  The usual (Bunch-Davies) Feynman propagator of a massless field is not well
defined in an expanding universe due to the presence of infrared divergences.
We propose a new propagator which yields infrared finite answers to any
correlation function. The key point is that in a de Sitter spacetime there is
an ambiguity in the zero-mode of the propagator. This ambiguity can be used to
cancel the apparent divergences which arise in some loop calculations in
eternally (or semi-eternally) inflating spacetime. We refer to this process as
zero-mode modification. The residual ambiguity is fixed by observational
measurement. We also discuss the application of this method to calculations
involving the graviton propagator.
