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There’s a significant difference between astrophysical black holes and primordial ones. The former occur when huge stars collapse to create a region of space in which gravity is so strong that nothing can escape (which is why they are black).

And they are huge. The one sitting at the centre of our galaxy is thought to be about 4 million times more massive than the Sun.

By contrast, primordial black holes are tiny, with masses measured in tonnes. Astrophysicists believe these objects must have formed in great numbers during the Big Bang. They also think that primordial black holes slowly evaporate, finally disappearing in a puff of powerful gamma rays.

However, nobody has conclusively seen the death of a primordial black hole, leaving open the possibility that something else may be going on.

So today, Pace VanDevender at Sandia National Labs and Aaron VanDevender put forward an alternative idea. Perhaps primordial black holes don’t evaporate. Instead, these objects interact with nearby particles to form the gravitational equivalent of atoms.

That’s an interesting idea (and certainly no crazier than cosmologist usually contemplate). Normally, gravity is so weak that it can effectively be ignored at the scale of atoms. But that’s not the case for mini black holes, which should generate forces capable of trapping atoms into orbit around them.

That immediately raises the big fear associated with black holes: that they consume all matter in their path while growing rapidly into planet-eating monsters. Won’t these mini black holes simply suck any nearby atoms into oblivion?

The VanDevenders say this is unlikely. And they make a pretty convincing stab at explaining why. Their argument is similar to that which Planck and others used to develop the theory of the atom early in the last century.

The problem then was that, in classical theories, an electron orbiting an atom ought to spiral into the nucleus. So in theory, atoms shouldn’t exist.

The new theory of quantum mechanics solved this by introducing the idea of quantisation in which the probability of the electron being absorbed by the nucleus is not impossible but vanishingly small.

The VanDevenders say a similar situation ought to exist for primordial black holes, provided they are small enough. These objects must have a gravitational field powerful enough to attract objects such as neutral atoms into orbit around them. But they must also have a radius that is so small that the chances of the orbiting atom encountering the black hole is vanishingly small.

The VanDevenders say that this should be true for black holes with a mass significantly smaller than a few hundred billion kilograms. And they go on to give a detailed study of some of the properties of these gravitational atoms.

For example, some black holes will be so small that the thermal energy of nearby particles will easily overcome the gravitational attraction. These black holes will scatter matter but cannot bind it into shells. Apparently, the black holes that might be formed in experiments like the LHC fall into this category.

Larger mini black holes of about 10 to1000 tonnes, however, can trap neutral atoms and so ought to be be surrounded by shells of atoms such as silicon or iron.

These objects ought to be detectable as they strike the Earth. The VanDevenders calculate that such a gravitational atom would be stripped of its orbiting atoms as it passed through Earth, creating radiofrequency emissions.

“Therefore, a search for electromagnetic signals from gravitational equivalent atoms should focus on fast moving, unidentifified rf sources in the space surrounding Earth,” they say.

That’s something we could look for now with relative ease. There may even be extant data that could place limits on the possibility that gravitational atoms exist.

Probably worth somebody taking a look.

Ref: arxiv.org/abs/1105.0265: Structure and Mass Absorption of Hypothetical Terrestrial Black Holes

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