Constructing Infrared Finite Propagators in Inflating Space-time

TL;DR

This paper introduces a new infrared finite propagator for massless fields in inflating spacetime by modifying the zero-mode, resolving divergences in loop calculations and applicable to graviton propagators.

Contribution

It proposes a zero-mode modification method to construct infrared finite propagators in de Sitter space, addressing divergence issues in quantum field calculations during inflation.

Findings

The new propagator yields finite correlation functions in de Sitter space.

Zero-mode ambiguity can be fixed by observational data.

Method applicable to graviton propagator calculations.

Abstract

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.