Anisotropic fluid for a set of non-diagonal tetrads in f(T) gravity

We consider $f(T)$ gravity for a Weitzenbock spherically symmetric and static spacetime, where the metric is projected in the tangent space to the manifold for a set of non-diagonal tetrads. The matter content is coupled through the energy momentum tensor of an anisotropic fluid, generating various classes of new black hole and wormhole solutions. One of these classes is that of cold black holes. We also perform the reconstruction scheme of the algebraic function $f(T)$ for two cases where the radial pressure is proportional to $f(T)$ and its first derivative.Comment: 12 pages. Accepted for publication in Physics Letters B (PLB

Fonctionnement et gestion des écosystèmes forestiers contractés sahéliens

La perception des formations végétales contractées par télédétection, dépend de la taille et de la forme des structures au sol, et des résolutions spatiales et spectrales des capteurs utilisés. L'objectif du présent travail est de comparer les capacités des satellites à haute définition actuels (résolution de 10 à 80 m), et des données aériennes (photographie et vidéographie) appliquées aux plateaux de l'ouest nigérien. Un indice de végétation combinant les données satellitaires SPOT multispectrales et panchromatiques est proposé. Il représente le meilleur compromis entre résolution spatiale et résolution spectrale. Une cartographie automatique des différentes formations contractées a été testée, à partir de la définition de fenêtres glissantes de 10 à 20 pixels de côté et des proportions de pixels en "sols nus" et en "végétation" caractéristiques de chaque type. Une validation au sol, réalisée par ailleurs sur 6 transects perpendiculaires à l'allongement des structures, a permis de proposer une relation entre indice et végétation et recouvrement. Un test effectué sur une zone de 25 km2 montre qu'il est possible de réaliser aisément et de façon fiable une extension spatiale des résultats. La méthode proposée constitue pour les services forestiers nationaux, un outil de suivi et de gestion des aires de production de bois. (Résumé d'auteur

Impulsive Fractional Differential Inclusions Involving the Caputo Fractional Derivative

Mathematics Subject Classification: 26A33, 34A37.In this paper, we establish sufficient conditions for the existence of solutions for a class of initial value problem for impulsive fractional differential inclusions involving the Caputo fractional derivative. Both cases of convex and nonconvex valued right-hand side are considered. The topological structure of the set of solutions is also considered

Late time cosmological approach in mimetic f(R,T)f(R,T) gravity

In this paper, we investigate the late-time cosmic acceleration in mimetic $f(R,T)$ gravity with Lagrange multiplier and potential in a Universe containing, besides radiation and dark energy, a self-interacting (collisional) matter. We obtain through the modified Friedmann equations, the main equation that can describe the cosmological evolution and with several models from $Q(z)$ and the well known particular model $f(R, T)$, we perform an analysis of the late-time evolution. We examine the behavior of the Hubble parameter, the dark energy equation of state and the total effective equation of state and we compare in each case the resulting picture with the non-collisional matter (assumed as dust) and also with the collisional matter in mimetic $f(R, T)$ gravity. The results obtained are in good agreement with the observational data and show that in presence of the collisional matter the dark energy oscillations in mimetic f(R, T) gravity can be damped.Comment: 18 pages, 2 figure

Functional differential inclusions with integral boundary conditions

In this paper, we investigate the existence of solutions for a class of second order functional differential inclusions with integral boundary conditions. By using suitable fixed point theorems, we study the case when the right hand side has convex as well as nonconvex values

Late time cosmological approach in mimetic f(R,T)f(R,T) gravity

In this paper, we investigate the late-time cosmic acceleration in mimetic $f(R,T)$ gravity with Lagrange multiplier and potential in a Universe containing, besides radiation and dark energy, a self-interacting (collisional) matter. We obtain through the modified Friedmann equations, the main equation that can describe the cosmological evolution and with several models from $Q(z)$ and the well known particular model $f(R, T)$, we perform an analysis of the late-time evolution. We examine the behavior of the Hubble parameter, the dark energy equation of state and the total effective equation of state and we compare in each case the resulting picture with the non-collisional matter (assumed as dust) and also with the collisional matter in mimetic $f(R, T)$ gravity. The results obtained are in good agreement with the observational data and show that in presence of the collisional matter the dark energy oscillations in mimetic f(R, T) gravity can be damped.Comment: 18 pages, 2 figure

Extended Birkhoff's Theorem in the f(T) Gravity

The f(T) theory, a generally modified teleparallel gravity, has been proposed as an alternative gravity model to account for the dark energy phenomena. Following our previous work [Xin-he Meng and Ying-bin Wang, EPJC(2011), arXiv:1107.0629v1], we prove that the Birkhoff's theorem holds in a more general context, specifically with the off diagonal tetrad case, in this communication letter. Then, we discuss respectively the results of the external vacuum and internal gravitational field in the f(T) gravity framework, as well as the extended meaning of this theorem. We also investigate the validity of the Birkhoff's theorem in the frame of f(T) gravity via conformal transformation by regarding the Brans-Dicke-like scalar as effective matter, and study the equivalence between both Einstein frame and Jordan frame.Comment: 7 pages, 1 figure, submitted to EPJ-C. arXiv admin note: substantial text overlap with arXiv:1107.062

Birkhoff's Theorem in f(T) Gravity up to the Perturbative Order

f(T) gravity, a generally modified teleparallel gravity, has become very popular in recent times as it is able to reproduce the unification of inflation and late-time acceleration without the need of a dark energy component or an inflation field. In this present work, we investigate specifically the range of validity of Birkhoff's theorem with the general tetrad field via perturbative approach. At zero order, Birkhoff's theorem is valid and the solution is the well known Schwarzschild-(A)dS metric. Then considering the special case of the diagonal tetrad field, we present a new spherically symmetric solution in the frame of f(T) gravity up to the perturbative order. The results with the diagonal tetrad field satisfy the physical equivalence between the Jordan and the so-called Einstein frames, which are realized via conformal transformation, at least up to the first perturbative order.Comment: 8 pages, no figure. Final version, accepted for publication in EPJ

Reconstruction of some cosmological models in f(R,T) gravity

In this paper, we reconstruct cosmological models in the framework of $f(R,T)$ gravity, where $R$ is the Ricci scalar and $T$ is the trace of the stress-energy tensor. We show that the dust fluid reproduces $\Lambda$CDM, phantom-non-phantom era and the phantom cosmology. Further, we reconstruct different cosmological models including, Chaplygin gas, scalar field with some specific forms of $f(R,T)$. Our numerical simulation for Hubble parameter shows good agreement with the BAO observational data for low redshifts $z<2$.Comment: 12 pages, 2 figure

New Black Holes Solutions in a Modified Gravity

We present a theory of modified gravity, inspired by the gauge theories, where the commutator algebra of covariant derivative gives us an added term with respect to the General Relativity, which represents the interaction of gravity with a substratum. New spherically symmetric solutions of this theory are obtained and can be view as solutions that reproduce, the mass, the charge, the cosmological constant and the Rindler acceleration, without coupling with the matter content, i.e., in the vacuum.Comment: 15 pages, accepted for publication in ISRN Astronomy and Astrophysic