Edge-of-Universe Blast Poses a Cosmic Mystery

Billions of years ago, a tiny region in a faint galaxy near the edge of the universe erupted in a ferocious explosion.

Crossing 12 billion light-years, radiation from the blast and its afterglow finally reached Earth in December, giving astronomers a look at what they now call the most powerful cosmic explosion ever witnessed.

The cataclysm released as much gamma radiation in its first two seconds as the entire universe releases now, researchers say. Conditions inside the fireball briefly rivaled those believed to have existed 1/1,000th of a second after the big bang, which scientists say formed the universe some 15 billion years ago.

"The amount of energy released is staggering. We've never seen this kind of energy released before," says George Djorgovski, an astronomy professor at the California Institute of Technology in Pasadena, Calif., and one of the researchers of an international team studying the explosion.

What the finding means - and whether it will eventually lead to greater understanding about conditions early in the history of the universe - is anybody's guess. A number of theories persist about what might cause such a power burst - such as a neutron star falling onto another neutron star.

BUT even this theory about what causes such events, known as gamma-ray bursters, fails to predict the large amount of energy in the December sighting, he adds.

The team's observations are detailed in today's issue of the journal Nature. Information about the explosion's strength come even as astronomers are progressing in their understanding of the puzzling gamma-ray bursters.

These bursters were first seen in 1967, picked up by satellites that had been launched to verify compliance with nuclear test-ban treaties. The satellites detected sudden flashes of gamma-rays, but operators could find no evidence of nuclear tests. The bursts had originated in space.

In 1991, NASA launched the Compton Gamma Ray Observatory, a satellite that would map the distribution of these events across the sky. It found them evenly spread throughout the cosmos, but it couldn't spot an event with enough precision to allow astronomers to look for the source and determine its distance - a factor critical to determining how much energy the bursters release and what caused them.

That changed with the 1996 launch of an Italian-Dutch satellite, which allowed astronomers to determine the location, and later the distance, of five bursters by charting the bursts' X-ray afterglow.

When astronomers working with the satellite spotted the huge afterglow of the burst in December, they notified colleagues in the United States, who took aim at the spot (located in the Big Dipper) with telescopes in Arizona and Hawaii.

Once the afterglow faded, the team could locate the burster's host galaxy and determine its distance - findings confirmed by the Hubble Space Telescope.

Scientists now know the distances for bursters are measured in terms of billions of light years, says Dale Frail at the National Radio Astronomy Observatory in Socorro, N.M.

That, in turn, has narrowed the number of theories from more than 100 to a handful.

The amount of energy released, he explains, implies a dense, compact source. Indeed, astronomers estimate that the region in which the explosion began was only 100 miles across.

One notion holds that the bursts result from a neutron star falling onto another neutron star or into a black hole. Dropping something as mundane as a ball-point pen onto a neutron star would release as much energy as all of the nuclear weapons ever detonated, says Dr. Frail, because of the intense gravitational field surrounding the star.

But, he continues, the calculations behind that explanation don't yield enough energy to match the amounts seen in December's event.

Instead, a "dark horse" idea has emerged, he says. It holds that the burst resulted from a "hypernova," when a star perhaps 100 times as massive as the sun ran out of fuel and collapsed to become a black hole. Smaller stars, ones with at least 10 times the sun's mass, explode in an event known as a supernova, leaving behind a dense core, or neutron star - and perhaps leaving behind a black hole.

On the other hand, "we could be dealing with something truly new," adds Dr. Djorgovski. "What causes gamma-ray bursters is truly a mystery."