  We study the quantum propagation of particles in cosmological backgrounds, by
considering a doublet of massive scalar fields propagating in an expanding
universe, possibly filled with radiation. We focus on the dissipative effects
related to the expansion rate. At first order, we recover the expected result
that the decay rate is determined by the local temperature. Beyond linear
order, the decay rate has an additional contribution governed by the expansion
parameter. This latter contribution is present even for stable particles in the
vacuum. Finally, we analyze the long time behaviour of the propagator and
briefly discuss applications to the trans-Planckian question.
