Is the Gravitational-Wave Ringdown a Probe of the Event Horizon?

TLDR

The ringdown signal from binary mergers is widely thought to confirm event‑horizon formation, based on the assumption that intermediate‑time waveforms are dominated by the final object’s quasinormal modes. However, very compact objects with a light ring can produce similar ringdown stages even with different quasinormal‑mode spectra, so universal ringdown waveforms indicate light rings rather than horizons; only late‑time precision observations can distinguish exotic alternatives and test quantum effects at the horizon scale.

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.