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

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

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

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

Cosmological Constraints on a Power Law Universe

Linearly coasting cosmology is comfortably concordant with a host of cosmological observations. It is surprisingly an excellent fit to SNe Ia observations and constraints arising from age of old quasars. In this article we highlight the overall viability of an open linear coasting cosmological model.The model is consistent with the latest SNe Ia ``gold'' sample and accommodates a very old high-redshift quasar, which the standard cold-dark model fails to do.Comment: 10 pages, 2 figure

Variable Chaplygin Gas: Constraints from CMBR and SNe Ia

We constrain the parameters of the variable Chaplygin gas model, using the location of peaks of the CMBR spectrum and SNe Ia ``gold '' data set. Equation of state of the model is $P=-A(a)/\rho$, where $A(a)=A_0 a^{-n}$ is a positive function of the cosmological scale factor $a$, $A_0$ and $n$ being constants. The variable Chaplygin gas interpolates from dust-dominated era to quintessence dominated era. The model is found to be compatible with current type Ia Supernovae data and location of first peak if the values of $\Omega_m$ and $n$ lie in the interval $[0.017,~0.117]$ and $[-1.3,~2.6]$ respectively.Comment: 9 pages,4 figure