Reacting fluids in the expanding Universe: A new mechanism for entropy production

It is shown that two reacting cosmological fluids, each of them perfect on its own, which exchange energy and momentum without preserving particle numbers, give rise to an entropy producing `reactive' bulk stress of the system as a whole, as soon as the detailed balance between decay and inverse decay processes is perturbed. This demonstrates explicitly that particle generation is dynamically equivalent to an effective bulk pressure. We derive a semiquantitative formula for the corresponding new kinetic coefficient and evaluate the latter for the out-of-equilibrium decay of heavy, nonrelativistic 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 viscosity, calculated, e.g., in radiative hydrodynamics.