Black Holes as Exotic Particle Honeypots?


Forget the Large Hadron Collider (LHC) near Geneva, Switzerland, if you really want to unravel the mysteries behind the most elusive subatomic particles in the Universe, you may be better off detecting the gravitational waves radiating from a black hole.

This sci-fi-sounding notion comes from the fertile minds of Vienna University of Technology scientists who argue that the extreme gravitational dominance of black holes may be honeypots for hypothetical exotic particles called axions.


In a study he carried out with Gabriela Mocanu, Daniel Grumiller said: “The existence of axions is not proven, but it is considered to be quite likely.” And what better way than to ask a black hole for some help in the axion hunt?

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In the strange world of quantum mechanics, particles can act like a wave and a particle. More massive particles — like protons — have very short wavelengths; very light particles — like photons — have longer wavelengths. Axions are believed to have an extremely low mass and therefore have a very long wavelengths.

Usually, when anything gets too close to a black hole — spaceships, subatomic particles, light, socks — it’s a one-way trip. Any matter that gets pulled into a black hole undergoes an extreme mass-energy conversion, never to be seen in the known Universe again.

But some particles may treat the black hole as a supersized atom. Like an electron orbits a proton in a hydrogen atom, axions may be created around a black hole only to stay in orbit.

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As axions are thought to have very long wavelengths (of several kilometers), they may remain in stable atom-like orbits. The key difference, apart from the extreme difference in scales, is that electrons around an atom are kept in place by the electromagnetic force. The axions would be kept in place by the gravitational force.

Also, electrons are part of a quantum family called “leptons.” Leptons are restricted by some very strict rules; one of them being that no two leptons can occupy the same location at the same time — they’re antisocial. Axions, on the other hand, are “bosons” and their rules are more flexible. Unlimited bosons can occupy the same state at the same time — it’s just one big boson party.

Mocanu and Grumiller have now taken this idea to a new level. Should many of these axions accumulate around the black hole, a busy “boson-cloud” may form — like a huge swarm of bees around their hive. Although individual axions carry close to zero mass, through sheer numbers, their collective mass could carry some serious heft. (Indeed, the collective mass of axions throughout the Universe are thought to carry at least some of the mass locked in cold dark matter.)

OK, black holes might be surrounded by a huge cloud of axions, so what?

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“Just like a loose pile of sand, which can suddenly slide, triggered by one single additional grain of sand, this boson cloud can suddenly collapse,” Grumiller said in a press release.

As the boson cloud will likely be carrying some significant mass, a sudden collapse — known as a “bose-nova” — would generate a huge amount of energy, rippling the fabric of space-time. These ripples are known as gravitational waves and we have detectors that may be able to observe them by 2016.

Image credit: Vienna University of Technology

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