Cosmology of f(T) gravity in a holographic dark energy and nonisotropic background

It is shown that the acceleration of the universe can be understood by considering a f(T) gravity models. Modified teleparallel gravity theory with the torsion scalar has recently gained a lot of attention as a possible explanation of dark energy. For these f(T) gravity models, a variant of the accelerating cosmology reconstruction program is developed. We consider spatially homogenous and anisotropic Bianchi type I universe in the context of f(T) gravity. The de Sitter, power-law and general exponential solutions are assumed for the scale factor in each spatial direction and the corresponding cosmological models are reconstructed. We reconstruct f(T) theories from two different holographic dark energy models in different time durations. For the holographic dark energy model, the dark energy dominated era with new setting up is chosen for reconstruction, and the Ricci dark energy model, radiation, matter and dark energy dominated time durations are all investigated. Finally we have obtained a modified gravity action consistent with the holographic dark energy scenario.     

Reconstruction of f(T) gravity from the Holographic Dark Energy

Among different candidates to play the role of Dark Energy (DE), modified gravity has emerged as offering a possible unification of Dark Matter (DM) and DE. The purpose of this work is to develop a reconstruction scheme for the modified gravity with f(T) action using holographic energy density. In the framework of the said modified gravity we have considered the equation of state of the Holographic DE (HDE) density. Subsequently we have developed a reconstruction scheme for modified gravity with f(T) action. Finally we have obtained a modified gravity action consistent with the HDE scenario.     

Viability bounds in f(R,G) gravity with energy conditions

In the present work, we discuss the viability bounds arising from the energy conditions in the context of an extended gravitational theory namely f(R, G) gravity, where R stands for the curvature scalar and G represents the Gauss-Bonnet invariant. Two specific forms of f(R, G) have been taken into account to examine the validity of the general inequalities obtained by the weak energy condition (WEC). More specifically, we consider the constraints imposed by the weak energy condition (WEC) and verify whether the parameter range of the recent proposed models are consistent with the energy conditions.     

Variable Chaplygin gas cosmologies in f(R,T) gravity with particle creation

A flat FLRW (Friedmann–Lemaitre–Robertson–Walker) cosmological model with perfect fluid comprising of variable Chaplygin gas (VCG) has been studied in the context of f(R, T) gravity with particle creation. The solutions of the modified field equations are obtained through three different considered form of scale factors. The effective pressure is negative throughout the evolution of universe, which leads to accelerated expansion of the universe. In addition to that we have also discussed the importance of particle creation pressure on the cosmological parameters, energy conditions and state-finder diagnostic parameters. It is noticed that the time evolution of source function yields almost constant particle production at late times.     

Existence of wormhole solutions and energy conditions in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>, <Emphasis Type="Italic">T</Emphasis>) gravity

This paper is devoted to investigate the spherically symmetric wormhole models in f(R, T) gravity, where T and R are trace of stress energy tensor and the Ricci scalar, respectively. In this context, we discuss three distinct cases of fluid distributions viz, anisotropic, barotropic and isotropic matter contents. After considering the exponential f(R, T) model, the behavior of energy conditions are analyzed that will help us to explore the general conditions for wormhole geometries in this gravity. It is inferred that the usual matter in the throat could obey the energy conditions but the gravitational field emerging from higher order terms of modified gravity favor the existence of the non-standard geometries of wormholes. The stability as well as the existence of wormholes are also analyzed in this theory.     

Holographic f(T) gravity model

We try to study the corresponding relation between f(T) gravity and holographic dark energy (HDE). A kind of energy density from f(T) is introduced which has the same role as HDE density. A f(T) model according to the HDE model is calculated. We find out a torsion scalar T based on the scalar factor is assumed by Capoziello et al. (Phys. Lett. B 639:135, 2006). The effective torsion equation of state, deceleration parameter of the holographic f(T)-gravity model are calculated.     

The study of Gödel type solutions in f(R, ϕ) gravity

The aim of this paper is to study the Gödel type universe in modified f(R, ϕ) theory of gravity, where R stands for Ricci scalar and ϕ be the scalar potential. We investigate the modified field equations by using anisotropic and perfect fluid distributions. In particular, we consider two proposed models with some fixed values of parameters and investigate the exact solutions. The behaviour of energy conditions can be seen by a detailed graphical analysis. Furthermore, Tolman-Oppenheimer-Volkoff equation has been studied for both models in this theory. We have also discussed some exact solutions using perfect fluid. It is concluded that f(R, ϕ) theory of gravity support the phenomenon of cosmic expansion of the universe through Gödel type universe for both models.     

Anisotropic cosmological models in f(R,T) gravity according to holographic and new agegraphic dark energy

The paper deals with a spatially homogeneous and anisotropic universe filled with perfect fluid and dark energy components. We consider the f(R,T) theory according to holographic and new agegraphic dark energy in the Bianchi type I universe. In this study, we concentrate on two particular models of f(R,T) gravity namely, R+2f(T) and f(R)+λT. We conclude that the derived f(R,T) models can represent phantom or quintessence regimes of the universe.     

Cosmological evolution for dark energy models in f(T) gravity

In this paper, we investigate the behavior of equation of state parameter and energy density for dark energy in the framework of f(T) gravity. For this purpose, we use anisotropic LRS Bianchi type I universe model. The behavior of accelerating universe is discussed for some well-known f(T) models. It is found that the universe takes a transition between phantom and non-phantom phases for f(T) models except exponential and logarithmic models. We conclude that our results are relativity analogous to the results of FRW universe.     

Reconstruction of f(R,T) gravity in anisotropic cosmological models of accelerating universe

In this paper, we search the existence of Bianchi type I cosmological model in f(R,T) gravity, where the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar R and of the trace of the stress-energy tensor T. We obtain the gravitational field equations in the metric formalism, and reconstruct the corresponding f(R,T) functions. Attention is attached to the special case, f(R,T)=f ₁(R)+f ₂(T) and two examples are assumed for this model. In the first example, we consider the unification of matter dominated and accelerated phases with f(R) gravity in anisotropic universe, and in the second instance, model of f(R,T) gravity with transition of matter dominated phase to the acceleration phase is obtained. In both cases, f(R,T) is proportional to a power of R with exponents depending on the input parameters.     

Reconstruction of f(R), f(T) and f(\mathcal{G}) models inspired by variable deceleration parameter

We study an special law for the deceleration parameter, recently proposed by Akarsu and Dereli, in the context of f(R), f(T) and $f(\mathcal{G})$ theories of modified gravity. This law covers the law of Berman for obtaining exact cosmological models to account for the current acceleration of the universe, and also gives the opportunity to generalize many of the dark energy models having better consistency with the cosmological observations. Our aim is to reconstruct the f(R), f(T) and $f(\mathcal{G})$ models inspired by this law of variable deceleration parameter. Such models may then exhibit better consistency with the cosmological observations.     

Correspondence between f(G) gravity and holographic dark energy via power-law solution

In this paper, we discuss cosmological application of holographic Dark Energy (HDE) in the framework of f(G) modified gravity. For this purpose, we construct f(G) model with the inclusion of HDE and a well-known power law form of the scale factor a(t). The reconstructed f(G) is found to satisfy a sufficient condition for a realistic modified gravity model. We find quintessence behavior of effective equation of state (EoS) parameter ω _DE through energy conditions in this context. Moreover, we observe that the squared speed of sound $v_{s}^{2}$ remains negative, which indicates the instability of HDE f(G) model.     

Cosmology from induced matter model applied to 5D f(R,T) theory

It is well known that the universe is undergoing a phase of accelerated expansion. Plenty of models have already been created with the purpose of describing what causes this non-expected cosmic feature. Among them, one could quote the extradimensional and the f(R,T) gravity models. In this work, in the scope of unifying Kaluza-Klein extradimensional model with f(R,T) gravity, cosmological solutions for density and pressure of the universe are obtained from the induced matter model application. Particular solutions for vacuum quantum energy and radiation are also shown.     

Bianchi type-VI0 perfect fluid cosmological model in a modified theory of gravity

A spatially homogeneous and anisotropic Bianchi type-VI₀ space-time filled with perfect fluid in general relativity and also in the framework of f(R,T) gravity proposed by Harko et al. (in arXiv:1104.2669 [gr-qc], 2011) has been studied with an appropriate choice of the function f(R,T). The field equations have been solved by using the anisotropy feature of the universe in Bianchi type-VI₀ space time. Some important features of the models, thus obtained, have been discussed. We noticed that the involvement of new function f(R,T) doesn’t affect the geometry of the space-time but slightly changes the matter distribution.     

Generalized teleparallel gravity via some scalar field dark energy models

We consider generalized teleparallel gravity in the flat FRW universe with a viable power-law f(T) model. We construct its equation of state and deceleration parameters which give accelerated expansion of the universe in quintessence era for the obtained scale factor. Further, we develop correspondence of f(T) model with scalar field models such as, quintessence, tachyon, K-essence and dilaton. The dynamics of scalar field as well as scalar potential of these models indicate the expansion of the universe with acceleration in the f(T) gravity scenario.     

Study of Bianchi I anisotropic model in f(R,T) gravity

We discuss the Bianchi type I model with perfect fluid as matter content in f(R,T) gravity, where R is the Ricci scalar and T is the trace of the energy-momentum tensor. We obtain exact solutions of the field equations employing the anisotropic feature of spacetime for two expansion laws namely exponential and power expansions. The physical and kinematical quantities are examined for both cases in future evolution of the universe. We also explore the validity of null energy condition and conclude that our solutions are consistent with the current observations.