arXiv subjects

Abha Dev

Publications and source records attributed to Abha Dev.

At least 19 recordsLinked to original sources

arXiv2009

Constraints on dark energy from the lookback time versus redshift test

We use lookback time versus redshift data from galaxy clusters (Capozziello et al., 2004) and passively evolving galaxies (Simon et al., 2005), and apply a bayesian prior on the total age of the Universe based on WMAP measurements, to constrain dark energy cosmological model parameters. Current lookback time data provide interesting and moderately restrictive constraints on cosmological parameters. When used jointly with current baryon acoustic peak and Type Ia supernovae apparent magnitude versus redshift data, lookback time data tighten the constraints on parameters and favor slightly smaller values of the nonrelativistic matter energy density.

astro-ph.CO↗

arXiv2008

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 $Λ$CDM at high redshift. However, for some parameter values there are variations in the expansion history of the universe at low redshift.

hep-th↗

arXiv2008

Power law cosmology - a viable alternative

A power law cosmology is defined by the cosmological scale factor evolving as $t^α$. In this work, we put bounds on $α$ by using the joint test of the SNe Ia data from Supernova Legacy Survey (SNLS) and H(z) data with curvature constant $ k = 0, \pm 1$. We observe that the combined analysis with SNLS and H(z) data favours the open power cosmology with $α= 1.31^{+0.06}_{-0.05}. It is also interesting to note that an Einstein - de Sitter model $(α= 2/3)$ is ruled out at $2σ$ level.

astro-ph↗

arXiv2007

Cosmological bounds on oscillating dark energy models

We study some cosmological constraints on the two phenomenological models of oscillating dark energy. In these scenarios, the equation of state of dark energy varies periodically and may provide a way to unify the early acceleration (inflation) and the late time acceleration of the universe. These models give also an effective way to tackle the so-called cosmic coincidence problem. We examine observational constraints on this class of models from the latest observational data including the \emph{gold} sample of 182 type Ia supernovae, the CMB shift parameter $R$ and the BAO measurements from the Sloan Digital Sky Survey.

astro-ph↗

arXiv2007

Age constraints on the cosmic equation of state

Dark energy is the invisible fuel that seems to drive the current acceleration of the Universe. Its presence, which is inferred from an impressive convergence of high-quality observational results along with some sucessful theoretical predictions, is also supported by the current estimates of the age of the Universe from dating of local and high-$z$ objects. In this work we study observational constraints on the dark energy equation of state ($w$) from lookback time measurements of high-$z$ galaxies, as recently published by the Gemini Deep Deep Survey (GDDS). In order to build up our lookback time sample from these observations we use 8 high-$z$ galaxies in the redshift interval $1.3 \leq z \leq 2.2$ and assume the total expanding age of the Universe to be $t_{0}^{obs} = 13.6^{+0.4}_{-0.3}$ Gyr, as obtained from current cosmic microwave background data. We show that these age measurements are compatible with values of $w$ close to -1, although there is still space for quintessence ($w > -1$) and phantom ($w < -1$) behaviors. In order to break possible degeneracies in the $Ω_{\rm{m}} - w$ plane, we also discuss the bounds on this parametric space when GDDS lookback time measurements are combined with the most recent SNe Ia, CMB and LSS data.

astro-ph↗

arXiv2006

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 $P=-A(a)/ρ$, 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 $Ω_m$ and $n$ lie in the interval $[0.017,~0.117]$ and $[-1.3,~2.6]$ respectively.

astro-ph↗

arXiv2005

Age of High Redshift Objects - a Litmus Test for the Dark Energy Models

The discovery of the quasar, the APM 08279+5255 at z = 3.91 whose age is 2-3 Gyr has once again led to ``age crisis''. The noticeable fact about this object is that it cannot be accommodated in a universe with $Ω_m = 0.27$, currently accepted value of matter density parameter and $ω= \mathrm{constant}$. In this work, we explore the concordance of various dark energy parameterizations ($w(z)$ models) with the age estimates of the old high redshift objects. It is alarming to note that the quasar cannot be accommodated in any dark energy model even for $Ω_m = 0.23$, which corresponds to $1 σ$ deviation below the best fit value provided by WMAP. There is a need to look for alternative cosmologies or some other dark energy parameterizations which allow the existence of the high redshift objects.

astro-ph↗

arXiv2005

Observational constraints on the time-dependence of dark energy

One of the most important questions nowadays in physics concerns the nature of the so-called dark energy. It is also a consensus among cosmologists that such a question will not be answered on the basis only of observational data. However, it is possible to diminish the range of possibilities for this dark component by comparing different dark energy scenarios and finding which models can be ruled out by current observations. In this paper, by asssuming three distinct parametrizations for the low-redshift evolution of the dark energy equation of state (EOS), we consider the possibility of discriminating between evolving dark energy and $Λ$CDM models from a joint analysis involving the most recent radio sources gravitational lensing sample, namely, the Cosmic All Sky Survey (CLASS) statistical data and the recently published \emph{gold} SNe Ia sample. It is shown that this particular combination of observational data restricts considerably the dark energy parameter space, which enables possible distinctions between time-dependent and constant EOS's.

astro-ph↗

arXiv2005

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.

astro-ph↗

arXiv2005

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 $H^{2} = g(ρ_m, n, q)$ where $g(ρ_m, n, q)$ is a new function of the energy density $ρ_m$, 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.

astro-ph↗

arXiv2004

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 $p = -A/ρ_{Cg}^α$ where $A$ and $α$ 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.

astro-ph↗

arXiv2003

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 $n(Δθ)$ 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.

astro-ph↗

arXiv2003

Gravitational lensing constraints on dark energy from modified Friedmann equations

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 $H^{2} = g(ρ_m, n, q)$ where $g(ρ_m, n, q)$ is a new function of the energy density $ρ_m$, the so-called generalized Cardassian models. We mainly focus our attention on the constraints from statistical properties of gravitationally lensed quasars on the parameters $n$ and $q$ that fully characterize the models. We show that these models are in agreement with the current gravitationally lensed quasar data for a large interval of the $q - n$ parametric space. The dependence of the acceleration redshift (the redshift at which the universe begins to accelerate) on these parameters is also briefly discussed.

astro-ph↗

arXiv2003

The angular size - redshift relation in power-law cosmologies

A linear evolution of the cosmological scale factor is a feature in several models designed to solve the cosmological constant problem via a coupling between scalar or tensor classical fields to the space-time curvature as well as in some alternative gravity theories. In this paper, by assuming a general time dependence of the scale factor, $R \sim t^α$, we investigate observational constraints on the dimensionless parameter $α$ from measurements of the angular size for a large sample of milliarcsecond compact radio sources. In particular, we find that a strictly linear evolution, i.e., $α\simeq 1$ is favoured by these data, which is also in agreement with limits obtained from other independent cosmological tests. The dependence of the critical redshift $z_m$ (at which a given angular size takes its minimal value) with the index $α$ is briefly discussed.

astro-ph↗

arXiv2002

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 $p = -A/ρ^α$ (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.

astro-ph↗

arXiv2002

Cosmological consequences of a Chaplygin gas dark energy

A combination of recent observational results has given rise to what is currently known as the dark energy problem. Although several possible candidates have been extensively discussed in the literature to date the nature of this dark energy component is not well understood at present. In this paper we investigate some cosmological implications of another dark energy candidate: an exotic fluid known as the Chaplygin gas, which is characterized by an equation of state $p = -A/ρ$, where $A$ is a positive constant. By assuming a flat scenario driven by non-relativistic matter plus a Chaplygin gas dark energy we study the influence of such a component on the statistical properties of gravitational lenses. A comparison between the predicted age of the universe and the latest age estimates of globular clusters is also included and the results briefly discussed. In general, we find that the behavior of this class of models may be interpreted as an intermediary case between the standard and $Λ$CDM scenarios.

astro-ph↗

arXiv2002

Age Constraints on Brane Models of Dark Energy

Inspired by recent developments in particle physics, the so-called brane world cosmology seems to provide an alternative explanation for the present dark energy problem. In this paper, we use the estimated age of high-$z$ objects to constrain the value of the cosmological parameters in some particular scenarios based on this large scale modification of gravity. We show that such models are compatible with these observations for values of the crossover distance between the 4 and 5 dimensions of the order of $r_c \leq 1.67H_o^{-1}$.

astro-ph↗

arXiv2002

Cosmological Tests for a Linear Coasting Cosmology

A strictly linear evolution of the scale factor is a characteristic feature in several classes of alternative gravity theories. In this article we investigate the overall viability of an open linear coasting cosmological model. We report that this model is consistent with gravitational lensing statistics (within $1σ$) and accomodates old high-redshift galaxies. We finally conclude that such a linear coasting, $\a(t) = t$, is not ruled out on basis of these observational tests.

astro-ph↗