Spherically symmetric loop quantum gravity: analysis of improved dynamics

TL;DR

This paper analyzes an improved loop quantum gravity approach for spherically symmetric space-times, showing it replaces classical singularities with regular, finite-curvature quantum geometries and aligns with known black hole models.

Contribution

It introduces a well-motivated variable-dependent polymerization parameter in spherically symmetric loop quantum gravity, avoiding undesirable properties seen in earlier models.

Findings

Classical singularities are replaced by regular quantum geometries.

Effective metrics approach Schwarzschild geometry at low curvatures.

Quantum effects induce effective violations of the null energy condition.

Abstract

We study the "improved dynamics" for the treatment of spherically symmetric space-times in loop quantum gravity introduced by Chiou {\em et al.} in analogy with the one that has been constructed by Ashtekar, Pawlowski and Singh for the homogeneous space-times. In this dynamics the polymerization parameter is a well motivated function of the dynamical variables, reflecting the fact that the quantum of area depends on them. Contrary to the homogeneous case, its implementation does not trigger undesirable physical properties. We identify semiclassical physical states in the quantum theory and derive the corresponding effective semiclassical metrics. We then discuss some of their properties. Concretely, the space-time approaches sufficiently fast the Schwarzschild geometry at low curvatures. Besides, regions where the singularity is in the classical theory get replaced by a regular but discrete effective geometry with finite and Planck order curvature, regardless of the mass of the black hole. This circumvents trans-Planckian curvatures that appeared for astrophysical black holes in the quantization scheme without the improvement. It makes the resolution of the singularity more in line with the one observed in models that use the isometry of the interior of a Schwarzschild black hole with the Kantowski--Sachs loop quantum cosmologies. One can observe the emergence of effective violations of the null energy condition in the interior of the black hole as part of the mechanism of the elimination of the singularity.