Non-Existence of Five Dimensional Perfect Fluid Cosmological Model in Lyra Manifold

In this paper we have taken an attempt to construct a five dimensional perfect fluid cosmological model within the framework of Lyra manifold. It is found that neither perfect fluid nor dust distributions survive. Finally the exact solutions of the vacuum field equations are obtained.     

Holographic Dark Energy Cosmological Models in f(G) Theory

In this paper, Locally Rotationally Symmetric (LRS) Bianchi type-I models with holographic dark energy within the framework of f(G) theory of gravitation are thought about. So as to get determinate solutions, volumetric exponential expansion, power law expansion and hybrid expansion law are mentioned. The physical interpretations of the solution have been studied by using some physical quantizes. Additionally to make the interpretation more clear for that the statefinder diagnostic pair {r, s} and jerk parameter are analyzed to characterize completely different phases of the universe.     

Bianchi Type I Anisotropic Magnetized Cosmological Model in General Relativity

Bianchi Type I magnetized Cosmological model for perfect fluid distribution is investigated. The magnetic field is due to an electric current produced along x-axis. The distribution consists of an electrically neutral perfect fluid with an infinite electrical conductivity. To get a determinate solution, a supplementary conditionA = BC between metric potentials is used. The behaviour of the model in presence and absence of magnetic field is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

The property of dark energy and the physical reason for the acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-la supemovae (SNe Ia) and anisotropy of cosmic microwave background (CMB). The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the "Gold" sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measureΩM = 0.28-0.05 0.04, which is consistent with Riess et al. The transition redshift is ZT = 0.60-0.08 0.06. We also discuss several dark energy models that define w(z) of the parameterized equation of state of dark energy including one parameter and two parameters (w(z) being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from ZT = 0.29-0.06 0.07 to zT = 0.60-0.08 0.06 across these models. We also calculate the minimum redshift zc at which the current observations need the universe to accelerate.     

Inflationary Solutions for Higher Dimensional Anisotropic Cosmological Model in Scalar-Tensor Theory II

This is the second paper of the series where we have considered Brans-Dicke (B-D) theory as well as general scalar tensor theory of gravitation in higher dimensional space-time model in the false vacuum state. We have examined whether inflationary solutions are possible both for constant or variable coupling parameter ω. Also the nature of the scalar field and the coupling parameter are discussed in the asymptotic limit. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reconstruction of f(T) gravity from the Holographic Dark Energy

Among different candidates to play the role of Dark Energy (DE), modified gravity has emerged as offering a possible unification of Dark Matter (DM) and DE. The purpose of this work is to develop a reconstruction scheme for the modified gravity with f(T) action using holographic energy density. In the framework of the said modified gravity we have considered the equation of state of the Holographic DE (HDE) density. Subsequently we have developed a reconstruction scheme for modified gravity with f(T) action. Finally we have obtained a modified gravity action consistent with the HDE scenario.     

基于γ暴等观测限制宇宙学参数与暗能量模型

利用Union2 557个Ia型超新星数据限制宇宙学参数qo、jo和so,在红移z≤1.4范围内校准5个γ暴(gamma-ray burst,GRB)光度关系.假设γ暴光度关系不随红移演化,得到66个高红移γ暴的距离模数.最后综合利用宇宙微波背景(Cosmic Microwave Background,CMB)辐射观测数据、重子声波震荡(Baryon AcousticOscillations,BAO)观测数据与116个具有红移的γ暴数据限制几个常见的暗能量模型.根据贝叶斯判据(Bayesian Information Criterion,BIC),发现ACDM模型是最好的模型;根据Akaike判据(Akaike Information Criterion,AIC),发现JBP模型是最好的模型.     

Generalized Expanding and Shearing Anisotropic Bianchi Type I Magnetofluid Cosmological Model in General Relativity

The behaviour of magnetic field in anisotropic Bianchi type I cosmological model for perfect fluid distribution in General Relativity, is investigated. The distribution consists of an electrically neutral perfect fluid with an infinite electrical conductivity. It is assumed that the component ¹ ₁ of shear tensor ^j _i is proportional to the expansion () which leads to A = (BC)ⁿ. The other physical and geometrical aspects of the model are also discussed, Bali (1986) obtained the cosmological model for n = 1 in presence of magnetic field. We have investigated the model for general values of n and discussed the particular case and general behaviour of the model.     

Gravitational Mesoscopic Constraints in Cosmological Dark Matter Halos

We present an analysis of the behaviour of the ‘coarse-grained’ (‘mesoscopic’) rank partitioning of the mean energy of collections of particles composing virialized dark matter halos in a Λ-CDM cosmological simulation. We find evidence that rank preservation depends on halo mass, in the sense that more massive halos show more rank preservation than less massive ones. We find that the most massive halos obey Arnold’s theorem (on the ordering of the characteristic frequencies of the system) more frequently than less massive halos. This method may be useful to evaluate the coarse-graining level (minimum number of particles per energy cell) necessary to reasonably measure signatures of ‘mesoscopic’ rank orderings in a gravitational system.     

Anisotropic Bianchi Type-II Cosmological Models in Self-Creation Cosmology

Totally anisotropic Bianchi type-II cosmological solutions are presented in Barber's second self-creation theory of gravitation both in vacuum and in the presence of stiff-matter. The corresponding cosmological models have no finite singularity. The stiff-matter model gives essentially an empty universe for large time. Some physical and kinematical properties of the models are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

An Interacting Model of Dark Energy in Brans-Dicke Theory

In this paper it is shown that in non-minimally coupled Brans-Dicke theory containing a self-interacting potential, a suitable conformal transformation can automatically give rise to an interaction between the normal matter and the Brans-Dicke scalar field. Considering the scalar field in the Einstein frame as the quintessence matter, it has been shown that such a non-minimal coupling between the matter and the scalar field can give rise to a late time accelerated expansion for the universe preceded by a decelerated expansion for very high values of the Brans-Dicke parameter ω. We have also studied the observational constraints on the model parameters considering the Hubble and Supernova data.     

Exact Bianchi Type-I Cosmological Micro Model in Modified Theory of General Relativity

An anisotropic, homogeneous Bianchi type-I cosmological micro model is obtained in Barber's modified theory of general relativity. Some properties of the model are discussed. Further, it is found that this theory leads to Einstein theory as the coupling parameterλ → 0 in micro level (i.e., quantum level) in general. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dark Energy in the Universe

I 24 Studying the Nature of Dark Energy with Galaxy Clusters I 50 Constraining Dark Energy via Baryon Acoustic Oscillations I 65 Constraining Dark Energy with Redshift Surveys I 103 Dark Energy: Necessity, Models and Expectations I 177 Searching for galaxy clusters through weak lensing, X‐rays and the SZ observations I 181 SNIa and Dark Energy     

A Study of an Inhomogeneous Bianchi-I Model in Lyra Geometry

The inhomogeneous Bianchi-I model based on Lyra's geometry has been studied in the cosmological theory in presence of a massless inhomogeneous scalar field whose potential has a flat part. The field equations are solved using separation of variables and it is shown that one of the time part of the field equations are solvable for any arbitrary other cosmic scale function. Solutions for a particular form of cosmic scale (time part) is presented. This revised version was published online in July 2006 with corrections to the Cover Date.