Observable Chiral Gravitational Waves from Inflation in String Theory

TL;DR

This paper shows that in string theory inflation models, gauge field excitations coupled with axions can produce observable, maximally chiral gravitational waves, even in small field inflation scenarios.

Contribution

It demonstrates a mechanism for generating observable chiral gravitational waves during string theory inflation involving D7 branes and axion couplings, extending to monodromy cases.

Findings

Observable chiral gravitational waves can be produced in small field inflation.

The mechanism is compatible with moduli stabilization and various inflationary dynamics.

The gravitational wave spectrum is maximally chiral.

Abstract

We consider gravitational wave production during inflation in type IIB string theory, and the possibility of observable gravitational waves in small field inflation. We show that the gauge field excitations on a set of coincident D7 branes, itself critical for moduli stabilization and hence intrinsic to inflation in string theory, coupled with axion fields from bulk fluxes, can act as a spectator sector during inflation. This results in a large production of chiral gravitational waves, even for relatively small values of the axion-gauge field coupling. We extend this to include a monodromy for the axion, and demonstrate that in both cases an observable level of gravitational waves is produced in small field inflation in string theory, with a spectrum that is maximally chiral. Finally, we demonstrate the consistency with moduli stabilization and with arbitrary (large or small field) inflationary dynamics of the host model, considering as an explicit example Kahler Moduli Inflation.