Quantum Cross-section of Near-extremal Black Holes

TL;DR

This paper investigates how quantum fluctuations in near-extremal black holes affect their absorption cross-section, revealing that strong quantum effects lead to an increase beyond classical predictions, serving as a signature of quantum geometry fluctuations.

Contribution

It provides a detailed calculation of the absorption cross-section in different coupling regimes, highlighting the impact of quantum fluctuations on black hole scattering properties.

Findings

In the weak coupling regime, the cross-section equals the horizon area.

In the strong coupling regime, the cross-section exceeds the semiclassical prediction.

Quantum fluctuations enhance absorption and suppress stimulated emission.

Abstract

We explore how to detect the large quantum fluctuations in the throat of a near-extremal black hole, where the dynamics are governed by the Schwarzian theory. To this end, we scatter a low-frequency wave of a massless, minimal scalar off the black hole and calculate the absorption cross-section. In the semiclassical regime, where the Schwarzian is weakly coupled, we recover the universal result that the cross-section equals the horizon area. However, in the strongly coupled regime, where quantum fluctuations dominate, we find that the absorption cross-section exceeds the semiclassical prediction. This result may seem counterintuitive, given that the density of black hole states is suppressed in this regime. Nevertheless, two effects outweigh this suppression. First, quantum fluctuations enhance absorption transitions between individual states, with the effect becoming stronger closer to the ground state. Second, these fluctuations significantly reduce stimulated emission. We conclude that a measurement showing an enhanced absorption cross-section serves as a clear signature of the large quantum fluctuations in the geometry.