  Starting from a Caldeira-Leggett model for the interaction of a system with
an environment, Joichi, Matsumoto, and Yoshimura have reconsidered the
derivation of the quantum Boltzmann equation. They find an extra term that
accounts for the effects of virtual particles, and which drastically changes
the results for relic densities of stable, weakly interacting massive particles
(WIMPs), and for the decay products of unstable particles. We show, however,
that this modified Boltzmann equation does not properly account for the
interaction energy between the massive particles (which are decaying or
annihilating) and the thermal bath of light particles. We argue that the
conventional Boltzmann equation gives the correct result.
