“…Israel and Stewart (IS) put forward an approach [35] where linearized disturbances around global equilibrium can be causal and stable [36], if certain conditions for the fluid's equation of state and transport coefficients (e.g., bulk and shear viscosities) are fulfilled. However, despite recent progress [37,38], very little is known about the properties and the constraints that must be fulfilled in these theories in the nonlinear regime, which can be important in simulations already in flat spacetime [39] and, also, when embedding such fluid models in dynamical spacetimes. In fact, in the context of viscous dark fluid modeling, it is not known how the recently found nonlinear constraints [37,38] coming from causality affect previous conclusions drawn from such Israel-Stewart-like models (e.g., [10,26,27]).…”