Studies support an accelerating universe.

New findings support the bizarre notion that the universe will not only expand forever but will do so at an ever increasing rate.

Early this year, two teams studying the brightness of a collection of distant, exploded stars--called type Ia supernovas--reported preliminary evidence that the expansion of the cosmos is accelerating (SN: 3/21/98, p. 185).

Although his team has only begun analyzing a dozen or so new supernovas, "the additional data set is reinforcing our conclusion that the acceleration [of the universe] appears to be nonzero," says Alex V. Filippenko of the University of California, Berkeley. Filippenko and his colleagues had initially studied 16 other supernovas.

"We have found no systematic errors that could explain why it is that it looks like we have an accelerating universe," notes Saul Perlmutter of the Lawrence Berkeley (Calif.) National Laboratory, a member of the second team, which has analyzed 42 supernovas. Both Filippenko and Perlmutter reported their latest results on Oct. 29 at a meeting at the University of Chicago on type Ia supernovas.

Type Ia supernovas can illuminate the universe's expansion rate because they all have roughly the same luminosity. The astronomers record each supernova's brightness and redshift, the amount by which cosmic expansion has stretched the wavelength of the light it emits. Redshift also indicates how many billions of years ago the light now reaching Earth left a supernova. The most distant supernovas studied by the astronomers come from a time when the universe was half its current age.

If the universe has revved up its rate of expansion, a supernova at a given redshift would lie farther away than expected, and so it would appear dimmer. That's exactly what both teams continue to find. The supernovas examined are about 15 percent fainter than astronomers can account for in a standard model of the universe with no acceleration.

Because gravity always acts to slow expansion, the findings are forcing theorists to grapple with the existence of an antigravity force or some other exotic source of energy in the cosmos (SN: 2/28/98, p. 139).

Caveats about the findings abound. Astronomers worry that masking by dust, rather than an accelerating cosmos, may explain the supernova results. Another concern is that supernovas in the distant past may not have been as bright as they are now.

Perlmutter reports that his team did an additional analysis in which they discarded the reddest supernovas. Red coloration can be a signpost of fine dust, which absorbs more blue light than red. The scientists still found evidence of an accelerating universe.

Surprisingly, because dust seems to be ubiquitous, Filippenko's group finds that some of the distant supernovas in their survey are less red than those nearby. This doesn't rule out the possibility that large particles of dust, which absorb all wavelengths of light uniformly, cause some of the dimming, critics say.

"I'm reaching the point that I'm beginning to believe the two teams," says Jeremiah P. Ostriker of Princeton University. "In another year, we will know much more, but they've come a long way in the last year."

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