Decreasing entropy of dynamical black holes in critical gravity

TL;DR

This paper demonstrates that in critical gravity, black hole horizons can shrink and entropy can become negative, revealing non-perturbative pathologies of the theory through an exact Vaidya-type solution.

Contribution

It provides an exact Vaidya-type solution in critical gravity showing horizon area and entropy decrease, highlighting non-perturbative instabilities and pathologies.

Findings

Black hole horizon area can decrease with positive energy flux

Black hole entropy can become negative and decrease over time

Reveals non-perturbative instabilities in critical gravity

Abstract

Critical gravity is a quadratic curvature gravity in four dimensions which is ghost-free around the AdS background. Constructing a Vaidya-type exact solution, we show that the area of a black hole defined by a future outer trapping horizon can shrink by injecting a charged null fluid with positive energy density, so that a black hole is no more a one-way membrane even under the null energy condition. In addition, the solution shows that the Wald-Kodama dynamical entropy of a black hole is negative and can decrease. These properties expose the pathological aspects of critical gravity at the non-perturbative level.