Gabriel Denicol (Universidade Federal Fluminense, Niterói, Brazil)

The applicability of hydrodynamics in describing the extreme conditions produced in heavy ion collisions has still not been properly justified from a theoretical point of view. Even more, the gradient expansion, commonly used to derive hydrodynamics from microscopic theory, has been recently shown to diverge for conformal fluids or relativistic gases undergoing Bjorken flow, putting under question the definition of hydrodynamics itself. Alternative derivations of the hydrodynamic series have been proposed recently, such as the slow-roll expansion or  a generalized Chapman-Enskog expansion, and can be promising candidates to define hydrodynamics.

In this talk, I discuss and present general analytical and semi-analytical solutions of the hydrodynamic attractor of Israel-Stewart theory and kinetic theory for Bjorken expanding fluids. We show that the gradient expansion diverges and, for Israel-Stewart theory, we show that even the slow-roll expansion, a commonly used approach to characterize the attractor, diverges. We find that there are examples in which the gradient expansion converges, but only for parameters choices which violate causality.