Gravitational waves from p-form inflation
  Recently it was shown that an inflationary background can be realized by any
$p$-form field non-minimally coupled to gravity. In this paper, we study
gravitational waves generated during p-form inflation. Even though the
background evolution is identical to that in conventional scalar field
inflation, the behavior of gravitational waves is different in p-form
inflation. In particular, we find that the propagation speed of gravitational
waves differs from unity in 2- and 3-form inflationary models. We point out
that the squared speed becomes negative in the large field models. The small
field models are free from pathologies and the correction to the spectrum of
gravitational waves turns out to be very small.
