Astronomers have so far discovered two types of black hole: supermassive ones at the centre of galaxies and stellar-mass black holes, which form when giant stars die.
But there’s no reason why black holes of any size cannot form. In fact, many astronomers think that the variations in density in the early universe would have led to the natural formation of relatively small black holes.
The smallest of these ought to have evaporated by now. But black holes with the mass of an asteroid, say, ought to be floating round as I write. They may even make up the mysterious dark matter that fills the universe. The question is how to find them.
Various theorists have suggested that we could spot primordial black holes by phenomena such as lensing effects as they pass in front of distant stars or by the gamma ray bursts they create as they flicker out of existence. But none of these approaches has produced definitive results.
Now Michael Kesden at New York University and Shravan Hanasoge at Princeton University in New Jersey say that the effect of a primordial black hole hitting the Sun ought to be easily observable.
Such an event wouldn’t be as catastrophic as it sounds. The likelihood is that a primordial black hole with mass of an asteroid or comet (about 10^21 g) would pass straight through the Sun, generating a small puff of X-rays in the process. Such a burst would be less even than the background rate of X-rays, so it would be impossible for astronomers to see.
Instead, Kesden and Hanasoge say that the collision would generate supersonic turbulence that would set the Sun ringing like a bell. Today, they calculate what these oscillations would look like.
Their conclusion is that the oscillations ought to be visible with today’s solar observatories as a kind of solar hiccup. So we might have seen them already.
That should generate a scramble. You can bet that solar scientists will be pouring over their data right now to see if they’ve missed the telltale signs of a black hole collision. If they find any, we should hear soon.
However, primordial black holes are likely to be rare, which means that collisions with Sun will be few and far between. So a more promising approach, say Kesden and Hanasoge, is to look at the way other stars oscillate.
And as luck would have it, astroseismology is an infant science that is rapidly maturing thanks to the observations being made by spacecraft such as CoRot and Kepler which can see other stars oscillating. One way or another, we’re going to learn a lot more about the way stars vibrate.
Ref: arxiv.org/abs/1106.0011: Transient Solar Oscillations Driven By Primordial Black Holes
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