Gravitational Atom Spectroscopy

TL;DR

This paper investigates the properties of scalar field clouds around black holes, called gravitational atoms, focusing on their quasi-normal modes and potential detectability via gravitational waves.

Contribution

It provides the first fully relativistic calculations of axial quasi-normal modes of gravitational atoms, revealing frequency shifts that could be observed.

Findings

Frequency shifts depend on the compactness of the gravitational atom.

Sufficiently compact configurations produce detectable gravitational wave signals.

Relativistic effects are significant in the quasi-normal mode spectrum.

Abstract

Black holes in our Universe are rarely truly isolated, being instead embedded in astrophysical environments such as plasma or dark matter. A particularly intriguing possibility is that light scalar fields form bound states around black holes, producing extended ''clouds'' known as gravitational atoms. When these clouds become sufficiently compact, the spacetime can no longer be described by a vacuum solution of General Relativity. In this regime, one can construct quasi-stationary, spherically symmetric, self-gravitating scalar gravitational-atom configurations. Here, we explore an observationally relevant aspect of these systems by computing their fundamental quasi-normal mode. We present a fully relativistic calculation of the axial modes in both the time and frequency domains, finding frequency shifts relative to the vacuum case that depends mostly on the compactness of the gravitational atom. For sufficiently compact configurations, these shifts may be detectable by current or future gravitational wave detectors.