  We study the dissipation of oscillating scalar backgrounds in a spatially
flat Friedmann-Lema\^{i}tre-Robertson-Walker universe using non-equilibrium
quantum field theory. To be concrete, a $Z_2$-symmetric two-scalar model with
quartic interactions is used. For quasi-harmonic oscillations, we adopt the
multi-scale analysis to obtain analytical approximate expressions for the
evolution of the scalar background in terms of the retarded self-energy and
retarded proper four-vertex function. Different from the case in flat
spacetime, we find that in an expanding universe the condensate decay in this
model can be complete only if the imaginary part of the retarded self-energy is
not negligibly small. The microphysical interpretation of the imaginary parts
of the retarded self-energy and retarded proper four-vertex function in terms
of particle production is also discussed.
