Slower burn. An amateur astronomer captured supernova 1998bu on 2 May 1998, 18.5 days before its peak brightness.

Flicker Found in Cosmological 'Candles'

Astronomers say they have found slight variations among the exploding stars called type Ia supernovae. These explosions, thought to flare up to roughly the same brightness each time, have served the crucial role of cosmic "standard candles" whose apparent brightness, as seen from Earth, can serve as a measure of their distance. Such variability could, in theory, call into question the cosmic measurements based on the explosions--among them the dramatic finding that the expansion rate of the universe is speeding up over time.

Because supernovae are generally not spotted until they are well under way, astronomers had never examined in detail the weeks between a supernova's appearance and its peak. Now Adam Riess of the University of California, Berkeley, and his colleagues have filled that gap by drawing on a robotic telescope, as well as on observations by a team of supernova watchers at the Beijing Astronomical Observatory and by amateurs. The team looked closely at the early phase of 10 nearby type Ia explosions. As the researchers reported at a workshop in Aspen, Colorado, on 17 June, they found that the time it took the explosions to reach their peak brightness was more than 2 days longer than the average for supernovae billions of light-years away as measured by another group.

This delay, if it is meaningful, could indicate that distant supernovae are intrinsically different from nearby ones. And that could raise questions about the most celebrated use of type Ia supernovae: studies of how the expansion rate of the universe has changed over time. But astronomers on the Supernova Cosmology Project, the team that studied the distant supernovae, emphasize that they have not completed their own analysis, let alone published it. "We're in a very early state," says Saul Perlmutter of Berkeley, leader of the Supernova Cosmology Project. That caution is echoed by Philip Pinto, an astrophysicist at the University of Arizona, Tucson: "There is so much uncertainty regarding this result that I don't either believe or disbelieve it."