  Fluctuations in the mass and decay rate of a heavy particle which for some
period dominates the energy density of the universe are known to lead to
adiabatic density perturbations. We show that generically the annihilation
cross section of the same particle also receives fluctuations, which leads to
entropy perturbations at freeze-out. If the particle comes to dominate the
energy density of the universe and subsequently decays, this leads to an
additional source of adiabatic density perturbations. On the other hand,
non-adiabatic density perturbations result when the particle does not decay but
contributes to the observed dark matter.
