  A Schroedinger picture analysis of time dependent quantum oscillators, in a
manner of Guth and Pi, clearly identifies two physical mechanisms for possible
decoherence of vacuum fluctuations in early universe: turning of quantum
oscillators upside-down, and rapid squeezing of upside-right oscillators so
that certain squeezing factor diverges. In inflationary cosmology the former
mechanism explains the stochastic evolution of light inflatons and the
classical nature of density perturbations in most of inflationary models, while
the later one is responsible for the classical evolution of relatively heavy
fields, with masses in a narrow range above the Hubble parameter: 2 < (m/H_0)^2
< 9/4. The same method may be applied to study of the decoherence of quantum
fluctuations in any Robertson-Walker cosmology.
