# Exotic Compact Objects and How to Quench their Ergoregion Instability

## TL;DR

This paper examines how the ergoregion instability affects exotic compact objects with reflective surfaces and finds that even minimal absorption can prevent the instability, challenging previous assumptions about their viability.

## Contribution

It demonstrates that a small amount of absorption at the surface of exotic compact objects can effectively quench the ergoregion instability, providing new insights into their stability.

## Key findings

- A tiny absorption rate (~0.4%) can suppress the ergoregion instability.
- Perfectly reflecting objects are prone to rapid instability.
- Absorption at the surface stabilizes the object against the instability.

## Abstract

Gravitational-wave astronomy can give us access to the structure of black holes, potentially probing microscopic or even Planckian corrections at the horizon scale, as those predicted by some quantum-gravity models of exotic compact objects. A generic feature of these models is the replacement of the horizon by a reflective surface. Objects with these properties are prone to the so-called ergoregion instability when they spin sufficiently fast. We investigate in detail a simple model consisting of scalar perturbations of a Kerr geometry with a reflective surface near the horizon. The instability depends on the spin, on the compactness, and on the reflectivity at the surface. The instability time scale increases only logarithmically in the black-hole limit and, for a perfectly reflecting object, this is not enough to prevent the instability from occurring on dynamical time scales. However, we find that an absorption rate at the surface as small as 0.4% (reflectivity coefficient as large as $|{\cal R}|^2=0.996$) is sufficient to quench the instability completely. Our results suggest that exotic compact objects are not necessarily ruled out by the ergoregion instability.