Is the gravitational-wave ringdown a probe of the event horizon?

TL;DR

The paper argues that the gravitational-wave ringdown signal alone cannot definitively confirm the presence of an event horizon, as similar signals can arise from ultra-compact objects without horizons, emphasizing the need for late-time observations.

Contribution

It challenges the assumption that intermediate-time ringdown waveforms uniquely indicate black hole horizons, highlighting the importance of late-time data for definitive tests.

Findings

Light rings produce similar ringdown signals to horizons.

Universal ringdown waveforms are linked to light rings, not horizons.

Late-time observations are crucial to distinguish black holes from exotic compact objects.

Abstract

It is commonly believed that the ringdown signal from a binary coalescence provides a conclusive proof for the formation of an event horizon after the merger. This expectation is based on the assumption that the ringdown waveform at intermediate times is dominated by the quasinormal modes of the final object. We point out that this assumption should be taken with great care, and that very compact objects with a light ring will display a similar ringdown stage, even when their quasinormal-mode spectrum is completely different from that of a black hole. In other words, universal ringdown waveforms indicate the presence of light rings, rather than of horizons. Only precision observations of the late-time ringdown signal, where the differences in the quasinormal-mode spectrum eventually show up, can be used to rule out exotic alternatives to black holes and to test quantum effects at the horizon scale.