  A calculation of the one-mode occupation numbers for vacuum fluctuations of
massive fields in De Sitter space shows that their use for the generation of
classical density perturbations in inflationary cosmology very much depends on
their masses and conformal couplings. A new mechanism for the decoherence of
relatively massive fields has been identified. A similar analysis of the power
law inflation shows that production of adiabatic density perturbations may not
take place in models with power p < 3.
