Hydrodynamic representation of the Klein-Gordon-Einstein equations in the weak field limit: I. General formalism and perturbations analysis

TL;DR

This paper develops a hydrodynamic formalism for the Klein-Gordon-Einstein equations in the weak field limit, analyzing scalar field perturbations and their growth, with implications for cosmological structure formation.

Contribution

It introduces a generalized Madelung transformation to derive hydrodynamic equations for scalar fields in a relativistic setting, including perturbation analysis in cosmology.

Findings

Perturbations below the Jeans length oscillate, above grow linearly with scale factor.

Scalar field perturbations grow faster than cold dark matter.

Relativistic effects can suppress perturbation growth near the horizon.

Abstract

Using a generalization of the Madelung transformation, we derive the hydrodynamic representation of the Klein-Gordon-Einstein equations in the weak field limit. We consider a complex self-interacting scalar field with a $\lambda|\varphi|^4$ potential. We study the evolution of the homogeneous background in the fluid representation and derive the linearized equations describing the evolution of small perturbations in a static and in an expanding universe. We compare the results with simplified models in which the gravitational potential is introduced by hand in the Klein-Gordon equation, and assumed to satisfy a (generalized) Poisson equation. We study the evolution of the perturbations in the matter era using the nonrelativistic limit of our formalism. Perturbations whose wavelength is below the Jeans length oscillate in time while pertubations whose wavelength is above the Jeans length grow linearly with the scale factor as in the cold dark matter model. The growth of perturbations in the scalar field model is substantially faster than in the cold dark matter model. When the wavelength of the pertubations approaches the cosmological horizon (Hubble length), a relativistic treatment is mandatory. In that case, we find that relativistic effects attenuate or even prevent the growth of pertubations. This paper exposes the general formalism and provides illustrations in simple cases. Other applications of our formalism will be considered in companion papers.