On wormhole throats in f (R) gravity theory

We study the existence and properties of wormhole throats in modified f (R) gravity theory. Specifically, we concentrate on the cases where the lapse is not necessarily constant, and hence are not limited to the zero tidal force scenarios. In the class of theories whose actions are generated by Lagrangians of the form f (R)?=?∑ α _n ?R ⁿ we find parameters which allow for the existence of energy condition respecting throats, which do not exist in Einstein gravity. We also consider the effect of the modified action on the anisotropy of the models, and find that modified gravity can minimize the amount of anisotropy required to support the existence of a throat. In both these respects, the sector containing theories with positive n is more promising than the negative n sector in comparison to Einstein gravity alone, with large n being most favorable.     

Vacuum spherically symmetric solutions in f(T) gravity

Spherically symmetric static vacuum solutions have been built in f(T) models of gravity theory. We apply some conditions on the metric components; then new vacuum spherically symmetric solutions are obtained. Also, by extracting metric coefficients we determine the analytical form of f(T).     

Non-exotic static spherically symmetric thin-shell wormhole solution in f(Q,T ) gravity

In this study, we conduct an analysis of traversable wormhole solutions within the framework of linear f(Q, T) = αQ + βT gravity, ensuring that all energy conditions hold for the entire spacetime. The solutions presented in this paper are derived through a comprehensive analytical examination of the parameter space associated with the wormhole model. This involves considering the exponents governing the redshift and shape functions, as well as the radius of the wormhole throat( r₀), t...     

Pilgrim dark energy in f(T) gravity

We discuss the interacting f(T) gravity with pressureless matter in an FRW spacetime. We construct an f(T) model by following the correspondence scheme incorporating a recently developed pilgrim dark energy model and taking the Hubble horizon as the IR cutoff. We use constructed model to discuss the evolution trajectories of the equation-of-state parameter, the ω _T -ω′ _T phase plane, and state-finder parameters in the evolving universe. It is found that the equation-of-state parameter gives a phantom era of the accelerated universe for some particular range of the pilgrim parameter. The ω _T -ω′ _T plane represents freezing regions only for an interacting framework, while the ΛCDM limit is attained in the state-finder plane. We also investigate the first and second laws of thermodynamics assuming equal temperatures at and inside the horizon in this scenario. Due to the violation of the first law of thermodynamics in f(T) gravity, we explore the behavior of the entropy production term. The validity of a generalized second law of thermodynamics depends on the present-day value of the Hubble parameter.     

Slow-roll inflation in f(T,T ) modified gravity

In this study, we explore the concept of cosmological inflation within the framework of the f(T, T)theory of gravity, where f is a general function of the torsion scalar T and the trace T of the energy-momentum tensor.It is assumed that the conditions of slow-roll inflation are applicable in f(T, T) gravity. To determine different observables related to inflation, such as the tensor-to-scalar ratio r, scalar spectral index ns, spectral index αs, and tensor spectral index nt, the Hubble slow-roll...     

Magnetized strange quark matter solutions in f(R,T) gravity with cosmological constant

In this research, we have studied magnetized strange quark matter (SQM) solutions for Friedmann-Robertson-Walker (FRW) universe in f(R, T) gravity. To obtain exact solutions of modified field equations we have used $f(R,T)=R+2f\left(T\right)$ and $f(R,T)=f_1\left(R\right)+f_2\left(T\right)$ models given by Harko et al. (Harko et al. in Phys. Rev. D 84:024020, 2011) and $f(R,T)=R+f_3\left(T\right)$ model (here f₃ is an arbitrary function) with cosmological constant Λ. For t → ∞ we obtain $p=?\rho$ dark energy situation with small constant values of cosmological constant in three different f(R, T) gravitation models. In our solutions magnetic field does not observe also we have transformed our solutions from FRW universe to Static Einstein Universe (SEU) and we get f(R, T) gravity results for SEU universe. Finally we discussed our physical solutions.     

Attractor solutions in f(T) cosmology

We present the results of three-dimensional simulations of quasar polarizations in the presence of pseudoscalar?Cphoton mixing in the intergalactic medium. The intergalactic magnetic field is assumed to be uncorrelated in wave vector space but correlated in real space. Such a field may be obtained if its origin is primordial. Furthermore we assume that the quasars, located at cosmological distances, have negligible initial polarization. In the presence of pseudoscalar?Cphoton mixing we show, through a direct comparison with observations, that this may explain the observed large scale alignments in quasar polarizations within the framework of big bang cosmology. We find that the simulation results give a reasonably good fit to the observed data.     

Non-static plane symmetric perfect fluid solutions and Killing symmetries in f(R,T)gravity

In this paper, the non-static solutions for perfect fluid distribution with plane symmetry in f（R, T）gravitational theory are obtained. Firstly, using the Lie symmetries, symmetry reductions are performed for considered vector fields to reduce the number of independent variables. Then,corresponding to each reduction, exact solutions are obtained. Killing vectors lead to different conserved quantities. Therefore, we figure out the Killing vector fields corresponding to all derived solutions. The derived solutions are further studied and it is observed that all of the obtained spacetimes, at least admit to the minimal symmetry group which consists of ?_y, ?_z and-z?_y+ y?_z. The obtained metrics, admit to 3, 4, 6, and 10, Killing vector fields. Conservation of linear momentum in the direction of y and z, and angular momentum along the x axis is provided by all derived solutions.     

Thin-shell gravastar model in f(Q,T) gravity

In the last few decades,gravastars have been proposed as an alternative to black holes.The stability of a gravastar has been examined in many modified theories of gravity along with Einstein s GR.The f(Q,T) gravity,a successfully modified theory of gravity for describing the current accelerated expansion of the universe,has been used in this study to examine gravastar in different aspects.According to Mazur and Mottola [Proc.Natl.Acad.Sci.101,9545(2004);Gravitational condensate stars:An alternat...     

Geodesic deviation equation in f(R,T) gravity

In this paper, we investigate the modified Geodesic Deviation Equation (GDE) in the framework of f(R, T) theory of gravity where R and T are the curvature scalar and the trace of the energy-momentum tensor, respectively, using the FLRW background. In this way, we obtain the GR equivalent (GDE) in f(R, T) metric formalism. We also extend our work to the generalization of the Matting relation and perform the numerical analysis with GDE for null vector.     

Bianchi type-I cosmology in f(R,T) gravity

We investigate the exact solutions of a Bianchi type-I space-time in the context of f(R, T) gravity [1], where f(R, T) is an arbitrary function of the Ricci scalar R and the trace of the energy-momentum tensor T. For this purpose, we find two exact solutions using the assumption of a constant deceleration parameter and the variation law of the Hubble parameter. The obtained solutions correspond to two different models of the Universe. The physical behavior of these models is also discussed.     

A special exact spherically symmetric solution in f(T) gravity theories

A non-diagonal spherically symmetric tetrad field, involving four unknown functions of radial coordinate $r$ r , is applied to the equations of motion of f(T) gravity theory. A special exact vacuum solution with one constant of integration is obtained. The scalar torsion related to this special solution vanishes. To understand the physical meaning of the constant of integration we calculate the energy associated with this solution and show how it is related to the gravitational mass of the system.     

Magnetic strings in f(R) gravity

We construct a new class of spinning magnetic string solutions in f(R) gravity with constant scalar curvature. These solutions which produce a longitudinal magnetic field have no curvature singularity and no horizon, but have a conic geometry with a deficit angle. We also generalize this class of solutions to the case of spinning magnetic solutions with one rotation parameter. We find that the spinning string has a net electric charge which is proportional to the rotation parameter. With choosing a suitable counterterm, we remove the divergences of the action. The conserved quantities of the solutions are also calculated by using the counterterm method.     

Constrains on f(T) gravity with the strong gravitational lensing data

Strong lensing is an effective way to probing the properties of dark energy.In this paper,we use the strong lensing data to constrain the f(T)theory,which is a new modified gravity to explain the present accelerating cosmic expansion without the need of dark energy.In our discussion,the CMB and BAO data are also added to constrain model parameters tightly and three different f(T)models are studied.We find that strong lensing has an important role on constraining f(T)models,and once the CMB+BAO data is added,a tighter constraint is obtained.However,the consistency of our result with what is obtained from SNIa+CMB+BAO is actually model-dependent.     

Anisotropic compact stars in f(R) gravity

The European Physical Journal C - We generalize our previous studies on the Maxwell quasinormal modes around Schwarzschild-anti-de-Sitter black holes with Robin type vanishing energy flux boundary...     

Cloud of strings in f(R) gravity

We derive the solution for a spherically symmetric string cloud configuration in a d-dimensional spacetime in the framework of f(R) theories of gravity. We also analyze some thermodynamic properties of the joint black hole-cloud of strings solution. For its Hawking temperature, we found that the dependence of the mass with the horizon is significantly different in both theories. For the interaction of a black hole with thermal radiation, we found that the shapes of the curves are similar, but shifted. Our analysis generalizes some known results in the literature.