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

Understand

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.