Backscattering Caused by the Expansion of the Universe

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Abstract
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.