Scale invariance of the primordial tensor power spectrum
  Future cosmic microwave background polarization experiments will search for
evidence of primordial tensor modes at large angular scales, in the multipole
range $4 \leq \ell \leq 50.$ Because in that range there is some mild evidence
of departures from scale invariance in the power spectrum of primordial
curvature perturbations, one may wonder about the possibility of similar
deviations appearing in the primordial power spectrum of tensor modes. Here we
address this issue and analyze the possible presence of features in the tensor
spectrum resulting from the dynamics of primordial fluctuations during
inflation. We derive a general, model independent, relation linking features in
the spectra of curvature and tensor perturbations. We conclude that even with
large deviations from scale invariance in the curvature power spectrum, the
tensor spectrum remains scale invariant for all observational purposes.
