We calculate the exact frequency redshift for fields coupled to gravity in Robertson-Walker backgrounds. The exact redshift factorizes and is proportional to the naive Doppler shift times a term representing diffractive effects. These diffractive corrections can be large for field modes with wavelengths on the order of the horizon size. Implications for cosmological density perturbations are discussed.

• Received 1 February 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.2553