  At momenta much higher than the Hubble scale, the cosmological expansion can
be thought of as a weak gravitational field. We consider QFT in a particularly
convenient set of coordinates that makes this manifest, so that, for those high
momenta, the effects of the cosmological expansion can be dealt with using the
standard tools of perturbation theory in Minkwoski space. In this way, we
re-derive standard results of QFT in a cosmological background, such as
mode-stretching and gravitational particle production. We discuss the
implications of our results for the trans-Planckian problem.
