Nature consists of two types of particles, one physicists call fermions – the type of particle that makes up solid matter – and another called bosons – the type of particle that can propagate interactions. Ultralight bosons can form large condensates around rapidly rotating black holes through a process called superradiance. A black hole carrying such a boson cloud is sometimes called a ‘gravitational atom’, because its configuration closely resembles – at a much larger scale – the proton-electron structure in a hydrogen atom. For example, just like the electron in the hydrogen atom, the boson cloud around a black hole can be in a number of different states, each with a particular energy.
In the case of the hydrogen atom, transitions between these different energy levels can be induced by shinning a laser onto the atom. When the energy of the laser is exactly right, the electron can ‘jump’ from one state into another. A similar effect can happen for the gravitational atom if it is part of a pair of black holes orbiting one another. In that case, the gravitational influence of the second black hole will play the role of the ‘laser’ and induce transitions between the energy states of the boson cloud.
In recent years, physicists have been able to measure gravitational waves – ripples in the gravitational field – that occur when pairs of black holes violently merge into a single one. As Baumann, Chia and Porto now show, the presence of energy level transitions in the hypothetical boson cloud would induce a characteristic ‘fingerprint’ in the gravitational wave signals produced by such merging black holes. Observing such a fingerprint would be an important test for theories that predict ultralight bosonic particles. While current gravitational wave observations aren't yet sensitive enough to observe the effect, this will certainly become an important target of future experiments.
Daniel Baumann, Horng Sheng Chia, and Rafael A. Porto, ‘Probing Ultralight Bosons with Binary Black Holes’ in Physical Review D 99, 044001.