Backscattering caused by the expansion of the Universe

We show that in a Robertson-Walker space-time, expansion-induced backscattering of electromagnetic waves is rigorously absent, contrary to earlier expectations. The absence of backscattering is a direct consequence of the conformal invariance of the equations of motion, and therefore extends to neutrinos and gravitational waves. Any non-null observation of back-scattering caused by the expansion would indicate that there exists expansion anisotropy.