Extending Israel-Stewart theory: Causal bulk viscosity at large gradients

We present a class of relativistic fluid models for cold and dense matter with bulk viscosity, whose equilibrium equation of state is polytropic. These models reduce to Israel-Stewart theory for small values of the viscous stress $\mathrm{\ensuremath{\Pi}}$. However, when $\mathrm{\ensuremath{\Pi}}$ becomes comparable to the equilibrium pressure $P$, the evolution equations ``adjust'' to prevent the onset of far-from-equilibrium pathologies that would otherwise plague Israel-Stewart. Specifically, the equations of motion remain symmetric hyperbolic and causal at all times along any continuously differentiable flow, and across the whole thermodynamic state space. This means that, no matter how fast the fluid expands or contracts, the hydrodynamic equations are always well behaved (away from singularities). The second law of thermodynamics is enforced exactly. Near equilibrium, these models can accommodate an arbitrarily complicated dependence of the bulk viscosity coefficient $\ensuremath{\zeta}$ on both density and temperature.