Symmetry and Equivalence in Teleparallel Gravity

TL;DR

This paper develops an invariant approach to analyze symmetries in teleparallel gravity, introduces a modified classification algorithm, and demonstrates that Minkowski space is the only maximally symmetric solution with non-zero torsion.

Contribution

It proposes a torsion-based symmetry classification algorithm and proves Minkowski space as the unique maximally symmetric solution with torsion in teleparallel gravity.

Findings

Symmetry groups of teleparallel solutions can be smaller than in GR.

The modified Cartan-Karlhede algorithm effectively classifies torsion geometries.

Minkowski space is the only maximally symmetric solution with non-zero torsion.

Abstract

In theories such as teleparallel gravity and its extensions, the frame basis replaces the metric tensor as the primary object of study. A choice of coordinate system, frame basis and spin-connection must be made to obtain a solution from the field equations of a given teleparallel gravity theory. It is worthwhile to express solutions in an invariant manner in terms of torsion invariants to distinguish between different solutions. In this paper we discuss the symmetries of teleparallel gravity theories, describe the classification of the torsion tensor and its covariant derivative and define scalar invariants in terms of the torsion. In particular, we propose a modification of the Cartan-Karlhede algorithm for geometries with torsion (and no curvature or nonmetricity). The algorithm determines the dimension of the symmetry group for a solution and suggests an alternative frame-based approach to calculating symmetries. We prove that the only maximally symmetric solution to any theory of gravitation admitting a non-zero torsion tensor is Minkowski space. As an illustration we apply the algorithm to six particular exact teleparallel geometries. From these examples we notice that the symmetry group of the solutions of a teleparallel gravity theory is potentially smaller than their metric-based analogues in General Relativity.