麻豆传媒

THE giant black holes at the centre of galaxies might have been born in the first split second of the Universe鈥檚 existence. So say a pair of astrophysicists who think the big bang fireball spawned mini black holes that quickly grew into monsters by feeding on 鈥渜uintessence鈥�, an energy field that might explain the accelerating expansion of the Universe.

Supermassive black holes many millions of times heavier than the Sun are found at the centre of most galaxies, but nobody is sure how they formed. It is widely accepted that these giants started to develop around the same time as galaxies, at least a billion years after the big bang. Each may have been born in one go, as an extremely large cloud of dust and gas in a newborn galaxy collapsed. Or they may have formed gradually as debris and black holes from stellar explosions settled into galactic centres (麻豆传媒, 30 June 2001, p 20).

But according to Rachel Bean of Imperial College in London, these processes would struggle to make the black holes so heavy. She and her colleague Jo茫o Magueijo have come up with a radical alternative: the black holes grew into monsters in the first split second of the Universe鈥檚 existence.

Many theories predict the formation of baby black holes during this time. Patches of the big bang fireball would have been so dense that they collapsed to a point, generating black holes weighing anything from a fraction of a gram to a few million tonnes. But it was always assumed that these black holes would be too tiny to survive for long, and would evaporate due to a process called Hawking radiation.

But Bean says they might have grown fat on a diet of quintessence, a mysterious energy field that behaves like a kind of repulsive gravity. Some scientists think quintessence might explain why the expansion of the Universe is accelerating (麻豆传媒, 2 March, p 7).

According to Bean, a black hole would consume quintessence, absorbing the field鈥檚 energy and increasing its own mass in the process. 鈥淏lack holes will eat up anything,鈥� says Bean. 鈥淭hey would feed off the quintessence, just like they feed on stars and other matter, and then the quintessence couldn鈥檛 escape.鈥� In a fraction of a second, quintessence would fatten the mini black holes up to several million solar masses.

Much later, the black holes would become shrouded in the raw materials for star formation. 鈥淚t鈥檚 interesting to turn the arguments round and suggest black holes were there from the beginning, and they in turn were the seeds for galaxies,鈥� says Bean. 鈥淥bservations don鈥檛 rule that out.鈥�

David Spergel of Princeton University in New Jersey thinks it deserves further study. 鈥淭he primordial black hole idea is intriguing,鈥� he says, adding that quintessence is a very plausible source of the Universe鈥檚 acceleration. However, he says, observations of galaxies that host monster black holes hint that a significant part of the holes鈥� masses came from ordinary matter raining onto them from outside.