REACTING FLUIDS IN THE EXPANDING UNIVERSE - A NEW MECHANISM FOR ENTROPY PRODUCTION

It is shown that two reacting cosmological fluids, each of them perfec t on its own, which exchange energy and momentum without preserving pa rticle numbers, give rise to an entropy producing 'reactive' bulk stre ss pf the system as a whole, as soon as the detailed balance between d ecay and inverse decay processes is perturbed. This demonstrates expli citly that particle generation is dynamically equivalent to an effecti ve bulk pressure. We derive a semiquantitative formula for the corresp onding new kinetic coefficient and evaluate the latter for the out-of- equilibrium decay of heavy, non-relativistic particles into radiation. It turns out that the associated reactive bulk viscosity may be more than one order of magnitude larger than the conventional bulk viscosit y, calculated, e.g., in radiative hydrodynamics.