Callan-Giddings-Harvey-Strominger vacuum in loop quantum gravity and singularity resolution

TL;DR

This paper presents a complete loop quantum gravity quantization of the CGHS vacuum model, demonstrating that quantum effects eliminate classical singularities and identifying key observables like black hole mass.

Contribution

It introduces a novel loop quantum gravity quantization of the CGHS model with a new variable formulation and explicit solution space construction, showing singularity resolution.

Findings

Singular geometries are excluded in the quantum theory.

The physical Hilbert space is constructed via group averaging.

The black hole mass observable is identified and matches classical expectations.

Abstract

We study here a complete quantization of a Callan-Giddings-Harvey-Strominger (CGHS) vacuum model following loop quantum gravity techniques. Concretely, we adopt a formulation of the model in terms of a set of new variables that resemble the ones commonly employed in spherically symmetric loop quantum gravity. The classical theory consists of two pairs of canonical variables plus a scalar and diffeomorphism (first class) constraints. We consider a suitable redefinition of the Hamiltonian constraint such that the new constraint algebra (with structure constants) is well adapted to the Dirac quantization approach. For it, we adopt a polymeric representation for both the geometry and the dilaton field. On the one hand, we find a suitable invariant domain of the scalar constraint operator, and we construct explicitly its solution space. There, the eigenvalues of the dilaton and the metric operators cannot vanish locally, allowing us to conclude that singular geometries are ruled out in the quantum theory. On the other hand, the physical Hilbert space is constructed out of them, after group averaging the previous states with the diffeomorphism constraint. In turn, we identify the standard observable corresponding to the mass of the black hole at the boundary, in agreement with the classical theory. We also construct an additional observable on the bulk associated with the square of the dilaton field, with no direct classical analog.