The Singularity Problem for Space-Times with Torsion

TL;DR

This paper develops a rigorous framework for analyzing singularities in space-times with torsion, proposing a new definition of geodesics and extending Hawking's singularity theorem to Einstein-Cartan theory, showing singularities are less generic in such models.

Contribution

It introduces a new definition of geodesics in torsion space-times and extends Hawking's singularity theorem to ECSK theory without causality assumptions.

Findings

Singularities are less generic in ECSK cosmological models.

A new definition of geodesics in torsion space-times is proposed.

Hawking's singularity theorem is extended to ECSK theory under certain conditions.

Abstract

The problem of a rigorous theory of singularities in space-times with torsion is addressed. We define geodesics as curves whose tangent vector moves by parallel transport. This is different from what other authors have done, because their definition of geodesics only involves the Christoffel connection, though studying theories with torsion. We propose a preliminary definition of singularities which is based on timelike or null geodesic incompleteness, even though for theories with torsion the paths of particles are not geodesics. The study of the geodesic equation for cosmological models with torsion shows that the definition has a physical relevance. It can also be motivated, as done in the literature, remarking that the causal structure of a space-time with torsion does not get changed with respect to general relativity. We then prove how to extend Hawking's singularity theorem without causality assumptions to the space-time of the ECSK theory. This is achieved studying the generalized Raychaudhuri equation in the ECSK theory, the conditions for the existence of conjugate points and properties of maximal timelike geodesics. Hawking's theorem can be generalized, provided the torsion tensor obeys some conditions. Thus our result can also be interpreted as a no-singularity theorem if these additional conditions are not satisfied. In other words, it turns out that the occurrence of singularities in closed cosmological models based on the ECSK theory is less generic than in general relativity. Our work is to be compared with previous papers in the literature. There are some relevant differences, because we rely on a different definition of geodesics, we keep the field equations of the ECSK theory in their original form rather than casting them in a form similar to general relativity with a modified energy momentum tensor,