  This article presents the derivation of the stress-energy tensor of a free
scalar field with a general non-linear dispersion relation in curved spacetime.
This dispersion relation is used as a phenomelogical description of the short
distance structure of spacetime following the conventional approach of
trans-Planckian modes in black hole physics and in cosmology. This
stress-energy tensor is then used to discuss both the equation of state of
trans-Planckian modes in cosmology and the magnitude of their backreaction
during inflation. It is shown that gravitational waves of trans-Planckian
momenta but subhorizon frequencies cannot account for the form of cosmic vacuum
energy density observed at present, contrary to a recent claim. The
backreaction effects during inflation are confirmed to be important and generic
for those dispersion relations that are liable to induce changes in the power
spectrum of metric fluctuations. Finally, it is shown that in pure de Sitter
inflation there is no modification of the power spectrum except for a possible
magnification of its overall amplitude independently of the dispersion
relation.
