Viscosity and matter creation in the early universe

Mechanical and thermodynamic aspects of the early universe are discussed. Adopting an isotropic and imperfect fluid model, we can introduce one single viscosity coefficient, viz. the bulk viscosity zeta. Allowing for particle creation or annihilation there is room for one additional coefficient, viz. the creation rate psi. Specializing to the FRW metric we consider the question, discussed in the recent literature, whether the viscosity/creation concepts describe after all one and the same physical process. We conclude that they do not. Thereafter considering the limitations on psi set by the second law of thermodynamics, we find that it is possible to account for the large nondimensional entropy in the universe (sigma similar to 10(9)) by ignoring viscosity altogether, and allowing for a particle sink (psi < 0) of large magnitude being operative during a brief time period. Numerical examples are given.