Higher Derivative Gravities and Negative Entropy

TL;DR

This paper explores black hole solutions in R^2 gravity, analyzing their thermodynamics, phase transitions, and the possibility of negative entropy, with implications for AdS/CFT and brane cosmology.

Contribution

It provides exact solutions for R^2 gravity without Riemann tensor squares, investigates their thermodynamics, and discusses negative entropy and brane cosmology applications.

Findings

Discovery of Hawking-Page like phase transition in R^2 gravity.

Identification of conditions leading to negative black hole entropy.

Behavior of brane matter transitions from CFT-like to dust or curvature dominated at different scales.

Abstract

We investigate the black hole solutions in the $R^2$-gravity, where the action contains the square of the curvature. In case that the action does not contain the square of the Riemann tensor and in case that the $R^2$-terms are the Gauss-Bonnet (GB) combination, we find exact solutions. We investigate the thermodynamics of these theories and find the Hawking-Page like phase transition, which is the phase transition between the black hole (BH) spacetime and the pure anti-deSitter (AdS) spacetime. From the viewpoint of the AdS/CFT correspondence, such a phase transition may correspond to thermal transition of dual CFT. An interesting feature of $R^2$-gravity is the possibility of the negative (or zero) dS (or AdS) BH entropy, which depends on the parameters of the $R^2$-terms. We speculate that the appearence of negative entropy may indicate a new type instability where a transition between dS (AdS) BH with negative entropy and AdS (dS) BH with positive entropy occurs. We also apply the GB gravity to the brane cosmology, where the brane moves in the bulk AdS BH spacetime. By investigating the FRW-like equation, which describes the motion of the brane, we find the behavior of the matter on the brane. When the radius of the brane is large, the matter fields behave as CFT but when the radius is small, the brane universe behaves as the universe with dust or curvature dominant universe, depending on the parameters.