Effective perfect fluids in cosmology

TL;DR

This paper develops an effective field theory framework for describing perfect fluids in cosmology, capturing their dynamics and perturbations with symmetry principles, and providing a flexible tool for higher-order analysis.

Contribution

It introduces a symmetry-based effective action approach for perfect fluids in cosmology, unifying fluid and metric perturbations and enabling advanced perturbation calculations.

Findings

Framework describes fluid and metric perturbations coherently.

Perturbations linked to adiabatic and vorticity modes.

Flexible formalism suitable for higher-order cosmological analysis.

Abstract

We describe the cosmological dynamics of perfect fluids within the framework of effective field theories. The effective action is a derivative expansion whose terms are selected by the symmetry requirements on the relevant long-distance degrees of freedom, which are identified with comoving coordinates. The perfect fluid is defined by requiring invariance of the action under internal volume-preserving diffeomorphisms and general covariance. At lowest order in derivatives, the dynamics is encoded in a single function of the entropy density that characterizes the properties of the fluid, such as the equation of state and the speed of sound. This framework allows a neat simultaneous description of fluid and metric perturbations. Longitudinal fluid perturbations are closely related to the adiabatic modes, while the transverse modes mix with vector metric perturbations as a consequence of vorticity conservation. This formalism features a large flexibility which can be of practical use for higher order perturbation theory and cosmological parameter estimation.