Nonlinear Dynamics of Tensor Modes in Conformal Real Relativistic Fluids

TL;DR

This paper investigates the nonlinear behavior of tensor modes in conformal relativistic fluids using Divergence Type Theories, revealing stochastic excitation mechanisms and scale-invariant spectra that could influence entropy cascades.

Contribution

It introduces a second order, non-linear framework for tensor modes in conformal fluids, incorporating stochastic effects and deriving correlation functions.

Findings

Tensor modes are excited by thermal fluctuations with a white spectrum at large scales.

The correlation functions show a scale-invariant spectrum at short scales.

Tensor modes may sustain an entropy cascade in the fluid.

Abstract

In the Second Order Theories (SOT) of real relativistic fluids, the non-ideal properties are described by a new set of dynamical tensor variables. In this work we explore the non-linear dynamics of those modes in a conformal fluid. Among all possible SOTs, we choose to work with the Divergence Type Theories (DTT) formalism, which ensures that the second law of thermodynamics is satisfied non-perturbatively. In considering a perturbative scheme within this formalism, at next to leading order a set of Maxwell-Cattaneo equations is obtained, as in e.g. Israel-Stewart theories. The tensor modes include two divergence-free modes which have no analog in theories based on covariant Navier-Stokes equations, and that are particularly relevant because they may couple linearly to a gravitational field. To study the dynamics of this irreducible tensor sector, we observe that in causal theories such as DTTs, thermal fluctuations induce a stochastic stirring force in the equations of motion, which excites the tensor modes while preserving energy momentum conservation. From fluctuation-dissipation considerations, it follows that the random force is Gaussian with a white spectrum. The irreducible tensor modes in turn excite vector modes, which back-react on the tensor sector, thus producing a consistent non-linear, second order description of the divergence-free tensor dynamics. Using the Martin-Siggia-Rose (MSR) formalism we obtain the two-point correlation function for these tensor modes at next to leading order, and the induced stochastic component of the energy-momentum tensor. We find that the thermal fluctuations induce a scale invariant spectrum at short scales, while preserving a white spectrum at large scales. This result suggests that tensor modes could sustain an entropy cascade.