scholarly journals Anisotropic Universe in f(G,T) Gravity

2017 ◽  
Vol 2017 ◽  
pp. 1-10 ◽  
Author(s):  
M. Farasat Shamir
Keyword(s):  
Equation Of State ◽  
Energy Density ◽  
Energy Momentum Tensor ◽  
State Parameter ◽  
Momentum Tensor ◽  
Anisotropic Universe ◽  
Field Equations ◽  
Equation Of State Parameter ◽  
Physical Quantities ◽  
Bonnet Term

This paper is devoted to investigating the recently introduced f(G,T) theory of gravity, where G is the Gauss-Bonnet term and T is the trace of the energy-momentum tensor. For this purpose, anisotropic background is chosen and a power law f(G,T) gravity model is used to find the exact solutions of field equations. In particular, a general solution is obtained which is further used to reconstruct some important solutions in cosmological contexts. The physical quantities like energy density, pressure, and equation of state parameter are calculated. A Starobinsky-like f2(T) model is proposed which is used to analyze the behavior of universe for different values of equation of state parameter. It is concluded that presence of term T in the bivariate function f(G,T) may give many cosmologically important solutions of the field equations.

scholarly journals Spinor Quintom Cosmology with Intrinsic Spin

2016 ◽  
Vol 2016 ◽  
pp. 1-10 ◽  
Author(s):  
Emre Dil
Keyword(s):  
Dark Energy ◽  
Energy Density ◽  
Dark Energy Model ◽  
Energy Momentum Tensor ◽  
State Parameter ◽  
Momentum Tensor ◽  
Big Rip ◽  
Equation Of State Parameter ◽  
Spin Contribution ◽  
Distance Parameter

We consider a spinor quintom dark energy model with intrinsic spin, in the framework of Einstein-Cartan-Sciama-Kibble theory. After constructing the mathematical formalism of the model, we obtain the spin contributed total energy-momentum tensor giving the energy density and the pressure of the quintom model, and then we find the equation of state parameter, Hubble parameter, deceleration parameter, state finder parameter, and some distance parameter in terms of the spinor potential. Choosing suitable potentials leads to the quintom scenario crossing between quintessence and phantom epochs, or vice versa. Analyzing three quintom scenarios provides stable expansion phases avoiding Big Rip singularities and yielding matter dominated era through the stabilization of the spinor pressure via spin contribution. The stabilization in spinor pressure leads to neglecting it as compared to the increasing energy density and constituting a matter dominated stable expansion epoch.

scholarly journals Stability analysis of Einstein universe in f(𝒢,T) gravity

2017 ◽  
Vol 26 (08) ◽  
pp. 1750084 ◽  
Author(s):  
M. Sharif ◽  
Ayesha Ikram
Keyword(s):  
Stability Analysis ◽  
Energy Momentum Tensor ◽  
State Parameter ◽  
Graphical Analysis ◽  
Momentum Tensor ◽  
Field Equations ◽  
Stability Regions ◽  
Einstein Universe ◽  
Equation Of State Parameter ◽  
The Stability

This paper explores the stability of the Einstein universe against linear homogeneous perturbations in the background of [Formula: see text] gravity. We construct static as well as perturbed field equations and investigate stability regions for the specific forms of generic function [Formula: see text] corresponding to conserved as well as nonconserved energy-momentum tensor. We use the equation-of-state parameter to parameterize the stability regions. The graphical analysis shows that the suitable choice of parameters lead to stable regions of the Einstein universe.

FRW viscous fluid cosmological model with time-dependent inhomogeneous equation of state

2017 ◽  
Vol 15 (01) ◽  
pp. 1830001 ◽  
Author(s):  
G. S. Khadekar ◽  
Deepti Raut
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Quadratic Form ◽  
State Parameter ◽  
The Other ◽  
Time Dependent ◽  
Inhomogeneous Equation ◽  
Field Equations ◽  
Equation Of State Parameter ◽  
Viscous Models

In this paper, we present two viscous models of non-perfect fluid by avoiding the introduction of exotic dark energy. We consider the first model in terms of deceleration parameter [Formula: see text] has a viscosity of the form [Formula: see text] and the other model in quadratic form of [Formula: see text] of the type [Formula: see text]. In this framework we find the solutions of field equations by using inhomogeneous equation of state of form [Formula: see text] with equation of state parameter [Formula: see text] is constant and [Formula: see text].

Investigating cosmology with equation of state

2019 ◽  
Vol 97 (7) ◽  
pp. 752-760 ◽  
Author(s):  
M. Farasat Shamir ◽  
Adnan Malik
Keyword(s):  
Equation Of State ◽  
Exact Solutions ◽  
Scalar Potential ◽  
State Parameter ◽  
Field Equations ◽  
Equation Of State Parameter ◽  
Radiation Universe ◽  
Modified Formula ◽  
Friedmann Robertson Walker ◽  
Dust Universe

The aim of this paper is to investigate the field equations of modified [Formula: see text] theory of gravity, where R and [Formula: see text] represent the Ricci scalar and scalar potential, respectively. We consider the Friedmann–Robertson–Walker space–time for finding some exact solutions by using different values of equation of state parameter. In this regard, different possibilities of the exact solutions have been discussed for dust universe, radiation universe, ultra-relativistic universe, sub-relativistic universe, stiff universe, and dark energy universe. Mainly power law and exponential forms of the scale factor are chosen for the analysis.

scholarly journals GENERALIZED MODEL FOR Λ DARK ENERGY

2009 ◽  
Vol 18 (03) ◽  
pp. 389-396 ◽  
Author(s):  
UTPAL MUKHOPADHYAY ◽  
P. C. RAY ◽  
SAIBAL RAY ◽  
S. B. DUTTA CHOUDHURY
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Cosmological Model ◽  
A Priori ◽  
State Parameter ◽  
Spherically Symmetric ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Equation Of State Parameter ◽  
Two Fluid

Einstein field equations under spherically symmetric space–times are considered here in connection with dark energy investigation. A set of solutions is obtained for a kinematic Λ model, viz. [Formula: see text], without assuming any a priori value for the curvature constant and the equation-of-state parameter ω. Some interesting results, such as the nature of cosmic density Ω and deceleration parameter q, have been obtained with the consideration of two-fluid structure instead of the usual unifluid cosmological model.

scholarly journals TRAPPING PHOTONS BY A LINE SINGULARITY

2003 ◽  
Vol 12 (06) ◽  
pp. 1095-1112 ◽  
Author(s):  
METIN ARIK ◽  
OZGUR DELICE
Keyword(s):  
Energy Density ◽  
Thin Shell ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Cylindrically Symmetric ◽  
Static Solutions ◽  
Line Singularity ◽  
Thick Shells

We present cylindrically symmetric, static solutions of the Einstein field equations around a line singularity such that the energy momentum tensor corresponds to infinitely thin photonic shells. Positivity of the energy density of the thin shell and the line singularity is discussed. It is also shown that thick shells containing mostly radiation are possible in a numerical solution.

A study on the modified holographic Ricci dark energy in logarithmic f(T) gravity

2013 ◽  
Vol 91 (4) ◽  
pp. 351-354 ◽  
Author(s):  
Antonio Pasqua ◽  
Surajit Chattopadhyay
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Equation Of State ◽  
Energy Density ◽  
Modified Gravity ◽  
State Parameter ◽  
Ricci Dark Energy ◽  
Equation Of State Parameter ◽  
Interaction Term ◽  
The Universe

In this paper, we have studied and investigated the behavior of a modified holographic Ricci dark energy (DE) model interacting with pressureless dark matter (DM) under the theory of modified gravity, dubbed logarithmic f(T) gravity. We have chosen the interaction term between DE and DM in the form Q = 3γHρm and investigated the behavior of the torsion, T, the Hubble parameter, H, the equation of state parameter, ωDE, the energy density of DE, ρDE, and the energy density contribution due to torsion, ρT, as functions of the redshift, z. We have found that T increases with the redshift, z, H increases with the evolution of the universe, ωDE has a quintessence-like behavior, and both energy densities increase going from higher to lower redshifts.

scholarly journals Unavoidable shear from quantum fluctuations in contracting cosmologies

2021 ◽  
Vol 81 (2) ◽  
Author(s):  
Julien Grain ◽  
Vincent Vennin
Keyword(s):  
Scalar Field ◽  
Equation Of State ◽  
Energy Density ◽  
State Parameter ◽  
Quantum Fluctuations ◽  
Shear Instability ◽  
Initial Value ◽  
Anisotropic Stress ◽  
Equation Of State Parameter ◽  
Contracting Phase

AbstractContracting cosmologies are known to be flawed with a shear instability, where the contribution from the anisotropic stress to the overall energy density grows as $$a^{-6}$$ a - 6 , with a the scale factor. Classically, whether or not this contribution becomes important before the bounce depends on its initial value, which can always be sufficiently fine tuned to make it irrelevant. However, vacuum quantum fluctuations inevitably provide a non-vanishing source of anisotropic stress. In this work, we compute the minimum amount of shear that is obtained if one assumes that it vanishes initially, but lets quantum fluctuations build it up. In practice, we consider a massless test scalar field, and describe its quantum fluctuations by means of the stochastic “inflation” (though here applied to a contracting phase) formalism. We find that, if the equation-of-state parameter of the contraction satisfies $$w>-1/9$$ w > - 1 / 9 , regardless of when the contracting phase is initiated, the time at which the shear becomes sizeable is always when the Hubble scale approaches the Planck mass (which is also where the bounce is expected to take place). However, if $$w<-1/9$$ w < - 1 / 9 , the shear backreaction becomes important much earlier, at a point that depends on the overall amount of contraction.

Stability of anisotropic perturbed Einstein universe in f(R) gravity

2020 ◽  
Vol 35 (18) ◽  
pp. 2050152
Author(s):  
M. Sharif ◽  
Sana Saleem
Keyword(s):  
General Relativity ◽  
Equation Of State ◽  
State Parameter ◽  
Field Equations ◽  
Static Universe ◽  
Scale Factors ◽  
Static Solutions ◽  
Equation Of State Parameter ◽  
Specific Formula ◽  
Einstein Static Universe

The aim of this paper is to investigate the existence of stable modes of the Einstein static universe in the background of [Formula: see text] theory. For this purpose, we take homogeneous anisotropic perturbations in scale factors as well as matter contents. We construct static and perturbed field equations that are further parameterized by linear equation of state parameter. We obtain the Einstein static solutions for two specific [Formula: see text] models and graphically analyze their stable regions. It is concluded that contrary to general relativity, there exists stable Einstein static universe with anisotropic perturbations.

COSMOLOGY WITH NEGATIVE POTENTIALS WITH wϕ<-1

2004 ◽  
Vol 13 (09) ◽  
pp. 1939-1953 ◽  
Author(s):  
A. DE LA MACORRA ◽  
G. GERMÁN
Keyword(s):  
Equation Of State ◽  
Energy Density ◽  
Cosmological Constant ◽  
Particle Physics ◽  
Scalar Potential ◽  
Energy Condition ◽  
State Parameter ◽  
Scalar Fields ◽  
High Energy ◽  
Equation Of State Parameter

We study the cosmology of canonically normalized scalar fields that lead to an equation of state parameter of wϕ=pϕ/ρϕ<-1 without violating the weak energy condition: ρ=Σiρi≥0 and ρi+pi≥0. This kind of behavior requires a negative scalar potential V, widely predicted in particle physics. We show that the energy density ρϕ=Ek+V takes negative values with an equation of state with wϕ<-1. However, the net effect of the ϕ field on the scale factor is to decelerate it giving a total equation of state parameter w=p/ρ>wb=pb/ρb, where ρb stands for any kind of energy density with -1≤wb≤1, such as radiation, matter, cosmological constant or other scalar field with a potential V≥0. The fact that ρϕ<0 allows, at least in principle, to have a small cosmological constant or quintessence today as the cancellation of high energy scales such as the electroweak or susy breaking scale. While V is negative |ρϕ| is smaller than the sum of all other energy densities regardless of the functional form of the potential V. We show that the existence of a negative potential leads, inevitable, to a collapsing universe, i.e. to a would be "big crunch." In this picture we would still be living in the expanding universe.

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