Non-linear causal bulk viscosity in Unified Dark Matter Cosmologies

Abstract

We propose a bulk viscous unified dark matter scenario based on a nonlinear extension of the full causal Israel-Stewart theory. This framework allows the viscous fluid to remain far from equilibrium, an essential feature for a physically consistent description of viscosity-driven accelerated expansion. We adopt the standard parametrization for the bulk viscosity, $\xi = \xi_{0} \rho_{m}^{s}$, treating $s$ as a free parameter (in contrast to most previous works), and study the model in a spatially flat Friedmann-Robertson-Walker background. By reformulating the cosmological equations as an autonomous dynamical system, we obtain both asymptotic analytical solutions and a numerical characterization of the phase space. At early times, the viscous component can mimic a stiff fluid, while at intermediate epochs it behaves like dark matter. With a suitable choice of dynamical variables, the system admits three distinct classes of late-time attractors. Two of them are separated by a basin-boundary saddle point: (i) a generic quintessence solution for $s = 1/2$, which encompasses a de Sitter-like behavior when $\xi_{0}$ satisfies a specific relation involving the nonlinear parameters; (ii) a global exact de Sitter attractor for $s < 1/2$; and (iii) a phantom-like solution that emerges for $s \ge 1/2$. In contrast to the generic $s \ne 1/2$ case, the $s = 1/2$ scenario exhibits a qualitatively different stability structure, allowing de Sitter and phantom attractors to coexist. All solutions respect entropy production, and cosmic acceleration emerges independently of $\xi_{0}$, relaxing the strong bounds $\xi_{0} \sim \mathcal{O}(1)$ required in Eckart-based viscous models.