# On effective loop quantum geometry of Schwarzschild interior

## TL;DR

This paper develops a path-integral approach to loop quantum gravity for Schwarzschild black hole interiors, demonstrating that quantum effects resolve classical singularities by bounding key phase-space variables and invariants.

## Contribution

It establishes a link between quantum and effective descriptions using path integrals, showing that quantum effects lead to bounded, non-singular geometrical quantities.

## Key findings

- Phase-space variables are bounded by effective dynamics.
- Classical singularities are resolved with finite invariants.
- Raychaudhuri equation remains finite, indicating no singularities.

## Abstract

The success of loop quantum cosmology to resolve classical singularities of homogeneous models has led to its application to the classical Schwarszchild black hole interior, which takes the form of a homogeneous Kantowski-Sachs model. The first steps of this were done in pure quantum mechanical terms, hinting at the traversable character of the would-be classical singularity, and then others were performed using effective heuristic models capturing quantum effects that allowed a geometrical description closer to the classical one but avoided its singularity. However, the problem of establishing the link between the quantum and effective descriptions was left open. In this work, we propose to fill in this gap by considering the path-integral approach to the loop quantization of the Kantowski-Sachs model corresponding to the Schwarzschild black hole interior. We show that the transition amplitude can be expressed as a path integration over the imaginary exponential of an effective action which just coincides, under some simplifying assumptions, with the heuristic one. Additionally, we further explore the consequences of the effective dynamics. We prove first that such dynamics imply some rather simple bounds for phase-space variables, and in turn, remarkably, in an analytical way, they imply that various phase-space functions that were singular in the classical model are now well behaved. In particular, the expansion rate, its time derivative, and the shear become bounded, and hence the Raychaudhuri equation is finite term by term, thus resolving the singularities of classical geodesic congruences. Moreover, all effective scalar polynomial invariants turn out to be bounded.