Primordial Gravitational Wave Birefringence in a de Sitter Background with Chern-Simons Coupling

TL;DR

This paper explores how Chern-Simons modifications to gravity induce parity-violating birefringence in primordial gravitational waves within a de Sitter universe, linking it to axion-like dark matter.

Contribution

It derives the effects of Chern-Simons coupling on tensor perturbations, revealing chiral corrections and a permanent parity-violating imprint in primordial gravitational waves.

Findings

Birefringence causes amplitude and velocity differences in gravitational waves.

The phase difference between polarizations grows quadratically inside the horizon.

Connecting Chern-Simons field dynamics to axion-like dark matter phenomenology.

Abstract

In this work, we investigate tensor perturbations in a de Sitter background within the framework of Chern-Simons modified gravity. We introduce transverse-traceless perturbations and analyze how the Chern-Simons Cotton tensor induces parity-violating modifications to gravitational wave propagation, while the Pontryagin density vanishes at linear order. Using a mode decomposition of the scalar background field, we derive the sub- and super-horizon limits of the wave equations and uncover chiral corrections in the dispersion relations of tensor modes. The resulting birefringence exhibits both amplitude and velocity components, alternating with the phase of the scalar field. Particular solutions sourced by the scalar background show helicity-dependent amplification and a characteristic scaling of the radiated flux that reduces smoothly to the Minkowski limit. The accumulated phase difference between right- and left-handed modes grows quadratically inside the horizon and becomes frozen outside, leaving a permanent parity-violating imprint in the primordial tensor spectrum. Finally, by promoting the Chern-Simons field to a massive dark matter candidate, we demonstrate how its mass-dependent dynamics connect gravitational birefringence to axion-like dark matter phenomenology.