Vacuum polarization in asymptotically anti-de Sitter black hole geometries

TL;DR

This paper analyzes vacuum polarization effects for a scalar field in asymptotically anti-de Sitter black hole geometries using WKB and point-splitting methods, providing analytical and numerical tools for semi-classical gravity.

Contribution

It introduces a systematic approach combining WKB expansion and zeta-function regularization to compute vacuum polarization in AdS black holes, including numerical evaluation techniques.

Findings

Derived explicit expressions for <φ^2> in AdS black hole backgrounds.

Demonstrated the cancellation of divergences and regularity of the Green function.

Provided a method to incorporate quantum back-reaction in semi-classical Einstein equations.

Abstract

We study the polarization of the vacuum for a scalar field, $<\phi^2 >$, on an asymptotically anti-de Sitter black hole geometry. The method we follow uses the WKB analytic expansion and point-splitting regularization, similarly to previous calculations in the asymptotically flat case. Following standard procedures, we write the Green function, regularize the initial divergent expression by point-splitting, renormalize it by subtracting geometrical counter-terms, and take the coincidence limit in the end. After explicitly demonstrating the cancellation of the divergences and the regularity of the Green function, we express the result as a sum of two parts. One is calculated analytically and the result expressed in terms of some generalized zeta-functions, which appear in the computation of functional determinants of Laplacians on Riemann spheres. We also describe some systematic methods to evaluate these functions numerically. Interestingly, the WKB approximation naturally organizes $<\phi^2>$ as a series in such zeta-functions. We demonstrate this explicitly up to next-to-leading order in the WKB expansion. The other term represents the `remainder' of the WKB approximation and depends on the difference between an exact (numerical) expression and its WKB counterpart. This has to be dealt with by means of numerical approximation. The general results are specialized to the case of Schwarzschild-anti-de Sitter black hole geometries. The method is efficient enough to solve the semi-classical Einstein's equations taking into account the back-reaction from quantum fields on asymptotically anti-de Sitter black holes.