Cosmological structure formation with clustering quintessence

TL;DR

This paper investigates how clustering quintessence affects large-scale structure formation, modifying growth rates and bispectra, and explores potential observational signatures to distinguish it from standard LambdaCDM cosmology.

Contribution

It provides a theoretical framework for modeling clustering quintessence with zero sound speed, deriving explicit perturbation kernels, and analyzing observational signatures in the power spectrum and bispectrum.

Findings

Clustering quintessence enhances the growth rate at low redshifts.

Explicit expressions for second-order kernels F_2 and G_2 are derived.

The reduced bispectrum shows potential to detect or rule out clustering quintessence.

Abstract

We study large-scale structure formation in the presence of a quintessence component with zero speed of sound in the framework of Eulerian Perturbation Theory. Due to the absence of pressure gradients, quintessence and dark matter are comoving and can be studied as a unique fluid in terms of the total energy density contrast and the common velocity. In this description the clustering of quintessence enhances the linear term proportional to the velocity divergence in the continuity equation by a factor (1+w) Omega_Q / Omega_m. This is responsible for a rapid evolution of the growth rate at low redshifts, and modifies the standard relation between the velocity divergence and the growth factor. For the total fluid, the solutions for the linear growth function and growth rate can be written in integral forms and admit simple fitting formulae, as in the LambdaCDM case. At second order in perturbation theory, we derive an explicit expression for the kernels F_2 and G_2. They receive modifications of the order of the ratio between quintessence and total energy density perturbations, which affect the corresponding tree-level bispectra. We finally compute the cumulative signal-to-noise in the power spectrum, bispectrum and reduced bispectrum, expected for departures from a LambdaCDM cosmology both in the clustering and smooth quintessence scenarios. The reduced bispectrum, in particular, receives sensible modifications only in the clustering case and can potentially be used to detect or rule out the model.