Discrete quantum spectrum of black holes

TL;DR

This paper demonstrates that quantum black holes retain a discrete emission spectrum with a minimum frequency bound, challenging the idea that black hole radiation can appear continuous and thermal.

Contribution

It proves that large quantum black holes have a discrete emission spectrum with a lower frequency bound, regardless of the gravity theory considered.

Findings

Minimum emission frequency is of order 1/M.

Black hole radiation spectrum remains discrete for large black holes.

Emission can be either non-thermal or highly thermal with high temperature.

Abstract

The quantum genesis of Hawking radiation is a long-standing puzzle in black hole physics. Semi-classically one can argue that the spectrum of radiation emitted by a black hole look very much sparse unlike what is expected from a thermal object. It was demonstrated through a simple quantum model that a quantum black hole will retain a discrete profile, at least in the weak energy regime. However, it was suggested that this discreteness might be an artifact of the simplicity of eigen-spectrum of the model considered. Different quantum theories can, in principle, give rise to different complicated spectra and make the radiation from black hole dense enough in transition lines, to make them look continuous in profile. We show that such a hope from a geometry-quantized black hole is not realized as long as large enough black holes are dubbed with a classical mass area relation in any gravity theory ranging from GR, Lanczos-Lovelock to f(R) gravity. We show that the smallest frequency of emission from black hole in any quantum description, is bounded from below, to be of the order of its inverse mass. That leaves the emission with only two possibilities. It can either be non-thermal, or it can be thermal only with the temperature being much larger than 1/M.