Massive Blast in Deep Space Puzzles Experts
Tracking a colossal burst of energy to a faint galaxy at the far edge of the cosmos, Caltech astronomers reported Wednesday that the blast was the most powerful explosion since the birth of the universe--big enough to outshine all the stars in the sky.
If their calculations are correct, the 40-second flash, recorded last December, spewed out as much energy as 5 billion exploding stars. “It has more energy than even astronomers thought in their wildest imaginations,” said Caltech astronomer Shri Kulkarni, one of the researchers.
At this point, astronomers can only imagine what it is that exploded. One possible explanation for the discovery--reported in today’s issue of the journal Nature and announced at a NASA news conference--features giant black holes wolfing down matter and then spitting it out in enormous cosmic burps.
It was barely a year ago that astronomers first followed one of these so-called “gamma ray bursters”--an outpouring of extremely high-energy light not visible to the human eye--to its lair in a galaxy between 2 billion and 7 billion light-years away. This new burst--which was some 12 billion light-years away, and therefore occurred 12 billion years ago--may be as much as a hundred times brighter.
“I was astounded when I heard these results,” said theorist Stan Woosley of UC Santa Cruz. “At first I could hardly believe them. . . . It’s as bright as if you took all the bright objects in the visible universe and put them together at that distance.”
While the intensity of the light is staggering, according to astronomers, that in itself is not as difficult to explain as the fact that it was packed into an extremely brief explosion--like taking 50 years worth of sunshine and cramming it into one second.
“It’s not enough [merely] to produce the energy,” said Kulkarni. “[The explosion] needs to produce it very quickly.” Kulkarni and others felt that explaining the burst might require new physics. “It certainly [challenges] all existing theories,” he said. However, others, such as Princeton University astrophysicist John Bahcall, thought the burst might be explained by currently understood, if extreme and bizarre, events--such as collapsed stars falling into massive black holes.
Unexpected Discovery
Blasts of gamma rays go off somewhere in the universe at the rate of approximately once per day. They were unknown until the late 1960s, when U.S. spy satellites looking for Soviet nuclear blasts serendipitously picked them up. However, since the bursts last only seconds before fading, astronomers could not determine their source--and therefore, how far away they are.
The turning point came last year, when a new Italian satellite started alerting astronomers worldwide to the bursts quickly enough for the scientists to chase them down. When the December burst was picked up, astronomers zeroed in on its precise location.
Once the afterglow of the burst faded, they found a faint galaxy, Kulkarni said. Using the powerful Keck II telescope on Mauna Kea in Hawaii, they were able to analyze the light from the galaxy, putting its probable location at 12 billion light-years away.
In order to produce the light they detected from such a great distance, it would have to be more energetic than anything ever detected in the universe.
Theoretically, the blast could have spewed light in all directions, like a star, or concentrated it in a narrow beam, like a flashlight. The former would require much more energy, produced by a greater explosion.
If such blasts took the form of focused beams, on the other hand, it would mean Earth-orbiting satellites would see only bursts aimed toward our planet. Hundreds more bursts in other directions would go undetected.
Either case, said Kulkarni, would have still amounted to at least a 10-times bigger bang than anything seen before. In their calculations, however, Kulkarni and his colleagues assumed that the most likely scenario involved the spreading out of energy in all directions. “If you have an enormous explosion, it’s hard to channel things into a tight cone,” said Caltech theorist Roger Blanford.
Until now, the biggest known explosions were supernovae, or exploding stars. Supernovae release most of their energy in virtually undetectable particles called neutrinos.
Gamma rays, in contrast, are an intense form of light--more intense even than X-rays. Producing them probably requires some kind of electric dynamo in space, Kulkarni said. Most likely, the source is some kind of rotating black hole.
Converting so much energy so quickly, the black hole would have to be “fed” matter from another source. Woosley proposed a scenario in which a neutron star--or remnant of a collapsed star--orbits a bit too closely around a black hole. As the neutron star gets sucked in by the black hole’s enormous gravity, it would be “torn to pieces,” he said. Those pieces would then swirl into the black hole like dirt going down the bathtub drain.
The problem, Woosley said, is how to turn such an enormous influx of matter into a giant explosion in a matter of seconds. Such alchemy would require almost perfect efficiency. “That efficiency should be close to 100% is astounding,” he said. It would be analogous to converting one-sixth the mass of the sun into pure energy in seconds, he said.
Although the idea that the bursts were created by something falling into black holes in the centers of galaxies also appealed to Blanford, he said the bursts might also imply that there’s “some completely new sort of physics that’s out there to be understood.”
Another possibility is that the bursts are somehow associated with star formation. The galaxy that produced the December burst appeared to be reddened and dusty, implying it was producing stars at a prodigious rate. “We know that these kinds of galaxies form stars copiously,” Kulkarni said.
The gamma rays themselves are not formed directly by the explosion, but rather by a shock wave from the explosion crashing into matter in interstellar space.
Chances are slim that such a burst would go off in our galaxy. If it did, however, it would be like having a dozen 50-megaton hydrogen bombs going off at once, according to Woosley. Assuming such a hypothetical burst originated from the far side of the galaxy, “It would give people a bad sunburn, but it wouldn’t destroy life,” he said.
“We’re in far greater danger from comets and asteroids than we are from gamma ray bursts. And we’re in far greater danger from ourselves than from any of these.”
