Extra polarization states of cosmological gravitational waves in alternative theories of gravity

TL;DR

This paper explores additional polarization states of cosmological gravitational waves predicted by alternative gravity theories, deriving their dynamical equations and highlighting the broader spectrum of GW modes beyond general relativity.

Contribution

It introduces a general formalism to analyze extra GW polarization states in various metric theories of gravity, with specific applications to scalar-tensor and bimetric theories.

Findings

Scalar-tensor theory predicts four polarization states.

Massive bimetric theory predicts six polarization states.

The formalism is applicable to all metric theories of gravity.

Abstract

Cosmological Gravitational Waves (GWs) are usually associated with the transverse-traceless part of the metric perturbations in the context of the theory of cosmological perturbations. These modes are just the usual polarizations `+' and `x' which appear in the general relativity theory. However, in the majority of the alternative theories of gravity, GWs can present more than these two polarization states. In this context, the Newman-Penrose formalism is particularly suitable for evaluating the number of non-null GW modes. In the present work we intend to take into account these extra polarization states for cosmological GWs in alternative theories of gravity. As an application, we derive the dynamical equations for cosmological GWs for two specific theories, namely, a general scalar-tensor theory which presents four polarization states and a massive bimetric theory which is in the most general case with six polarization states for GWs. The mathematical tool presented here is quite general, so it can be used to study cosmological perturbations in all metric theories of gravity.