Tidal acceleration of black holes and superradiance

TL;DR

This paper explores how black hole horizons can cause tidal acceleration through superradiance, especially when coupled with scalar fields, potentially affecting gravitational-wave signals and observations.

Contribution

It introduces the concept of polarization acceleration via scalar fields as a stronger alternative to traditional tidal effects in black hole binaries.

Findings

Tidal acceleration in black holes is linked to superradiance.

Scalar field coupling can enhance tidal effects significantly.

Implications for gravitational-wave detection and astrophysical observations.

Abstract

Tidal effects have long ago locked the Moon in synchronous rotation with the Earth and progressively increase the Earth-Moon distance. This "tidal acceleration" hinges on dissipation. Binaries containing black holes may also be tidally accelerated, dissipation being caused by the event horizon - a flexible, viscous one-way membrane. In fact, this process is known for many years under a different guise: superradiance. In General Relativity, tidal acceleration is obscured by gravitational-wave emission. However, when coupling to light scalar degrees of freedom is allowed, an induced dipole moment produces a "polarization acceleration", which might be orders of magnitude stronger than tidal quadrupolar effects. Consequences for optical and gravitational-wave observations are intriguing and it is not impossible that imprints of such mechanism have already been observed.