Limits on the existence of totally reflective exotic compact objects with current and future gravitational-wave detectors

TL;DR

This paper uses gravitational-wave data to set upper limits on the population of exotic compact objects (ECOs), constraining their properties and abundance with current and future detectors, thus testing their viability as alternatives to black holes.

Contribution

It provides the first constraints on ECO populations based on spin measurements from gravitational-wave detections, improving previous bounds and exploring future detector capabilities.

Findings

ECOs cannot constitute more than 71% of CBCs for certain parameters.

Current data limits ECOs to less than 28% of CBCs if they are ultracompact.

Next-generation detectors can reduce ECO population limits to below 20% with minimal data.

Abstract

Exotic compact objects (ECOs) are a theorized class of compact objects that solve the paradoxes of black holes by replacing the event horizon with a physical surface located at $r=r_+(1+\epsilon)$ from the would-be horizon at $r_+$. Spinning horizonless objects are prone to the ergoregion instability, which would prevent their existence if their spin is higher than a critical threshold. In this paper, we set upper limits on the existence of a population of merging ECOs from the spin distribution of the population of compact binary coalescences (CBCs) detected by the LIGO, Virgo and KAGRA collaboration. Using spin measurements from 104 compact objects, we find that if ECOs have $\epsilon \in [10^{-42}-10^{-3}]$ and their surface is totally reflective, the population of CBCs cannot be composed (at 90% credible level) by more than 71% (59%) of ECOs for polar (axial) perturbations. If we restrict the ECOs to be ultracompact ($\epsilon<10^{-30}$), at 90% credible level, ECOs cannot compose more than 28% and 25% of the CBC population for polar and axial perturbations. The constraints from current data are a factor of two more precise than the ones obtained from a non-detection of a stochastic GW background due to spin loss. We also study how next generation gravitational-wave detectors, such as the Einstein Telescope, can constrain the ECO population. We find that 1 day of data taking would be enough to constrain the fraction of ECOs to be lower than 20% for $\epsilon \in [10^{-42}-10^{-3}]$.