scholarly journals A CAUSAL MODEL FOR A CLOSED UNIVERSE

2004 ◽  
Vol 13 (05) ◽  
pp. 871-883
Author(s):  
MAURICIO CATALDO ◽  
SERGIO DEL CAMPO ◽  
PAUL MINNING ◽  
FRANCISCO PEÑA
Keyword(s):  
Viscous Fluid ◽  
Entropy Production ◽  
Causal Model ◽  
Closed Universe ◽  
Frw Universe ◽  
Dynamical Equations ◽  
Closed Model ◽  
The Universe ◽  
Friedmann Robertson Walker ◽  
Causal Thermodynamics

We study a closed model of a universe filled with viscous fluid and quintessence matter components. The dynamical equations imply that the universe might look like an accelerated flat Friedmann–Robertson–Walker (FRW) universe at low redshift. We consider here dissipative processes which obey a causal thermodynamics. Here, we account for the entropy production via causal dissipative inflation.

scholarly journals The cosmological behavior and the statefinder diagnosis for the New Tsallis agegraphic dark energy

2020 ◽  
Vol 35 (38) ◽  
pp. 2050318
Author(s):  
Umesh Kumar Sharma ◽  
Shikha Srivastava
Keyword(s):  
Dark Energy ◽  
Dynamical Analysis ◽  
Current Phase ◽  
Statefinder Parameters ◽  
Frw Universe ◽  
Eos Parameter ◽  
Agegraphic Dark Energy ◽  
Evolutionary Trajectories ◽  
The Universe ◽  
Friedmann Robertson Walker

In this work, we have considered the recently proposed new Tsallis agegraphic dark energy (NTADE) model [Mod. Phys. Lett. A 34, 1950086 (2019)] within the framework of a flat Friedmann–Robertson–Walker (FRW) Universe by taking various values of the parameter [Formula: see text]. The NTADE model shows the current phase transition of the Universe from decelerated to accelerated phase. The NTADE equation of state (EoS) parameter shows a rich behavior as it can be quintessence-like or phantom-like depending on the value of [Formula: see text]. For discriminating the NTADE model from [Formula: see text]CDM, we have plotted the statefinder parameters [Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text] pair. The NTADE model shows distinct evolutionary trajectories of their evolution in ([Formula: see text]) and ([Formula: see text]) plane. An analysis using the snap parameter and the [Formula: see text] pair dynamical analysis have also been performed.

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 Inflation in terms of a viscous van der Waals coupled fluid

2018 ◽  
Vol 15 (09) ◽  
pp. 1850150 ◽  
Author(s):  
I. Brevik ◽  
V. V. Obukhov ◽  
A. V. Timoshkin
Keyword(s):  
Van Der Waals ◽  
Accelerated Expansion ◽  
Frw Universe ◽  
Slow Roll ◽  
Analytic Expressions ◽  
Acceleration Of The Universe ◽  
The Universe ◽  
Gravitational Equations ◽  
Friedmann Robertson Walker ◽  
Planck Satellite

We propose to describe the acceleration of the universe by introducing a model of two coupled fluids. We focus on the accelerated expansion at the early stages. The inflationary expansion is described in terms of a van der Waals equation of state for the cosmic fluid, when account is taken of bulk viscosity. We assume that there is a weak interaction between the van der Waals fluid and the second component (matter). The gravitational equations for the energy densities of the two components are solved for a homogeneous and isotropic Friedmann–Robertson–Walker (FRW) universe, and analytic expressions for the Hubble parameter are obtained. The slow-roll parameters, the spectral index, and the tensor-to-scalar ratio are calculated and compared with the most recent astronomical data from the Planck satellite. Given reasonable restriction on the parameters, the agreement with observations is favorable.

scholarly journals Quantum Cosmologies Under Geometrical Unification of Gravity and Dark Energy

Symmetry ◽  
2019 ◽  
Vol 11 (7) ◽  
pp. 860 ◽  
Author(s):  
Carlos A. Rubio ◽  
Felipe A. Asenjo ◽  
Sergio A. Hojman
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Field Potential ◽  
Constraint Equation ◽  
Spinless Particle ◽  
Quantization Scheme ◽  
Dynamical Equations ◽  
Order Constraint ◽  
The Universe ◽  
Friedmann Robertson Walker

A Friedmann–Robertson–Walker Universe was studied with a dark energy component represented by a quintessence field. The Lagrangian for this system, hereafter called the Friedmann–Robertson–Walker–quintessence (FRWq) system, was presented. It was shown that the classical Lagrangian reproduces the usual two (second order) dynamical equations for the radius of the Universe and for the quintessence scalar field, as well as a (first order) constraint equation. Our approach naturally unified gravity and dark energy, as it was obtained that the Lagrangian and the equations of motion are those of a relativistic particle moving on a two-dimensional, conformally flat spacetime. The conformal metric factor was related to the dark energy scalar field potential. We proceeded to quantize the system in three different schemes. First, we assumed the Universe was a spinless particle (as it is common in literature), obtaining a quantum theory for a Universe described by the Klein–Gordon equation. Second, we pushed the quantization scheme further, assuming the Universe as a Dirac particle, and therefore constructing its corresponding Dirac and Majorana theories. With the different theories, we calculated the expected values for the scale factor of the Universe. They depend on the type of quantization scheme used. The differences between the Dirac and Majorana schemes are highlighted here. The implications of the different quantization procedures are discussed. Finally, the possible consequences for a multiverse theory of the Dirac and Majorana quantized Universe are briefly considered.

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.

COSMOLOGICAL MODELS IN GENERALIZED SCALAR–TENSOR THEORY WITH CAUSAL VISCOUS FLUID

1998 ◽  
Vol 07 (06) ◽  
pp. 849-856
Author(s):  
G. P. SINGH ◽  
S. G. GHOSH ◽  
AROONKUMAR BEESHAM
Keyword(s):  
Scalar Field ◽  
Viscous Fluid ◽  
Nonequilibrium Thermodynamics ◽  
Coupling Parameter ◽  
Scalar Tensor Theory ◽  
Bulk Viscous Fluid ◽  
Tensor Theory ◽  
Bulk Viscous ◽  
Friedmann Robertson Walker ◽  
Causal Thermodynamics

We consider generalized Brans–Dicke theory in which the coupling parameter is not constant but a function of the scalar field. Friedmann–Robertson–Walker models with bulk viscous fluid source described by full (i.e., nontruncated) causal nonequilibrium thermodynamics are analyzed. Exact solutions for the flat case have been obtained. Our results are compared with those in the usual Brans–Dicke theory with causal thermodynamics.

scholarly journals THERMODYNAMIC OF THE GHOST DARK ENERGY UNIVERSE

2012 ◽  
Vol 27 (31) ◽  
pp. 1250182 ◽  
Author(s):  
CHAO-JUN FENG ◽  
XIN-ZHOU LI ◽  
XIAN-YONG SHEN
Keyword(s):  
Dark Energy ◽  
Vacuum Energy ◽  
Hawking Temperature ◽  
Frw Universe ◽  
Energy Component ◽  
Ghost Dark Energy ◽  
Dark Energy Component ◽  
Acceleration Of The Universe ◽  
The Universe ◽  
Friedmann Robertson Walker

Recently, the vacuum energy of the QCD ghost in a time-dependent background is proposed as a kind of dark energy candidate to explain the acceleration of the Universe. In this model, the energy density of the dark energy is proportional to the Hubble parameter H, which is the Hawking temperature on the Hubble horizon of the Friedmann–Robertson–Walker (FRW) Universe. In this paper, we generalized this model and chose the Hawking temperature on the so-called trapping horizon, which will coincide with the Hubble temperature in the context of flat FRW Universe dominated by the dark energy component. We study the thermodynamics of Universe with this kind of dark energy and find that the entropy-area relation is modified, namely, there is another new term besides the area term.

Generalized ghost pilgrim dark energy in f(G,T) gravity

2019 ◽  
Vol 28 (06) ◽  
pp. 1950077 ◽  
Author(s):  
M. Sharif ◽  
Saadia Saba
Keyword(s):  
Dark Energy ◽  
State Parameter ◽  
Momentum Tensor ◽  
Chaplygin Gas ◽  
Frw Universe ◽  
Pilgrim Dark Energy ◽  
Energy Formula ◽  
Text Model ◽  
The Universe ◽  
Friedmann Robertson Walker

In this paper, we explore the reconstruction paradigm for generalized ghost pilgrim dark energy model with [Formula: see text] gravity ([Formula: see text] is the Gauss–Bonnet invariant and [Formula: see text] is the trace of the energy–momentum tensor) depending upon the speculation of black hole-free universe. To accomplish this, we adopt correspondence scheme for dust fluid configuration with flat Friedmann-Robertson-Walker (FRW) universe. The cosmic behavior of reconstructed models is examined through cosmological diagnostic parameters and phase planes. It is found that the deceleration parameter indicates accelerated phase while equation-of-state parameter represents phantom regime for some specific range of free parameters. The squared speed of sound parameter gives stable models for analyzing evolutionary paradigm of the universe. The trajectories of [Formula: see text]–[Formula: see text] plane gives thawing region, whereas [Formula: see text]–[Formula: see text] phase plane corresponds to Chaplygin gas model. We conclude that the resulting model represents self-consistent phantom-like universe for [Formula: see text] cannot be fraction) as well as stable generalized ghost pilgrim dark energy [Formula: see text] model.

scholarly journals Particle number and current in Dirac field in the closed universe

1970 ◽  
Vol 9 (9) ◽  
pp. 4-7
Author(s):  
PR Dhungel ◽  
U Khanal
Keyword(s):  
Particle Number ◽  
Particle Number Density ◽  
Dirac Field ◽  
Closed Universe ◽  
Frw Universe ◽  
Frw Model ◽  
The Universe ◽  
Particle Current ◽  
Massless Fields ◽  
Finite Particle

The behavior of the particle number and particle current of the free Dirac field in the closed FRW universe has been explored. Although the particle number is conserved for the massless fields as expected, it is not for the massive ones. So, there is a finite particle current in any finite sized volume of the universe. Such currents tend to enhance the density contrast over time. It is seen that the momenta of the Dirac particle is quantized in a closed FRW model. The particles distribute themselves in such a way as to resemble that required for the flattened rotation curves of galaxies. Key words: Closed Universe; Dirac field; Comoving particle number density DOI: http://dx.doi.org/10.3126/sw.v9i9.5508 SW 2011; 9(9): 4-7

scholarly journals Tsallis Holographic Dark Energy with Granda-Oliveros Scale in (n+1)-Dimensional FRW Universe

2019 ◽  
Vol 2019 ◽  
pp. 1-7 ◽  
Author(s):  
Ayman A. Aly
Keyword(s):  
Dark Energy ◽  
Speed Of Sound ◽  
Dark Energy Model ◽  
Holographic Dark Energy ◽  
Holographic Dark Energy Model ◽  
Frw Universe ◽  
Accelerating Expansion ◽  
The Stability ◽  
The Universe ◽  
Friedmann Robertson Walker

Based on Tsallis holographic dark energy model recently proposed by using the general model of the Tsallis entropy expression, we reconstruct cosmographic parameters, q,j,κ,l, and we study their evolution in spatially flat (n+1)-dimensional Friedmann-Robertson-Walker universe using Granda-Oliveros scale. Our results show that the universe is in an accelerating expansion mode described by phantom-like behavior. We go further and study the state finder operators and the Om diagnostic to understand the behavior of our model. The stability of the system is also studied by using the square of speed of sound showing that our model is stable over the low range of red-shift considered. The results indicate that the entropy formalism will play an important role in understanding the dynamics of our universe.

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