Hydrodynamic Gradient Expansion Diverges beyond Bjorken Flow

Understand

The gradient expansion is the fundamental organising principle underlying relativistic hydrodynamics, yet understanding its convergence properties for general nonlinear flows has posed a major challenge.

• We introduce a simple method to address this question in a class of fluids modelled by Israel-Stewart--type relaxation equations.
• We apply it to (1+1)-dimensional flows and provide numerical evidence for factorially divergent gradient expansions.
• This generalises results previously only obtained for (0+1)-dimensional comoving flows, notably Bjorken flow.