Reacting fluids in the expanding universe: a new mechanism for entropy production

It is shown that two reacting cosmological fluids, each of them perfect onits own, which exchange energy and momentum without preserving particlenumbers, give rise to an entropy producing `reactive' bulk stress of the systemas a whole, as soon as the detailed balance between decay and inverse decayprocesses is perturbed. This demonstrates explicitly that particle generationis dynamically equivalent to an effective bulk pressure. We derive asemiquantitative formula for the corresponding new kinetic coefficient andevaluate the latter for the out-of-equilibrium decay of heavy, nonrelativisticparticles into radiation. It turns out that the associated reactive bulkviscosity may be more than one order of magnitude larger than the conventionalbulk viscosity, calculated, e.g., in radiative hydrodynamics.Comment: 13 pages, Revtex, submitted to MNRA