The Association of Compact Groups of Galaxies with Large-scale Structures

We use various samples of compact groups (CGs) to examine the types of association CGs have with rich and poor clusters of galaxies at low (z~0.04) and intermediate (z~0.1) redshifts. We find that ~10-20 % of CGs are associated with rich clusters and a much larger fraction with poorer clusters or loose groups. Considering the incompleteness of catalogs of poorer systems at intermediate redshift, our result is consistent with all CGs at intermediate redshift being associated with larger-scale systems. The richness of the clusters associated with CGs significantly increases from z~0.04 to z~0.1, while their Bautz-Morgan type changes from early to late type for the same range in z. Neither trend is compatible with a selection effect in the cluster catalogs used. We find earlier morphological types of galaxies to be more frequent in CGs associated with larger-scale structures, compared to those in CGs not associated to such structures. We consider this as new evidence that CGs are part of the large-scale structure formation process and that they may play an important role in the evolution of galaxies in these structures.Comment: 5 pages, no figures, Proc. ESO Workshop "Groups of galaxies in the nearby Universe", Santiago, Chile, 5-9 Dec. 2005, ESO Astrophysics Symposia, eds. I. Saviane, V. Ivanov & J. Borissova, Springer-Verlag; very minor revision of text on 15 Mar 2006, added one referenc

Production of entanglement in Raman three-level systems using feedback

We examine the theoretical limits of the generation of entanglement in a damped coupled ion-cavity system using jump-based feedback. Using Raman transitions to produce entanglement between ground states reduces the necessary feedback bandwidth, but does not improve the overall effect of the spontaneous emission on the final entanglement. We find that the fidelity of the resulting entanglement will be limited by the asymmetries produced by vibrations in the trap, but that the concurrence remains above 0.88 for realistic ion trap sizes.Comment: 8 pages, 8 figure

Radiative decays of light vector mesons in a quark level linear sigma model

We calculate the P0 to gamma gamma, V0 to P0 gamma and V0to V'0 gamma gamma decays in the framework of a U(3)xU(3) linear sigma model which includes constituent quarks. For the first two decays this approach improves results based on the anomalous Wess-Zumino term, with contributions due to SU(3) symmetry breaking and vector mixing. The phi to (omega,rho) gamma gamma decays are dominated by resonant eta' exchange . Our calculation for the later decays improves and update similar calculations in the -closely related- framework of vector meson dominance. We obtain BR(phi to rho gamma gamma)=2.5x10^{-5} and BR(phi to omega gamma gamma)=2.8x10^{-6} within the scope of the high-luminosity phi factories.Comment: 8 pages, submitted to Phys. Rev.

Constraining the dark energy with galaxy clusters X-ray data

The equation of state characterizing the dark energy component is constrained by combining Chandra observations of the X-ray luminosity of galaxy clusters with independent measurements of the baryonic matter density and the latest measurements of the Hubble parameter as given by the HST key project. By assuming a spatially flat scenario driven by a "quintessence" component with an equation of state $p_x = \omega \rho_x$ we place the following limits on the cosmological parameters $\omega$ and $\Omega_{\rm{m}}$: (i) $-1 \leq \omega \leq -0.55$ and $\Omega_{\rm m} = 0.32^{+0.027}_{-0.014}$ (1$\sigma$) if the equation of state of the dark energy is restricted to the interval $-1 \leq \omega < 0$ (\emph{usual} quintessence) and (ii) $\omega = -1.29^{+0.686}_{-0.792}$ and $\Omega_{\rm{m}} = 0.31^{+0.037}_{-0.034}$ ($1\sigma$) if $\omega$ violates the null energy condition and assume values $< -1$ (\emph{extended} quintessence or ``phantom'' energy). These results are in good agreement with independent studies based on supernovae observations, large-scale structure and the anisotropies of the cosmic background radiation.Comment: 6 pages, 4 figures, LaTe

Bianchi Type I Cosmology in Generalized Saez-Ballester Theory via Noether Gauge Symmetry

In this paper, we investigate the generalized Saez-Ballester scalar-tensor theory of gravity via Noether gauge symmetry (NGS) in the background of Bianchi type I cosmological spacetime. We start with the Lagrangian of our model and calculate its gauge symmetries and corresponding invariant quantities. We obtain the potential function for the scalar field in the exponential form. For all the symmetries obtained, we determine the gauge functions corresponding to each gauge symmmetry which include constant and dynamic gauge. We discuss cosmological implications of our model and show that it is compatible with the observational data.Comment: 13 pages, 2 figures, accepted for publication in 'European Physical Journal C

Alternatives to Quintessence Model Building

We discuss the issue of toy model building for the dark energy component of the universe. Specifically, we consider two generic toy models recently proposed as alternatives to quintessence models, known as Cardassian expansion and the Chaplygin gas. We show that the former is enteriely equivalent to a class of quintessence models. We determine the observational constraints on the latter, coming from recent supernovae results and from the shape of the matter power spectrum. As expected, these restrict the model to a behaviour that closely matches that of a standard cosmological constant $\Lambda$.Comment: RevTex4; 7 pages, 4 figures. v2: Improved discussion of constraints on Chaplygin gas models. Other clarifications added. Phys Rev. D (in press

A Better Way to Reconstruct Dark Energy Models ?

To reconstruct dark energy models the redshift $z_{eq}$, marking the end of radiation era and the beginning of matter-dominated era, can play a role as important as $z_{t}$, the redshift at which deceleration parameter experiences a signature flip. To implement the idea we propose a variable equation of state for matter that can bring a smooth transition from radiation to matter-dominated era in a single model. A popular $\Lambda \propto \rho$ dark energy model is chosen for demonstration but found to be unacceptable. An alternative $\Lambda \propto \rho a^{3}$ model is proposed and found to be more close to observation.Comment: 17 pages, 5 figures Accepted for publication in `Astrophysics and Space Science