Navegando por Autor "Cuzinatto, R.R."
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Artigo Analytic study of cosmological perturbations in a unified model of dark matter and dark energy with a sharp transition(Elsevier, 2018-12) Medeiros, Léo Gouvêa; Cuzinatto, R.R.; Morais, Eduardo M. de; Brandenberger, Robert H.We study cosmological perturbations in a model of unified dark matter and dark energy with a sharp transition in the late-time universe. The dark sector is described by a dark fluid which evolves from an early stage at redshifts z > zC when it behaves as cold dark matter (CDM) to a late time dark energy (DE) phase (z < zC ) when the equation of state parameter is w = −1 + ,with a constant which must be in the range 0 < < 2/3. We show that fluctuations in the dark energy phase suffer from an exponential instability, the mode functions growing both as a function of comoving momentum k and of conformal time η. In order that this exponential instability does not lead to distortions of the energy density power spectrum on scales for which we have good observational results, the redshift zC of transition between the two phases is constrained to be so close to zero that the model is unable to explain the supernova dataArtigo 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-timesArtigo Cosmic acceleration from second order gauge gravity(Springer, 2010-09-21) Medeiros, Léo Gouvêa; Cuzinatto, R.R.; Melo, C.A.M. de; Pompeia, P. J.We construct a phenomenological theory of gravitation based on a second order gauge formulation for the Lorentz group. The model presents a long-range modification for the gravitational field leading to a cosmological model provided with an accelerated expansion at recent times. We estimate the model parameters using observational data and verify that our estimative for the age of the Universe is of the same magnitude than the one predicted by the standard model. The transition from the decelerated expansion regime to the accelerated one occurs recently (at ∼9.3 Gyr)Artigo De Broglie-Proca and Bopp-Podolsky massive photon gases in cosmology(IOP Publishing, 2017-04-01) Medeiros, Léo Gouvêa; Cuzinatto, R.R.; Morais, Eduardo M. de; Souza, Clicia Naldoni de; Pimentel, B. M.We investigate the influence of massive photons on the evolution of the expanding universe. Two particular models for generalized electrodynamics are considered, namely de Broglie-Proca and Bopp-Podolsky electrodynamics. We obtain the equation of state (EOS) $P=P(\varepsilon)$ for each case using dispersion relations derived from both theories. The EOS are inputted into the Friedmann equations of a homogeneous and isotropic space-time to determine the cosmic scale factor a(t). It is shown that the photon non-null mass does not significantly alter the result $a\propto t^{1/2}$ valid for a massless photon gas; this is true either in de Broglie-Proca's case (where the photon mass m is extremely small) or in Bopp-Podolsky theory (for which m is extremely large)Artigo How can one probe podolsky electrodynamics?(World Scientific Publishing, 2011-08-20) Medeiros, Léo Gouvêa; Cuzinatto, R.R.; Melo, C. A. M. de; Pompeia, P. J.We investigate the possibility of detecting the Podolsky generalized electrodynamics constant a. First we analyze an ion interferometry apparatus proposed by B. Neyenhuis et al. (Phys. Rev. Lett. 99, 200401 (2007)), who looked for deviations from Coulomb’s inverse-square law in the context of Proca model. Our results show that this experiment has not enough precision for measurements of a. In order to set up bounds for a, we investigate the influence of Podolsky’s electrostatic potential on the ground state of the Hydrogen atom. The value of the ground state energy of the Hydrogen atom requires Podolsky’s constant to be smaller than 5.6 fm, or in energy scales larger than 35.51 MeVArtigo Non-linear effects on radiation propagation around a charged compact object(Springer, 2015-09-23) Medeiros, Léo Gouvêa; Cuzinatto, R.R.; Melo, C. A. M. de; Vasconcelos, K. C. de; Pompeia, P. J.The propagation of non-linear electromagnetic waves is carefully analyzed on a curved spacetime created by static spherically symmetric mass and charge distribution. We compute how non-linear electrodynamics affects the geodesic deviation and the redshift of photons propagating near this massive charged object. In the first order approximation, the effects of electromagnetic self-interaction can be distinguished from the usual Reissner–Nordström terms. In the particular case of Euler–Heisenberg effective Lagrangian, we find that these self-interaction effects might be important near extremal compact charged objectsArtigo 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 structureArtigo Observational constraints to a unified cosmological model(Elsevier, 2016-01-15) Medeiros, Léo Gouvêa; Cuzinatto, R.R.; Morais, Eduardo Messias deWe propose a phenomenological unified model (UM) for dark matter and dark energy based on an equation of state parameter w that scales with the of the redshift. The free parameters of the model are three constants: Ωb0, α and β. Parameter α dictates the transition rate between the matter dominated era and the accelerated expansion period. The ratio β/α gives the redshift of the equivalence between both regimes. Cosmological parameters are fixed by observational data from primordial nucleosynthesis (PN), supernovae of the type Ia (SNIa), gamma-ray bursts (GRBs) and baryon acoustic oscillations (BAOs). The calibration of the 138 GRB events is performed using the 580 SNIa of the Union2.1 data set and a new set of 79 high-redshift GRB is obtained. The various sets of data are used in different combinations to constraint the parameters through statistical analysis. The UM is compared to the ΛCDM model and their differences are emphasized