Local equilibrium solutions in simple anisotropic cosmological models,
  as described by relativistic fluid dynamics
  We test the physical relevance of the full and the truncated versions of the
Israel-Stewart theory of irreversible thermodynamics in a cosmological setting.
Using a dynamical systems method, we determine the asymptotic future of plane
symmetric Bianchi type I spacetimes with a viscous mathematical fluid, keeping
track of the magnitude of the relative dissipative fluxes, which determines the
applicability of the Israel-Stewart theory. We consider the situations where
the dissipative mechanisms of shear and bulk viscosity are involved separately
and simultaneously. It is demonstrated that the only case in the given model
when the fluid asymptotically approaches local thermal equilibrium, and the
underlying assumptions of the Israel-Stewart theory are therefore not violated,
is that of a dissipative fluid with vanishing bulk viscosity. The truncated
Israel-Stewart equations for shear viscosity are found to produce solutions
which manifest pathological dynamical features and, in addition, to be strongly
sensitive to the choice of initial conditions. Since these features are
observed already in the case of an oversimplified mathematical fluid model, we
have no reason to assume that the truncation of the Israel-Stewart transport
equations will produce relevant results for physically more realistic fluids.
The possible role of bulk and shear viscosity in cosmological evolution is also
discussed.
