Gas clouds may have created biggest cosmic explosions

THEY would make supernovae look like firecrackers. Giant gas clouds in the early universe could have powered the most energetic eruptions since the big bang.

Evidence for supermassive black holes - weighing millions or billions of suns - has been found in the early universe, but no one knows how they grew so big so fast. Tiny black holes, weighing as much as a star, simply didn't have enough time to clump together into the behemoths.

One theory suggests huge gas clouds around at the time collapsed into middleweight "seed" black holes. These could then have attracted more matter and become supermassive.

Now Pedro Montero of the Max Planck Institute for Astrophysics in Garching, Germany, and colleagues have calculated how these gas clouds, weighing a million suns, might have evolved into seed black holes. They also found that clouds don't always form black holes, but either way they would have created powerful explosions.

The clouds are so massive that they begin to contract under their own weight, eventually becoming dense enough to trigger nuclear reactions. These provide an outward pressure that counteracts the clouds' collapse.

What happens next depends on the clouds' chemical composition. Heavy elements such as oxygen and nitrogen - spat out by dying stars - boost the rate of nuclear reactions. If a giant gas cloud had at least 10 per cent of the sun's proportion of these elements, they would set off enough reactions to overwhelm gravity's inward pull. This would blow a cloud apart in an explosion with 100 times the electromagnetic energy of any supernova today, the team says (arxiv.org/1108.3090).

Such gigantic, bright bursts might be detected with future observatories that could search for fleeting events, says Mitch Begelman of the University of Colorado, Boulder.

If the cloud contained fewer heavy elements providing outward pressure, gravity would win out and the cloud would collapse into a seed black hole. But that would release even more energy than in the detonation scenario because high pressures and temperatures in the cloud's core would lead energetic photons there to turn into pairs of electrons and their antimatter counterparts. These would annihilate, releasing about 10,000 times the electromagnetic energy of the brightest supernovae in the form of neutrinos. These particles, which rarely interact with normal matter, are invisible, making the bursts ultra-powerful but not unusually bright.

Begelman says future models could be made more realistic if they allowed different parts of the cloud to rotate at different rates.