How clustering dark energy affects matter perturbations

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 |$c^{2}_{\rm eff}=0$| dark energy clusters in a similar fashion to dark matter while for |$c^{2}_{\rm eff}=1$| it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, wd = const and |$w_{\rm d}=w_0+w_1\left(\frac{z}{1+z}\right)$| with |$c^{2}_{\rm eff}$| as a free parameter and we try to constrain the dark energy effective sound speed using current available data including Type Ia supernovae, baryon acoustic oscillation, cosmic microwave background shift parameter (Planck and WMAP), Hubble parameter, big bang nucleosynthesis and the growth rate of structures fσ8(z). At first we derive the most general form of the equations governing dark matter and dark energy clustering under the assumption that |$c^{2}_{\rm eff}={\rm const}$|⁠. Finally, performing an overall likelihood analysis we find that the likelihood function peaks at |$c^{2}_{\rm eff}=0$|⁠; however, the dark energy sound speed is degenerate with respect to the cosmological parameters, namely Ωm and wd.