J. P. Morais Graça

Generalized entropy formalism and a new holographic dark energy model

Abstract Recently, the Renyi and Tsallis generalized entropies have extensively been used in order to study various cosmological and gravitational setups. Here, using a special type of generalized entropy, a generalization of both the Renyi and Tsallis entropy, together with holographic principle, we build a new model for holographic dark energy. Thereinafter, considering a flat FRW universe, filled by a pressureless component and the new obtained dark energy model, the evolution of cosmos has been investigated showing satisfactory results and behavior. In our model, the Hubble horizon plays the role of IR cutoff, and there is no mutual interaction between the cosmos components. Our results indicate that the generalized entropy formalism may open a new window to become more familiar with the nature of spacetime and its properties.

Quasinormal modes of a black hole with a cloud of strings in Einstein-Gauss-Bonnet gravity

The quasinormal modes for a scalar field in the background spacetime corresponding to a black hole, with a cloud of strings, in Einstein-Gauss-Bonnet gravity, and the tensor quasinormal modes corresponding to perturbations in such spacetime, were both calculated using the WKB approximation. In the obtained results we emphasize the role played by the parameter associated with the string cloud, comparing them with the results already obtained for the Boulware-Deser metric. We also study how the Gauss-Bonnet correction to general relativity affects the results for the quasinormal modes, comparing them with the same background in general relativity.

Thermodynamics of black holes in Rastall gravity

A promising theory in modifying general relativity by violating the ordinary energy-momentum conservation law in curved spacetime is the Rastall theory of gravity. In this theory, geometry and matter fields are coupled to each other in a non-minimal way. Here, we study thermodynamic properties of some black hole solutions in this framework, and compare our results with those of general relativity. We demonstrate how the presence of these matter sources may amplify effects caused by the Rastall parameter in thermodynamic quantities. Our investigation also shows that black holes with radius smaller than a certain amount (

Scalar and spinor QNMs of a black hole with a global monopole in f ( R ) gravity

equiv r_0

Scalar QNMs for higher dimensional black holes surrounded by quintessence in Rastall gravity

) have negative heat capacity in the Rastall framework. In fact, it is a lower bound for the possible values of horizon radius satisfied by stable black holes.

Cloud of strings in

The quasinormal modes for scalar and spinor fields in the background spacetime corresponding to an f(R) global monopole are calculated using the WKB approximation. In the obtained results we emphasize the role played by the parameter ψ0, associated with the f(R) gravity. We discuss the appropriate limit ψ0 → 0, in which case the results concerning to the Schwarzschild black hole are obtained, as it should be expected. ar X iv :1 51 0. 07 18 4v 1 [ gr -q c] 2 4 O ct 2 01 5 Scalar and spinor quasinormal modes of an f(R) global monopole 2

f(R)

The spacetime solution for a black hole, surrounded by an exotic matter field, in Rastall gravity, is calculated in an arbitrary d-dimensional spacetime. After this, we calculate the scalar quasinormal modes of such solution, and study the shift on the modes caused by the modification of the theory of gravity, i.e., by the introduction of a new term due to Rastall. We conclude that the shift strongly depends on the kind of exotic field one is studying, but for a low density matter that supposedly pervades the universe, it is unlikely that Rastall gravity will cause an instability for the probe field.

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.

Non-Abelian cosmic string in the Starobinsky model of gravity

In this paper, we analyze numerically the behaviour of the solutions corresponding to a non-Abelian cosmic string in the framework of the Starobinsky model, i.e. where

Effects of a string cloud on the criticality and efficiency of AdS black holes as heat engines

f(R)=R + zeta R^2