Probing the Purely Ingoing Nature of the Black-hole Event Horizon

TL;DR

This paper proposes a novel gravitational-wave-based test to verify the purely ingoing nature of black-hole event horizons, by analyzing potential outgoing gravitational waves and their effects on quasinormal modes.

Contribution

It introduces a new phenomenological boundary condition at the black hole horizon to test the ingoing assumption using gravitational-wave data.

Findings

Reanalysis of GW150914 constrains outgoing horizon flux to less than 10^{40} W.

Additional outgoing gravitational waves would excite extra quasinormal modes.

The study provides a method to empirically test fundamental black hole properties.

Abstract

One of the most fundamental results of general relativity is that the event horizon of black hole is purely ingoing. On the other hand, semiclassical-gravity effects, such as particle creation and the quantization of black-hole area, suggest that black holes can emit energy. Since a black hole is characterized by the presence of the event horizon, the emitted energy must be extracted from the black hole through its horizon. These considerations provide a motivation to test the validity of the purely ingoing nature of black-hole horizon. In this paper, we propose a novel test of the purely ingoing nature of black-hole horizon through gravitational-wave detection. We study the effects of hypothetical out-going gravitational waves to a perturbed black hole by supplementing the boundary condition of gravitational waves at the horizon with a phenomenological outgoing part. We show that this leads to extra excitation of the usual quasinormal modes of a perturbed black hole and continuous emission of gravitational waves. These additional signatures enable us to test the boundary condition(s) of the black-hole event horizon through gravitational-wave detection. Reanalysing the merger remnant of GW150914, we constrain the intensity of the outgoing gravitational-horizon flux to be $ < 10^{40} \rm W$, which is roughly $ 10^{-9} $ of peak luminosity of GW150914.