  To model a realistic situation for the beginning we consider massive real
scalar $\phi^4$ theory in a (1+1)-dimensional asymptotically static Minkowski
spacetime with an intermediate stage of expansion. To have an analytic headway
we assume that scalars have a big mass. At past and future infinities of the
background we have flat Minkowski regions which are joint by the inflationary
expansion region. We use the tree-level Keldysh propagator in the theory in
question to calculate the expectation value of the stress-energy tensor which
is, thus, due to the excitations of the zero-point fluctuations. Then we show
that even for large mass, if the de Sitter expansion stage is long enough, the
quantum loop corrections to the expectation value of the stress-energy tensor
are not negligible in comparison with the tree-level contribution. That is
revealed itself via the excitation of the higher-point fluctuations of the
exact modes: During the expansion stage a non-zero particle number density for
the exact modes is generated. This density is not Plankian and serves as a
quench which leads to a thermalization in the out Minkowski stage.
