We investigate the relic density n χ of non-relativistic long-lived or stable particles χ in cosmological scenarios in which the temperature T is too low for χ to achieve full chemical equilibrium. The case with a heavier particle decaying into χ is also investigated. We derive approximate solutions for n χ (T ) which accurately reproduce numerical results when full thermal equilibrium is not achieved. If full equilibrium is reached, our ansatz no longer reproduces the correct temperature

more »

... ence of the χ number density. However, it does give the correct final relic density, to an accuracy of about 3% or better, for all cross sections and initial temperatures. This talk is based on our work in Ref. [1] .