Bounce conditions in Kantowski–Sachs and Bianchi cosmologies in modified gravity theories

2015 ◽  
Vol 30 (14) ◽  
pp. 1550073 ◽  
Author(s):  
T. Singh ◽  
R. Chaubey ◽  
Ashutosh Singh
Keyword(s):  
Bianchi Type ◽  
Lyra Geometry ◽  
Energy Conditions ◽  
Modified Theories Of Gravity ◽  
Variable Cosmological Term ◽  
Bouncing Universe ◽  
Modified Gravity Theories ◽  
General Relativistic ◽  
Theories Of Gravity ◽  
The Universe

Many cosmological scenarios envisage either a bounce of the universe at early times, or a collapse locally to form a black hole which re-expands into a new expanding universe region. Energy conditions preclude this phenomena for ordinary matter in general relativistic universe, but scalar or other types of fields can violate some of these conditions, and so can, possibly, provide conditions for a bouncing universe. In this paper, we have investigated the necessary conditions for a bounce in Kantowski–Sachs (KS) and Bianchi type models in some modified theories of gravity like Hoyle–Narlikar creation field theory, Lyra geometry, general class of scalar–tensor theories, Einstein's theory with variable cosmological term.

scholarly journals A logarithmic correction in the entropy functional formalism

2016 ◽  
Vol 25 (07) ◽  
pp. 1650080 ◽  
Author(s):  
Fayçal Hammad ◽  
Mir Faizal
Keyword(s):  
Quantum Gravity ◽  
Modified Gravity ◽  
Correction Term ◽  
Modified Theories Of Gravity ◽  
Logarithmic Correction ◽  
Functional Formalism ◽  
Entropy Formula ◽  
Entropy Functional ◽  
Modified Gravity Theories ◽  
Theories Of Gravity

The entropy functional formalism allows one to recover general relativity, modified gravity theories, as well as the Bekenstein–Hawking entropy formula. In most approaches to quantum gravity, the Bekenstein–Hawking’s entropy formula acquires a logarithmic correction term. As such terms occur almost universally in most approaches to quantum gravity, we analyze the effect of such terms on the entropy functional formalism. We demonstrate that the leading correction to the micro-canonical entropy in the entropy functional formalism can be used to recover modified theories of gravity already obtained with an uncorrected micro-canonical entropy. Furthermore, since the entropy functional formalism reproduces modified gravity, the rise of gravity-dependent logarithmic corrections turns out to be one way to impose constraints on these theories of modified gravity. The constraints found here for the simple case of an [Formula: see text]-gravity are the same as those obtained in the literature from cosmological considerations.

PALATINI f(R) COSMOLOGY

2008 ◽  
Vol 23 (17n20) ◽  
pp. 1388-1396 ◽  
Author(s):  
SEOKCHEON LEE
Keyword(s):  
Linear Growth ◽  
Matter Density ◽  
Modified Theories Of Gravity ◽  
Cosmic Expansion ◽  
Energy Models ◽  
Modified Gravity Theories ◽  
Theories Of Gravity ◽  
The Stability ◽  
Cosmic Expansion History ◽  
General Gauge

We investigate the modified gravity theories in terms of the effective dark energy models. We compare the cosmic expansion history and the linear growth in different models. We also study the evolution of linear cosmological perturbations in modified theories of gravity assuming the Palatini formalism. We find the stability of the superhorizon metric evolution depends on models. We also study the matter density fluctuation in the general gauge and show the differential equations in super and sub-horizon scales.

scholarly journals Phantom energy-dominated universe as a transient stage in f(R) cosmology

2019 ◽  
Vol 28 (10) ◽  
pp. 1950124 ◽  
Author(s):  
P. H. R. S. Moraes ◽  
P. K. Sahoo ◽  
Barkha Taori ◽  
Parbati Sahoo
Keyword(s):  
Quantum Cosmology ◽  
Bianchi Type ◽  
Phantom Energy ◽  
Type I ◽  
Dark Sector ◽  
Transient Stage ◽  
Standard Cosmology ◽  
Theories Of Gravity ◽  
The Universe ◽  
Einstein’S General Relativity

The [Formula: see text] theories of gravity are the most popular, simple and well-succeeded extension of Einstein’s General Relativity. They can account for some observational issues of standard cosmology with no need for evoking the dark sector of the universe. In the present paper, we will investigate LRS Bianchi type-I spacetime in [Formula: see text] gravity theory within the phantom energy-dominated era. We show that in this formalism the phantom energy-dominated universe is a transient stage and in the further stage of the universe dynamics, it is dominated, once again, by dark energy. Such an important feature is obtained from the model, rather than imposed to it, and may have a relation to loop quantum cosmology.

Cosmological and thermodynamics consequences of viscous modified theories of gravity

2019 ◽  
Vol 34 (35) ◽  
pp. 1950287
Author(s):  
Abdul Jawad ◽  
Saba Qummer ◽  
Shamaila Rani ◽  
M. Younas
Keyword(s):  
Cosmic Acceleration ◽  
Accelerated Expansion ◽  
Modified Theories Of Gravity ◽  
Thermal Equilibrium Condition ◽  
Entropy Variation ◽  
Expansion Of The Universe ◽  
Theories Of Gravity ◽  
Chern Simons ◽  
The Universe ◽  
Dgp Braneworld

The illustration of cosmic acceleration is being presented in the framework of DGP braneworld and dynamical Chern–Simons modified gravity in the presence of casual Israel–Stewart formalism. In this way, we discuss the evolution parameter which leads to the accelerated expansion of the universe in the phantom as well as quintessence region for both gravities. The squared speed of sound [Formula: see text] leads to the stable behavior of the current physical system in both gravities in the later epoch. Also, the entropy variation, as well as thermal equilibrium condition, remains valid in both frameworks at the present and later epoch.

scholarly journals INFLATION IN F(R, T) GRAVITATION WITH f-ESSENCE

2020 ◽  
Vol 5 (333) ◽  
pp. 106-112
Author(s):  
S.R. Myrzakul ◽  
◽  
Y.M. Myrzakulov ◽  
М. Arzimbetova ◽  
◽  
...  
Keyword(s):  
Equations Of Motion ◽  
Accelerated Expansion ◽  
Modified Theories Of Gravity ◽  
Fermion Field ◽  
Theory Of Relativity ◽  
Modern Physics ◽  
Expansion Of The Universe ◽  
Isotropic Cosmological Model ◽  
Theories Of Gravity ◽  
The Universe

. Modified theories of gravity have become a kind of paradigm in modern physics because they seem to solve several shortcomings of the standard General Theory of Relativity (GTR) related to cosmology, astrophysics and quantum field theory. The most famous modified theories of gravity are F(R) and F(T) theories of gravity. A generalization of these two modified theories and gravitations, which was first proposed by Myrzakulov Ratbay. In this paper, we study an inhomogeneous isotropic cosmological model with a fermion field f-essence whose action has the form , where R is the scalar of curvature, and T is the torsion scalar, and Lm is the Lagrangian f-essence. A particular case is studied in detail when parameters are obtained that describe the current accelerated expansion of the Universe. The type of Lagrangian f-essence of this model is determined. The presented results show that gravity with f-essence can describe inflation in the early evolution of the Universe. A modified F(R, T) gravity with f-essence is considered. Equations of motion were obtained and the inflationary period of the early Universe was considered. To describe the inflationary period, the form of the Hubble parameter and the slow-roll parameter were determined.

SOME BIANCHI TYPE COSMOLOGICAL MODELS IN LYRA GEOMETRY

2008 ◽  
Vol 23 (30) ◽  
pp. 4925-4931 ◽  
Author(s):  
J. K. SINGH
Keyword(s):  
Bianchi Type ◽  
Lyra Geometry ◽  
Energy Conditions ◽  
Geometrical Parameters ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Massive Scalar Field ◽  
Normal Gauge ◽  
Type V ◽  
Bianchi Type V

A class of nonstatic solutions for Einstein field equations has been investigated in the context of Bianchi type-V space–time in the theory based on Lyra's geometry in Gauss normal gauge and in the presence of an attractive massive scalar field. The results have been studied through various physical and geometrical parameters. The energy conditions of the model have also been verified.

BIANCHI TYPE-I STRING DUST COSMOLOGICAL MODELS IN LYRA GEOMETRY

2010 ◽  
Vol 25 (15) ◽  
pp. 3043-3054 ◽  
Author(s):  
RAJ BALI ◽  
LOKESH KUMAR GUPTA
Keyword(s):  
Bianchi Type ◽  
Deterministic Model ◽  
Cosmic Time ◽  
Lyra Geometry ◽  
Cosmological Models ◽  
Type I ◽  
Bianchi Type I ◽  
Tensor Formula ◽  
The Universe ◽  
Special Case

Bianchi type-I string cosmological models with time dependent gauge function (β) within the framework of Lyra geometry is presented. To get the deterministic model of the universe, we assume that the eigenvalue [Formula: see text] of shear tensor [Formula: see text] is proportional to the expansion (θ). This leads to A = (BC)nwhere A, B, C are metric potential and n is a constant. The physical and geometrical aspects of the model are discussed in detail. In a special case, the behavior of the model in terms of cosmic time t for [Formula: see text] is also discussed.

scholarly journals Bianchi Type—IX Barotropic Fluid Model with Time-Dependent Displacement Vector in Lyra Geometry

2012 ◽  
Vol 2012 ◽  
pp. 1-9
Author(s):  
Raj Bali ◽  
Rajendra Vadhwani ◽  
Jagdish Prasad Dhanka
Keyword(s):  
Bianchi Type ◽  
Deterministic Model ◽  
Displacement Vector ◽  
Fluid Model ◽  
Cosmic Time ◽  
Big Bang ◽  
Lyra Geometry ◽  
Barotropic Fluid ◽  
Frame Work ◽  
The Universe

Bianchi Type IX barotropic fluid cosmological model in the frame work of Lyra geometry is investigated. To get the deterministic model of the universe, it is assumed that shear () is proportional to expansion (). This leads to , where and are metric potentials and is a constant. To get the results in terms of cosmic time , we have also considered a special case (dust filled universe) and . We find that the model starts with a big bang initially and the displacement vector () is initially large but decreases due to lapse of time. The models and have point-type singularity at and , respectively. The physical and geometrical aspects of the models are also discussed.

scholarly journals Anisotropic Bianchi Type-III Bulk Viscous Fluid Universe in Lyra Geometry

2013 ◽  
Vol 2013 ◽  
pp. 1-5 ◽  
Author(s):  
Priyanka Kumari ◽  
M. K. Singh ◽  
Shri Ram
Keyword(s):  
Cosmological Model ◽  
Viscous Fluid ◽  
Bianchi Type ◽  
Lyra Geometry ◽  
Scale Factor ◽  
Field Equations ◽  
Type Iii ◽  
Bulk Viscous Fluid ◽  
Bulk Viscous ◽  
The Universe

An anisotropic Bianchi type-III cosmological model is investigated in the presence of a bulk viscous fluid within the framework of Lyra geometry with time-dependent displacement vector. It is shown that the field equations are solvable for any arbitrary function of a scale factor. To get the deterministic model of the universe, we have assumed that (i) a simple power-law form of a scale factor and (ii) the bulk viscosity coefficient are proportional to the energy density of the matter. The exact solutions of the Einstein’s field equations are obtained which represent an expanding, shearing, and decelerating model of the universe. Some physical and kinematical behaviors of the cosmological model are briefly discussed.

scholarly journals From the Flamm–Einstein–Rosen bridge to the modern renaissance of traversable wormholes

2016 ◽  
Vol 25 (07) ◽  
pp. 1630017 ◽  
Author(s):  
Francisco S. N. Lobo
Keyword(s):  
Energy Condition ◽  
Einstein Field Equation ◽  
Fundamental Property ◽  
Null Energy Condition ◽  
Energy Conditions ◽  
Modified Theories Of Gravity ◽  
Multiply Connected ◽  
Traversable Wormholes ◽  
Normal Matter ◽  
Theories Of Gravity

We consider the possibility of multiply-connected spacetimes, ranging from the Flamm–Einstein–Rosen bridge, geons, and the modern renaissance of traversable wormholes. A fundamental property in wormhole physics is the flaring-out condition of the throat, which through the Einstein field equation entails the violation of the null energy condition (NEC). In the context of modified theories of gravity, it has also been shown that the normal matter can be imposed to satisfy the energy conditions, and it is the higher order curvature terms, interpreted as a gravitational fluid, that sustain these nonstandard wormhole geometries, fundamentally different from their counterparts in general relativity (GR). We explore interesting features of these geometries, in particular, the physical properties and characteristics of these ‘exotic spacetimes’.

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