Navegando por Autor "Pompeia, P.J."

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Bopp–Podolsky black holes and the no-hair theorem
(SpringerOpen, 2018-01) Medeiros, Léo Gouvêa; Cuzinatto, R.R.; Melo, C.A.M.; Pimentel, B.M.; Pompeia, P.J.
Bopp–Podolsky electrodynamics is generalized to curved space-times. The equations of motion are writ- ten for the case of static spherically symmetric black holes and their exterior solutions are analyzed using Bekenstein’s method. It is shown that the solutions split up into two parts, namely a non-homogeneous (asymptotically massless) regime and a homogeneous (asymptotically massive) sector which is null outside the event horizon. In addition, in the simplest approach to Bopp–Podolsky black holes, the non-homogeneous solutions are found to be Maxwell’s solutions leading to a Reissner–Nordström black hole. It is also demonstrated that the only exterior solution consistent with the weak and null energy conditions is the Maxwell one. Thus, in the light of the energy conditions, it is concluded that only Maxwell modes propagate outside the horizon and, therefore, the no-hair theorem is satisfied in the case of Bopp–Podolsky fields in spherically symmetric space-times
Causal structure and birefringence in nonlinear electrodynamics
(World Scientific Publishing, 2015-02-16) Medeiros, Léo Gouvêa; Melo, C.A.M.; Pompeia, P.J.
We investigate the causal structure of general nonlinear electrodynamics and determine which Lagrangians generate an effective metric conformal to Minkowski. We also prove that there is only one analytic nonlinear electrodynamics not presenting birefringence
f(R,∇μ1R,…,∇μ1…∇μnR) theories of gravity in Einstein frame: a higher order modified Starobinsky inflation model in the Palatini approach
(American Physical Society, 2019-04-30) Medeiros, Léo Gouvêa; Cuzinatto, R.R; Melo, C.A.M.; Pompeia, P.J.
In Cuzinatto et al. [Phys. Rev. D 93, 124034 (2016)], it has been demonstrated that theories of gravity in which the Lagrangian includes terms depending on the scalar curvature R and its derivatives up to order n, i.e., fðR; ∇μR; ∇μ1∇μ2R; ...; ∇μ1 ∇μnRÞ theories of gravity, are equivalent to scalar-multitensorial theories in the Jordan frame. In particular, in the metric and Palatini formalisms, this scalar-multitensorial equivalent scenario shows a structure that resembles that of the Brans-Dicke theories with a kinetic term for the scalar field with ω0 1⁄4 0 or ω0 1⁄4 −3=2, respectively. In the present work, the aforementioned analysis is extended to the Einstein frame. The conformal transformation of the metric characterizing the transformation from Jordan’s to Einstein’s frame is responsible for decoupling the scalar field from the scalar curvature and also for introducing a usual kinetic term for the scalar field in the metric formalism. In the Palatini approach, this kinetic term is absent in the action. Concerning the other tensorial auxiliary fields, they appear in the theory through a generalized potential. As an example, the analysis of an extension of the Starobinsky model (with an extra term proportional to ∇μR∇μR) is performed and the fluid representation for the energy-momentum tensor is considered. In the metric formalism, the presence of the extra term causes the fluid to be an imperfect fluid with a heat flux contribution; on the other hand, in the Palatini formalism the effective energy-momentum tensor for the extended Starobinsky gravity is that of a perfect fluid type. Finally, it is also shown that the extra term in the Palatini formalism represents a dynamical field which is able to generate an inflationary regime without a graceful exit
Observational constraints on a phenomenological f (R,∂ R)-model
(General Relativity And Gravitation, 2015-02-24) Medeiros, Léo Gouvêa; Melo, C.A.M.; Cuzinatto, R.R.; Pompeia, P.J.
This paper analyses the cosmological consequences of a modified theory of gravity whose action integral is built from a linear combination of the Ricci scalar R and a quadratic term in the covariant derivative of R. The resulting Friedmann equations are of the fifth-order in the Hubble function. These equations are solved numerically for a flat space section geometry and pressureless matter. The cosmological parameters of the higher-order model are fit using SN Ia data and X-ray gas mass fraction in galaxy clusters. The best-fit present-day t0 values for the deceleration parameter, jerk and snap are given. The coupling constant β of the model is not univocally determined by the data fit, but partially constrained by it. Density parameter Ωm0 is also determined and shows weak correlation with the other parameters. The model allows for two possible future scenarios: there may be either an eternal expansion or a Rebouncing event depending on the set of values in the space of parameters. The analysis towards the past performed with the best-fit parameters shows that the model is not able to accommodate a matter-dominated stage required to the formation of structure
Scalar-multi-tensorial equivalence for higher order f(R , ∇ μ R , ∇ μ 1 ∇ μ 2 R , … , ∇ μ 1 … ∇ μ n R) theories of gravity
(American Physical Society, 2016-06-13) Medeiros, Léo Gouvêa; Cuzinatto, R.R; Melo, C.A.M.; Pompeia, P.J.
The equivalence between theories depending on the derivatives of R,i.e. f(R,∇ R,…,∇nR), and scalar-multi-tensorial theories is verified. The analysis is done in both metric and Palatini formalisms. It is shown that f(R,∇R,…,∇nR) theories are equivalent to scalar-multi-tensorial ones resembling Brans-Dicke theories with kinetic terms ω0=0 and ω0=−32 for metric and Palatini formalisms respectively. This result is analogous to what happens for f(R) theories. It is worth emphasizing that the scalar-multi-tensorial theories obtained here differ from Brans-Dicke ones due to the presence of multiple tensorial fields absent in the last. Furthermore, sufficient conditions are established for f(R,∇R,…,∇nR) theories to be written as scalar-multi-tensorial theories. Finally, some examples are studied and the comparison of f(R∇R,…,∇nR) theories to f (R,□R,…,□nR) theories is performed

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Navegando por Autor "Pompeia, P.J."