Chiral primordial blue tensor spectra from the axion-gauge couplings

TL;DR

This paper proposes a model where axion-gauge couplings during inflation produce primordial blue-tilted gravitational waves with small chirality, which could be detected by future space-based gravitational wave observatories.

Contribution

It introduces a novel inflationary scenario with axion-gauge couplings inspired by string theory, generating distinctive primordial gravitational wave spectra.

Findings

Primordial tensor spectra can be blue-tilted with small chirality.

Both polarization modes of gauge fields are amplified during inflation.

Potential detectability of these signals by DECIGO and BBO.

Abstract

We suggest the new feature of primordial gravitational waves sourced by the axion-gauge couplings, whose forms are motivated by the dimensional reduction of the form field in the string theory. In our inflationary model, as an inflaton we adopt two types of axion, dubbed the model-independent axion and the model-dependent axion, which couple with two gauge groups with different sign combination each other. Due to these forms both polarization modes of gauge fields are amplified and enhance both helicies of tensor modes during inflation. We point out the possibility that a primordial blue-tilted tensor power spectra with small chirality are provided by the combination of these axion-gauge couplings, intriguingly both amplitudes and chirality are potentially testable by future space-based gravitational wave interferometers such as DECIGO and BBO project.