Scale-dependent gravitational waves from a rolling axion

TL;DR

This paper explores a model where a rolling pseudo-scalar field during inflation sources gravitational waves and scalar perturbations, potentially explaining observable B-modes while remaining consistent with current scalar perturbation limits.

Contribution

It introduces a scenario where a rolling axion-like field produces scale-dependent gravitational waves and scalar perturbations through gravitational coupling during inflation.

Findings

Potential to generate observable B-modes in CMB polarization.

Scalar perturbations remain within current observational limits.

Parameter choices can lead to localized non-Gaussian features.

Abstract

We consider a model in which a pseudo-scalar field $\sigma$ rolls for some e-folds during inflation, sourcing one helicity of a gauge field. These fields are only gravitationally coupled to the inflaton, and therefore produce scalar and tensor primordial perturbations only through gravitational interactions. These sourced signals are localized on modes that exit the horizon while the roll of $\sigma$ is significant. We focus our study on cases in which the model can simultaneously produce (i) a large gravitational wave signal, resulting in observable B-modes of the CMB polarizations, and (ii) sufficiently small scalar perturbations, so to be in agreement with the current limits from temperature anisotropies. Different choice of parameters can instead lead to a localized and visible departure from gaussianity in the scalar sector, either at CMB or LSS scales.