scholarly journals Friedmann equations for deformed entropic gravity

2020 ◽  
Vol 29 (06) ◽  
pp. 2050042
Author(s):  
Salih Kibaroğlu ◽  
Mustafa Senay
Keyword(s):  
Early Universe ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Frw Universe ◽  
Friedmann Equations ◽  
Entropic Gravity ◽  
The One ◽  
Two Parameters ◽  
Friedmann Robertson Walker

In this study, we investigate the effects of the one- and two-parameters deformed systems on the Friedmann equations of the Friedmann–Robertson–Walker (FRW) universe by using the entropic gravity approach in the framework of the early universe era. We give simplified forms for the deformed Unruh temperature and Einstein field equations for three different deformed systems. Based on these compact equations, we derive the Friedmann equations with the effective gravitational and cosmological terms.

EINSTEIN FIELD EQUATIONS: AN ALTERNATE APPROACH TOWARDS EXACT SOLUTIONS FOR AN FRW UNIVERSE

2005 ◽  
Vol 20 (11) ◽  
pp. 2481-2484 ◽  
Author(s):  
F. L. WILLIAMS
Keyword(s):  
Exact Solutions ◽  
Recent Work ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Frw Universe ◽  
Friedmann Robertson Walker

We show that the Einstein field equations for a Friedmann-Robertson-Walker(FRW) universe are completely equivalent to a generalized Ermakov-Milne-Pinney system. This extends recent work of R. Hawkins and J. Lidsey, and provides an alternate method for deriving exact solutions of the field equations.

Various dark energy models for variables G and Λ in f(R, T) modified theory

2019 ◽  
Vol 34 (13) ◽  
pp. 1950098 ◽  
Author(s):  
Can Aktaş
Keyword(s):  
Dark Energy ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Tachyon Field ◽  
Field Equations ◽  
Frw Universe ◽  
Linearly Varying Deceleration Parameter ◽  
Energy Models ◽  
Modified Theory ◽  
Friedmann Robertson Walker

In this paper, we have researched tachyon field, k-essence and quintessence dark energy (DE) models for Friedmann–Robertson–Walker (FRW) universe with varying G and [Formula: see text] in f(R, T) gravitation theory. The theory of f(R, T) is proposed by Harko et al. [Phys. Rev. D 84, 024020, 2011]. In this theory, R is the Ricci scalar and T is the trace of energy–momentum tensor. For the solutions of field equations, we have used linearly varying deceleration parameter (LVDP), the equation of state (EoS) and the ratio between [Formula: see text] and Hubble parameter. Also, we have discussed some physical behavior of the models with various graphics.

A unified picture of cosmological entropy on apparent horizon in F(R, G) gravity

2017 ◽  
Vol 32 (33) ◽  
pp. 1750182 ◽  
Author(s):  
Ali İhsan Keskin ◽  
Irfan Acikgoz
Keyword(s):  
Apparent Horizon ◽  
Second Law Of Thermodynamics ◽  
Hawking Temperature ◽  
Second Law ◽  
Field Equations ◽  
Frw Universe ◽  
Generalized Second Law ◽  
History Of ◽  
The Universe ◽  
Friedmann Robertson Walker

In this study, the validity of the generalized second law of thermodynamics (GSLT) has been investigated in F(R, G) gravity. We consider that the boundary of the universe is surrounded by an apparent horizon in the spatially flat Friedmann–Robertson–Walker (FRW) universe, and we take into account the Hawking temperature on the horizons. The unified solutions of the field equations corresponding to gravity theory have been applied to the validity of the GSLT frame, and in this way, both the solutions have been verified and all the expansion history of the universe has been shown in a unified picture.

scholarly journals Quantum Mixmaster as a Model of the Primordial Universe

Universe ◽  
2019 ◽  
Vol 6 (1) ◽  
pp. 7 ◽  
Author(s):  
Hervé Bergeron ◽  
Ewa Czuchry ◽  
Jean Pierre Gazeau ◽  
Przemysław Małkiewicz
Keyword(s):  
Early Universe ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Molecular Physics ◽  
Local Structures ◽  
Cosmological Scenario ◽  
Quantum Version ◽  
Formation And Evolution ◽  
Physical Features ◽  
Mixmaster Model

The Mixmaster solution to Einstein field equations was examined by C. Misner in an effort to better understand the dynamics of the early universe. We highlight the importance of the quantum version of this model for the early universe. This quantum version and its semi-classical portraits are yielded through affine and standard coherent state quantizations and more generally affine and Weyl–Heisenberg covariant integral quantizations. The adiabatic and vibronic approximations widely used in molecular physics can be employed to qualitatively study the dynamics of the model on both quantum and semi-classical levels. Moreover, the semi-classical approach with the exact anisotropy potential can be effective in the numerical integration of some solutions. Some promising physical features such as the singularity resolution, smooth bouncing, the excitation of anisotropic oscillations and a substantial amount of post-bounce inflation as the backreaction to the latter are pointed out. Finally, a realistic cosmological scenario based on the quantum mixmaster model, which includes the formation and evolution of local structures is outlined.

Thermodynamics of apparent horizon in mimetic cosmology

2019 ◽  
Vol 28 (03) ◽  
pp. 1950057 ◽  
Author(s):  
Ahmad Sheykhi
Keyword(s):  
Apparent Horizon ◽  
Local Equilibrium ◽  
Second Law Of Thermodynamics ◽  
High Energy ◽  
Frw Universe ◽  
Generalized Second Law ◽  
Friedmann Equations ◽  
New Perspective ◽  
Friedmann Robertson Walker ◽  
Mimetic Gravity

A new perspective toward Einstein’s theory of general relativity, called mimetic gravity, was suggested in [A. H. Chamseddine and V. Mukhanov, J. High Energy Phys. 1311 (2013) 135] by isolating the conformal degree of freedom in a covariant fashion through a re-parametrization of the physical metric in terms of an auxiliary metric and a mimetic field. In this paper, we first derive the Friedmann equations of the Friedmann–Robertson–Walker (FRW) universe with any spatial curvature in mimetic gravity. Then, we disclose that one can always rewrite the Friedmann equations of mimetic cosmology in the form of the first law of thermodynamics, [Formula: see text], on the apparent horizon. We confirm that the entropy associated with the apparent horizon in mimetic cosmology still obeys the area law of entropy which is useful in studying the thermodynamical properties of the black holes in mimetic gravity. We also examine the time evolution of the total entropy in mimetic cosmology and show that, with the local equilibrium assumption, the generalized second law of thermodynamics is fulfilled in a region enclosed by the apparent horizon. Our study further supports the viability of the mimetic gravity from a thermodynamic viewpoint and provides a strong consistency check of this model.

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.

The spatially flat Friedmann equation for early rainbow universe

2015 ◽  
Vol 30 (31) ◽  
pp. 1550165
Author(s):  
Han Siong Ch’ng ◽  
Geri Gopir ◽  
Shahidan Radiman
Keyword(s):  
Early Universe ◽  
Quantum Cosmology ◽  
Gravitational Constant ◽  
Friedmann Equation ◽  
Late Time ◽  
Friedmann Equations ◽  
Bohm Interpretation ◽  
The One ◽  
Canonical Quantum ◽  
De Broglie Bohm

We derive the spatially flat rainbow-Friedmann equation from de Broglie–Bohm interpretation in canonical quantum cosmology. Our result shows that the spatially flat rainbow-Friedmann equations of early and late-time universe are having different forms. The spatially flat rainbow-Friedmann equation of early universe which is obtained in this paper is quite different from the one which was initially derived by Magueijo and Smolin [Class. Quantum Grav. 21, 1725 (2004)]. However, the spatially flat rainbow-Friedmann equation for late-time universe obtained in this paper is found to be the same as the one derived by Magueijo and Smolin (for the case [Formula: see text] and Newton’s gravitational constant [Formula: see text]. The new spatially flat rainbow-Friedmann equation obtained in this paper could provide an alternative way in understanding the evolution of the early rainbow universe.

Two-fluid Friedmann–Robertson–Walker cosmologies and their numerical predictions

1986 ◽  
Vol 64 (2) ◽  
pp. 204-209 ◽  
Author(s):  
A. A. Coley ◽  
B. O. J. Tupper
Keyword(s):  
Cosmic Microwave Background ◽  
Perfect Fluid ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Microwave Background ◽  
Two Fluids ◽  
Numerical Predictions ◽  
Imperfect Fluid ◽  
State Formula ◽  
Friedmann Robertson Walker

Friedmann–Robertson–Walker models satisfying the Einstein field equations for a combination of two fluids, one of which is a comoving perfect fluid with the radiation equation of state [Formula: see text], representing the cosmic microwave background, are discussed. Existing models, in which the second fluid is a comoving perfect fluid, are reviewed and their numerical predictions calculated. These models are generalized by considering the case in which the second fluid is an imperfect fluid. This fluid is necessarily noncomoving, the tilt representing the motion of the local supercluster of galaxies relative to the cosmic microwave background. The numerical predictions of one such model are calculated and are found to be in excellent agreement with observation.

CREATION OF A WORMHOLE DUE TO NONLINEAR ELECTRODYNAMICS

2002 ◽  
Vol 17 (20) ◽  
pp. 1305-1314 ◽  
Author(s):  
CARLA FONSECA-BARBATTI ◽  
M. NOVELLO ◽  
J. M. SALIM ◽  
REGINA C. ARCURI
Keyword(s):  
Magnetic Field ◽  
Quantum Electrodynamics ◽  
Nonlinear Electrodynamics ◽  
Einstein Field Equations ◽  
Field Equations ◽  
New Class ◽  
Effective Lagrangian ◽  
The One

We obtain a new class of solutions for the Einstein field equations which describe wormholes by using the one-loop effective Lagrangian of quantum electrodynamics. We also show that the derived wormholes can be maintained only by means of a magnetic field.

scholarly journals Thermodynamics in the viscous early universe

2010 ◽  
Vol 88 (11) ◽  
pp. 825-831 ◽  
Author(s):  
A. Tawfik
Keyword(s):  
Early Universe ◽  
Bulk Viscosity ◽  
Viscosity Coefficient ◽  
Matter Content ◽  
Einstein Field Equations ◽  
Field Equations ◽  
Curvature Parameter ◽  
Radical Revision ◽  
Bulk Viscosity Coefficient ◽  
The Universe

Assuming that the matter in the background geometry is a free gas and that no phase transitions were occurring in the early Universe, we discuss the thermodynamics of this closed system using classical approaches. We find that essential cosmological quantities, such as the Hubble parameter H, the scaling factor a, and the curvature parameter k, can be derived from this simple model, which on one hand fulfills and entirely obeys the laws of thermodynamics, and on the other hand, its results are compatible with the Friedmann–Robertson–Walker model and the Einstein field equations. Including a finite bulk viscosity coefficient leads to important changes in all these cosmological quantities. Accordingly, our picture about the evolution of the Universe and its astrophysical consequences seems to undergoing a radical revision. We find that k strongly depends on the thermodynamics of background matter. The time scale at which negative curvature might take place depends on the relation between the matter content and the total energy. Using quantum and statistical approaches, we introduce expressions for H and the bulk viscosity coefficient ξ.

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