String theoretic axion coupling and the evolution of cosmic structures

TL;DR

This paper investigates how axion coupling to gravitational terms influences cosmic structure evolution, revealing polarization asymmetries in gravitational waves that could be observable in the cosmic microwave background.

Contribution

It demonstrates that axion coupling causes polarization asymmetry in tensor perturbations without affecting scalar or vector perturbations, highlighting potential observational signatures.

Findings

Scalar and vector perturbations unaffected by axion coupling

Tensor polarization asymmetry generated during inflation

Polarization asymmetry conserved in large-scale evolution

Abstract

We examine the effects of the axion coupling to $R\tilde{R}$ on the evolution of cosmic structures. It is shown that the evolutions of the scalar- and vector-type perturbations are not affected by this axion coupling. However the axion coupling causes an asymmetric evolution of the two polarization states of the tensor-type perturbation, which may lead to a sizable polarization asymmetry in the cosmological gravitational wave if inflation involves a period in which the axion coupling is important. The polarization asymmetry produced during inflation are conserved over the subsequent evolution as long as the scales remain in the large-scale limit, and thus this may lead to an observable trace in the cosmic microwave background radiation.