Vector field instability and the primordial tensor spectrum
  It has recently been shown that the presence of a spectator pseudoscalar
field, coupled to photons through a Chern-Simons term, can amplify the
primordial tensor spectrum without observationally disrupting the primordial
scalar spectrum. The amplification occurs due to an instability that develops
for the vector fields. We consider the extension of previous studies to account
for the contribution arising from an inhomogeneous vector background which is
generated prior to the onset of inflation. We find that there may be
contributions in which net momentum is transferred between the inhomogeneous
vector background and the gravitons, which would give rise to a signature
different than in the absence of the semiclassical corrections. We discuss the
properties the classical vector field form must have in order for these
signatures to leave observable imprints, though we were unable to construct a
model for generating such a vector field.
