Chromo-natural inflation supported by enhanced friction from Horndeski gravity

TL;DR

This paper extends chromo-natural inflation by incorporating Horndeski gravity to enhance friction on the axion, delaying gauge field activation, reducing gravitational wave overproduction, and improving agreement with CMB data.

Contribution

It introduces a general framework for enhanced friction in chromo-natural inflation using Horndeski gravity, identifying a novel model with better observational fit.

Findings

New enhanced friction model aligns within 1σ of CMB measurements.

Universal analytic formula for gravitational-wave spectrum derived.

Delayed gauge field activation suppresses early gravitational wave overproduction.

Abstract

We study the extension of the chromo-natural inflation model by incorporating nonminimal coupling between the axion field and gravity. Nonminimal coupling is introduced so that it enhances friction in the axion's equation of motion and thus supports slow-roll inflation. This enhanced friction effectively delays the activation of the gauge field, thereby preventing the overproduction of gravitational waves in the CMB scale. We extend previous results by describing the nonminimal coupling in a general and unifying way utilizing Horndeski gravity. This allows us to explore systematically and comprehensively possible enhanced friction models of chromo-natural inflation consistent with observations. We find a novel enhanced friction model that shows better agreement (within 1$\sigma$) with CMB measurements than the previous nonminimally coupled chromo-natural inflation model. The gravitational-wave spectrum starts to rise at some wavenumber due to retarded activation of the gauge field in the late stage of inflation. We show how one can identify the wavenumber at which this occurs based on the background evolution and present a universal analytic formula for the gravitational-wave spectrum that can be used for any enhanced friction model of chromo-natural inflation.