W. Zimdahl, REACTING FLUIDS IN THE EXPANDING UNIVERSE - A NEW MECHANISM FOR ENTROPY PRODUCTION, Monthly Notices of the Royal Astronomical Society, 288(3), 1997, pp. 665-673
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.