Blue running of the primordial tensor spectrum
  We examine the possibility of positive spectral index of the power spectrum
of the primordial tensor perturbation produced during inflation in the light of
the detection of the B-mode polarization by the BICEP2 collaboration. We find a
blue tilt is in general possible when the slow-roll parameter decays rapidly.
We present two known examples in which a positive spectral index for the tensor
power spectrum can be obtained. We also briefly discuss other consistency tests
for further studies on inflationary dynamics.
