Near-Horizon Geometry and the Entropy of a Minimally Coupled Scalar Field in the Kerr Black Hole

TL;DR

This paper calculates the entropy of a scalar field near the Kerr black hole horizon using the brick wall model, revealing proportionality to horizon area, divergence behavior, and effects of rotation, with comparisons to Schwarzschild black holes.

Contribution

It extends the brick wall model to Kerr black holes, analyzing near-horizon scalar field entropy including leading and subleading terms, and compares results with Schwarzschild black holes.

Findings

Entropy proportional to horizon area and logarithmically divergent.

Leading order entropy in Kerr is half of Schwarzschild due to rotation.

Subleading terms connect Kerr and Schwarzschild entropy expressions.

Abstract

In this article we will discuss a Lorentzian sector calculation of the entropy of a minimally coupled scalar field in the Kerr black hole background. We will use the brick wall model of G.'t Hooft. In the Kerr black hole, complications arise due to the absence of a global timelike Killing field and the presence of the ergosphere. Nevertheless, it is possible to calculate the entropy of a thin shell of matter field in the near-horizon region using the brick wall model. The corresponding leading order entropy of the nonsuperradiant modes is found to be proportional to the area of the horizon and is logarithmically divergent. Thus, to the leading order, the entropy of a three-dimensional system in the near-horizon region is proportional to the boundary surface. This aspect is also valid in the Schwarzschild black hole. The corresponding internal energy remains finite if the entropy is chosen to be of the order of the black hole entropy itself. For a fixed value of the brick wall cut-off, the leading order entropy in the Kerr black hole is found to be half of the corresponding term in the Schwarzschild black hole. This is due to rotation and is consistent with the preferential emission of particles in the Kerr black hole with azimuthal angular momentum in the same direction as that of the black hole itself. However, we can obtain the Schwarzschild case expression by including a subleading term and taking the appropriate slow rotation limit. In this version, we will discuss a sub-leading term for the scalar field entropy in Schwarzschild black hole. This term reduces to an expression similar to the corresponding flat spacetime entropy in the limit when radius of event horizon is vanishing.