scholarly journals Realistic f(T) model describing the de Sitter epoch of the dark energy dominated universe

2015 ◽  
Vol 93 (10) ◽  
pp. 1050-1056 ◽  
Author(s):  
S.B. Nassur ◽  
A.V. Kpadonou ◽  
M.E. Rodrigues ◽  
M.J.S. Houndjo ◽  
J. Tossa
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Model Checking ◽  
Exponential Model ◽  
Effective Parameter ◽  
Perturbation Function ◽  
De Sitter ◽  
Torsion Scalar ◽  
Theory Of Gravity

We consider an exponential model within the so-called f(T) theory of gravity, where T denotes the torsion scalar. We focus our attention on a cosmological feature of a f(T) model, checking whether it may describe the de Sitter stage of the current universe according to the evolution of the redshift, z. Our results show that the model reproduces the de Sitter stage only for low redshifts, where the perturbation function approached zero, whereas the effective parameter of the equation of state goes to –1, which is the expected behavior for any model able to reproduce the de Sitter stage.

scholarly journals ATTRACTOR SOLUTION IN COUPLED YANG–MILLS FIELD DARK ENERGY MODELS

2009 ◽  
Vol 18 (09) ◽  
pp. 1331-1342 ◽  
Author(s):  
WEN ZHAO
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
De Sitter ◽  
Yang Mills ◽  
Constraint Equations ◽  
Energy Models ◽  
Big Rip ◽  
Attractor Solution ◽  
The Universe ◽  
Mills Field

We investigate the attractor solution in the coupled Yang–Mills field dark energy models with the general interaction term, and obtain the constraint equations for the interaction if the attractor solution exists. The research also shows that, if the attractor solution exists, the equation of state of dark energy must evolve from wy > 0 to wy ≤ -1, which is slightly suggested by the observation. At the same time, the total equation of state in the attractor solution is w tot = -1, the universe is a de Sitter expansion, and the cosmic big rip is naturally avoided. These features are all independent of the interacting forms.

scholarly journals LITTLE RIP COSMOLOGICAL MODELS WITH TIME-DEPENDENT EQUATION OF STATE

2012 ◽  
Vol 27 (36) ◽  
pp. 1250210 ◽  
Author(s):  
I. BREVIK ◽  
V. V. OBUKHOV ◽  
K. E. OSETRIN ◽  
A. V. TIMOSHKIN
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Cosmological Models ◽  
Time Dependent ◽  
De Sitter Spacetime ◽  
De Sitter ◽  
Energy Models ◽  
Sitter Spacetime ◽  
The Universe ◽  
Far Future

Specific dark energy models, leading to the Little Rip (LR) cosmology in the far future, are investigated. Conditions for the occurrence of LR in terms of the parameters present in the proposed equation of state for the dark energy cosmic fluid are studied. Estimates about the time needed before the occurrence of the small singularity in the standard LR model in which the universe approaches the de Sitter spacetime asymptotically, are given.

scholarly journals DIRECT DETERMINATIONS OF THE REDSHIFT BEHAVIOR OF THE PRESSURE, ENERGY DENSITY, AND EQUATION OF STATE OF THE DARK ENERGY AND THE ACCELERATION OF THE UNIVERSE

2005 ◽  
Vol 20 (06) ◽  
pp. 1113-1120 ◽  
Author(s):  
RUTH A. DALY ◽  
S. G. DJORGOVSKI
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Energy Density ◽  
Radio Galaxy ◽  
A Priori ◽  
Acceleration Rate ◽  
Acceleration Of The Universe ◽  
Theory Of Gravity ◽  
The Universe

One of the goals of current cosmological studies is the determination of the expansion and acceleration rates of the universe as functions of redshift, and the determination of the properties of the dark energy that can explain these observations. Here the expansion and acceleration rates are determined directly from the data, without the need for the specification of a theory of gravity, and without adopting an a priori parameterization of the form or redshift evolution of the dark energy. We use the latest set of distances to SN standard candles from Riess et al. (2004), supplemented by data on radio galaxy standard ruler sizes, as described by Daly & Djorgovski (2003, 2004). We find that the universe transitions from acceleration to deceleration at a redshift of zT≈0.4, with the present value of q0=-0.35±0.15. The standard "concordance model" with Ω0=0.3 and Λ=0.7 provides a reasonably good fit to the dimensionless expansion rate as a function of redshift, though it fits the dimensionless acceleration rate as a function of redshift less well. The expansion and acceleration rates are then combined with a theory of gravity to determine the pressure, energy density, and equation of state of the dark energy as functions of redshift. Adopting General Relativity as the correct theory of gravity, the redshift trends for the pressure, energy density, and equation of state of the dark energy out to z~1 are determined, and are found to be generally consistent with the concordance model; they have zero redshift values of p0=-0.6±0.15, f0=0.62±0.05, and w0=-0.9±0.1.

FRIEDMANN COSMOLOGY ON CODIMENSION 2 BRANE WITH TIME-DEPENDENT TENSION

2006 ◽  
Vol 21 (02) ◽  
pp. 159-167 ◽  
Author(s):  
HONGSHENG ZHANG ◽  
QI GUO ◽  
RONG-GEN CAI
Keyword(s):  
Equation Of State ◽  
Einstein Frame ◽  
Time Dependent ◽  
Effective Parameter ◽  
De Sitter

A solution of codimension 2 brane is found for which four-dimensional Friedmann cosmology is recovered on the brane with time-dependent tension, in the Einstein frame. The effective parameter p/ρ of equation of state on the brane can be quintessence like, de Sitter like or phantom like, depending on the integration constants of the solution.

scholarly journals Embedding with Vaidya geometry

2020 ◽  
Vol 80 (7) ◽  
Author(s):  
A. V. Nikolaev ◽  
S. D. Maharaj
Keyword(s):  
Equation Of State ◽  
Euclidean Space ◽  
Mass Function ◽  
Energy Conditions ◽  
Dimensional Euclidean Space ◽  
De Sitter ◽  
Radiating Star ◽  
Theory Of Gravity ◽  
Null String ◽  
Vaidya Metric

Abstract The Vaidya metric is important in describing the exterior spacetime of a radiating star and for describing astrophysical processes. In this paper we study embedding properties of the generalized Vaidya metric. We had obtained embedding conditions, for embedding into 5-dimensional Euclidean space, by two different methods and solved them in general. As a result we found the form of the mass function which generates a subclass of the generalized Vaidya metric. Our result is purely geometrical and may be applied to any theory of gravity. When we apply Einstein’s equations we find that the embedding generates an equation of state relating the null string density to the null string pressure. The energy conditions lead to particular metrics including the anti/de Sitter spacetimes.

scholarly journals Aspects of late-time evolution in mimetic F(R) gravity

2016 ◽  
Vol 31 (33) ◽  
pp. 1650191 ◽  
Author(s):  
V. K. Oikonomou
Keyword(s):  
Dark Energy ◽  
Growth Factor ◽  
Equation Of State ◽  
Time Evolution ◽  
Early Time ◽  
Exponential Model ◽  
Late Time ◽  
Effective Equation ◽  
Dark Energy Density ◽  
Evolution Analysis

We demonstrate how to describe in an unified way early and late-time acceleration in the context of mimetic F(R) gravity. As we show, an exponential F(R) gravity model has appealing features, with regard to unification and we perform an analysis of the late-time evolution. The resulting picture is interesting since in the mimetic case, certain pathologies of some ordinary F(R) models are remedied in a consistent way, owing to the presence of the mimetic potential and the Lagrange multiplier. We quantify the late-time evolution analysis by studying the scaled dark energy density, the dark energy equation of state and the total effective equation of state, and as we show the late-time evolution is crucially affected by the functional form of the F(R) gravity. It is intriguing that the most appealing case corresponds to the exponential F(R) gravity which unifies late- and early-time acceleration. Finally, we study the behavior of the effective gravitational constant and the growth factor, and as we show, significant differences between the mimetic and ordinary F(R) exponential model are spotted in the growth factor.

scholarly journals Modified cosmology through Kaniadakis horizon entropy

2021 ◽  
Vol 81 (11) ◽  
Author(s):  
Andreas Lymperis ◽  
Spyros Basilakos ◽  
Emmanuel N. Saridakis
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Energy Equation ◽  
State Parameter ◽  
De Sitter ◽  
Single Model ◽  
Dark Energy Density ◽  
Horizon Entropy ◽  
Analytical Expressions ◽  
The Universe

AbstractWe apply the gravity-thermodynamics conjecture, namely the first law of thermodynamics on the Universe horizon, but using the generalized Kaniadakis entropy instead of the standard Bekenstein–Hawking one. The former is a one-parameter generalization of the classical Boltzmann–Gibbs–Shannon entropy, arising from a coherent and self-consistent relativistic statistical theory. We obtain new modified cosmological scenarios, namely modified Friedmann equations, which contain new extra terms that constitute an effective dark energy sector depending on the single model Kaniadakis parameter K. We investigate the cosmological evolution, by extracting analytical expressions for the dark energy density and equation-of-state parameters and we show that the Universe exhibits the usual thermal history, with a transition redshift from deceleration to acceleration at around 0.6. Furthermore, depending on the value of K, the dark energy equation-of-state parameter deviates from $$\Lambda $$ Λ CDM cosmology at small redshifts, while lying always in the phantom regime, and at asymptotically large times the Universe always results in a dark-energy dominated, de Sitter phase. Finally, even in the case where we do not consider an explicit cosmological constant the resulting cosmology is very interesting and in agreement with the observed behavior.

scholarly journals Phase space analysis and singularity classification for linearly interacting dark energy models

2020 ◽  
Vol 80 (2) ◽  
Author(s):  
Muhsin Aljaf ◽  
Daniele Gregoris ◽  
Martiros Khurshudyan
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Cosmological Parameters ◽  
State Parameter ◽  
Late Time ◽  
De Sitter ◽  
Effective Equation ◽  
De Sitter Universe ◽  
Friedmann Cosmological Models ◽  
Interacting Dark Energy Models

Abstract In this paper, applying the Hartman–Grobman theorem we carry out a qualitative late-time analysis of some unified dark energy-matter Friedmann cosmological models, where the two interact through linear energy exchanges, and the dark energy fluid obeys to the dynamical equation of state of Redlich–Kwong, Modified Berthelot, and Dieterici respectively. The identification of appropriate late-time attractors allows to restrict the range of validity of the free parameters of the models under investigation. In particular, we prove that the late-time attractors which support a negative deceleration parameter correspond to a de Sitter universe. We show that the strength of deviation from an ideal fluid for the dark energy does not influence the stability of the late-time attractors, as well as the values of all the cosmological parameters at equilibrium, but for the Hubble function (which represents the age of the universe). Our analysis also shows that a singularity in the effective equation of state parameter for the dark energy fluid is not possible within this class of models.

scholarly journals Interacting generalized ghost dark energy in a non-flat universe

Open Physics ◽  
2013 ◽  
Vol 11 (7) ◽  
Author(s):  
Esmaeil Ebrahimi ◽  
Ahmad Sheykhi ◽  
Hamzeh Alavirad
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
State Parameter ◽  
Model Parameters ◽  
De Sitter ◽  
Frw Universe ◽  
Ghost Dark Energy ◽  
Equation Of State Parameter ◽  
The Universe ◽  
Generalized Ghost Dark Energy

AbstractWe investigate the generalized Quantum Chromodynamics (QCD) ghost model of dark energy in the framework of Einstein gravity. First, we study the non-interacting generalized ghost dark energy in a flat Friedmann-Robertson-Walker (FRW) background. We obtain the equation of state parameter, w D = p/ρ, the deceleration parameter, and the evolution equation of the generalized ghost dark energy. We find that, in this case, w D cannot cross the phantom line (w D > −1) and eventually the universe approaches a de-Sitter phase of expansion (w D → −1). Then, we extend the study to the interacting ghost dark energy in both a flat and non-flat FRW universe. We find that the equation of state parameter of the interacting generalized ghost dark energy can cross the phantom line (w D < −1) provided the parameters of the model are chosen suitably. Finally, we constrain the model parameters by using the Markov Chain Monte Carlo (MCMC) method and a combined dataset of SNIa, CMB, BAO and X-ray gas mass fraction.

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