In this article we give sufficient and necessary conditions for the validity of the Huygens' principle for the Dirac equation in the non-constant curvature spacetime of the Friedmann-Lemaître-Robertson-Walker models of cosmology. The Huygens' principle discussed for the equation of a field with mass m=0 as well as a massive spin-12 field undergoing a red shift of its wavelength as the universe expands.
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