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 $c^{2}_{rm eff}$ 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 $w_{rm d}=w_0+w_1left(frac{z}{1+z}right)$ 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 $fsigma_{8}(z)$. 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 $w_{rm d}$.

Dynamics and constraints of the Unified Dark Matter flat cosmologies

We study the dynamics of the scalar field FLRW flat cosmological models within the framework of the Unified Dark Matter (UDM) scenario. In this model we find that the main cosmological functions such as the scale factor of the Universe, the scalar field, the Hubble flow and the equation of state parameter are defined in terms of hyperbolic functions. These analytical solutions can accommodate an accelerated expansion, equivalent to either the dark energy or the standard $Lambda$ models. Performing a joint likelihood analysis of the recent supernovae type Ia data and the Baryonic Acoustic Oscillations traced by the SDSS galaxies, we place tight constraints on the main cosmological parameters of the UDM cosmological scenario. Finally, we compare the UDM scenario with various dark energy models namely $Lambda$ cosmology, parametric dark energy model and variable Chaplygin gas. We find that the UDM scalar field model provides a large and small scale dynamics which are in fair agreement with the predictions by the above dark energy models although there are some differences especially at high redshifts.