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Breton N.,Research Center Estudios Avanzados Del | Bergliaffa S.E.P.,State University of Rio de Janeiro
Annals of Physics | Year: 2015

We analyze the thermodynamical stability of static and spherically symmetric black hole solutions with nonlinear electromagnetism as a source, and show that any sequence of such black holes in isolation that includes the Schwarzschild black hole is stable in the (βM, M) plane for any nonlinear Lagrangian describing the electromagnetic field. The study of three exact solutions (which include the Schwarzschild solution in some limit) in the (βQ, Q) plane show that they are stable in the microcanonical ensemble, and unstable or less unstable (due to the existence of a turning point) in the canonical ensemble. If the less unstable configurations are stable, our results indicate that they would be in equilibrium with a reservoir at a higher temperature than the corresponding Reissner-Nordstrom configuration. An expression for the heat capacity at constant charge valid for any Lagrangian describing nonlinear electromagnetism is also presented. It displays a divergence with a change of sign that occurs precisely at the turning point obtained by the Poincarè method. © 2015 Elsevier Inc.

Torres Martinez D.Y.,CINVESTAV | Castanedo Perez R.,CINVESTAV | Torres Delgado G.,CINVESTAV | Zelaya Angel O.,Research Center Estudios Avanzados Del
Journal of Photochemistry and Photobiology A: Chemistry | Year: 2012

Thin films of the mixed ZnO-SnO 2 system were obtained by employing the sol-gel technique, starting from the mixture of zinc oxide and tin oxide precursor solutions. The tin atomic concentration percentages (X) in the precursor solutions with respect to the ZnO + SnO 2 total growing solution were: 0, 10, 20, 30, 40 and 100 at.%, here, the ZnO atomic percentage is (100-X). The films were deposited by the dip coating method on glass substrates and were sintered at 450, 500 and 550 °C, in an open atmosphere. The X-ray diffraction patterns show the presence of zinc oxide and tin oxide crystals. The optical properties of the films were studied by UV-vis spectroscopy. The photocatalytic activity was quantified by means of the bleaching of methylene blue. The films of the mixed ZnO-SnO 2 system show higher photocatalytic activity than the SnO 2 and ZnO films. © 2012 Elsevier B.V.

Lazkoz R.,University of the Basque Country | Montiel A.,Research Center Estudios Avanzados Del | Salzano V.,University of the Basque Country
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

In this paper we set observational constraints of the superfluid Chaplygin gas model, which gives a unified description of the dark sector of the Universe as a Bose-Einstein condensate that behaves as dark energy while it is in the ground state and as dark matter when it is in the excited state. We first show and perform the various steps leading to a form of the equations suitable for the observational tests to be carried out. Then, by using a Markov Chain MonteCarlo code, we constrain the model with a sample of cosmology-independent long gamma-ray bursts calibrated using their type I fundamental plane, as well as the Union2.1 set and observational Hubble parameter data. In this analysis, using our cosmological constraints, we sketch the effective equation of state parameter and the deceleration parameter, and we also obtain the redshift of the transition from deceleration to acceleration z t. © 2012 American Physical Society.

Montiel A.,Research Center Estudios Avanzados Del | Lazkoz R.,University of the Basque Country | Sendra I.,University of the Basque Country | Escamilla-Rivera C.,University of the Basque Country | Salzano V.,University of the Basque Country
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

In this work we present a nonparametric approach, which works on minimal assumptions, to reconstruct the cosmic expansion of the Universe. We propose to combine a locally weighted scatterplot smoothing method and a simulation-extrapolation method. The first one (LOESS) is a nonparametric approach that allows us to obtain smoothed curves with no prior knowledge of the functional relationship between variables or of the cosmological quantities. The second one (SIMEX) takes into account the effect of measurement errors on a variable via a simulation process. For the reconstructions we use as raw data the Union2.1 type Ia supernovae compilation, as well as recent Hubble parameter measurements. This work aims to illustrate the approach, which turns out to be a self-sufficient technique in the sense that we do not have to choose anything by hand. We examine the details of the method, among them the amount of observational data needed to perform the locally weighted fit which will define the robustness of our reconstruction. In view of our results, we believe that our proposal offers a promising alternative for reconstructing global trends of cosmological data when there is little intuition on the relationship between the variables and we also think it even presents good prospects to generate reliable mock data points where the original sample is poor. © 2014 American Physical Society.

Velten H.,Bielefeld University | Montiel A.,Research Center Estudios Avanzados Del | Carneiro S.,Federal University of Bahia
Monthly Notices of the Royal Astronomical Society | Year: 2013

In previous papers, a cosmological model with constant rate particle creation and vacuum term decaying linearly with the Hubble parameter was shown to lead to a good concordance when tested against precise observations: the position of the first peak in the spectrum of anisotropies of the cosmic microwave background (CMB), the Hubble diagram for Type Ia supernovae (SNe Ia), the distribution of large-scale structures and the distance to the baryonic acoustic oscillations (BAO). That model has the same number of parameters as the spatially flat standard model and seems to alleviate some observational/theoretical tensions appearing in the latter. In this paper, we complement those tests with 109 gamma-ray bursts (GRBs), 59 of them with redshifts above z = 1.4, which permits to extend the Hubble diagram to redshifts up to z ≈ 8. For the calibration of the 50 GRBs with z < 1.4, we use the 288 supernovae of the Sloan Digital Sky Survey project, calibrated with the MLCS2k2 fitter, less model-dependent than other samples like Union2. Our results show a good concordance with the previous tests and, again, less tensions between SNe Ia and GRB best fits as compared to the standard model. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

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