  I present the procedure for integrating out quantum fields whose mass M is
well above the Hubble scale H in de Sitter space. The effective interaction and
density matrix are explicitly computed for a simple example and are found to be
of order H/M, are nonlocally distributed in time and contain a highly
oscillatory phase. This radical departure from the static space-time result
demonstrates that these new tools are necessary for constructing effective
inflationary potentials. The effective coupling naturally forms a perturbative
series in the relative energy transferred \Delta E/M, whose coefficients can
then be compared to observation.
