Primordial black hole ringdown: The irreducible stochastic gravitational wave background

TL;DR

This paper discusses the unavoidable gravitational wave background produced by primordial black holes during their ringdown phase, which could be detected by current or future experiments, especially for very massive black holes.

Contribution

It introduces the concept of an irreducible stochastic gravitational wave background from primordial black hole ringdowns, linking it to observational prospects for very massive black holes.

Findings

The gravitational wave background is unavoidable and directly linked to primordial black hole formation.

For black holes over 10^{14} solar masses, the background is detectable by current and future CMB experiments.

This provides a new observational window into the existence of extremely massive primordial black holes.

Abstract

Independently from the formation mechanism of primordial black holes in the early Universe, their generation is accompanied by a ringdown phase during which they relax to a stationary configuration and gravitational waves under the form of quasinormal modes are emitted. Such gravitational waves generate an irreducible and unavoidable stochastic background which is testable by current and future experiments. In particular, for primordial black holes with masses exceeding $10^{14}\,M_{\odot}$, the associated stochastic background lies within the frequency range accessible to current and upcoming cosmic microwave background experiments, thereby providing a direct observational way to probe the existence of such extremely massive objects.