Ultralight scalars and resonances in black-hole physics

TL;DR

This paper explores how ultralight scalar fields interacting with matter can cause resonances in black-hole systems, leading to observable phenomena like floating orbits that stall orbital decay.

Contribution

It demonstrates that ultralight scalars induce resonances causing floating orbits in black-hole binaries, a novel effect with potential observational signatures.

Findings

Resonances occur when scalar field wavelength matches dynamical scales.

Floating orbits cause orbital stalling despite gravitational-wave emission.

Finite-size effects do not suppress the resonance phenomenon.

Abstract

Ultralight degrees of freedom coupled to matter lead to resonances, which can be excited when the Compton wavelength of the field equals a dynamical scale in the problem. For binaries composed of a star orbiting a supermassive black hole, these resonances lead to a smoking-gun effect: a periastron distance which {\it stalls}, even in the presence of gravitational-wave dissipation. This effect, also called a {\it floating orbit}, occurs for generic equatorial but eccentric orbits and we argue that finite-size effects are not enough to suppress it.