A Modified Dynamical Model of Cosmology

Arxiv preprint astro-ph/9605176, 1996

Astroparticle Physics, 2009

We propose a new class of accelerating world models unifying the cosmological dark sector (dark matter and dark energy). All the models are described by a simplified version of the Chaplygin gas Quartessence cosmology. It is found that even for Ω k = 0, this Quartessence scenario depends only on a pair of parameters which can severely be constrained by the cosmological tests. As an example we perform a joint analysis involving the latest SNe type Ia data and the recent Sloan Digital Sky Survey measurement of baryon acoustic oscillations. In our analysis we have considered separately the SNe type Ia gold sample measured by Riess et al. (2004) and the Supernova Legacy Survey (SNLS) from Astier et al. (2006). At 95.4% (c.l.), we find for BAO + gold sample, α = 0.82 +0.04 −0.06 and ΩQ4 = 1.08 +0.25 −0.31 while BAO + SNLS analysis provides α = 0.83 +0.03 −0.05 and ΩQ4 = 1.11 +0.21 −0.26. The best fit for this simplified Quartessence scenario is a spatially closed Universe and with the same number of parameters, the χ 2 = 174.6 is slightly smaller than the one of the flat concordance model (ΛCDM).

1997

Supernovae are bright luminous stellar objects observable up to redshifts close to z~1. They are used to probe the geometry of the Universe and its expansion rate by applying different methods. In this article, I review various approaches used to measure the present expansion rate of the Universe, $H_{0}$, and the paths to determine its matter density $\Omega_{M}$ and the possible contribution of a non-zero cosmological constant $\Lambda$. An account is given of the numerical estimates of those cosmological parameters according to the present status of the research.

This paper contains the study of a six-dimensional space time, in which there are determined the scale factors A(t), B(t), C(t), D(t) and E(t) with the help of the Einstein field equation and of the energy momentum tensor for the case of the perfect fluid. We have also evaluated some cosmological terms, like energy density ρ, energy pressure p, the Hubble parameter H, the deceleration parameter q, the anistropic mean A, and proved that all the parameters diverges at t = − c1 nl , which emphasize this point as being a singular one. Plots for the comparative study of scale factors and cosmological terms are provided. M.S.C. 2010: 85A40, 83F05.

Physical Review D, 2014

The European Physical Journal Plus, 2013

We suggest an approach for the description of gravitation fields based on the weak equivalence principle, where the rest mass of an object is decreased by the field as much as its static binding energy. The law of modification of the rest mass we introduce, in fact, serves as the energy conservation law in the gravitation field, as pointed out by the first author in his previous publications. Thus, this approach allows avoiding known ambiguities of the General Theory of Relativity (GTR) with respect to the energy of the gravitation field. We further indicate ways toward a covariant formulation of our approach; however, in the present contribution we use the limit of a weak gravitation field, in order to describe the evolution of the Universe at times sufficiently far from the classically presumed "Big Bang". Even along with this limitation, we demonstrate the efficiency of our approach, and determine a number of essential properties of the behavior of the Universe, which are thence based on just the law of energy conservation, and have a general character. In particular, we find out a very small positive (outward) acceleration for the expansion of the Modern Universe, which therefore constitutes a clue for the dark energy quest without involving the cosmological constant (whose value in quantum gravity has anyway a huge difference from that furnished by GTR). We also show that at the earlier stages of the Universe evolution, the acceleration might be negative. In addition, we find mainly a radially non-uniform exfoliation of the Universe (versus the classically assumed directionally uniform expanding Universe); thus we come out with a way of formation of galaxies along directions perpendicular to the direction of the expansion of the Universe. Finally, we derive the Hubble law along with a satisfactory calculation of the Hubble constant, and show that the linear dependence of the velocity v on a distance R represents, in effect, an approximation, which is not fulfilled for very large R. We also come up with an explanation, regarding the so-far-unanswered dispersion of data around the classical Hubble plot approximations.

Physics Letters B, 2011

Motivated by the fact that any nonzero Λ can introduce a length scale or a time scale into Einstein's theory, rΛ = ctΛ = 3/|Λ|. Conversely, any cosmological length scale or time scale can introduce a Λ(t), Λ(t) = 3/r 2 Λ (t) = 3/(c 2 t 2 Λ (t)). In this letter, we investigate the time varying Λ(t) corresponding to the length scales, including the Hubble horizon, the particle horizon and the future event horizon, and the time scales, including the age of the universe and the conformal time. It is found out that, in this scenario, the Λ(t)CDM model can be taken as the unified origin of the holographic and agegraphic dark energy models with interaction between the matter and the dark energy, where the interacting term is determined by Q = −ρΛ. We place observational constraints on the Λ(t)CDM models originating from different cosmological length scales and time scales with the recently compiled "Union2 compilation" which consists of 557 Type Ia supernovae (SNIa) covering a redshift range 0.015 ≤ z ≤ 1.4. In conclusion, an accelerating expansion universe can be derived in the cases taking the Hubble horizon, the future event horizon, the age of the universe and the conformal time as the length scale or the time scale.

2005

In these lectures I review the present status of the so-called Standard Cosmological Model, based on the hot Big Bang Theory and the Inflationary Paradigm. I will make special emphasis on the recent developments in observational cosmology, mainly the acceleration of the universe, the precise measurements of the microwave background anisotropies, and the formation of structure like galaxies and clusters of galaxies from tiny primodial fluctuations generated during inflation. 1 One parallax second (1 pc), parsec for short, corresponds to a distance of about 3.26 light-years or 3.09 × 10 18 cm. 2 I am using c = 1 everywhere, unless specified, and a metric signature (−, +, +, +).

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.

Pramana

We investigate a new class of LRS Bianchi type-II cosmological models by revisiting in the paper of Mishra et al (2013) by considering a new deceleration parameter (DP) depending on the time in string cosmology for the modified gravity theory suggested by Sáez & Ballester (1986). We have considered the energy-momentum tensor proposed by Leteliar (1983) for bulk viscous and perfect fluid under some assumptions. To make our models consistent with recent astronomical observations, we have used scale factor (Sharma et al 2018; Garg et al 2019) a(t) = exp [ 1 β √ 2βt + k], where β and k are positive constants and it provides a time-varying DP. By using the recent constraints (H 0 = 73.8, and q 0 = −0.54) from SN Ia data in combination with BAO and CMB observations (Giostri et al, arXiv:1203.3213v2[astro-ph.CO]), we affirm β = 0.0062 and k = 0.000016. For these constraints, we have substantiated a new class of cosmological transit models for which the expansion takes place from early decelerated phase to the current accelerated phase. Also, we have studied some physical, kinematic and geometric behavior of the models, and have found them consistent with observations and well established theoretical results. We have also compared our present results with those of Mishra et al (2013) and observed that the results in this paper are much better, stable under perturbation and in good agreement with cosmological reflections.