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

The angular correlation function of the ROSAT All Sky Survey Bright Source Catalogue

We have derived the angular correlation function of a sample of 2096 sources detected in the ROSAT All Sky Survey Bright Source Catalogue, in order to investigate the clustering properties of AGN in the local Universe. Our sample is constructed by rejecting all known stars, as well as extended X-ray sources. Areas with |b|<30 deg. and declination <-30 deg. are also rejected due to the high or uncertain neutral hydrogen absorption. Cross-correlation of our sample with the Hamburg/RASS optical identification catalogue, suggests that the vast majority of our sources are indeed AGN. A 4.1 sigma correlation signal between 0 and 8 degrees was detected with w(theta<8 deg.)=0.025 +- 0.006. Assuming the usual power-law form of the 2-point correlation function we find an angular correlationlength of 0.062 degrees. Deprojection on 3 dimensions, using the Limber's equation, yields a spatial correlation length of 6.0+- 1.6 h^-1 Mpc. This is consistent with the AGN clustering results derived at higher redshifts in optical surveys and suggests a comoving model for the clustering evolution.Comment: 5 pages, revised version accepted in MNRA

Comparison of the linear bias models in the light of the Dark Energy Survey

The evolution of the linear and scale independent bias, based on the most popular dark matter bias models within the $\Lambda$CDM cosmology, is confronted to that of the Dark Energy Survey (DES) Luminous Red Galaxies (LRGs). Applying a $\chi^2$ minimization procedure between models and data we find that all the considered linear bias models reproduce well the LRG bias data. The differences among the bias models are absorbed in the predicted mass of the dark-matter halo in which LRGs live and which ranges between $\sim 6 \times 10^{12} h^{-1} M_{\odot}$ and $1.4 \times 10^{13} h^{-1} M_{\odot}$, for the different bias models. Similar results, reaching however a maximum value of $\sim 2\times 10^{13} h^{-1} M_{\odot}$, are found by confronting the SDSS (2SLAQ) Large Red Galaxies clustering with theoretical clustering models, which also include the evolution of bias. This later analysis also provides a value of $\Omega_{m}=0.30\pm 0.01$, which is in excellent agreement with recent joint analyses of different cosmological probes and the reanalysis of the Planck data.Comment: 10 page, 5 figures. Accepted for publication by MNRAS (new discussion and corrections added

Reconstructing Positions \& Peculiar Velocities of Galaxy Clusters within 25000 km/sec: The Bulk Velocity

Using a dynamical 3-D reconstruction procedure we estimate the peculiar velocities of $R\ge0$ Abell/ACO galaxy clusters from their measured redshift within 25000 km/sec. The reconstruction algorithm relies on the linear gravitational instability hypothesis, assumes linear biasing and requires an input value of the cluster $\beta$-parameter ($\beta_c \equiv \Omega_{\circ}^{0.6}/b_c$), which we estimated in Branchini \& Plionis (1995) to be $\beta_c\simeq 0.21$. The resulting cluster velocity field is dominated by a large scale streaming motion along the Perseus Pisces--Great Attractor base-line directed towards the Shapley concentration, in qualitative agreement with the galaxy velocity field on smaller scales. Fitting the predicted cluster peculiar velocities to a dipole term, in the local group frame and within a distance of $\sim 18000$ km/sec, we recover extremely well both the local group velocity and direction, in disagreement with the Lauer \& Postman (1994) observation. However, we find a $\sim 6\%$ probability that their observed velocity field could be a realization of our corresponding one, if the latter is convolved with their large distance dependent errors. Our predicted cluster bulk velocity amplitude agrees well with that deduced by the POTENT and the da Costa et al. (1995) analyses of observed galaxy motions at $\sim 5000 - 6000$ km/sec; it decreases thereafter while at the Lauer \& Postman limiting depth ($\sim 15000$ km/sec) its amplitude is $\sim 150$ km/sec, in comfortable agreement with most cosmological models.Comment: 8 pages, uuencoded compressed tarred postscript file uncluding text and 3 figures. Accepted in ApJ Letter