TINY black holes may be detonating like time bombs all over our cosmic
backyard. The explosions could explain a class of unusual energy bursts that
have been picked up here on Earth, say physicists in the US.
Gamma-ray bursts are spectacular releases of energy that for a few seconds
can outshine everything else in the Universe. They can come from any part of the
sky, and from this astronomers have concluded that they are produced at the very
edge of the Universe, most likely in the super-bright fireballs created by the
birth or merger of star-sized black holes.
But David Cline, Christina Matthey and Stanislaw Otwinowski of the University
of California in Los Angeles say this is not true of bursts that last for less
than 100 milliseconds, which make up 42 of the 3000 bursts observed so far.
These short gamma-ray bursts don’t come from all directions. Nor are they
concentrated in the disc of our Galaxy or at its centre, as would be expected if
they were associated with stars. But there is a hint of a concentration towards
the Orion spiral arm of our Galaxy, the next spiral arm out from the Sun, and
this, says Cline, “suggests they are local”.
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Most gamma-ray bursts are quite varied. But the short bursts identified by
Cline share the same characteristics, suggesting they are all caused by similar
events. Cline suggests that we are seeing the detonation of microscopic black
holes formed in the big bang.
Black holes are thought to end their lives in a violent burst of energy
called Hawking radiation. The bigger their mass, the longer they survive before
this happens. For black holes to be dying today, about 14 billion years after
the big bang, they would have to have a mass of 100 million tonnes—about
that of a small mountain—and have been created when the Universe was less
than a billion-billion-billionth of a second old.
The researchers are preparing a paper on their work for the journal
Astroparticle Physics. If they are right, every cube of space a
few light years across in our cosmic neighbourhood could contain some 10 billion
primordial black holes waiting to go off. This puts the closest hardly further
away than Pluto. Every year, in the same volume, a few will detonate, each with
the energy of 100 billion 1-megaton H-bombs. Dramatic as that sounds, we are
quite safe, says Cline: “One would have to go off closer than the Sun to affect
the Earth.”
Other astronomers are cautious about the theory. “The idea isn’t completely
mad,” says Martin Rees of the University of Cambridge.
Roger Blandford of the California Institute of Technology in Pasadena says he
has yet to be convinced that the short bursts are in a class of their own. “But
Cline’s proposal has the merit of being testable in the near future.” To gain
more evidence, Cline and his team suggest looking more closely at the way the
brightness changes during the bursts. Fluctuations that lasted a matter of
nanoseconds would support the idea that the source is a very small object such
as a primordial black hole.