The stochastic gravitational-wave background in the absence of horizons

TL;DR

This paper demonstrates that the ergoregion instability in horizonless ultracompact objects produces a detectable stochastic gravitational-wave background, allowing constraints on alternative models to classical black holes using current and future detectors.

Contribution

It introduces the connection between ergoregion instability and gravitational-wave background, providing the first observational constraints on horizonless black-hole mimickers.

Findings

Current LIGO data constrains horizonless objects to be a small fraction of black-hole population.

The predicted gravitational-wave background from these objects is within the detection capabilities of existing and upcoming detectors.

Future LISA observations will further tighten constraints on supermassive black-hole mimickers.

Abstract

Gravitational-wave astronomy has the potential to explore one of the deepest and most puzzling aspects of Einstein's theory: the existence of black holes. A plethora of ultracompact, horizonless objects have been proposed to arise in models inspired by quantum gravity. These objects may solve Hawking's information-loss paradox and the singularity problem associated with black holes, while mimicking almost all of their classical properties. They are, however, generically unstable on relatively short timescales. Here, we show that this "ergoregion instability" leads to a strong stochastic background of gravitational waves, at a level detectable by current and future gravitational-wave detectors. The absence of such background in the first observation run of Advanced LIGO already imposes the most stringent limits to date on black-hole alternatives, showing that certain models of "quantum-dressed" stellar black holes can be at most a small percentage of the total population. The future LISA mission will allow for similar constraints on supermassive black-hole mimickers.