Anisotropic universe models in f(T) gravity

Pramana, 2017

The general class of anisotropic Bianchi cosmological models in f (R, T) modified theories of gravity with (T) has been considered. This paper deals with f (R, T) modified theories of gravity, where the gravitational Lagrangian is given by an arbitrary function of Ricci scalar R and the trace of the stress-energy tensor T has been investigated for a specific choice of f (R, T) = f 1 (R) + f 2 (T). The exact solutions to the corresponding field equations are obtained in quadrature form. We have discussed three types of solutions of the average scale factor for the general class of Bianchi cosmological models by using a special law for deceleration parameter which is linear in time with a negative slope. The solutions to the Einstein field equations are obtained for three different physical viable cosmologies. All physical parameters are calculated and discussed in each model.

Pramana, 2013

A class of non-singular bouncing cosmological models of a general class of Bianchi models filled with perfect fluid in the framework of f (R, T) gravity is presented. The model initially accelerates for a certain period of time and decelerates thereafter. The physical behaviour of the model is also studied.

arXiv (Cornell University), 2022

Despite having a reasonably successful account of accelerated cosmology, understanding the early evolution of Universe has always been difficult for mankind. Our promising strategy is based on a novel class of symmetric teleparallel theories of gravity called f (Q), in which the gravitational interaction is caused by the non-metricity scalar Q, which may help to solve some problems. We consider the locally rotationally symmetric (LRS) Bianchi type-I spacetime cosmological models and derive the motion of equations to study the early evolution of the cosmos. By assuming the Hybrid Expansion Law (HEL) for the average scale factor, we are able to determine the solutions to the field equations of Bianchi type-I spacetime. We discuss the energy density profile, the equation of state, and the skewness parameter and conclude that our models preserve anisotropic spatial geometry during the early stages of the Universe with the possibility of an anisotropic fluid present. However, as time goes on, even in the presence of an anisotropic fluid, the Universe may move towards isotropy due to inflation while the anisotropy of the fluid dims away at the same time. It is seen from the squared speed of sound that Universe shows phantom nature at the beginning then approaches to dark energy at present epoch. We analyze both geometrical and physical behaviors of the derived model.

Astrophysics and Space Science, 2015

We try to study the theory of modified Gauss-Bonnet gravity in non-isotope universe. It is considered the exact power-law solution in modified gravity models. A f (G) function corresponding with power law solutions for given scale factor are calculated. We show that BI-like power-law solutions only exist for a very special class of f (G) theories. It is shown that transition to phantom phase is happened by applied some bound on free parameters. We also explore the stability issue of modified gravitational models.

Universal Journal of Physics and Application, 2016

Following the recognition of a positive value for the vacuum energy density and the realization that a simple Kantowski-Sachs model might fit the classical tests of cosmology, we study the qualitative behavior of three anisotropic and homogeneous models: Kantowski-Sachs, Bianchi type-I and Bianchi type-III universes, with dust and a cosmological constant, in order to find out which are physically permitted. We find that these models undergo isotropization up to the point that the observations will not be able to distinguish between them and the standard model, except for the Kantowski-Sachs model (Ω k0 < 0) and for the Bianchi type-III (Ω k0 > 0) with Ω Λ0 smaller than some critical value Ω ΛM. Even if one imposes that the Universe should be nearly isotropic since the last scattering epoch (z ≈ 1000), meaning that the Universe should have approximately the same Hubble parameter in all directions (considering the COBE 4-Year data), there is still a large range for the matter density parameter compatible with Kantowsky-Sachs and Bianchi type-III if |Ω 0 + Ω Λ0 − 1| ≤ δ, for a very small δ. The Bianchi type-I model becomes exactly isotropic owing to our restrictions and we have Ω 0 + Ω Λ0 = 1 in this case. Of course, all these models approach locally an exponential expanding state provided the cosmological constant Ω Λ > Ω ΛM .

International Journal of Theoretical Physics, 2014

A new class of cosmological models in f (R, T ) modified theories of gravity proposed by Harko et al. (Phys. Rev. D 84:024020, 2011), where the gravitational Lagrangian is given by an arbitrary function of Ricci scalar R and the trace of the stress-energy tensor T , have been investigated for a specific choice of f (R, T ) = f 1 (R) + f 2 (T ) by considering time dependent deceleration parameter. The concept of time dependent deceleration parameter (DP) with some proper assumptions yield the average scale factor a(t) = sinh 1 n (αt), where n and α are positive constants. For 0 < n ≤ 1, this generates a class of accelerating models while for n > 1, the models of universe exhibit phase transition from early decelerating phase to present accelerating phase which is in good agreement with the results from recent astrophysical observations. Our intention is to reconstruct f (R, T ) models inspired by this special law for the deceleration parameter in connection with the theories of modified gravity. In the present study we consider the cosmological constant Λ as a function of the trace of the stress energy-momentum-tensor, and dub such a model "Λ(T ) gravity" where we have specified a certain form of Λ(T ). Such models may display better uniformity with the cosmological observations. The statefinder diagnostic pair {r, s} parameter has been embraced to characterize different phases of the universe. We also discuss the physical consequences of the derived models.

Advances in High Energy Physics, 2016

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.

Central European Journal of Physics, 2011

The exact solutions of the Einstein field equations for dark energy in Kantowski-Sachs metric under the assumption on the anisotropy of the fluid are obtained for exponential and power-law volumetric expansions. The isotropy of the fluid, space and expansion are examined.

Physics of Particles and Nuclei, 2014

In this review we discuss the evolution of the universe filled with dark energy with or without perfect fluid. In doing so we consider a number of cosmological models, namely Bianchi type I, III, V, VI 0 , VI and FRW ones. For the anisotropic cosmological models we have used proportionality condition as an additional constrain. The exact solutions to the field equations in quadrature are found in case of a BVI model. It was found that the proportionality condition used here imposed severe restriction on the energy-momentum tensor, namely it leads to isotropic distribution of matter. Anisotropic BVI 0 , BV, BIII and BI DE models with variable EoS parameter ω have been the dark energy parameter, has been investigated. In doing so we have used three different ansatzs regarding the scale factor that gives rise to a variable decelerating parameter. It is observed that, in the non-interacting case, both the open and flat universes can cross the phantom region whereas in the interacting case only the open universe can cross the phantom region. The stability and acceptability of the obtained solution are also investigated.

International Journal of Geometric Methods in Modern Physics, 2017

The objective of this work enclosed with the study of spatially homogeneous anisotropic Bianchi type-I universe in [Formula: see text] gravity (where [Formula: see text] is the Ricci scalar and [Formula: see text] is the trace of stress energy momentum tensor) in two different cases viz. [Formula: see text] and [Formula: see text] with bulk viscosity matter content. In this study, we consider a time varying deceleration parameter (DP), which generates an accelerating universe to obtain the exact solution of the field equations. The physical and kinematical properties of both the models are discussed in detail for the future evolution of the universe. We have explored the nature of WEC, DEC, SEC and energy density for both the cases. We have found that both the models, with bulk viscosity matter component, show an acceleration of the universe. We have also shown that the cosmic jerk parameter is compatible with the three kinematical data sets.