scholarly journals STRING COSMOLOGY IN ANISOTROPIC BIANCHI-II SPACETIME

2011 ◽  
Vol 26 (11) ◽  
pp. 779-793 ◽  
Author(s):  
SURESH KUMAR
Keyword(s):  
Cosmological Model ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Hubble’S Parameter ◽  
Massive String ◽  
Field Equations ◽  
Expansion Scalar ◽  
Spatially Homogeneous ◽  
The Universe ◽  
Shear Tensor

The present study deals with a spatially homogeneous and anisotropic Bianchi-II cosmological model representing massive strings. The energy–momentum tensor, as formulated by Letelier,10 has been used to construct a massive string cosmological model for which the expansion scalar is proportional to one of the components of shear tensor. The Einstein's field equations have been solved by applying a variation law for generalized Hubble's parameter that yields a constant value of deceleration parameter in Bianchi-II spacetime. A comparative study of accelerating and decelerating modes of the evolution of universe has been carried out in the presence of string scenario. The study reveals that massive strings dominate the early Universe. The strings eventually disappear from the Universe for sufficiently large times, which is in agreement with the current astronomical observations.

Gravity, stability and cosmological models

2017 ◽  
Vol 26 (12) ◽  
pp. 1743026
Author(s):  
Asher Yahalom
Keyword(s):  
Stability Analysis ◽  
Cosmological Model ◽  
Cosmological Constant ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Lorentzian Metric ◽  
Standard Cosmological Model ◽  
Flat Spacetimes ◽  
Stable Solutions ◽  
The Universe

Stability analysis plays a major rule in our understanding of nature. For example it was shown that among empty flat spacetimes only those with a Lorentzian metric are stable [A. Yahalom, Found Phys. 38 (2008) 489–497; Int. J. Mod. Phys. D 18(4) (2009) 2155–2158]. However, the universe is not empty and the energy momentum tensor is metric dependent an thus effects stability. In this essay we concentrate on simple perturbations of the standard cosmological model with and without cosmological constant which is based on a uniform mass distribution. The results suggest that while Euclidean, open or closed section models are valid solutions, the choice of stable solutions is limited. In particular, the popular Lambda-CDM model is unstable.

FRW universe in f(R,T) gravity

2021 ◽  
pp. 2150104
Author(s):  
R. K. Tiwari ◽  
D. Sofuoğlu ◽  
A. Beesham
Keyword(s):  
Phase Transition ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Field Equations ◽  
Frw Universe ◽  
Expansion Phase ◽  
Age Of The Universe ◽  
The Universe ◽  
Modified Theory ◽  
Friedmann Robertson Walker

In this study, Friedmann–Robertson–Walker space-time filled with a perfect fluid in [Formula: see text] modified theory, where [Formula: see text] is the Ricci scalar and [Formula: see text] is the trace of the energy–momentum tensor of matter, has been considered. The investigation of the phase transition of the universe from the decelerating expansion phase to the accelerating one has been made by adopting a special form of the varying deceleration parameter that is inversely proportional to the Hubble parameter. The exact solution of the field equations has been derived. The kinematic and dynamical quantities of the model have been obtained and their evolutions have been discussed by means of their graphs. The statefinder diagnostic has been used and the age of the universe has been computed for testing the validity of the model. It has been shown that the dominant energy of the model is ordinary matter which behaves as the SCDM model at the beginning and it is a quintessence like fluid which behaves as the [Formula: see text]CDM model at late times.

scholarly journals Bianchi Type I Cosmological Models with Expansion Driven by Particle Creation

2018 ◽  
Author(s):  
Kalyani Desikan
Keyword(s):  
Bianchi Type ◽  
Energy Momentum Tensor ◽  
Particle Creation ◽  
Momentum Tensor ◽  
Conservation Equation ◽  
Type I ◽  
Field Equations ◽  
Bianchi Type I ◽  
Energy Conservation Equation ◽  
The Universe

A study of Bianchi Type I cosmological model is undertaken in the framework of creation of particles. To accommodate the creation of new particles, the universe is regarded as an Open thermodynamical system. The energy conservation equation is modified with the incorporation of a creation pressure in the energy momentum tensor. Exact solutions of the field equations are obtained (i) for a particular choice of the particle creation function and (ii) by considering the deceleration parameter to be constant. In the first model the behavior of the solution at late times is investigated. The physical aspects of the model have also been discussed. In the case of the second model we have restricted our analysis to the power law behaviour for the average scale factor. This leads to a particular form for the particle creation function. The behavior of the solution is investigated and the physical aspects of the model have also been discussed for the matter dominated era.

scholarly journals Energy-Momentum Tensor and Parameters in Cosmological Model

2020 ◽  
Author(s):  
Ying-Qiu Gu
Keyword(s):  
Cosmological Model ◽  
Cosmological Constant ◽  
Energy Momentum Tensor ◽  
Equations Of State ◽  
Mass Density ◽  
Big Bang ◽  
Momentum Tensor ◽  
Ideal Gas ◽  
Dimensional Sphere ◽  
The Universe

In cosmology, the cosmic curvature $K$ and the cosmological constant $\Lambda$ are two most important parameters, whose values have strong influence on the behavior of the universe. By analyzing the energy-momentum tensor and equations of state of ideal gas, scalar, spinor and vector potential in detail, we find that the total mass density of all matter is always positive, and the initial total pressure is negative. Under these conditions, by qualitatively analyzing the global behavior of the dynamical equation of cosmological model, we get the following results: (i) $K= 1$, namely, the global spatial structure of the universe should be a 3-dimensional sphere $S^3$. (ii) $0\le\Lambda < 10 ^ {-24} {\rm ly} ^ {-2}$, the cosmological constant should be zero or an infinitesimal. (iii) $a(t)>0$, the initial singularity of the universe is unreachable, and the evolution of universe should be cyclic in time. This means that the initial Big Bang is impossible at all. Since the matter components considered are quite complete and the proof is very elementary and strict, these logical conclusions should be quite reliable. Obviously, these conclusions will be much helpful to correct some popular misconceptions and bring great convenience to further research other problems in cosmology such as property of dark matter and dark energy.

Comparative study of transit FLRW and axially symmetric cosmological structures with domain walls in f(R, T)-gravity

2020 ◽  
Author(s):  
Umesh Kumar Sharma ◽  
Ambuj Kumar Mishra ◽  
Anirudh Pradhan
Keyword(s):  
Domain Walls ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Axially Symmetric ◽  
Field Equations ◽  
Stress Energy ◽  
Thick Domain Walls ◽  
Theory Of Gravitation ◽  
The Universe ◽  
Stress Energy Momentum Tensor

In the present article, we study the physical and geometric scene of the inflection of the Friedmann- Lemaitre-Robertson-Walker (FLRW) and an axially symmetric (AS) perfect fluid Universe with thick domain walls in f(R, T) theory of gravitation [Harko et al., Phys. Rev. D {84} (2011) 024020], where R and T represent Ricci scalar and trace of the stress energy-momentum tensor respectively in the scenario of decelerating-accelerating transition phases. To ascertain the exact solution of the corresponding field equations, we use the concept of a time-subordinate deceleration parameter (DP) which brings forth the scale factor a(t) = sinh^{\frac{1}{n}}(\alpha t), where n and \alpha are positive parameters. For n\in (0.27, 1], a class of accelerating phase is ensured while for n > 1, the Universe attains a phase transition from positive (decelerating) to negative (accelerating) which is uniform with recent observations. The models have been tested for physically acceptable by using stability. More or less physical and geometric behavior of the models are also devoted.

scholarly journals Models of Collapsing and Expanding Cylindrical Source in f(R,T) Theory

2019 ◽  
Vol 2019 ◽  
pp. 1-16
Author(s):  
M. Sharif ◽  
Aisha Siddiqa
Keyword(s):  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Maxwell Field ◽  
Energy Conditions ◽  
Field Equations ◽  
Anisotropic Parameter ◽  
Model Parameter ◽  
Expansion Scalar ◽  
Cylindrical Source ◽  
Charged Cylinder

We discuss the collapsing and expanding solutions of anisotropic charged cylinder in the context of f(R,T) theory (R represents the Ricci scalar and T denotes the trace of energy-momentum tensor). For this purpose, we take an auxiliary solution of Einstein-Maxwell field equations and evaluate expansion scalar whose negative values lead to collapse and positive values give expansion. For both cases, the behavior of density, pressure, anisotropic parameter, and mass is explored and the effects of charge as well as model parameter on these quantities are examined. The energy conditions are found to be satisfied for both solutions.

Interaction of viscous modified Chaplygin gas with f(R, T) gravity

2017 ◽  
Vol 32 (28) ◽  
pp. 1750151 ◽  
Author(s):  
M. Sharif ◽  
Aisha Siddiqa
Keyword(s):  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Chaplygin Gas ◽  
Modified Chaplygin Gas ◽  
Field Equations ◽  
Frw Universe ◽  
Eos Parameter ◽  
Total Energy Density ◽  
Conservation Of Energy ◽  
The Universe

We study the evolution of viscous modified Chaplygin gas (MCG) interacting with f(R, T) gravity in flat FRW universe, where T is the trace of energy–momentum tensor. The field equations are formulated for a particular model f(R, T) = R + 2[Formula: see text]T and constraints for the conservation of energy–momentum tensor are obtained. We investigate the behavior of total energy density, pressure and equation of state (EoS) parameter for emergent, intermediate as well as logamediate scenarios of the universe with two interacting models. It is found that the EoS parameter lies in the matter-dominated or quintessence era for all the three scenarios while the bulk viscosity enhances the expansion for the intermediate and logamediate scenarios.

scholarly journals ENERGY–MOMENTUM DISTRIBUTION: A CRUCIAL PROBLEM IN GENERAL RELATIVITY

2005 ◽  
Vol 20 (18) ◽  
pp. 4309-4330 ◽  
Author(s):  
M. SHARIF ◽  
TASNIM FATIMA
Keyword(s):  
General Relativity ◽  
Cosmological Model ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Hamiltonian Approach ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Special Cases ◽  
Crucial Problem ◽  
Electromagnetic Generalization

This paper is aimed to elaborate the problem of energy–momentum in general relativity. In this connection, we use the prescriptions of Einstein, Landau–Lifshitz, Papapetrou and Möller to compute the energy–momentum densities for two exact solutions of Einstein field equations. The space–times under consideration are the nonnull Einstein–Maxwell solutions and the singularity-free cosmological model. The electromagnetic generalization of the Gödel solution and the Gödel metric become special cases of the nonnull Einstein–Maxwell solutions. It turns out that these prescriptions do not provide consistent results for any of these space–times. These inconsistent results verify the well-known proposal that the idea of localization does not follow the lines of pseudotensorial construction but instead follows from the energy–momentum tensor itself. These differences can also be understood with the help of the Hamiltonian approach.

scholarly journals A cosmology with variable c

2006 ◽  
Vol 84 (10) ◽  
pp. 933-944 ◽  
Author(s):  
H Shojaie ◽  
M Farhoudi
Keyword(s):  
Cosmological Model ◽  
Dynamic Equation ◽  
Deceleration Parameter ◽  
Energy Momentum Tensor ◽  
Energy Content ◽  
Big Bang ◽  
Momentum Tensor ◽  
Constant Deceleration Parameter ◽  
Extra Energy ◽  
The Universe

A new varying-c cosmological model, constructed using two additional assumptions, which was introduced in our previous work, is briefly reviewed and the dynamic equation of the model is derived distinctly from a semi-Newtonian approach. The results of this model, using a [Formula: see text] term and an extra energy-momentum tensor, are considered separately. It is shown that the Universe began from a hot Big Bang and expands forever with a constant deceleration parameter regardless of its curvature. Finally, the age, the radius, and the energy content of the Universe are estimated and some discussion about the type of the geometry of the Universe is provided. PACS Nos.: 98.80.Bp, 98.80.Jk

scholarly journals LRS Bianchi type V perfect fluid cosmological model in f(R, T) theory

2019 ◽  
Vol 97 (4) ◽  
pp. 443-449 ◽  
Author(s):  
A. Nath ◽  
S.K. Sahu
Keyword(s):  
Cosmological Model ◽  
Perfect Fluid ◽  
Bianchi Type ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Ricci Scalar ◽  
Field Equations ◽  
Exponential Law ◽  
Type V ◽  
Bianchi Type V

This article deals with the study of Bianchi type V cosmological model filled with perfect fluid in the context of f(R, T) gravity. R and T represent the Ricci scalar and trace of the energy–momentum tensor, respectively. The modified Einstein’s field equations are solved under the consideration of hybrid exponential law for two cases of f(R, T). The presented models are shear free.

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