Hunting high and low: Disentangling primordial and late-time non-Gaussianity with cosmic densities in spheres

TL;DR

This paper develops an analytical framework using large deviation statistics and spherical collapse dynamics to disentangle primordial non-Gaussianity from late-time effects in cosmic density data, validated against simulations.

Contribution

It introduces a novel analytical approach to separate primordial non-Gaussian signals from late-time effects using densities in spheres, extending bias concepts to underdense regions.

Findings

Analytical formulas agree within 2% with simulations for local non-Gaussianity with $f_{NL}$ between -100 and 100.

The formalism is valid for densities in [0.5,3] and scales of 15 Mpc/h down to redshift 0.35.

Estimators for $\sigma_8$ and $f_{NL}$ are validated for joint likelihood analysis.

Abstract

Non-Gaussianities of dynamical origin are disentangled from primordial ones using the formalism of large deviation statistics with spherical collapse dynamics. This is achieved by relying on accurate analytical predictions for the one-point probability distribution function (PDF) and the two-point clustering of spherically-averaged cosmic densities (sphere bias). Sphere bias extends the idea of halo bias to intermediate density environments and voids as underdense regions. In the presence of primordial non-Gaussianity, sphere bias displays a strong scale dependence relevant for both high and low density regions, which is predicted analytically. The statistics of densities in spheres are built to model primordial non-Gaussianity via an initial skewness with a scale-dependence that depends on the bispectrum of the underlying model. The analytical formulas with the measured nonlinear dark matter variance as input are successfully tested against numerical simulations. For local non-Gaussianity with a range from $f_{\rm NL}=-100$ to $+100$ they are found to agree within 2\% or better for densities $\rho\in[0.5,3]$ in spheres of radius 15 Mpc$/h$ down to $z=0.35$. The validity of the large deviation statistics formalism is thereby established for all observationally relevant local-type departures from perfectly Gaussian initial conditions. The corresponding estimators for the amplitude of the nonlinear variance $\sigma_8$ and primordial skewness $f_{\rm NL}$ are validated using a fiducial joint maximum likelihood experiment. The influence of observational effects and the prospects for a future detection of primordial non-Gaussianity from joint one- and two-point densities-in-spheres statistics are discussed.