  We consider dynamics of the massive minimally coupled scalar field theory in
an expanding Friedmann-Lemaitre-Robertson-Walker universe. We consider the
standard toy model of the conformally flat space-time where the conformal
factor becomes constant at the distant past and the distant future. Employing
Schwinger-Keldysh diagrammatic technique, we compute infrared loop corrections
to the occupation number and anomalous quantum average of the scalar field and
show that these corrections are growing with time. Using these observations, we
demonstrate that the regularized stress-energy tensor at the distant future
acquires substantial quantum corrections which exceed the long known tree-level
contributions to the particle flux.
