Dynamic Field Theory and Equations of Motion in Cosmology

TL;DR

This paper develops a covariant, field-theoretical framework using variational principles to derive equations of motion for cosmological perturbations, including dark matter, dark energy, and baryonic matter, expanding around the FLRW background.

Contribution

It introduces a gauge-invariant, asymptotic Lagrangian expansion for cosmological perturbations that does not require small perturbations, enabling successive post-Friedmannian approximations.

Findings

Derived covariant field equations for perturbations.

Formulated equations of motion for large and small scale inhomogeneities.

Applied framework to baryonic matter dynamics in cosmology.

Abstract

We discuss a field-theoretical approach based on variational principle to derive the field and hydrodynamic equations of motion of baryonic matter governed by cosmological perturbations of dark matter and dark energy. The action depends on the gravitational and matter Lagrangian. The gravitational Lagrangian depends on the metric tensor and its first and second derivatives. The matter Lagrangian includes dark matter, dark energy and the ordinary baryonic matter. The total Lagrangian is expanded in an asymptotic Taylor series around the background manifold defined as a solution of Einstein's equations in the form of the Friedmann-Lemaitre-Robertson-Walker (FLRW) metric tensor. The small parameter of the decomposition is the magnitude of the metric tensor perturbation. Each term of the series expansion is gauge-invariant and all of them together form a basis for the successive post-Friedmannian approximations. The approximation scheme is covariant and the asymptotic nature of the Lagrangian decomposition does not require the post-Friedmannian perturbations to be small though computationally it works the most effectively when the perturbed metric is close enough to the background FLRW metric. The temporal evolution of the background metric is governed by dark matter and dark energy and we associate the large scale inhomogeneities in these two components as those generated by the primordial cosmological perturbations. The small scale inhomogeneities are generated by the condensations of baryonic matter considered as the bare perturbations. We explicitly work out the covariant field equations of the successive post-Friedmannian approximations of Einstein's equations and derive equations of motion of large and small scale inhomogeneities of dark matter and dark energy. We apply these equations to derive the post-Friedmannian equations of motion of the baryonic matter.