Large scale structure from viscous dark matter

TL;DR

This paper develops a fluid dynamic model incorporating effective viscosity and pressure to describe large-scale cosmological structures from dark matter, achieving accurate predictions aligned with simulations up to certain scales.

Contribution

It introduces a novel effective viscous fluid approach for mildly non-linear cosmological scales, with parameter-free expressions derived from perturbation theory matching simulations.

Findings

Excellent agreement with N-body simulations up to k=0.2 h/Mpc

Improved convergence over standard perturbation theory

Results are robust against variations in the matching scale

Abstract

Cosmological perturbations of sufficiently long wavelength admit a fluid dynamic description. We consider modes with wavevectors below a scale $k_m$ for which the dynamics is only mildly non-linear. The leading effect of modes above that scale can be accounted for by effective non-equilibrium viscosity and pressure terms. For mildly non-linear scales, these mainly arise from momentum transport within the ideal and cold but inhomogeneous fluid, while momentum transport due to more microscopic degrees of freedom is suppressed. As a consequence, concrete expressions with no free parameters, except the matching scale $k_m$, can be derived from matching evolution equations to standard cosmological perturbation theory. Two-loop calculations of the matter power spectrum in the viscous theory lead to excellent agreement with $N$-body simulations up to scales $k=0.2 \, h/$Mpc. The convergence properties in the ultraviolet are better than for standard perturbation theory and the results are robust with respect to variations of the matching scale.