of California, Berkeley, says the new raft of findings testify to the power of supernovae as cosmic probes. "We have a tool that can be used to approach cosmology from another angle," he says.

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