  In extended models of gravity a nonminimal coupling to matter has been
assumed to lead to irreversible particle creation. In this paper we challenge
this assumption. We argue that a non-minimal coupling of the matter and
gravitational sectors results in a change in particle-momentum on a
cosmological timescale, irrespective of particle creation or decay. We further
argue that particle creation or decay associated with a non-minimal coupling to
gravity could only happen as a result of significant deviations from a
homogeneous Friedmann-Robertson-Walker geometry on microscopic scales, and
provide a phenomenological description of the impact of particle creation or
decay on the cosmological evolution of the density of the matter fields.
