Luminal Propagation of Gravitational Waves in Scalar-tensor Theories: The Case for Torsion

TL;DR

This paper demonstrates that in scalar-tensor theories with nonminimal Gauss-Bonnet coupling, allowing for spacetime torsion restores the standard gravitational wave propagation speed, challenging previous constraints based on torsionless assumptions.

Contribution

It introduces a new first-order formulation approach to analyze gravitational wave propagation in Riemann-Cartan geometries with torsion, revealing the impact of torsion on wave speed.

Findings

Removing the torsionless condition restores the canonical dispersion relation.

Torsion presence can eliminate anomalous gravitational wave speeds.

The approach applies to scalar-tensor theories with nonminimal Gauss-Bonnet coupling.

Abstract

Scalar-tensor gravity theories with a nonminimal Gauss-Bonnet coupling typically lead to an anomalous propagation speed for gravitational waves, and have therefore been tightly constrained by multimessenger observations such as GW170817/GRB170817A. In this paper we show that this is not a general feature of scalar-tensor theories, but rather a consequence of assuming that spacetime torsion vanishes identically. At least for the case of a nonminimal Gauss-Bonnet coupling, removing the torsionless condition restores the canonical dispersion relation and therefore the correct propagation speed for gravitational waves. To achieve this result we develop a new approach, based on the first-order formulation of gravity, to deal with perturbations on these Riemann-Cartan geometries.