scholarly journals Kasner type magnetized string cosmological models in f(R,T) gravity

2018 ◽  
pp. 1-11
Author(s):  
S.P. Hatkar ◽  
S.V. Gore ◽  
S.D. Katore
Keyword(s):  
Magnetic Field ◽  
Equation Of State ◽  
Energy Density ◽  
String Tension ◽  
Eos Parameter ◽  
Rest Energy ◽  
Bianchi I ◽  
Functional Forms ◽  
Theory Of Gravitation ◽  
The Universe

The Bianchi-I Kasner type metric with cosmic string and magnetic field in the framework of the f(R,T) theory of gravitation is considered. Three different functional forms of the function f(R,T) are chosen for investigation. We found that the strings exist in early stages of evolution of the Universe and they disappear as time increases. The variation of the equation of state (EoS) parameter ? = p/?<-1 may come from the effect of the string. We find that the string tension and rest energy density reduce in presence of magnetic field. The Universe is expanding and accelerating.

scholarly journals Dynamical system analysis of FLRW models with Modified Chaplygin gas

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Ali Osman Yılmaz ◽  
Ertan Güdekli
Keyword(s):  
Dynamical System ◽  
Equation Of State ◽  
Energy Density ◽  
System Analysis ◽  
Chaplygin Gas ◽  
Modified Chaplygin Gas ◽  
State Spaces ◽  
The Universe ◽  
Matter Energy ◽  
Far Future

AbstractWe investigate Friedmann–Lamaitre–Robertson–Walker (FLRW) models with modified Chaplygin gas and cosmological constant, using dynamical system methods. We assume $$p=(\gamma -1)\mu -\dfrac{A}{\mu ^\alpha }$$ p = ( γ - 1 ) μ - A μ α as equation of state where $$\mu$$ μ is the matter-energy density, p is the pressure, $$\alpha$$ α is a parameter which can take on values $$0<\alpha \le 1$$ 0 < α ≤ 1 as well as A and $$\gamma$$ γ are positive constants. We draw the state spaces and analyze the nature of the singularity at the beginning, as well as the fate of the universe in the far future. In particular, we address the question whether there is a solution which is stable for all the cases.

scholarly journals DARK VISCOUS FLUID DESCRIBED BY A UNIFIED EQUATION OF STATE IN COSMOLOGY

2007 ◽  
Vol 16 (08) ◽  
pp. 1341-1348 ◽  
Author(s):  
JIE REN ◽  
XIN-HE MENG
Keyword(s):  
Equation Of State ◽  
Viscous Fluid ◽  
Late Time ◽  
Dissipative Effect ◽  
Eos Parameter ◽  
Friedmann Equations ◽  
Λcdm Model ◽  
Unique Framework ◽  
The Universe ◽  
Single Constant

We generalize the ΛCDM model by introducing a unified EOS to describe the Universe contents modeled as dark viscous fluid, motivated by the fact that a single constant equation of state (EOS) p = -p0 (p0 > 0) reproduces the ΛCDM model exactly. This EOS describes the perfect fluid term, the dissipative effect, and the cosmological constant in a unique framework and the Friedmann equations can be analytically solved. Especially, we find a relation between the EOS parameter and the renormalizable condition of a scalar field. We develop a completely numerical method to perform a χ2 minimization to constrain the parameters in a cosmological model directly from the Friedmann equations, and employ the SNe data with the parameter [Formula: see text] measured from the SDSS data to constrain our model. The result indicates that the dissipative effect is rather small in the late-time Universe.

A study on the modified holographic Ricci dark energy in logarithmic f(T) gravity

2013 ◽  
Vol 91 (4) ◽  
pp. 351-354 ◽  
Author(s):  
Antonio Pasqua ◽  
Surajit Chattopadhyay
Keyword(s):  
Dark Matter ◽  
Dark Energy ◽  
Equation Of State ◽  
Energy Density ◽  
Modified Gravity ◽  
State Parameter ◽  
Ricci Dark Energy ◽  
Equation Of State Parameter ◽  
Interaction Term ◽  
The Universe

In this paper, we have studied and investigated the behavior of a modified holographic Ricci dark energy (DE) model interacting with pressureless dark matter (DM) under the theory of modified gravity, dubbed logarithmic f(T) gravity. We have chosen the interaction term between DE and DM in the form Q = 3γHρm and investigated the behavior of the torsion, T, the Hubble parameter, H, the equation of state parameter, ωDE, the energy density of DE, ρDE, and the energy density contribution due to torsion, ρT, as functions of the redshift, z. We have found that T increases with the redshift, z, H increases with the evolution of the universe, ωDE has a quintessence-like behavior, and both energy densities increase going from higher to lower redshifts.

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.

scholarly journals Cosmological aspects of sound speed parameterizations in fractal universe

2019 ◽  
Vol 79 (11) ◽  
Author(s):  
Abdul Jawad ◽  
Sadaf Butt ◽  
Shamaila Rani ◽  
Khadija Asif
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Speed Of Sound ◽  
Cosmological Parameters ◽  
State Parameter ◽  
Graphical Analysis ◽  
Eos Parameter ◽  
Energy Models ◽  
Equation Of State Parameter ◽  
The Universe

AbstractIn the framework of fractal universe, the unified models of dark energy and dark matter are being presented with the background of homogenous and isotropic FLRW geometry. The aspects of fractal cosmology helps in better understanding of the universe in different dimensions. Relationship between the squared speed of the sound and the equation of state parameter is the key feature of these models. We have used constant as well as variable forms of speed of sound and express it as a function of equation of state parameter. By utilizing the four different forms of speed of sound, we construct the energy densities and pressures for these models and then various cosmological parameters like hubble parameter, EoS parameter, deceleration parameter and Om- diagnostic are investigated. Graphical analysis of these parameters show that in most of the cases EoS parameters and trajectories of Om-diagnostic corresponds to the quintessence like nature of the universe and the deceleration parameters represent accelerated and decelerated phase. In the end, we remark that cosmological analysis of these models indicates that these models correspond to different well known dark energy models.

Anisotropic MHRDE model in BD theory of gravitation

2019 ◽  
Vol 16 (12) ◽  
pp. 1950185
Author(s):  
Anirudh Pradhan ◽  
Archana Dixit ◽  
Shilpi Singhal
Keyword(s):  
Dark Energy ◽  
Energy Density ◽  
Deceleration Parameter ◽  
Big Bang ◽  
Smooth Transition ◽  
Complete Agreement ◽  
Symmetric Model ◽  
Axially Symmetric ◽  
Theory Of Gravitation ◽  
The Universe

In this paper, in the framework of the Brans–Dicke [Phys. Rev. 124 (1961) 925] Gravitation theory, we propose to study the spatially homogeneous, anisotropic and axially symmetric model filled with dark matter and dark energy. Here, we consider the modified holographic Ricci dark energy proposed by Chen and Jing [Phys. Rev. B 679 (2009) 144] as a feasible state of darkness. To achieve a solution, we consider the time-dependent deceleration parameter, which contributes to the average scale factor of [Formula: see text], where [Formula: see text] and [Formula: see text] are arbitrary constants. We have derived field equations of Brans–Dicke theory of gravitation with the help of an axially symmetric anisotropic Bianchi-type space-time. We have determined the cosmological parameters, namely, deceleration parameter, matter energy density, anisotropic dark energy density, BD scalar field, skewness parameter, EoS parameter and jerk parameter. Here, the various phenomena like the Big Bang, expanding the universe, and shift from anisotropy to isotropy are observed in the model. A comprehensive physical debate of these dynamic parameters is provided through a graphical representation. We observe that we have a quintessence model that exhibits a smooth transition from decelerated stage to an accelerated phase of the universe. This situation is in complete agreement with the modern cosmology scenario. Some physical and geometric behaviors are also discussed and discovered to be in excellent agreement with SNe Ia Supernova’s latest observations.

Kantowski–Sachs cosmological solutions in the generalized teleparallel gravity via Noether symmetry

2016 ◽  
Vol 31 (15) ◽  
pp. 1650095 ◽  
Author(s):  
H. Motavalli ◽  
A. Rezaei Akbarieh ◽  
M. Nasiry
Keyword(s):  
Equation Of State ◽  
Teleparallel Gravity ◽  
Noether Symmetry ◽  
Anisotropic Model ◽  
Eos Parameter ◽  
Scale Factors ◽  
Cosmological Solutions ◽  
Frw Metric ◽  
The Universe ◽  
Friedmann Robertson Walker

We study the f(T) theory as an extension of teleparallel gravity and consider the Noether symmetry of Kantowski–Sachs (KS) anisotropic model for this theory. We specify the explicit teleparallel form of f(T) and find the corresponding exact cosmological solutions under the assumption that the Lagrangian admits the Noether symmetry. It is found that the universe experiences a power law expansion for the scale factors in the context of f(T) theory. By deriving equation of state (EOS) parameter, we show that the universe passes through the phantom and [Formula: see text]CDM theoretical scenarios. In this way, we estimate a lower limit age for the universe in excellent agreement with the value reported from recent observations. When KS model reduces to the flat Friedmann–Robertson–Walker (FRW) metric, our results are properly transformed into the corresponding values.

Holographic dark energy models in higher derivative torsion corrected modified teleparallel gravity

2018 ◽  
Vol 15 (04) ◽  
pp. 1850067 ◽  
Author(s):  
Shamaila Rani ◽  
Abdul Jawad
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Holographic Dark Energy ◽  
Teleparallel Gravity ◽  
Eos Parameter ◽  
Phantom Divide ◽  
Energy Models ◽  
Higher Derivative ◽  
Phantom Divide Line ◽  
The Universe

We consider the recently proposed higher derivative torsion corrected modified teleparallel gravity and holographic dark energy (HDE) models. We apply the correspondence scheme to construct models in underlying scenario using various scale factor forms. We investigate the reconstructed functions through equation of state (EoS) parameter. It is demonstrated that the EoS parameter provides quintom-like nature of the Universe in most of the cases, i.e. it drives the Universe from vacuum dark energy era toward phantom era of the Universe by crossing the phantom divide line. We also demonstrate that the consistency with the observational data can be achieved.

scholarly journals A Study on Ricci Dark Energy in Bulk-Brane Interaction

2013 ◽  
Vol 2013 ◽  
pp. 1-6
Author(s):  
Surajit Chattopadhyay
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Energy Density ◽  
Power Law ◽  
Ricci Dark Energy ◽  
Dark Energy Density ◽  
Evolution Of The Universe ◽  
The Impact ◽  
The Universe ◽  
Matter Fields

We have investigated the effects of the interaction between a brane universe and the bulk in which it is embedded. Considering the effects of the interaction between a brane universe and the bulk, we have obtained the equation of state for the interacting holographic Ricci dark energy density ρΛ=3c2(H˙+2H2) in the flat universe. We have investigated the impact of c2 on the equation of state ωΛ. Also, considering the power law for of the scale factor, we have observed that nontrivial contributions of dark energy which differ from the standard matter fields confined to the brane are increasing with the evolution of the universe.

Bouncing scenario in f(R,T) gravity

2020 ◽  
Vol 35 (13) ◽  
pp. 2050095 ◽  
Author(s):  
Parbati Sahoo ◽  
Snehasish Bhattacharjee ◽  
S. K. Tripathy ◽  
P. K. Sahoo
Keyword(s):  
Equation Of State ◽  
Initial Conditions ◽  
Scale Factor ◽  
Energy Conditions ◽  
Geometrical Parameters ◽  
Singularity Problem ◽  
Eos Parameter ◽  
Qualitative Understanding ◽  
The Stability ◽  
The Universe

This paper presents modeling of matter bounce in the framework of [Formula: see text] gravity, where [Formula: see text]. We start by defining a parametrization of scale factor which is non-vanishing. The geometrical parameters such as the Hubble parameter and deceleration parameter are derived, from which expressions of pressure, density and Equation of State (EoS) parameter and a qualitative understanding of the initial conditions of the universe at the bounce are ascertained. We found that the initial conditions of the universe are finite owing to the non-vanishing nature of the scale factor thus eliminates the initial singularity problem. Furthermore, we show the violation of energy conditions near the bouncing region and analyze the stability of our model with respect to linear homogeneous perturbations in Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime. We found that our model and hence matter bounce scenarios in general are highly unstable at the bounce in the framework of [Formula: see text] gravity but the perturbations decay out rapidly away from the bounce safeguarding its stability at late-times.

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