Non-equilibrium effects on particle freeze-out in the early universe

We investigate the possible effects that deviations from kinetic equilibrium can have on massive particles as they decouple from chemical equilibrium. Different methods of solving the Boltzmann equation yield significantly different relic number densities of such particles. General considerations concerning the Dirac or Majorana structure of the particles are discussed. It is shown that non-equilibrium effects are small for particles decoupling while strongly non-relativistic, as will be the case for most cold dark matter candidates.