  Motivated by a recent finding of an exact solution of the relativistic
Boltzmann equation in a Friedmann-Robertson-Walker spacetime, we implement this
metric into the newly developed transport approach Simulating Many Accelerated
Strongly-interacting Hadrons (SMASH). We study the numerical solution of the
transport equation and compare it to this exact solution for massless
particles. We also compare a different initial condition, for which the
transport equation can be independently solved numerically. Very nice agreement
is observed in both cases. Having passed these checks for the SMASH code, we
study a gas of massive particles within the same spacetime, where the particle
decoupling is forced by the Hubble expansion. In this simple scenario we
present an analysis of the freeze-out times, as function of the masses and
cross sections of the particles. The results might be of interest for their
potential application to relativistic heavy-ion collisions, for the
characterization of the freeze-out process in terms of hadron properties.
