The cosmos' fate: world without end.

Will the universe expand forever, or will it eventually contract into a Big Crunch? Observing the fireworks from distant, exploding stars, two independent research groups have found evidence suggesting that the cosmos will balloon indefinitely.

The density of matter may simply be too low for gravity to halt the expansion that began with the birth of the universe--the Big Bang--some 15 billion years ago. Although other studies over the past several years have hinted at the same conclusion, the technique employed in the two new reports is a particularly promising method of determining the fate of the cosmos, researchers say.

If further observations support the notion of perpetual expansion, cosmologists may have to rethink their options. The widely accepted theory known as inflation explains why the structure of the universe looks the same in all directions, but it also predicts that the cosmos has exactly the right density to bring expansion to an eventual halt (SN: 6/7/97, p. 354). Reconciling this theory with endless expansion may require cosmologists to resurrect the so-called cosmological constant--an antigravity term in the equations of general relativity.

The new results are based on measurements of the brightness and recession velocity of a special class of supernovas, or exploded stars. Known as type la, these supernovas are the brightest in the cosmos and can be seen from great distances. Moreover, they all have about the same intrinsic brightness.

The most distant la supernova observed with the Hubble Space Telescope provides a glimpse of the cosmos when it was half its current age. The rate at which the universe was expanding in the past determines how far away a supernova with a given recession velocity lies. By comparing observations of faraway supernovas to those of nearby ones, astronomers can infer the rate at which cosmic expansion may be slowing down.

Saul Perlmutter of the Lawrence Berkeley (Calif.) National Laboratory and his colleagues find little or no sign of deceleration, they report in the Jan. 1 NATURE.

Their analysis of five nearby supernovas and a newly discovered distant one suggests that the universe will expand indefinitely. A preliminary analysis of about 35 additional supernovas, both nearby and distant, to be presented next week at a meeting of the American Astronomical Society in Washington, D.C., supports that finding, Perlmutter says.

Observations of three other distant supernovas, to be described by Peter M. Garnavich and Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and their colleagues in an upcoming ASTROPHYSICAL JOURNAL LETTERS, yield a similar conclusion.

"We cannot make much of a conclusion from a single farthest supernova... but when we average it with several others, we find, to a 95 percent level of confidence, that the density of matter is insufficient to halt the expansion of the universe," Garnavich says. In a year or two, adds Kirshner, when both teams will have completed studies of several dozen supernovas, the fun will begin.

"This is a hard subject, and the data do not [yet] constrain the imagination very much," says Kirshner. "What's new here is that we're beginning to show that we have the potential to [distinguish among models of the cosmos] and that it isn't just going to be a matter of what you like, what you think is beautiful. It's going to be a matter of looking into the real answer book of nature and finding out what it says."

"It's very exciting--not from the results, but from the promise of what's in the future," says Michael S. Turner of the University of Chicago and the Fermi National Accelerator Laboratory in Batavia, Ill.