Transition from decelerated to accelerated cosmic expansion in braneworld universes

Braneworld theory provides a natural setting to treat, at a classical level, the cosmological effects of vacuum energy. Non-static extra dimensions can generally lead to a variable vacuum energy, which in turn may explain the present accelerated cosmic expansion. We concentrate our attention in models where the vacuum energy decreases as an inverse power law of the scale factor. These models agree with the observed accelerating universe, while fitting simultaneously the observational data for the density and deceleration parameter. The redshift at which the vacuum energy can start to dominate depends on the mass density of ordinary matter. For Omega = 0.3, the transition from decelerated to accelerated cosmic expansion occurs at z approx 0.48 +/- 0.20, which is compatible with SNe data. We set a lower bound on the deceleration parameter today, namely q > - 1 + 3 Omega/2, i.e., q > - 0.55 for Omega = 0.3. The future evolution of the universe crucially depends on the time when vacuum starts to dominate over ordinary matter. If it dominates only recently, at an epoch z < 0.64, then the universe is accelerating today and will continue that way forever. If vacuum dominates earlier, at z > 0.64, then the deceleration comes back and the universe recollapses at some point in the distant future. In the first case, quintessence and Cardassian expansion can be formally interpreted as the low energy limit of our model, although they are entirely different in philosophy. In the second case there is no correspondence between these models and ours.Comment: In V2 typos are corrected and one reference is added for section 1. To appear in General Relativity and Gravitatio

Effective spacetime from multi-dimensional gravity

We study the effective spacetimes in lower dimensions that can be extracted from a multidimensional generalization of the Schwarzschild-Tangherlini spacetimes derived by Fadeev, Ivashchuk and Melnikov ({\it Phys. Lett,} {\bf A 161} (1991) 98). The higher-dimensional spacetime has $D = (4 + n + m)$ dimensions, where $n$ and $m$ are the number of "internal" and "external" extra dimensions, respectively. We analyze the effective $(4 + n)$ spacetime obtained after dimensional reduction of the $m$ external dimensions. We find that when the $m$ extra dimensions are compact (i) the physics in lower dimensions is independent of $m$ and the character of the singularities in higher dimensions, and (ii) the total gravitational mass $M$ of the effective matter distribution is less than the Schwarzshild mass. In contrast, when the $m$ extra dimensions are large this is not so; the physics in $(4 + n)$ does explicitly depend on $m$, as well as on the nature of the singularities in high dimensions, and the mass of the effective matter distribution (with the exception of wormhole-like distributions) is bigger than the Schwarzshild mass. These results may be relevant to observations for an experimental/observational test of the theory.Comment: A typo in Eq. (24) is fixe

Stars in five dimensional Kaluza Klein gravity

In the five dimensional Kaluza Klein (KK) theory there is a well known class of static and electromagnetic--free KK--equations characterized by a naked singularity behavior, namely the Generalized Schwarzschild solution (GSS). We present here a set of interior solutions of five dimensional KK--equations. These equations have been numerically integrated to match the GSS in the vacuum. The solutions are candidates to describe the possible interior perfect fluid source of the exterior GSS metric and thus they can be models for stars for static, neutral astrophysical objects in the ordinary (four dimensional) spacetime.Comment: 15 pages, 8 figures. To be published in EPJ

Ensembles de classificadores para bases de dados desbalanceadas: uma abordagem baseada em amostragem evolucionária

Em muitos problemas práticos de classificação, o conjunto de dados a ser utilizado para a indução do classificador é significativamente desbalanceado. Isso ocorre quando a quantidade de exemplos de determinada classe é muito inferior à(s) da(s) outra(s) classe(s). Conjuntos de dados desbalanceados podem comprometer o desempenho da maioria dos algoritmos clássicos de classificação, uma vez que estes assumem uma distribuição de exemplos equilibrada entre as classes. Por outro lado, em diferentes cenários de aplicação, a estratégia de combinar vários classificadores em estruturas conhecidas como ensembles tem se mostrado bastante eficaz, levando a uma acurácia preditiva estável e, muitas vezes, superior àquela obtida por um classificador isoladamente. Nesse contexto, este trabalho propõe uma nova abordagem para lidar com conjuntos de dados desbalanceados, a qual utiliza ensembles de classificadores induzidos a partir de amostras balanceadas do conjunto de dados original. Para tanto, utiliza-se algoritmo genético multiobjetivo, que evolui a combinação dos exemplos que compõe as amostras balanceadas, levando em consideração a diversidade e o valor da área sob a curva ROC (AUC) dos classificadores induzidos por estas amostras.FAPES

SO(n + 1) Symmetric Solutions of the Einstein Equations in Higher Dimensions

A method of solving the Einstein equations with a scalar field is presented. It is applied to find higher dimensional vacuum metrics invariant under the group SO(n + 1) acting on n-dimensional spheres.Comment: 11 page

Reconstruction of Five-dimensional Bounce cosmological Models From Deceleration Factor

In this paper, we consider a class of five-dimensional Ricci-flat vacuum solutions, which contain two arbitrary functions $\mu(t)$ and $\nu(t)$. It is shown that $\mu(t)$ can be rewritten as a new arbitrary function $f(z)$ in terms of redshift $z$ and the $f(z)$ can be determined by choosing particular deceleration parameters $q(z)$ which gives early deceleration and late time acceleration. In this way, the $5D$ cosmological model can be reconstructed and the evolution of the universe can be determined.Comment: 5 pages, 1 figure, to be published in IJT

Anisotropic Brane Cosmology with Variable GG and Λ\Lambda

In this work, the cosmological implications of brane world scenario are investigated when the gravitational coupling $G$ and the cosmological term $\Lambda$ are not constant but rather there are time variation of them. From observational point of view, these time variations are taken in the form $\frac{\dot{G}}{G}\sim H$ and $\Lambda \sim H^{2}$. The behavior of scale factors and different kinematical parameters are investigated for different possible scenarios where the bulk cosmological constant $\Lambda_{5}$ can be zero, positive or negative.Comment: RevTex, 7 figures, 16 page

Charged membrane as a source for repulsive gravity

We demonstrate an alternative (with respect to the ones existing in literature) and more habitual for physicists derivation of exact solution of the Einstein-Maxwell equations for the motion of a charged spherical membrane with tangential tension. We stress that the physically acceptable range of parameters for which the static and stable state of the membrane producing the Reissner-Nordstrom (RN) repulsive gravity effect exists. The concrete realization of such state for the Nambu-Goto membrane is described. The point is that membrane are able to cut out the central naked singularity region and at the same time to join in appropriate way the RN repulsive region. As result we have a model of an everywhere-regular material source exhibiting a repulsive gravitational force in the vicinity of its surface: this construction gives a more sensible physical status to the RN solution in the naked singularity case.Comment: Accepted for publication in IJMPD, 17-07-2008; 16 pages, 1 figur

An evolutionary sampling approach for classification with imbalanced data

In some practical classification problems in which the number of instances of a particular class is much lower/higher than the instances of the other classes, one commonly adopted strategy is to train the classifier over a small, balanced portion of the training data set. Although straightforward, this procedure may discard instances that could be important for the better discrimination of the classes, affecting the performance of the resulting classifier. To address this problem more properly, in this paper we present MOGASamp (after Multiobjective Genetic Sampling) as an adaptive approach that evolves a set of samples of the training data set to induce classifiers with optimized predictive performance. More specifically, MOGASamp evolves balanced portions of the data set as individuals of a multi objective genetic algorithm aiming at achieving a set of induced classifiers with high levels of diversity and accuracy. Through experiments involving eight binary classification problems with varying levels of class imbalancement, the performance of MOGASamp is compared against the performance of six traditional methods. The overall results show that the proposed method have achieved a noticeable performance in terms of accuracy measures.FAPESPCAPESCNP

Self-similar cosmologies in 5D: Our universe as a topological separation from an empty 5D Minkowski space

In this paper we find the most general self-similar, homogeneous and isotropic, Ricci flat cosmologies in 5D. These cosmologies show a number of interesting features: (i) the field equations allow a complete integration in terms of one arbitrary function of the similarity variable, and a free parameter; (ii) the three-dimensional spatial surfaces are flat; (iii) the extra dimension is spacelike; (iv) the general solution is Riemann-flat in 5D but curved in 4D, which means that an observer confined to 4D spacetime can relate this curvature to the presence of matter, as determined by the Einstein equations in 4D. We show that these cosmologies can be interpreted, or used, as 5D Riemann-flat embeddings for spatially-flat FRW cosmologies in 4D. In this interpretation our universe arises as a topological separation from an empty 5D Minkowski space, as envisioned by Zeldovich