scholarly journals Gödel-type solution in f(R,T) modified gravity

2015 ◽  
Vol 30 (40) ◽  
pp. 1550214 ◽  
Author(s):  
A. F. Santos ◽  
C. J. Ferst
Keyword(s):  
Scalar Field ◽  
Perfect Fluid ◽  
Modified Gravity ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Matter Content ◽  
Type Solution ◽  
Special Cases ◽  
Tensor Formula ◽  
The Universe

In this paper, we will examine the problem of violation of causality in [Formula: see text] modified gravity, where [Formula: see text] is the Ricci scalar and [Formula: see text] is the trace of the energy–momentum tensor [Formula: see text]. We investigate the causality problem in two special cases, in the first we consider the matter content of the universe as a perfect fluid and in the second case, the matter content is a perfect fluid plus a scalar field.

scholarly journals Cosmological reconstruction of f(T, 𝒯) gravity

2014 ◽  
Vol 11 (08) ◽  
pp. 1450077 ◽  
Author(s):  
Davood Momeni ◽  
Ratbay Myrzakulov
Keyword(s):  
Scalar Field ◽  
Modified Gravity ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Chaplygin Gas ◽  
Massless Scalar ◽  
Massless Scalar Field ◽  
Two Fluids ◽  
Consistent Model ◽  
The Universe

Motivated by the newly proposal for gravity as the effect of the torsion scalar T and trace of the energy momentum tensor 𝒯, we investigate the cosmological reconstruction of different models of the Universe. Our aim here is to show that how this modified gravity model, f(T, 𝒯) is able to reproduce different epochs of the cosmological history. We explicitly show that f(T, 𝒯) can be reconstructed for ΛCDM as the most popular and consistent model. Also we study the mathematical reconstruction of f(T, 𝒯) for a flat cosmological background filled by two fluids mixture. Such model describes phantom–non-phantom era as well as the purely phantom cosmology. We extend our investigation to more cosmological models like perfect fluid, Chaplygin gas and massless scalar field. In each case we obtain some specific forms of f(T, 𝒯). These families of f(T, 𝒯) contain arbitrary function of torsion and trace of the energy momentum.

Cosmic acceleration with tachyon field

2018 ◽  
Vol 33 (34) ◽  
pp. 1850199 ◽  
Author(s):  
A. I. Keskin
Keyword(s):  
Scalar Field ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Cosmic Acceleration ◽  
Late Time ◽  
De Sitter ◽  
Tachyon Field ◽  
Minimal Coupling ◽  
Acceleration Of The Universe ◽  
The Universe

In this study, we examine two models of the scalar field, that is, a normal scalar field and a tachyon scalar field in [Formula: see text] gravity to describe cosmic acceleration of the universe, where [Formula: see text], [Formula: see text] and [Formula: see text] are Ricci curvature scalar, trace of energy–momentum tensor and kinetic energy of scalar field [Formula: see text], respectively. Using the minimal-coupling Lagrangian [Formula: see text], for both the scalar models we obtain a viable cosmological system, where [Formula: see text] and [Formula: see text] are real constants. While a normal scalar field gives a system describing expansion from the deceleration to the late-time acceleration, tachyon field together with [Formula: see text] in the system produces a quintessential expansion which is very close to de Sitter point, where we find a new condition [Formula: see text] for inflation.

scholarly journals Dynamics of an Anisotropic Universe inf(R,T)Theory

2016 ◽  
Vol 2016 ◽  
pp. 1-8 ◽  
Author(s):  
B. Mishra ◽  
Sankarsan Tarai ◽  
S. K. Tripathy
Keyword(s):  
Power Law ◽  
Modified Gravity ◽  
Bianchi Type ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Model Parameters ◽  
Anisotropic Universe ◽  
Expansion Of The Universe ◽  
Physical Features ◽  
The Universe

Dynamics of an anisotropic universe is studied inf(R,T)gravity using a rescaled functionalf(R,T), whereRis the Ricci Scalar andTis the trace of energy-momentum tensor. Three models have been constructed assuming a power law expansion of the universe. Physical features of the models are discussed. The model parameters are constrained from a dimensional analysis. It is found from the work that the anisotropic Bianchi typeVIh(BVIh) model in the modified gravity generally favours a quintessence phase when the parameterhis either-1or0. We may not get viable models in conformity with the present day observation forh=1.

scholarly journals EXOTIC LOW DENSITY FERMION STATES IN THE TWO MEASURES FIELD THEORY: NEUTRINO DARK ENERGY

2006 ◽  
Vol 21 (21) ◽  
pp. 4373-4406 ◽  
Author(s):  
E. I. GUENDELMAN ◽  
A. B. KAGANOVICH
Keyword(s):  
Field Theory ◽  
Dark Energy ◽  
Scalar Field ◽  
Energy Density ◽  
Energy Momentum Tensor ◽  
Neutrino Energy ◽  
Momentum Tensor ◽  
Dark Energy Density ◽  
Two Measures ◽  
The Universe

There exist field theory models where the fermionic energy–momentum tensor contains a term proportional to [Formula: see text] which may contribute to the dark energy. We show that this new field theory effect can be achieved in the Two Measures Field Theory (TMT) in the cosmological context. TMT is an alternative gravity and matter field theory where the gravitational interaction of fermionic matter is reduced to that of General Relativity when the energy density of the fermion matter is much larger than the dark energy density. In this case also the fifth force problem is solved automatically. In the opposite limit, where the magnitudes of fermionic energy density and scalar field dark energy density become comparable, nonrelativistic fermions can participate in the cosmological expansion in a very unusual manner. Some of the features of such Cosmo-Low-Energy-Physics (CLEP) states are studied in a toy model of the late time universe filled with homogeneous scalar field and uniformly distributed nonrelativistic neutrinos, and the following results are obtained: neutrino mass increases as m ∝ a3/2 (a is the scale factor); the proportionality factor in the noncanonical contribution to the neutrino energy–momentum tensor (proportional to the metric tensor) approaches a constant as a(t) → ∞ and therefore the noncanonical contribution to the neutrino energy density dominates over the canonical one ~ m/a3 ~ a-3/2 at the late enough universe; hence the neutrino gas equation-of-state approaches w = -1, i.e. neutrinos in the CLEP regime behave as a sort of dark energy as a → ∞; the equation-of-state for the total (scalar field + neutrino) energy density and pressure also approaches w = -1 in the CLEP regime; besides the total energy density of such universe is less than it would be in the universe filled with the scalar field alone. An analytic solution is presented. A domain structure of the dark energy seems to be possible. We speculate that decays of the CLEP state neutrinos may be both an origin of cosmic rays and responsible for a late super-acceleration of the universe. In this sense the CLEP states exhibit simultaneously new physics at very low densities and for very high particle masses.

scholarly journals TIMELIKE AND SPACELIKE MATTER INHERITANCE VECTORS IN SPECIFIC FORMS OF ENERGY–MOMENTUM TENSOR

2006 ◽  
Vol 21 (15) ◽  
pp. 3213-3234 ◽  
Author(s):  
M. SHARIF ◽  
UMBER SHEIKH
Keyword(s):  
Perfect Fluid ◽  
Energy Momentum Tensor ◽  
Killing Vector ◽  
Sufficient Conditions ◽  
Momentum Tensor ◽  
String Cosmology ◽  
Conformal Killing ◽  
Inheritance Vector ◽  
Special Cases ◽  
Inheritance Vectors

This paper is devoted to the investigation of the consequences of timelike and spacelike matter inheritance vectors in specific forms of energy–momentum tensor, i.e. for string cosmology (string cloud and string fluid) and perfect fluid. Necessary and sufficient conditions are developed for a space–time with string cosmology and perfect fluid to admit a timelike matter inheritance vector, parallel to ua and spacelike matter inheritance vector, parallel to xa. We compare the outcome with the conditions of conformal Killing vectors. This comparison provides us the conditions for the existence of matter inheritance vector when it is also a conformal Killing vector. Finally, we discuss these results for the existence of matter inheritance vector in the special cases of the above mentioned space–times.

scholarly journals Effects of Rastall parameter on perturbation of dark sectors of the Universe

2021 ◽  
pp. 2150082
Author(s):  
A. H. Ziaie ◽  
H. Shabani ◽  
S. Ghaffari
Keyword(s):  
Modified Gravity ◽  
Numerical Solutions ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Linear Regime ◽  
Minimal Coupling ◽  
Spherical Collapse ◽  
Modified Gravity Theory ◽  
Collapse Model ◽  
The Universe

In recent years, Rastall gravity is undergoing a considerable surge in popularity. This theory purports to be a modified gravity theory with a non-conserved energy–momentum tensor (EMT) and an unusual non-minimal coupling between matter and geometry. This work looks for the evolution of homogeneous spherical perturbations within the Universe in the context of Rastall gravity. Using the spherical Top-Hat collapse model, we seek for exact solutions in linear regime for density contrast of dark matter (DM) and dark energy (DE). We find that the Rastall parameter affects crucially the dynamics of density contrasts for DM and DE and the fate of spherical collapse is different in comparison to the case of general relativity (GR). Numerical solutions for perturbation equations in nonlinear regime reveal that DE perturbations could amplify the rate of growth of DM perturbations depending on the values of Rastall parameter.

FRW MODELS WITH PERFECT FLUID AND SCALAR FIELD IN HIGHER DERIVATIVE THEORY

2011 ◽  
Vol 26 (20) ◽  
pp. 1495-1507 ◽  
Author(s):  
C. P. SINGH ◽  
VIJAY SINGH
Keyword(s):  
Scalar Field ◽  
Perfect Fluid ◽  
Scalar Potential ◽  
Physical Parameters ◽  
Higher Derivative ◽  
Special Cases ◽  
Theory Of Gravitation ◽  
Frw Models ◽  
Nonzero Scalar ◽  
The Universe

In this paper we study the dynamics of the universe in Friedmann–Robertson–Walker models including perfect fluid and coupled scalar field with nonzero scalar potential in higher derivative theory of gravitation. We study the evolution of the universe by assuming the scalar potential and scale factor as functions of the scalar field. Exact cosmological solutions are obtained for flat, closed and open models, which are physically interesting for the description of the present-day universe. The properties of scalar field and other physical parameters are discussed in detail. Some special cases have been studied by imposing certain constraints on constants to discuss the decelerated and accelerated phases of the universe.

Thermodynamic consequences of specific modified gravity on the apparent horizon

2019 ◽  
Vol 16 (09) ◽  
pp. 1950144
Author(s):  
Abdul Jawad ◽  
Zoya Khan ◽  
Shamaila Rani
Keyword(s):  
Scalar Field ◽  
Modified Gravity ◽  
Energy Momentum Tensor ◽  
Apparent Horizon ◽  
Second Law Of Thermodynamics ◽  
Momentum Tensor ◽  
Isotropic Universe ◽  
First Order ◽  
Generalized Second Law ◽  
Thermodynamical Behavior

We discuss the thermodynamical behavior of homogeneous and isotropic universe (flat and non-flat) in the framework of [Formula: see text] gravity, where [Formula: see text] stands for Ricci scalar and [Formula: see text] signifies the trace of energy–momentum tensor of a scalar field [Formula: see text]. We follow through the first-order formalism that specifies the scalar field to the Hubble parameter which becomes [Formula: see text] By using Bekenstein–Hawking entropy, we analyze the validity of generalized second law of thermodynamics at apparent horizon for different values of [Formula: see text] and evaluate the equilibrium condition for all cases as well.

scholarly journals Kantowski–Sachs scalar field cosmological models in a modified theory of gravity

2017 ◽  
Vol 95 (2) ◽  
pp. 136-144 ◽  
Author(s):  
M. Vijaya Santhi ◽  
V.U.M. Rao ◽  
Y. Aditya
Keyword(s):  
Scalar Field ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Distance Measures ◽  
Field Equations ◽  
Scalar Expansion ◽  
Shear Scalar ◽  
Theory Of Gravity ◽  
The Universe ◽  
Modified Theory

In this paper, we investigate the anisotropic Kantowski–Sachs model in the f(R, T) theory of gravity proposed by Harko et al. (Phys. Rev. D, 84, 024020, 2011) with scalar field (quintessence or phantom). Here R is the Ricci scalar and T is the trace of the energy–momentum tensor. The field equations have been solved using the fact that scalar expansion is proportional to the shear scalar of the space–time. We explore the behavior of the deceleration parameter, which represents a transition of the universe from the early decelerating phase to the present accelerated phase. Some physical properties and various cosmological distance measures are also obtained and discussed.

CASIMIR EFFECT FOR THE ROBIN SURFACES IN STATIC ROBERTSON–WALKER SPACE–TIME

2011 ◽  
Vol 20 (02) ◽  
pp. 161-168 ◽  
Author(s):  
MOHAMMAD R. SETARE ◽  
M. DEHGHANI
Keyword(s):  
Scalar Field ◽  
Energy Momentum Tensor ◽  
Casimir Effect ◽  
Vacuum Expectation ◽  
Robin Boundary Condition ◽  
Momentum Tensor ◽  
Space Time ◽  
Conformally Invariant ◽  
Time Space ◽  
Robin Boundary

We investigate the energy–momentum tensor for a massless conformally coupled scalar field in the region between two curved surfaces in k = -1 static Robertson–Walker space–time. We assume that the scalar field satisfies the Robin boundary condition on the surfaces. Robertson–Walker space–time space is conformally related to Rindler space; as a result we can obtain vacuum expectation values of the energy–momentum tensor for a conformally invariant field in Robertson–Walker space–time space from the corresponding Rindler counterpart by the conformal transformation.

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