Breaking the sigma_8-Omega_m degeneracy using the clustering of high-z X-ray AGN

The clustering of X-ray selected AGN appears to be a valuable tool for extracting cosmological information. Using the recent high-precision angular clustering results of ~30000 XMM-Newton soft (0.5-2 keV) X-ray sources (Ebrero et al. 2009), which have a median redshift of $z\sim 1$, and assuming a flat geometry, a constant in comoving coordinates AGN clustering evolution and the AGN bias evolution model of Basilakos et al. (2008), we manage to break the Omega_m-sigma_8 degeneracy. The resulting cosmological constraints are: Omega_m=0.27 (+0.03 -0.05), w=-0.90 (+0.10 -0.16) and sigma_8=0.74 (+0.14 -0.12), while the dark matter host halo mass, in which the X-ray selected AGN are presumed to reside, is M=2.50 (+0.50 -1.50) X 10^13 h^{-1} M(solar). For the constant Lambda model (w=-1) we find Omega_m=0.24 (+- 0.06) and sigma_8=0.83 (+0.11 -0.16), in good agreement with recent studies based on cluster abundances, weak lensing and the CMB, but in disagreement with the recent bulk flow analysis.Comment: Accepted in ApJ. Lett

Constraining the dark energy models with H(z) data: an approach independent of H0H_{0}

We study the performance of the latest $H(z)$ data in constraining the cosmological parameters of different cosmological models, including that of Chevalier-Polarski-Linder $w_{0}w_{1}$ parametrization. First, we introduce a statistical procedure in which the chi-square estimator is not affected by the value of the Hubble constant. As a result, we find that the $H(z)$ data do not rule out the possibility of either non-flat models or dynamical dark energy cosmological models. However, we verify that the time varying equation of state parameter $w(z)$ is not constrained by the current expansion data. Combining the $H(z)$ and the Type Ia supernova data we find that the $H(z)$/SNIa overall statistical analysis provides a substantial improvement of the cosmological constraints with respect to those of the $H(z)$ analysis. Moreover, the $w_{0}-w_{1}$ parameter space provided by the $H(z)$/SNIa joint analysis is in a very good agreement with that of Planck 2015, which confirms that the present analysis with the $H(z)$ and SNIa probes correctly reveals the expansion of the Universe as found by the team of Planck. Finally, we generate sets of Monte Carlo realizations in order to quantify the ability of the $H(z)$ data to provide strong constraints on the dark energy model parameters. The Monte Carlo approach shows significant improvement of the constraints, when increasing the sample to 100 $H(z)$ measurements. Such a goal can be achieved in the future, especially in the light of the next generation of surveys.Comment: 11 pages, 9 figures, accepted for publication by Phys. Rev.

Interacting Dark Matter as an Alternative to Dark Energy

We investigate the global dynamics of the universe within the framework of the Interacting Dark Matter (IDM) scenario. Considering that the dark matter obeys the collisional Boltzmann equation, we can obtain analytical solutions of the global density evolution, which can accommodate an accelerated expansion, equivalent to either the {\em quintessence} or the standard $\Lambda$ models. This is possible if there is a disequilibrium between the DM particle creation and annihilation processes with the former process dominating, which creates an effective source term with negative pressure. Comparing the predicted Hubble expansion of one of the IDM models (the simplest) with observational data, we find that the effective annihilation term is quite small, as suggested by various experiments.Comment: 8 pages, 2 figures, Proceedings of 'Invisible Universe International Conference', Paris, June 29- July 3, 200