Primordial Black Holes

Click on Image to Enlarge. Image Credit: Sprott Physics

Were vast numbers of black holes spawned during the universe's earliest moments?

So far, there is no hard evidence that such primordial black holes (PBHs) ever existed, but new observations just around the corner could change that.

There are a variety of ways that PBHs might have formed in the early universe. Concentrations of energy associated with exotic energy fields could collapse under their own gravity – according to Einstein's relativity, energy exerts gravity just as matter does – to make black holes. One such energy field is thought to be responsible for the rapid expansion (inflation) of the early universe.

A wide variety of masses for PBHs are possible, depending on the formation scenario. The least massive ones, with less than about the mass of a comet, or 1 trillion kilograms, would quickly evaporate through a quantum process known as Hawking radiation.

More massive PBHs, born with up to 100,000 times the mass of the Sun, could survive to put an imprint on the CMB cosmic microwave background, radiation emitted by warm matter roughly 400,000 years after the big bang.

Normally black holes would emit X-rays as they swallow matter from their surroundings, and these X-rays can escape the vicinity of the black holes to break apart, or ionise, hydrogen atoms. This would subtly affect how matter distributes itself into regions of high and low density - a distribution reflected in the CMB radiation.

This effect might explain a puzzling discrepancy between results of the Wilkinson Microwave Anisotropy Probe (WMAP), which measures the CMB, and studies of how galaxies are clustered.

The two disagree on a parameter called sigma8, which describes how matter clumped together in the early universe. But according to a recent study led by Massimo Ricotti of the University of Maryland in College Park, US, the two measurements agree if PBHs are included in the models.
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But Ricotti himself says it is too soon to claim there is evidence for primordial black holes. "It is still possible that refining the measurements will bring them into agreement without invoking these exotic objects," he says.

The study also suggests that the ionising effect of PBHs would have helped spark the formation of the first stars in the universe. The presence of free electrons helps pairs of hydrogen atoms to join together to form molecular hydrogen. "You form a lot of molecular hydrogen – about 10 to 100 times more than you would form if you didn't have primordial black holes," Ricotti said.

Molecular hydrogen helps to cool gas clouds by emitting radiation, allowing the clouds to contract enough to condense into stars. Ricotti says the James Webb Space Telescope, scheduled to launch in 2013, just may be able to detect this enhancement of star formation.

Perhaps most intriguingly, if primordial black holes survive in great enough numbers today, then clouds of them could account for some or even all of the mysterious dark matter in the universe.

The main problem with this possibility is that it is not clear whether the conditions needed to form PBHs in large numbers ever occurred in our universe.

In the formation scenario involving the inflation field, for example, the number of PBHs formed depends on unknowns such as the size of fluctuations in the inflation field. In some inflationary models, you can form a lot of PBHs; in others you form very few of them.

It is possible that unusually large amounts of ionisation in the early universe - possibly due to the X-rays emitted by PBHs - could be detected by Europe's Planck satellite, scheduled to launch in mid-2008, says WMAP team member Rachel Bean of Cornell University in Ithaca, New York, US.

If convincing evidence of primordial black holes ever emerges, it would give scientists an extremely important window into the universe at very early times.

The mass of the black holes would reveal the time at which they formed, since the different scenarios for their formation occur at different times and give different masses. If they formed at the end of inflation, then their existence would reveal important information about the murky physics of this period of rapid expansion.

"You could rule out models of inflation that don't produce these black holes," says physicist James Chisholm of Southern Utah University. "Someone would probably get a Nobel prize."
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