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 $\frac{ H^2}{ M^2_P}$.
Categories: gr-qc hep-th