  We argue that in at least a portion of the history of the universe the relic
background neutrinos are spatially-extended, coherent superpositions of mass
states. We show that an appropriate quantum mechanical treatment affects the
neutrino mass values derived from cosmological data. The coherence scale of
these neutrino flavor wavepackets can be an appreciable fraction of the causal
horizon size, raising the possibility of spacetime curvature-induced
decoherence.
