Reconstruction of f(T) gravity in the context of standard Chaplygin gas as tachyon scalar field and study of the stability against gravitational perturbation

2019 ◽  
Vol 16 (07) ◽  
pp. 1950101
Author(s):  
Surajit Chattopadhyay ◽  
Soumyodipta Karmakar
Keyword(s):  
Scalar Field ◽  
Equation Of State ◽  
State Parameter ◽  
Chaplygin Gas ◽  
Gravitational Perturbation ◽  
Gravitational Perturbations ◽  
Equation Of State Parameter ◽  
Speed Parameter ◽  
The Stability ◽  
The Universe

The present paper reports a study on the [Formula: see text] gravity reconstruction scheme in the context of considering standard Chaplygin gas as tachyon scalar field model of dark energy. The solution for reconstructed [Formula: see text] gravity has been obtained from reconstructed potential and scalar field of tachyon based on the standard Chaplygin gas. It has been observed that the equation of state parameter due to the reconstructed torsion contribution to the density and pressure behaves like quintom and is consistent with the observational value of the equation of state parameter for the current universe. The reconstructed [Formula: see text] has then been tested for gravitational perturbation by deriving the frictional term, the effective mass and the sound speed parameter for the gravitational potential and it has been found to be stable against gravitational perturbations through positive value of the squared speed of sound. Finally, statefinder diagnostics has been carried out and the reconstructed [Formula: see text] gravity is found to interpolate between dust and [Formula: see text]CDM phases of the universe.

scholarly journals Chameleon field dynamics during inflation

2018 ◽  
Vol 27 (04) ◽  
pp. 1850041 ◽  
Author(s):  
Nasim Saba ◽  
Mehrdad Farhoudi
Keyword(s):  
Scalar Field ◽  
Equation Of State ◽  
State Parameter ◽  
Big Bang ◽  
Late Time ◽  
Inflationary Model ◽  
Slow Roll ◽  
Equation Of State Parameter ◽  
The Common ◽  
The Universe

By studying the chameleon model during inflation, we investigate whether it can be a successful inflationary model, wherein we employ the common typical potential usually used in the literature. Thus, in the context of the slow-roll approximations, we obtain the e-folding number for the model to verify the ability of resolving the problems of standard big bang cosmology. Meanwhile, we apply the constraints on the form of the chosen potential and also on the equation of state parameter coupled to the scalar field. However, the results of the present analysis show that there is not much chance of having the chameleonic inflation. Hence, we suggest that if through some mechanism the chameleon model can be reduced to the standard inflationary model, then it may cover the whole era of the universe from the inflation up to the late time.

scholarly journals Unified universe history through phantom extended Chaplygin gas

2016 ◽  
Vol 94 (7) ◽  
pp. 659-670 ◽  
Author(s):  
B. Pourhassan
Keyword(s):  
Scalar Field ◽  
State Parameter ◽  
Chaplygin Gas ◽  
Late Time ◽  
Two Component ◽  
Equation Of State Parameter ◽  
Phantom Scalar ◽  
The Universe ◽  
Phantom Scalar Field ◽  
Universe Evolution

The universe evolution from inflation to late-time acceleration is investigated in a unified way, using a two-component fluid constituted from extended Chaplygin gas alongside a phantom scalar field. We extract solutions for the various cosmological eras, focusing on the behavior of the scale factor, the various density parameters and the equation-of-state parameter. Furthermore, we extract and discuss bouncing solutions. Finally, we examine the perturbations of the model, ensuring their stability and extracting the predictions for the tensor-to-scalar ratio.

scholarly journals PHANTOM COSMOLOGY WITH A NONLINEAR BORN–INFELD TYPE SCALAR FIELD

2005 ◽  
Vol 14 (02) ◽  
pp. 355-362 ◽  
Author(s):  
H. Q. LU
Keyword(s):  
Dark Energy ◽  
Scalar Field ◽  
Equation Of State ◽  
Energy Condition ◽  
State Parameter ◽  
Strong Energy Condition ◽  
Strong Energy ◽  
Accelerating Expansion ◽  
Equation Of State Parameter ◽  
The Universe

Recent many physicists suggest that the dark energy in the universe might result from the Born–Infeld (B–I) type scalar field of string theory. The universe of B–I type scalar field with potential can undergo a phase of accelerating expansion. The corresponding equation of state parameter lies in the range of -1<ω<-⅓. The equation of state parameter of B–I type scalar field without potential lies in the range of 0≤ω≤1. We find that weak energy condition and strong energy condition are violated for phantom B–I type scalar field. The equation of state parameter lies in the range of ω<-1.

Thermodynamics in loop quantum cosmology with non-canonical scalar field model

2019 ◽  
Vol 16 (06) ◽  
pp. 1950081
Author(s):  
Ayesha Iqbal ◽  
Abdul Jawad
Keyword(s):  
Scalar Field ◽  
Equation Of State ◽  
Equilibrium Condition ◽  
Field Model ◽  
State Parameter ◽  
Second Law Of Thermodynamics ◽  
Second Law ◽  
Generalized Second Law ◽  
Equation Of State Parameter ◽  
The Universe

The cosmological scenario is built up within the framework of scalar field model possessing a noncanonical kinetic term in loop quantum gravity. The noncanonical scalar field is permitted to interact with dark matter field by assuming a specific form of coupling term. The equation of state parameter is set to be constant as well as variable (Chevallier–Polarski–Linder parametrization) and evaluated the behavior of universe through deceleration parameter and weak energy condition. These parameters favor the accelerated expansion of the universe for three values of equation of state parameter in both cases allowed by observational data. The squared speed of sound leads to the stability of the underlying models in both forms of equation of state parameter. Moreover, the validity of generalized second law of thermodynamics is analyzed by using first law of thermodynamics and assume the universe to be enclosed by apparent horizon. The Bekenstein, logarithmic and power-law entropy is being considered as entropy of horizon. The thermodynamic equilibrium condition is also discussed for all three cases of entropies. The generalized second law of thermodynamics and thermal equilibrium condition is satisfied for all the three types of entropies.

scholarly journals BOUNCING UNIVERSE WITH THE NON-MINIMALLY COUPLED SCALAR FIELD AND ITS RECONSTRUCTION

2009 ◽  
Vol 24 (29) ◽  
pp. 2363-2376 ◽  
Author(s):  
J. SADEGHI ◽  
F. MILANI ◽  
A. R. AMANI
Keyword(s):  
Scalar Field ◽  
Equation Of State ◽  
State Parameter ◽  
Stability Conditions ◽  
Bouncing Universe ◽  
Equation Of State Parameter ◽  
The Stability

In this paper we consider a non-minimally coupled scalar field, and show that its equation of state parameter can cross over -1, ω→-1, and bouncing condition. Also we obtain the stability conditions and consider reconstructing for our model.

Cosmological consequences and thermodynamics of modified gravity with extended nonminimal derivative couplings

2019 ◽  
Vol 28 (11) ◽  
pp. 1950137 ◽  
Author(s):  
Abdul Jawad ◽  
M. Bilal Amin ◽  
Shamaila Rani
Keyword(s):  
Equation Of State ◽  
Speed Of Sound ◽  
State Parameter ◽  
Second Law Of Thermodynamics ◽  
Frw Universe ◽  
Generalized Second Law ◽  
Horizon Entropy ◽  
Equation Of State Parameter ◽  
The Stability ◽  
The Universe

We consider the newly proposed gravitational modifications that go beyond Horndeski’s theory, named as theories with extended nonminimal derivative couplings. By these modifications, the coefficient functions depend on the scalar field and its kinetic energy. These theories become ghost-free in cosmological background. We consider the flat FRW universe and explore the equation-of-state parameter, [Formula: see text]–[Formula: see text] plane and the squared speed of sound. The equation-of-state parameter exhibits phantom behavior of the universe, [Formula: see text]–[Formula: see text] plane represents the freezing region of the universe while the squared speed of sound denotes the stability of the model for the specific choice of constant parameters. Also, we investigate the validity of generalized second law of thermodynamics on the Hubble horizon taking into account the Bekenstein, power-law, Renyi and logarithmic corrections to the horizon entropy.

Viability of specific reconstructed f(T,𝒯 ) models

2019 ◽  
Vol 34 (30) ◽  
pp. 1950184
Author(s):  
M. Umair Shahzad ◽  
Nadeem Azhar ◽  
Abdul Jawad ◽  
Shamaila Rani
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Dark Energy Model ◽  
State Parameter ◽  
Pilgrim Dark Energy ◽  
Ghost Dark Energy ◽  
Energy Models ◽  
Equation Of State Parameter ◽  
Hubble Horizon ◽  
The Universe

The reconstruction scenario of well-established dark energy models such as pilgrim dark energy model and generalized ghost dark energy with Hubble horizon and [Formula: see text] models is being considered. We have established [Formula: see text] models and analyzed their viability through equation of state parameter and [Formula: see text] (where prime denotes derivative with respect to [Formula: see text]) plane. The equation of state parameter evolutes the universe in three different phases such as quintessence, vacuum and phantom. However, the [Formula: see text] plane also describes the thawing as well as freezing region of the universe. The recent observational data also favor our results.

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 On a Slightly Different Power Law-Scaling for the Flat Universe

2020 ◽  
Vol 12 (4) ◽  
pp. 569-574
Author(s):  
C. Sivakumar ◽  
R. Francis
Keyword(s):  
Dark Energy ◽  
Equation Of State ◽  
Power Law ◽  
Hubble Parameter ◽  
State Parameter ◽  
Effective Equation ◽  
Flat Universe ◽  
Equation Of State Parameter ◽  
Age Of The Universe ◽  
The Universe

A slightly different power law-scaling fits to the picture of our 13.7 billion years old flat universe which is expanding presently at 67 km/s/Mpc with an acceleration. The model which is an attempt to retain power-law scaling in the light of the accepted facts about the universe we are living in, has a constant effective equation of state parameter as the cosmic fluid is a solution of matter, radiation and dark energy. It is successful in explaining the acceleration of universe which the normal power law fails if the present Hubble parameter is 67 km/s/Mpc and age of the universe is 13.7 billion years, and it is free from the defect of singularity.

scholarly journals Unavoidable shear from quantum fluctuations in contracting cosmologies

2021 ◽  
Vol 81 (2) ◽  
Author(s):  
Julien Grain ◽  
Vincent Vennin
Keyword(s):  
Scalar Field ◽  
Equation Of State ◽  
Energy Density ◽  
State Parameter ◽  
Quantum Fluctuations ◽  
Shear Instability ◽  
Initial Value ◽  
Anisotropic Stress ◽  
Equation Of State Parameter ◽  
Contracting Phase

AbstractContracting cosmologies are known to be flawed with a shear instability, where the contribution from the anisotropic stress to the overall energy density grows as $$a^{-6}$$ a - 6 , with a the scale factor. Classically, whether or not this contribution becomes important before the bounce depends on its initial value, which can always be sufficiently fine tuned to make it irrelevant. However, vacuum quantum fluctuations inevitably provide a non-vanishing source of anisotropic stress. In this work, we compute the minimum amount of shear that is obtained if one assumes that it vanishes initially, but lets quantum fluctuations build it up. In practice, we consider a massless test scalar field, and describe its quantum fluctuations by means of the stochastic “inflation” (though here applied to a contracting phase) formalism. We find that, if the equation-of-state parameter of the contraction satisfies $$w>-1/9$$ w > - 1 / 9 , regardless of when the contracting phase is initiated, the time at which the shear becomes sizeable is always when the Hubble scale approaches the Planck mass (which is also where the bounce is expected to take place). However, if $$w<-1/9$$ w < - 1 / 9 , the shear backreaction becomes important much earlier, at a point that depends on the overall amount of contraction.

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