Viscous dark matter growth in (neo-)Newtonian cosmology

TL;DR

This paper investigates how small bulk-viscous pressure in cold dark matter affects structure growth, showing that a neo-Newtonian approach is necessary for accurate modeling beyond the Hubble scale.

Contribution

It introduces a neo-Newtonian framework that properly incorporates pressure effects into cosmological perturbation analysis for viscous dark matter.

Findings

Viscous dark matter suppresses inhomogeneity growth.

Neo-Newtonian approach accurately reproduces relativistic dynamics.

Upper limits on viscous pressure ensure structure formation.

Abstract

We assume cold dark matter to possess a small bulk-viscous pressure which typically attenuates the growth of inhomogeneities. Explicit calculations, based on Eckart's theory of dissipative processes, reveal that for viscous cold dark matter the usual Newtonian approximation for perturbation scales smaller than the Hubble scale is no longer valid. We advocate the use of a neo-Newtonian approach which consistently incorporates pressure effects into the fluid dynamics and correctly reproduces the general relativistic dynamics. This result is of interest for numerical simulations of nonlinear structure formation involving nonstandard dark-matter fluids. We obtain upper limits on the magnitude of the viscous pressure by requiring that relevant perturbation amplitudes should grow sufficiently to enter the nonlinear stage.