How clustering dark energy affects matter perturbations

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 ceff2 and for ceff2 = 0 dark energy clusters in a similar fashion to dark matter while for ceff2 = 1 it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, wd = const and wd = w0 + w1 (z/1+z) with ceff2 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 ceff2 = const. Finally, performing an overall likelihood analysis we find that the likelihood function peaks at ceff2 = 0; however, the dark energy sound speed is degenerate with respect to the cosmological parameters, namely Ωm and wd.