Non-linear density-velocity divergence relation from phase space dynamics

TL;DR

This paper derives a non-linear relation between cosmological density and velocity perturbations using phase space dynamics, providing a new interpretation and a fitting formula that can help constrain dark energy parameters.

Contribution

It introduces a phase space approach to the non-linear density-velocity relation and generalizes existing formulas, with implications for observational constraints on dark energy.

Findings

The Zel'dovich curve acts as an attracting solution in phase space.

A generalized fitting formula for the non-linear relation is provided.

Phase space analysis can help break degeneracies in dark energy parameter estimation.

Abstract

We obtain the non-linear relation between cosmological density and velocity perturbations by examining their joint dynamics in a two dimensional density-velocity divergence phase space. We restrict to spatially flat cosmologies consisting of pressureless matter and non-clustering dark energy characterized by a constant equation of state $w$. Using the spherical top-hat model, we derive the coupled equations that govern the joint evolution of the perturbations and examine the flow generated by this system. In general, the initial density and velocity are independent, but requiring that the perturbations vanish at the big bang time sets a relation between the two. This relation, which we call the `Zel'dovich curve', acts like an attracting solution for the phase space dynamics and is the desired non-linear extension of the density-velocity divergence relation. We obtain a fitting formula for the curve, which is a generalization of the formulae by Bernardeau and Bilicki & Chodorowski, and find that as in the linear regime, the explicit dependence on the dark energy parameters stays weak even in the non-linear regime. Although the result itself is somewhat expected, the new feature of this work is the interpretation of the relation in the phase space picture and the generality of the method. Finally, as an observational implication, we examine the evolution of galaxy cluster profiles using the spherical infall model for different values of $w$. We demonstrate that using only the density or the velocity information to constrain $w$ is subject to degeneracies in other parameters such as $\sigma_8$ but plotting observations onto the joint density-velocity phase space can help lift this degeneracy.