Generalized equilibrium of cosmological fluids in second-order thermodynamics

Combining the second-order entropy flow vector of the causal Israel-Stewart theory with the conformal Killing-vector property of u(i)/T, where u(i) is the 4-velocity of the medium and T is its equilibrium temperature, we inve stigate generalized equilibrium states for cosmological fluids with non-con served particle number. We calculate the corresponding equilibrium particle production rate and show that this quantity is reduced compared with the r esults of the previously studied first-order theory. Generalized equilibriu m for massive particles turns out to be compatible with a dependence rho pr oportional to a(-2) of the fluid energy density rho on the scale factor a o f the Robertson-Walker metric and may be regarded as a realization of so-ca lled K-matter.