A new class of LRS Bianchi type-II dark energy models with variable EoS parameter (original) (raw)
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Journal of Scientific Research and Studies, 2014
In this work, 257 Type Ia supernovae (SNe Ia) which have been discovered from different space telescopes in the redshift range 0.01 < z ≤ 1.55 were investigated to realize the history of cosmic expansion over the last 13.5 Gyr. The Hubble parameter, H(z) was measured from plotted data. The observational and the new theoretical luminosity distances of these 257 SNe Ia were determined from the relations between light-curve shape and luminosity, and Hubble velocities, respectively. Using a model of the expansion history for a flat universe, the transition between the two epochs was constrained to be at z = 0.39±0.007 and the Hubble parameters were measured as ΩM (=Ω1) = 0.14±0.06 and ΩΛ (=Ω2) = 0.86±0.02. The radiation parameter did not take into account and accepted that the universe had started to expand from a spherical distribution at the beginning. The Minkowskian metric was used, nevertheless some assumptions were presented with the help of Neo-Newtonian cosmology which shows similar features to Minkowskian metric and enables to apply the Newtonian laws, to make the Minkowskian metric applicable to the shell model. In this paper, the astrophysical applications for the Minkowskian metric were mainly presented rather than relativistic calculations.
A Theoretical Study of the Characteristics of Time Evolution of Some Cosmological Parameters
World Scientific News, 2021
In the present study we have determined the nature of time dependence of various cosmological parameters with the help of four mathematical models. Our theoretical derivations are based on gravitational field equations (involving a dynamical cosmological constant) obtained for a homogeneous and isotropic space-time with zero curvature. In the models proposed here, we have assumed four ansatzes in the form of expressions involving the cosmological constant, Hubble parameter and time. Substituting these ansatzes into the field equations and solving them, we have derived expressions for the scale factor, Hubble parameter, deceleration parameter, energy density, gravitational constant, equation of state (EoS) parameter, cosmological constant and the relative rate of change of the gravitational constant. Using these expressions, the characteristics of time evolution of these cosmological parameters can be completely determined. Signature flip of the deceleration parameter, which is evident from astrophysical observations, has been obtained from all models of the present study. The gravitational constant has been found to be decreasing with time. The cosmological constant increases from negative to positive values while the EoS parameter decreases from positive to negative values with time. The rate of change for each of them decreases with time. It has been found that an accelerated expansion of the universe, preceded by a phase of deceleration, can also be caused by a timeindependent cosmological constant with a negative value.
2020
Abstract. We apply a parameterization-independent approach to fitting the dark energy equation-of-state (EOS). Utilizing the latest type Ia supernova data, combined with results from the cosmic microwave background and baryon acoustic oscillations, we find that the dark energy is consistent with a cosmological constant. We establish independent estimates of the evolution of the dark energy EOS by diagonalizing the covariance matrix. We find three independent constraints, which taken together imply that the equation of state is more negative than -0.2 at the 68% confidence level in the redshift range 0 < z < 1.8, independent of the flat universe assumption. Our estimates argue against previous claims of dark energy "metamorphosis," where the EOS was found to be strongly varying at low redshifts. Our results are inconsistent with extreme models of dynamical dark energy, both in the form of "freezing" models where the dark energy EOS begins with a value greate...
Dark energy, a cosmological constant, and type Ia supernovae
New Journal of Physics, 2007
We focus on uncertainties in supernova measurements, in particular of individual magnitudes and redshifts, to review to what extent supernovae measurements of the expansion history of the universe are likely to allow us to constrain a possibly redshift-dependent equation of state of dark energy, w(z). focus in particular on the central question of how well one might rule out the possibility of a cosmological constant w = −1. We argue that it is unlikely that we will be able to significantly reduce the uncertainty in the determination of w beyond its present bounds, without significant improvements in our ability to measure the cosmic distance scale as a function of redshift. Thus, unless the dark energy significantly deviates from w(z) = −1 at some redshift, very stringent control of the statistical and systematic errors will be necessary to have a realistic hope of empirically distinguishing exotic dark energy from a cosmological constant.
Astrophysics and Space Science, 2012
We review different dark energy cosmologies. In particular, we present the ΛCDM cosmology, Little Rip and Pseudo-Rip universes, the phantom and quintessence cosmologies with Type I, II, III and IV finite-time future singu-K. Bamba ( ) · S. Nojiri Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Astrophys Space Sci (2012) 342:155-228 z 0 dz /E(z ), where E(z) in Eq. (13) with Ω (0) K = 0. It follows from this relation, we find H (z) = {(d/dz)[d L (z)/(1 + z)]} −1 . Accordingly, for z < O(1), the cosmic expansion history can be obtained Author's personal copy Astrophys Space Sci (2012) 342:155-228 159
High-redshift cosmography: new results and implications for dark energy
Monthly Notices of the Royal Astronomical Society, 2012
The explanation of the accelerated expansion of the Universe poses one of the most fundamental questions in physics and cosmology today. If the acceleration is driven by some form of dark energy, and in the absence of a well-based theory to interpret the observations, one can try to constrain the parameters describing the kinematical state of the universe using a cosmographic approach, which is fundamental in that it requires only a minimal set of assumptions, namely to specify the metric, and it does not rely on the dynamical equations for gravity. Our high-redshift analysis allows us to put constraints on the cosmographic expansion up to the fifth order. It is based on the Union2 Type Ia Supernovae (SNIa) data set, the Hubble diagram constructed from some Gamma Ray Bursts luminosity distance indicators, and gaussian priors on the distance from the Baryon Acoustic Oscillations (BAO), and the Hubble constant h (these priors have been included in order to help break the degeneracies among model parameters). To perform our statistical analysis and to explore the probability distributions of the cosmographic parameters we use the Markov Chain Monte Carlo Method (MCMC). We finally investigate implications of our results for the dark energy, in particular, we focus on the parametrization of the dark energy equation of state (EOS). Actually, a possibility to investigate the nature of dark energy lies in measuring the dark energy equation of state, w, and its time (or redshift) dependence at high accuracy. However, since w(z) is not directly accessible to measurement, reconstruction methods are needed to extract it reliably from observations. Here we investigate different models of dark energy, described through several parametrizations of the equation of state, by comparing the cosmographic and the EOS series. The main results are: a) even if relying on a mathematical approximate assumption such as the scale factor series expansion in terms of time, cosmography can be extremely useful in assessing dynamical properties of the Universe; b) the deceleration parameter clearly confirms the present acceleration phase; c) the MCMC method provides stronger constraints for parameter estimation, in particular for higher order cosmographic parameters (the jerk and the snap), with respect to those presented in the literature; d) both the estimation of the jerk and the DE parameters, reflect the possibility of a deviation from the ΛCDM cosmological model; e) there are indications that the dark energy equation of state is evolving for all the parametrizations that we considered; f ) the q(z) reconstruction provided by our cosmographic analysis allows a transient acceleration.
Narrowing constraints with type Ia supernovae: converging on a cosmological constant
Journal of Cosmology and Astroparticle Physics, 2007
We apply a parameterization-independent approach to fitting the dark energy equation-of-state (EOS). Utilizing the latest type Ia supernova data, combined with results from the cosmic microwave background and baryon acoustic oscillations, we find that the dark energy is consistent with a cosmological constant. We establish independent estimates of the evolution of the dark energy EOS by diagonalizing the covariance matrix. We find three independent constraints, which taken together imply that the equation of state is more negative than -0.2 at the 68% confidence level in the redshift range 0<z<1.8, independent of the flat universe assumption. Our estimates argue against previous claims of dark energy ``metamorphosis,'' where the EOS was found to be strongly varying at low redshifts. Our results are inconsistent with extreme models of dynamical dark energy, both in the form of ``freezing'' models where the dark energy EOS begins with a value greater than -0.2 at z > 1.2 and rolls to a value of -1 today, and ``thawing'' models where the EOS is near -1 at high redshifts, but rapidly evolves to values greater than -0.85 at z < 0.2. Finally, we propose a parameterization-independent figure-of-merit, to help assess the ability of future surveys to constrain dark energy. While previous figures-of-merit presume specific dark energy parameterizations, we suggest a binning approach to evaluate dark energy constraints with a minimum number of assumptions.