Self-Gravity Effects of Ultralight Boson Clouds Formed by Black Hole Superradiance

TL;DR

This paper investigates how the self-gravity of ultralight boson clouds around black holes affects gravitational wave signals, using relativistic models to improve waveform accuracy for detectors like LIGO and LISA.

Contribution

It provides the first fully relativistic calculations of self-gravity effects in black hole-boson cloud systems, refining gravitational wave models for ultralight boson detection.

Findings

Self-gravity can double the frequency shift in relativistic regimes.

Improved waveform models reduce phase errors to a few cycles.

Clouds alter the innermost stable orbit and light-ring around black holes.

Abstract

Oscillating clouds of ultralight bosons can grow around spinning black holes through superradiance, extracting energy and angular momentum, and eventually dissipating through gravitational radiation. Gravitational wave detectors like LIGO, Virgo, KAGRA, and LISA can thus probe the existence of ultralight bosons. In this study, we use fully general-relativistic solutions of the black hole-boson cloud systems to study the self-gravity effects of scalar and vector boson clouds, making only the simplifying assumption that the spacetime is axisymmetric (essentially corresponding to taking an oscillation average). We calculate the self-gravity shift in the cloud oscillation frequency, which determines the frequency evolution of the gravitational wave signal, finding that this effect can be up to twice as large in the relativistic regime compared to non-relativistic estimates. We use this to improve the superrad waveform model, and estimate that this reduces the theoretical phase error to a few cycles over the characteristic timescale of the gravitational wave emission timescale for the louder vector boson signals. We also perform an analysis of the spacetime geometry of these systems, calculating how the cloud changes the innermost stable circular orbit and light-ring around the black hole.