Singularities and Quantum Gravity

TL;DR

This paper reviews the status of singularities in classical and quantum gravity, discusses potential ways quantum gravity might resolve them, and introduces a scheme called quantum hyperbolicity for singularity removal.

Contribution

It provides a comprehensive overview of classical and quantum singularities, proposing a general scheme for their removal through quantum hyperbolicity and semiclassical cosmological models.

Findings

Quantum hyperbolicity can resolve classical singularities.

Modified dynamics and geometrical structures offer routes to evade singularities.

Semiclassical models illustrate cosmological bounces as singularity resolutions.

Abstract

Although there is general agreement that a removal of classical gravitational singularities is not only a crucial conceptual test of any approach to quantum gravity but also a prerequisite for any fundamental theory, the precise criteria for non-singular behavior are often unclear or controversial. Often, only special types of singularities such as the curvature singularities found in isotropic cosmological models are discussed and it is far from clear what this implies for the very general singularities that arise according to the singularity theorems of general relativity. In these lectures we present an overview of the current status of singularities in classical and quantum gravity, starting with a review and interpretation of the classical singularity theorems. This suggests possible routes for quantum gravity to evade the devastating conclusion of the theorems by different means, including modified dynamics or modified geometrical structures underlying quantum gravity. The latter is most clearly present in canonical quantizations which are discussed in more detail. Finally, the results are used to propose a general scheme of singularity removal, quantum hyperbolicity, to show cases where it is realized and to derive intuitive semiclassical pictures of cosmological bounces.