  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
temperatures. 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.
