Interacting viscous mixtures

Gravitational and hydrodynamical perturbations are analysed in a relativistic plasma containing a mixture of interacting fluids characterized by a non-negligible bulk viscosity coefficient. The energy-momentum transfer between the cosmological fluids, as well as the fluctuations of the bulk viscosity coefficients, are analyzed simultaneously with the aim of deriving a generalized set of evolution equations for the entropy and curvature fluctuations. For typical length scales larger than the Hubble radius, the fluctuations of the bulk viscosity coefficients and of the decay rate provide source terms for the evolution of both the curvature and the entropy fluctuations. According to the functional dependence of the bulk viscosity coefficient on the energy densities of the fluids composing the system, the mixing of entropy and curvature perturbations is scrutinized both analytically and numerically.