FLUID COSMOLOGY WITH DECAY AND PRODUCTION OF PARTICLES

We investigate the influence of particle number nonconserving processe s on the thermal history and the dynamics of the universe. There occur s a backreaction both on the temperature and on the scale factor if pa rticles of one species decay into particles of another species, or if the overall particle number changes. This backreaction is determined b y the ratios of the relevant decay and production rates to the Hubble expansion rate. It results in reheating phenomena and modifications of the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) expansion law s. A high production rate at the Planck scale tends to avoid the cosmo logical singularity.