Gravitational Waves in Bianchi Type-I Universes I: The Classical Theory

TL;DR

This paper investigates how classical gravitational waves behave in anisotropic Bianchi Type-I universes, revealing their unique properties and interactions that differ from isotropic cosmologies.

Contribution

It demonstrates that gravitational waves in Bianchi Type-I universes are more sensitive to anisotropy and exhibit polarization coupling, unlike in Robertson-Walker universes.

Findings

Gravitational waves are not equivalent to scalar fields in Bianchi Type-I universes.

They gain an effective mass term due to anisotropy.

A coupling between polarization states is identified.

Abstract

The propagation of classical gravitational waves in Bianchi Type-I universes is studied. We find that gravitational waves in Bianchi Type-I universes are not equivalent to two minimally coupled massless scalar fields as it is for the Robertson-Walker universe. Due to its tensorial nature, the gravitational wave is much more sensitive to the anisotropy of the spacetime than the scalar field is and it gains an effective mass term. Moreover, we find a coupling between the two polarization states of the gravitational wave which is also not present in the Robertson-Walker universe.