Counting large black holes could help us find elusive dark matter particles, a new study led by the Centre for Gravitational Astrophysics has found.

Large black holes are formed when smaller ones collide – in dense regions of the universe this could happen multiple times. But if the colliding black holes are spinning fast then the new black hole could be kicked out of the dense area, preventing any further mergers.

But Dr Lilli Sun and her colleagues from CGA and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) realised in some scenarios dark matter clouds can form around fast spinning black holes. In the process the black holes lose their rotational energy and avoid ejection from the dense area.

“If certain ultralight particles exist, then a huge number of them could appear and get trapped in the black hole’s powerful gravity field, forming a cloud co-rotating with the black hole,” said Dr Sun, from CGA and OzGrav.

“The formation of the cloud could spin down a black hole very quickly, because the rotational energy of the black hole is extracted into the boson cloud.”

Since the first detection of gravitational waves in 2015 many black hole mergers have been found, some involving black holes larger than could be formed by stars collapsing at the end of their life – such behemoths must have come from smaller black holes merging into larger ones, perhaps multiple times.

But Dr Sun and her collaborators realised that dark matter could play a significant role in the cosmic game of dodgem cars that black holes play in regions packed with stars.

But that depends on what dark matter actually is – which to date is not known. Its existence is known from large scale gravitational effects, such as the rotation of galaxies, but the nature of it has remained elusive.

One theory proposes clouds of small particles, and it is this proposition that Dr Sun and her colleagues has explored, in a paper in the Astrophysical Journal.

“These hypothetical particles have been proposed as solutions to a number of astrophysics and particle physics problems,” said first author, Ethan Payne, formerly from CGA and OzGrav, now a PhD student at Caltech in the United States.

“The intricate physics of black-hole superradiance provides a bridge between novel observations of binary black-hole mergers and the possible impacts of ultralight bosons.”

If dark matter were made up of such small particles, it is possible they could interact with a black hole through a process called superradiance, that would sap the rotation of the black hole.

The superradiance effect is maximized if the Compton wavelength of the particles is comparable to the size of the black hole, which would create a resonance that would allow the energy to be efficiently coupled out of the black hole.

Such a multistep link – small particles slowing black holes, leading to more mergers and therefore on average larger black holes – is difficult to prove, Dr Sun said.

“It’s a conceptual proposition that might apply to populations of black holes in dense clusters, but it’s not straightforward – there are many subtleties,” she said.