Vienna University of Technology, scientists presented a method to prove the existence of hypothetical “axions” that could accumulate around a black hole and extract energy from it.

This process could emit gravity waves, which could then be measured.

Background

Finding new particles usually requires high energies — that is why huge accelerators have been built, which can accelerate particles to almost the speed of light.

Axions  are hypothetical particles with a very low mass. According to Einstein, mass is directly related to energy, and therefore very little energy is required to produce axions.

In quantum physics, every particle is described as a wave. The wavelength corresponds to the particle’s energy. Heavy particles have short wavelengths, but the low-energy axions can have wavelengths of many kilometers.

“The existence of axions is not proven, but it is considered to be quite likely,” says Daniel Grumiller. He and Gabriela Mocanu calculated how axions could be detected. They show that axions can circle a black hole, similar to electrons circling the nucleus of an atom. Instead of the electromagnetic force, which ties the electrons and the nucleus together, gravitational force is what acts between the axions and the black hole.

However, electrons are fermions — which means that two of them can never be in the same state. Axions on the other hand are bosons; many of them can occupy the same quantum state at the same time. They can create a “boson-cloud” surrounding the black hole. This cloud continuously sucks energy from the black hole and the number of axions in the cloud increases.

Such a cloud is not necessarily stable. “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”, says Grumiller.

The exciting thing about such a collapse is that this “bosenova” could be measured. This event would make space and time vibrate and emit gravity waves. Detectors for gravity waves have already been developed; in 2016 they may reach an accuracy at which gravity waves can be unambiguously detected.

Ref.: Gabriela Mocanu & Daniel Grumiller, Self-organized criticality in boson clouds around black holes, Physical Review D, 2012; DOI: 10.1103/PhysRevD.85.105022

Ref.: Gabriela Mocanu & Daniel Grumiller, Self-organized criticality in boson clouds around black holes, arXiv:1203.4681