One-point probability distribution function from spherical collapse: Early Dark Energy (EDE) vs. $\Lambda$CDM

TL;DR

This paper computes the one-point PDF of dark matter density fields using spherical collapse, compares models with Early Dark Energy and $ ext{Lambda}$CDM, and assesses the impact of cosmology on non-linear growth and PDF fitting accuracy.

Contribution

It introduces a spherical collapse-based method for the one-point PDF and compares cosmological effects in EDE and $ ext{Lambda}$CDM$, validating a density-velocity relation fit for EDE.

Findings

Skewed log-normal fits the non-linear PDF well for both cosmologies.

Differences in non-linear growth depend on initial conditions and are model-dependent.

The density-velocity divergence relation fit remains accurate for EDE within 4 ext%.

Abstract

We compute the one-point PDF of an initially Gaussian dark matter density field using spherical collapse (SC). We compare the results to other forms available in the literature and also compare the PDFs in the $\Lambda$CDM model with an early dark energy (EDE) model. We find that the skewed log-normal distribution provides the best fit to the non-linear PDF from SC for both cosmologies, from $a=0.1$ to 1 and for scales characterized by the comoving width of the Gaussian: $\sigma_G = 0.5, 1, 2$. To elucidate the effect of cosmology, we examine the linear and non-linear growth rates through test cases. For overdensities, when the two models have the same initial density contrast, the differences due to cosmology are amplified in the non-linear regime, whereas, if the two models have the same linear density contrast today, then the differences in cosmology are damped in the non-linear regime. This behaviour is in contrast with voids, where the non-linear growth becomes `self-regulatory' and is less sensitive to cosmology and initial conditions. To compare the PDFs, we examine the difference of the PDFs and evolution of the width of the PDF. The trends with scale and redshift are as expected. A tertiary aim of this paper was to check if the fitting form for the non-linear density-velocity divergence relation, derived for constant equation of state ($w$) models by Nadkarni-Ghosh holds for the EDE model. We find that it does with an accuracy of 4\%, thus increasing its range of validity.