A new effective theory for stochastic relativistic hydrodynamics

¹ Illinois Center for Advanced Studies of the Universe Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
² Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, RJ, 22290-180, Brasil

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Published online: 17 April 2026

Abstract

Thermal fluctuations are a fundamental feature of dissipative systems that are essential for understanding physics near the expected critical point of QCD and in small systems. When such fluctuations are modeled naively in relativistic systems, strange features can appear such as negative self-correlation functions. We construct an effective theory for nonlinear stochastic relativistic hydrodynamics that ensure a well-posed mathematical formulation. Using Crooks fluctuation theorem, we derive a symmetry of the effective action that incorporates fluctuations through a suitable free energy functional. For divergence type theories, the action can then be fully specified using a single vector generating current. The equations of motion obtained using this procedure are guaranteed to be flux conservative and symmetric hyperbolic when the dynamics is causal. This ensures that these equations are well-posed (for suitable initial data) and are in a form that can easily be simulated, including with Metropolis techniques.