09 December 2010

Evidence for Tiny Black Holes?

Last month some astronomical news came out that didn't get much play, but I was fascinated by it.  UCLA scientists are claiming they might have detected evidence for primordial black holes, tiny cosmic denizens first predicted by Stephen Hawking in the 1970s.  If that's true, it's pretty big news, as here would be a means to study what happens when general relativity meets quantum mechanics—the holy grail of physics these last few decades.

What Hawking did was ask how a black hole might affect its surroundings from the viewpoint of an atom.  He concluded that space-time gets so twisted near a black hole that it enables pairs of particles (a nuclear particle and its antimatter mate) to pop into existence just outside the black hole.  You could think of it as energy being extracted from the black hole's intense gravitational field and then converted into matter.  At times, one of the particles disappears into the black hole, never to return, while the remaining one flies off. As a result, the hole's total mass-energy is reduced a smidgen.  This means the black hole is actually evaporating!

Illustration of primordial black hole's final evaporation
For stellar-size holes, this bizarre quantum-mechanical process is just about meaningless.  It would take trillions upon trillions of years for a regular black hole to shrink away to nothingness.  But Hawking suggested that the early universe, in the first turbulent moments of the Big Bang, might have manufactured a multitude of tiny black holes.  The smallest would have vanished by now, but objects containing the mass of a mountain, yet compressed to the size of a proton, would be shedding the last of their mass at this very moment in a short and spectacular burst of gamma rays. 

That's what the team of UCLA scientists, led by David Cline, believe they are seeing.  Looking over data from a number of gamma-ray telescopes, they have detected gamma-ray bursts lasting less than 100 thousandths of a second.  That's the type of signal expected from the evaporation of primordial black holes.  Of course, such signals could be also be arriving from a more common stellar process not yet identified.  As Carl Sagan liked to say, "Extraordinary claims require extraordinary evidence."  Cline agrees.  He's urging others to start studying these events as well, to see if their claim holds up to scrutiny. 
Image Credit: Virginia Tech Department of Electrical and Computer Engineering

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