Dark Energy and the Statistical Study of the Observed Image Separations of the Multiply Imaged Systems in the CLASS Statistical Sample

The present day observations favour a universe which is flat, accelerated and composed of $\sim 1/3$ matter (baryonic + dark) and $\sim 2/3$ of a negative pressure component, usually referred to as dark energy or quintessence. The Cosmic Lens All Sky Survey (CLASS), the largest radio-selected galactic mass scale gravitational lens search project to date, has resulted in the largest sample suitable for statistical analyses. In the work presented here, we exploit observed image separations of the multiply imaged lensed radio sources in the sample. We use two different tests: (1) image separation distribution function $n(\Delta\theta)$ of the lensed radio sources and (2) {\dtheta}_{\mathrm{pred}} vs {\dtheta}_{\mathrm{obs}} as observational tools to constrain the cosmological parameters $w$ and \Om. The results are in concordance with the bounds imposed by other cosmological tests.Comment: 20 pages latex; Modified " Results and Discussion " section, new references adde

Delicate f(R) gravity models with disappearing cosmological constant and observational constraints on the model parameters

We study the $f(R)$ theory of gravity using metric approach. In particular we investigate the recently proposed model by Hu-Sawicki, Appleby $-$ Battye and Starobinsky. In this model, the cosmological constant is zero in flat space time. The model passes both the Solar system and the laboratory tests. But the model parameters need to be fine tuned to avoid the finite time singularity recently pointed in the literature. We check the concordance of this model with the $H(z)$ and baryon acoustic oscillation data. We find that the model resembles the $\Lambda$CDM at high redshift. However, for some parameter values there are variations in the expansion history of the universe at low redshift.Comment: 16 pages and 9 figures, typos corrected, few references and minor clarifications added, revised version to appera in PR

Linear Coasting in Cosmology and SNe Ia

A strictly linear evolution of the cosmological expansion scale factor is a characteristic feature in several classes of alternative gravity theories as also in the standard (big-bang) model with specially chosen equations of state of matter. Such an evolution has no free parameters as far as the classical cosmological tests are concerned and should therefore be easily falsifiable. In this article we demonstrate how such models present very good fits to the current supernovae 1a data. We discuss the overall viability of such models.Comment: 12 latex 2e pages including 5 ps figures. More references and Figuresinclude

High-redshift objects and the generalized Chaplygin gas

Motivated by recent developments in particle physics and cosmology, there has been growing interest in an unified description of dark matter and dark energy scenarios. In this paper we explore observational constraints from age estimates of high-$z$ objects on cosmological models dominated by an exotic fluid with equation of state $p = -A/\rho^{\alpha}$ (the so-called generalized Chaplygin gas) which has the interesting feature of interpolating between non-relativistic matter and negative-pressure dark energy regimes. As a general result we find that, if the age estimates of these objects are correct, they impose very restrictive limits on some of these scenarios.Comment: 5 pages, 3 figures, to appear in Phys. Rev.

Gravitational lensing constraint on the cosmic equation of state

Recent redshift-distance measurements of Type Ia supernovae (SNe Ia) at cosmological distances suggest that two-third of the energy density of the universe is dominated by dark energy component with an effective negative pressure. This dark energy component is described by the equation of state $p_{x} = w \rho_{x}$ $(w \geq -1)$. We use gravitational lensing statistics to constrain the equation of state of this dark energy. We use $n(\Delta\theta)$, image separation distribution function of lensed quasars, as a tool to probe $w$. We find that for the observed range of $\Omega_m \sim 0.2 - 0.4$, $w$ should lie between $-0.8 \leq w \leq -0.4$ in order to have five lensed quasars in a sample of 867 optical quasars. This limit is highly sensitive to lens and Schechter parameters and evolution of galaxies.Comment: Modified results and inclusion of calculations with new set of parameter

Constraints on the Cardassian Expansion from the Cosmic Lens All-Sky Survey Gravitational Lens Statistics

The existence of a dark energy component has usually been invoked as the most plausible way to explain the recent observational results. However, it is also well known that effects arising from new physics (e.g., extra dimensions) can mimic the gravitational effects of a dark energy through a modification of the Friedmann equation. In this paper we investigate some observational consequences of a flat, matter dominated and accelerating/decelerating scenario in which this modification is given by $H^{2} = g(\rho_m, n, q)$ where $g(\rho_m, n, q)$ is a new function of the energy density $\rho_m$, the so-called generalized Cardassian models. We mainly focus our attention on the constraints from the recent Cosmic All Sky Survey (CLASS) lensing sample on the parameters $n$ and $q$ that fully characterize the models. We show that, for a large interval of the $q - n$ parametric space, these models are in agreement with the current gravitational lenses data. The influence of these parameters on the acceleration redshift, i.e., the redshift at which the universe begins to accelerate, and on the age of the universe at high-redshift is also discussed.Comment: 6 pages, 4 figures, AAS late

Constraints on Chaplygin quartessence from the CLASS gravitational lens statistics and supernova data

The nature of the dark components (dark matter and dark energy) that dominate the current cosmic evolution is a completely open question at present. In reality, we do not even know if they really constitute two separated substances. In this paper we use the recent Cosmic All Sky Survey (CLASS) lensing sample to test the predictions of one of the candidates for a unified dark matter/energy scenario, the so-called generalized Chaplygin gas (Cg) which is parametrized by an equation of state $p = -A/\rho_{Cg}^{\alpha}$ where $A$ and $\alpha$ are arbitrary constants. We show that, although the model is in good agreement with this radio source gravitational lensing sample, the limits obtained from CLASS statistics are only marginally compatible with the ones obtained from other cosmological tests. We also investigate the constraints on the free parameters of the model from a joint analysis between CLASS and supernova data.Comment: 7 pages, 3 figures, to appear in Astronomy & Astrophysic