Quantum effects in the early universe. III. Dissipation of anisotropy by scalar particle production

The dissipation of small amounts of anisotropy by particle production in homogeneous, spatially flat cosmologies is studied using the effective-action method. As a particular model we consider the production of conformally invariant scalar particles in a universe which also contains some classical radiation. We find that there is significant dissipation of anisotropy near the singularity. Calculations of the classical geometry, the particle production and vacuum persistence amplitudes, the spectrum of produced particles and their back reaction on the geometry are all discussed to lowest nonvanishing order in the deviation from isotropy.