Non-linear causal bulk viscosity in unified dark matter cosmologies

We propose a bulk viscous unified dark matter scenario based on a non-linear extension of the full causal Israel-Stewart theory. This framework allows the viscous f luid to remain far from equilibrium- an essential feature for a physically consistent description of viscosity-driven accel erated expansion. We adopt the standard parametrization for the bulk viscosity, xi = xi(0) rho(s)(m) , 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 dynamicalsystem,weobtainbothasymptoticanalyticalsolu tions and a numerical characterization of the phase space. At early times, the viscous component can mimic a stiff fluid, while at intermediate epoch sit 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 Sit ter attractor for s < 1/2; and (iii) a phantom-like solution that emerges for s >= 1/2. In contrast to the generic s not equal 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 cos mic acceleration emerges independently of xi(0) , relaxing the strong bounds, xi(0) similar to O(1), required in Eckart-based viscous models.