Gravitational waves induced by massless vector fields with non-minimal coupling to gravity

TL;DR

This paper investigates how a non-minimally coupled massless vector field influences primordial gravitational waves, revealing scale-invariant corrections and potential amplification effects during the early universe.

Contribution

It introduces a calculation of the vector field's contribution to gravitational wave spectra considering non-minimal coupling, highlighting potential amplification mechanisms.

Findings

Correction is scale-invariant and of order H^4/M_P^4

Non-minimal coupling introduces a dependence on H^2/M^2

Correlation function can be amplified up to H^2/M_P^2

Abstract

In this paper, we calculate the contribution of the late time mode of a massless vector field to the power spectrum of the primordial gravitational wave using retarded Green's propagator. We consider a non-trivial coupling between gravity and the vector field. We find that the correction is scale-invariant and of order $\frac{H^4}{M_P^4}$. The non-minimal coupling leads to a dependence of $\frac{H^2}{M^2}$, which can amplify the correlation function up to the level of $\frac{H^2}{M^2_P}$.