  In this paper we analyze the possibility of detecting nontrivial quantum
phenomena in observations of the temperature anisotropy of the cosmic
background radiation (CBR), for example, if the Universe could be found in a
coherent superposition of two states corresponding to different CBR temperature
self-correlations. Such observations are sensitive to scalar primordial
fluctuations but insensitive to tensor fluctuations, which are therefore
converted into an environment for the former. Even for a free inflaton field
minimally coupled to gravity, scalar-tensor interactions induce enough
decoherence among histories of the scalar fluctuations as to render them
classical under any realistic probe of their amplitudes.
