Floating and sinking: the imprint of massive scalars around rotating black holes

TL;DR

This paper explores how massive scalar fields around rotating black holes can create 'floating orbits' where bodies hover due to superradiance, potentially revealing deviations from general relativity.

Contribution

It demonstrates the existence of floating orbits caused by superradiance and scalar field resonances, a novel effect in black hole physics.

Findings

Floating orbits can occur due to superradiance in rotating black holes.

Resonances at scalar field quasibound states can accelerate inspiral.

Floating orbits are unlikely for nonrotating black holes.

Abstract

We study the coupling of massive scalar fields to matter in orbit around rotating black holes. It is generally expected that orbiting bodies will lose energy in gravitational waves, slowly inspiralling into the black hole. Instead, we show that the coupling of the field to matter leads to a surprising effect: because of superradiance, matter can hover into "floating orbits" for which the net gravitational energy loss at infinity is entirely provided by the black hole's rotational energy. Orbiting bodies remain floating until they extract sufficient angular momentum from the black hole, or until perturbations or nonlinear effects disrupt the orbit. For slowly rotating and nonrotating black holes floating orbits are unlikely to exist, but resonances at orbital frequencies corresponding to quasibound states of the scalar field can speed up the inspiral, so that the orbiting body "sinks". These effects could be a smoking gun of deviations from general relativity.