scholarly journals Effects of Rastall parameter on perturbation of dark sectors of the Universe

2021 ◽  
pp. 2150082
Author(s):  
A. H. Ziaie ◽  
H. Shabani ◽  
S. Ghaffari
Keyword(s):  
Modified Gravity ◽  
Numerical Solutions ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Linear Regime ◽  
Minimal Coupling ◽  
Spherical Collapse ◽  
Modified Gravity Theory ◽  
Collapse Model ◽  
The Universe

In recent years, Rastall gravity is undergoing a considerable surge in popularity. This theory purports to be a modified gravity theory with a non-conserved energy–momentum tensor (EMT) and an unusual non-minimal coupling between matter and geometry. This work looks for the evolution of homogeneous spherical perturbations within the Universe in the context of Rastall gravity. Using the spherical Top-Hat collapse model, we seek for exact solutions in linear regime for density contrast of dark matter (DM) and dark energy (DE). We find that the Rastall parameter affects crucially the dynamics of density contrasts for DM and DE and the fate of spherical collapse is different in comparison to the case of general relativity (GR). Numerical solutions for perturbation equations in nonlinear regime reveal that DE perturbations could amplify the rate of growth of DM perturbations depending on the values of Rastall parameter.

Cosmic acceleration with tachyon field

2018 ◽  
Vol 33 (34) ◽  
pp. 1850199 ◽  
Author(s):  
A. I. Keskin
Keyword(s):  
Scalar Field ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Cosmic Acceleration ◽  
Late Time ◽  
De Sitter ◽  
Tachyon Field ◽  
Minimal Coupling ◽  
Acceleration Of The Universe ◽  
The Universe

In this study, we examine two models of the scalar field, that is, a normal scalar field and a tachyon scalar field in [Formula: see text] gravity to describe cosmic acceleration of the universe, where [Formula: see text], [Formula: see text] and [Formula: see text] are Ricci curvature scalar, trace of energy–momentum tensor and kinetic energy of scalar field [Formula: see text], respectively. Using the minimal-coupling Lagrangian [Formula: see text], for both the scalar models we obtain a viable cosmological system, where [Formula: see text] and [Formula: see text] are real constants. While a normal scalar field gives a system describing expansion from the deceleration to the late-time acceleration, tachyon field together with [Formula: see text] in the system produces a quintessential expansion which is very close to de Sitter point, where we find a new condition [Formula: see text] for inflation.

scholarly journals Dynamics of an Anisotropic Universe inf(R,T)Theory

2016 ◽  
Vol 2016 ◽  
pp. 1-8 ◽  
Author(s):  
B. Mishra ◽  
Sankarsan Tarai ◽  
S. K. Tripathy
Keyword(s):  
Power Law ◽  
Modified Gravity ◽  
Bianchi Type ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Model Parameters ◽  
Anisotropic Universe ◽  
Expansion Of The Universe ◽  
Physical Features ◽  
The Universe

Dynamics of an anisotropic universe is studied inf(R,T)gravity using a rescaled functionalf(R,T), whereRis the Ricci Scalar andTis the trace of energy-momentum tensor. Three models have been constructed assuming a power law expansion of the universe. Physical features of the models are discussed. The model parameters are constrained from a dimensional analysis. It is found from the work that the anisotropic Bianchi typeVIh(BVIh) model in the modified gravity generally favours a quintessence phase when the parameterhis either-1or0. We may not get viable models in conformity with the present day observation forh=1.

scholarly journals Cosmological reconstruction of f(T, 𝒯) gravity

2014 ◽  
Vol 11 (08) ◽  
pp. 1450077 ◽  
Author(s):  
Davood Momeni ◽  
Ratbay Myrzakulov
Keyword(s):  
Scalar Field ◽  
Modified Gravity ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Chaplygin Gas ◽  
Massless Scalar ◽  
Massless Scalar Field ◽  
Two Fluids ◽  
Consistent Model ◽  
The Universe

Motivated by the newly proposal for gravity as the effect of the torsion scalar T and trace of the energy momentum tensor 𝒯, we investigate the cosmological reconstruction of different models of the Universe. Our aim here is to show that how this modified gravity model, f(T, 𝒯) is able to reproduce different epochs of the cosmological history. We explicitly show that f(T, 𝒯) can be reconstructed for ΛCDM as the most popular and consistent model. Also we study the mathematical reconstruction of f(T, 𝒯) for a flat cosmological background filled by two fluids mixture. Such model describes phantom–non-phantom era as well as the purely phantom cosmology. We extend our investigation to more cosmological models like perfect fluid, Chaplygin gas and massless scalar field. In each case we obtain some specific forms of f(T, 𝒯). These families of f(T, 𝒯) contain arbitrary function of torsion and trace of the energy momentum.

scholarly journals Gödel-type solution in f(R,T) modified gravity

2015 ◽  
Vol 30 (40) ◽  
pp. 1550214 ◽  
Author(s):  
A. F. Santos ◽  
C. J. Ferst
Keyword(s):  
Scalar Field ◽  
Perfect Fluid ◽  
Modified Gravity ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Matter Content ◽  
Type Solution ◽  
Special Cases ◽  
Tensor Formula ◽  
The Universe

In this paper, we will examine the problem of violation of causality in [Formula: see text] modified gravity, where [Formula: see text] is the Ricci scalar and [Formula: see text] is the trace of the energy–momentum tensor [Formula: see text]. We investigate the causality problem in two special cases, in the first we consider the matter content of the universe as a perfect fluid and in the second case, the matter content is a perfect fluid plus a scalar field.

MODIFIED GRAVITY THEORY DUE TO POSITIVE AS WELL AS NEGATIVE CURVATURE INDUCED ACCELERATION

2012 ◽  
Vol 21 (02) ◽  
pp. 1250012
Author(s):  
PRIYADARSHI MAJUMDAR
Keyword(s):  
Dark Energy ◽  
Modified Gravity ◽  
Negative Curvature ◽  
Gravity Theory ◽  
Cosmic Acceleration ◽  
Late Time ◽  
Negative Power ◽  
Modified Gravity Theory ◽  
Phase Q ◽  
The Universe

We formulate a modified gravity theory that eliminates the need for dark energy and is stable for a Lagrangian containing R, R2 as well as 1/R terms (i.e. nonlinear contributions of the Ricci curvature with a non-analytic model of f(R) at R = 0) without considering any matter-dominated era. The terms with positive powers (1, 2) of the curvature support the inflationary epoch while the terms with negative power (-1) serves as effective dark energy, supporting current cosmic acceleration. We present a few analytical solutions of evolution equation for the deceleration parameter q as a function of Hubble parameter H and time t; specially in one solution, the universe evolves continuously from q = 1 (a radiation-dominated epoch) to q = -1/2 (dark-energy-dominated late-time accelerating phase) when the universe is sufficiently old. The solution is supported by numerical results. The transition from the decelerated (q > 0) to the accelerated phase (q < 0) of expansion takes place smoothly without having to resort to a study of asymptotic behavior.

Gravity, stability and cosmological models

2017 ◽  
Vol 26 (12) ◽  
pp. 1743026
Author(s):  
Asher Yahalom
Keyword(s):  
Stability Analysis ◽  
Cosmological Model ◽  
Cosmological Constant ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Lorentzian Metric ◽  
Standard Cosmological Model ◽  
Flat Spacetimes ◽  
Stable Solutions ◽  
The Universe

Stability analysis plays a major rule in our understanding of nature. For example it was shown that among empty flat spacetimes only those with a Lorentzian metric are stable [A. Yahalom, Found Phys. 38 (2008) 489–497; Int. J. Mod. Phys. D 18(4) (2009) 2155–2158]. However, the universe is not empty and the energy momentum tensor is metric dependent an thus effects stability. In this essay we concentrate on simple perturbations of the standard cosmological model with and without cosmological constant which is based on a uniform mass distribution. The results suggest that while Euclidean, open or closed section models are valid solutions, the choice of stable solutions is limited. In particular, the popular Lambda-CDM model is unstable.

scholarly journals Quantum cosmology with vector torsion

2021 ◽  
Author(s):  
Ammar Kasem ◽  
Shaaban Said Khalil
Keyword(s):  
Quantum Cosmology ◽  
Energy Momentum Tensor ◽  
Momentum Tensor ◽  
Two Stage ◽  
Cosmological Principle ◽  
The Universe ◽  
Universe Evolution

Abstract We extend the treatment of quantum cosmology to a manifold with torsion. We adopt a model of Einstein-Cartan-Sciama-Kibble compatible with the cosmological principle. The universe wavefunction is shown to be subject to a PT-symmetric Hamiltonian. With a vanishing energy-momentum tensor, the universe evolution in the semiclassical and classical regimes is shown to suggest a two-stage inflationary process induced by torsion.

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.

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