How clustering dark energy affects matter perturbations

TL;DR

This paper investigates how the clustering properties of dark energy, characterized by its effective sound speed, influence matter perturbations and structure formation, using current cosmological data to constrain dark energy models.

Contribution

It derives general equations for dark matter and dark energy clustering with constant sound speed and constrains this parameter using multiple observational datasets.

Findings

Likelihood peaks at zero sound speed for dark energy

Dark energy sound speed degenerates with matter density and equation of state

Current data favor clustered dark energy with low sound speed

Abstract

The rate of structure formation in the Universe is different in homogeneous and clustered dark energy models. The degree of dark energy clustering depends on the magnitude of its effective sound speed $c^{2}_{\rm eff}$ and for $c_{\rm eff}=0$ dark energy clusters in a similar fashion to dark matter while for $c_{\rm eff}=1$ it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, $w_{\rm d}=const$ and $w_{\rm d}=w_0+w_1\left(\frac{z}{1+z}\right)$ with $c_{\rm eff}$ as a free parameter and we try to constrain the dark energy effective sound speed using current available data including SnIa, Baryon Acoustic Oscillation, CMB shift parameter ({\em Planck} and {\em WMAP}), Hubble parameter, Big Bang Nucleosynthesis and the growth rate of structures $f\sigma_{8}(z)$. At first we derive the most general form of the equations governing dark matter and dark energy clustering under the assumption that $c_{\rm eff}=const$. Finally, performing an overall likelihood analysis we find that the likelihood function peaks at $c_{\rm eff}=0$, however the dark energy sound speed is degenerate with respect to the cosmological parameters, namely $\Omega_{\rm m}$ and $w_{\rm d}$.