Gauge Fields and Inflation: Chiral Gravitational Waves, Fluctuations and the Lyth Bound

TL;DR

This paper explores how non-Abelian gauge fields during inflation can produce observable, chiral gravitational waves without requiring super-Planckian field excursions, but finds scalar fluctuations render the model observationally unviable.

Contribution

It analyzes the generation of chiral gravitational waves in Chromo-Natural Inflation and assesses their observational implications, challenging previous assumptions about the Lyth bound.

Findings

Chiral gravitational waves can be produced at observable levels.

Scalar fluctuations make the model incompatible with observations.

The model evades the Lyth bound but remains observationally unviable.

Abstract

Models of inflation involving non-Abelian gauge field backgrounds can produce gravitational waves at an observable level with a preferred handedness. This asymmetry comes about because the non-Abelian background generates parity-violation in the action for perturbations. In the specific model we study, Chromo-Natural Inflation, these gravitational waves can be produced at observable levels even when no field makes a super-Planckian field excursion, thus evading a common formulation of the Lyth bound. Unfortunately, when considered in concert with the scalar fluctuations, this chiral enhancement of the gravitational waves makes the model observationally inviable.