The effective theory of fluids at NLO and implications for dark energy

TL;DR

This paper develops an advanced effective fluid theory at next-to-leading order, revealing new features relevant for cosmological acceleration and dark energy modeling, including anisotropic stress and modified wave propagation.

Contribution

It introduces a novel operator at NLO in the effective theory of fluids, linking high-derivative effects with cosmological perturbations and stability considerations.

Findings

The theory predicts anisotropic stress and non-adiabatic sound speeds in cosmological fluids.

Modifications to vector and tensor mode equations are identified.

A stability condition related to an energy scale controlling high-derivative terms is established.

Abstract

We present the effective theory of fluids at next-to-leading order in derivatives, including an operator that has not been considered until now. The power-counting scheme and its connection with the propagation of phonon and metric fluctuations are emphasized. In a perturbed FLRW geometry the theory presents a set of features that make it very rich for modelling the acceleration of the Universe. These include anisotropic stress, a non-adiabatic speed of sound and modifications to the standard equations of vector and tensor modes. These effects are determined by an energy scale which controls the size of the high derivative terms and ensures that no instabilities appear.