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It has recently been shown that the presence of a spectator pseudoscalarfield, coupled to photons through a Chern-Simons term, can amplify theprimordial tensor spectrum without observationally disrupting the primordialscalar spectrum. The amplification occurs due to an instability that developsfor the vector fields. We consider the extension of previous studies to accountfor the contribution arising from an inhomogeneous vector background which isgenerated prior to the onset of inflation. We find that there may becontributions in which net momentum is transferred between the inhomogeneousvector background and the gravitons, which would give rise to a signaturedifferent than in the absence of the semiclassical corrections. We discuss theproperties the classical vector field form must have in order for thesesignatures to leave observable imprints, though we were unable to construct amodel for generating such a vector field.

Vector field instability and the primordial tensor spectrum