Black holes in the early universe may have circumvented a law of physics to grow rapidly to colossal size. The finding could solve a longstanding puzzle over why such massive objects appeared so soon after the universe began.

The new analysis, by Marta Volonteri and Martin Rees, both at the University of Cambridge, UK, ties up all the important factors involved in the growth of a black hole and concludes rapid growth is possible. This might be because the black hole "swallows" the radiation generated as the hole gobbles up the matter around it, preventing a destructive explosion.

The puzzle first arose after astronomers spotted what appear to be monster black holes, with the mass of a billion Suns, near the edge of the visible universe. The black holes themselves are invisible, but matter falling into them is heated by friction and emits very powerful X-rays. These extreme emissions define the distant system as a quasar.

Because of the time it takes for the X-rays to travel from these extremely distant objects to Earth, astronomers see the quasars the way they were less than a billion years after the big bang.

All consuming
Until now, astronomers could not explain how the objects gathered such enormous amounts of matter in such a relatively brief time. One suggestion was that black holes in the early universe somehow overcame a law called the "Eddington limit", which normally restricts the growth of objects that are collecting matter.

The limit arises because if a black hole eats too quickly, the disc of matter feeding it radiates so much energy that it blows itself apart, leaving little for the object to absorb, so halting growth.

Some astronomers have suggested that early black holes managed to get around this law by swallowing the radiation in its vicinity before it had a chance to blow apart the disc of matter.

In the dense inner part of the disc, the X-rays might have a hard time travelling outward because of its frequent collisions with matter. If so, it could get pulled into the black hole along with the descending matter.

Eats, shoots out and leaves
But it was not clear whether even these "outlaw" black holes would grow fast enough, given other constraints that exist. For example, black holes can occasionally get kicked out of matter-rich clouds into intergalactic space, where there is nothing to eat. However, the analysis by Volonteri and Rees shows the growth can be fast enough.

"This growth is quite a challenge," says Stuart Shapiro, of the University of Illinois at Urbana-Champaign in the US. But the new study shows "the challenge can be met under special, but plausible, circumstances", he told New Scientist. Those circumstances include an assumption that the rate at which black holes get kicked out of host galaxies is relatively low.

Eventually, the black holes would go back to eating at less than the Eddington limit due to a lack of supply, says Rees: "Most holes in present-day galaxies are inconspicuous because they are starved of fuel."

[link to www.newscientistspace.com]