Disformally coupled inflation
  A disformal coupling between two scalar fields is considered in the context
of cosmological inflation. The coupling introduces novel derivative
interactions mixing the kinetic terms of the fields but without introducing
superluminal or unstable propagation of the two scalar fluctuation modes.
Though the typical effect of the disformal coupling is to inhibit one of the
fields from inflating the universe, the energy density of the other field can
drive viable near Sitter -inflation in the presence of nontrivial disformal
dynamics, in particular when one assumes exponential instead of power-law form
for the couplings. The linear perturbation equations are written for the
two-field system, its canonical degrees of freedom are quantised, their spectra
are derived and the inflationary predictions are reported for numerically
solved exponential models. A generic prediction is low tensor-to-scalar ratio.
