Stochastic gravitational-wave background from spin loss of black holes

TL;DR

This paper explores the potential for detecting a stochastic gravitational-wave background generated by the spin-down of black holes, linking it to black hole formation rates and emission spectra, and discusses its observability with future detectors.

Contribution

It introduces a method to relate the gravitational-wave background from black hole spin-down to formation rates and emission spectra, highlighting its detectability prospects.

Findings

Current LIGO-Virgo data are consistent with slow spin-down processes.

The stochastic background could be detectable by next-generation GW detectors.

A significant fraction of black hole spin energy might be emitted within the Hubble time.

Abstract

Although spinning black holes are shown to be stable in vacuum in general relativity, there exists exotic mechanisms that can convert the spin energy of black holes into gravitational waves. Such waves may be very weak in amplitude, since the spin-down could take a long time, and a direct search may not be feasible. We propose to search for the stochastic background associated with the spin-down, and we relate the level of this background to the formation rate of spinning black holes from the merger of binary black holes, as well as the energy spectrum of waves emitted by the spin-down process. We argue that current LIGO-Virgo observations are not inconsistent with the existence of a spin-down process, as long as it is slow enough. On the other hand, the background may still exist as long as a moderate fraction of spin energy is emitted within Hubble time. This stochastic background could be one interesting target of next generation GW detector network, such as LIGO Voyager, and could be extracted from total stochastic background.