Cosmic acceleration: Inhomogeneity versus vacuum energy

In this essay, I present an alternative explanation for the cosmic acceleration which appears as a consequence of recent high redshift Supernova data. In the usual interpretation, this cosmic acceleration is explained by the presence of a positive cosmological constant or vacuum energy, in the background of Friedmann models. Instead, I will consider a Local Rotational Symmetric (LRS) inhomogeneous spacetime, with a barotropic equation of state for the cosmic matter. Within this framework the kinematical acceleration of the cosmic fluid or, equivalently, the inhomogeneity of matter, is just the responsible of the SNe Ia measured cosmic acceleration. Although in our model the Cosmological Principle is relaxed, it maintains local isotropy about our worldline in agreement with the CBR experiments.Comment: LATEX, 7 pags, no figs, Honorable Mention in the 1999 Essay Competition of the Gravity Research Foundatio

Bright and dark breathers in Fermi-Pasta-Ulam lattices

In this paper we study the existence and linear stability of bright and dark breathers in one-dimensional FPU lattices. On the one hand, we test the range of validity of a recent breathers existence proof [G. James, {\em C. R. Acad. Sci. Paris}, 332, Ser. 1, pp. 581 (2001)] using numerical computations. Approximate analytical expressions for small amplitude bright and dark breathers are found to fit very well exact numerical solutions even far from the top of the phonon band. On the other hand, we study numerically large amplitude breathers non predicted in the above cited reference. In particular, for a class of asymmetric FPU potentials we find an energy threshold for the existence of exact discrete breathers, which is a relatively unexplored phenomenon in one-dimensional lattices. Bright and dark breathers superposed on a uniformly stressed static configuration are also investigated.Comment: 11 pages, 16 figure

Complex Scalar DM in a B-L Model

In this work, we implement a complex scalar Dark Matter (DM) candidate in a $U(1)_{B-L}$ gauge extension of the Standard Model. The model contains three right handed neutrinos with different quantum numbers and a rich scalar sector, with extra doublets and singlets. In principle, these extra scalars can have VEVs ($V_{\Phi}$ and $V_{\phi}$ for the extra doublets and singlets, respectively) belonging to different energy scales. In the context of $\zeta\equiv\frac{V_{\Phi}}{V_{\phi}}\ll1$, which allows to obtain naturally light active neutrino masses and mixing compatible with neutrino experiments, the DM candidate arises by imposing a $Z_{2}$ symmetry on a given complex singlet, $\phi_{2}$, in order to make it stable. After doing a study of the scalar potential and the gauge sector, we obtain all the DM dominant processes concerning the relic abundance and direct detection. Then, for a representative set of parameters, we found that a complex DM with mass around $200$ GeV, for example, is compatible with the current experimental constraints without resorting to resonances. However, additional compatible solutions with heavier masses can be found in vicinities of resonances. Finally, we address the issue of having a light CP-odd scalar in the model showing that it is safe concerning the Higgs and the $Z_{\mu}$ boson invisible decay widths, and also the energy loss in stars astrophysical constraints.Comment: 20 pages, 3 figure

Interpretation of HINODE SOT/SP asymmetric Stokes profiles observed in quiet Sun network and internetwork

We present the first interpretation of the Stokes profile asymmetries measured in the FeI 630 nm lines by SOT/SP, in both quiet Sun internetwork (IN) and network regions. The inversion is carried out under the hypothesis of MISMA, where the unresolved structure is assumed to be optically thin. We analyze a 29.52"x31.70" subfield carefully selected to be representative of the properties of a 302"x162" quiet Sun field-of-view at disk center. The inversion code is able to reproduce the observed asymmetries in a very satisfactory way. The inversion code interprets 25% of inverted profiles as emerging from pixels in which both positive and negative polarities coexist. kG field strengths are found at the base of the photosphere in both network and IN; in the case of the latter, both kG fields and hG fields are admixed. When considering the magnetic properties at the mid photosphere most kG fields are gone, and the statistics is dominated by hG fields. We constrain the magnetic field of only 4.5% of the analyzed photosphere (and this percentage reduces to 1.3% when referred to all pixels, including those with low polarization not analyzed). The rest of the plasma is consistent with the presence of weak fields not contributing to the detected polarization signals. The average flux densities derived in the full subfield and in IN regions are higher than the ones derived from the same dataset by Milne-Eddington inversion. The existence of large asymmetries in SOT/SP polarization profiles is uncovered. These are not negligible in quiet Sun data. The MISMA inversion code reproduces them in a satisfactory way, and provides a statistical description of the magnetized IN and network which partly differs and complements the results obtained so far. From this it follows the importance of having a complete interpretation of the line profile shapes.Comment: 11 pages, 9 figures, 1 table - Accepted for publication on A&

Preliminary data for a geoconservation strategy of impact structures in Brazil

Serie : Cuadernos del Museo Geominero, n.º 18Las estructuras de impacto son formaciones raras en el planeta. Son extremadamente importantes para la comprensión de la evolución geológica y biológica de la Tierra porque son eventos catastróficos que causaron las extinciones en masa y grandes cambios en la superficie del Planeta. En Brasil son ocho cráteres de impacto reconocidos y aceptados por la comunidad científica; cuatro de ellos ya están identificadas como sitios geológicos por el Servicio Geológico del Brasil. La geoconservación sigue siendo un tema novedoso en el país, pero se está avanzando. Es necesario hacer un inventario de las estructuras, seguido por la medida de su valor y el riesgo de degradación, con criterios e indicadores bien definidos. Este artículo presenta un proyecto para el inventario de las estructuras de impacto en Brasil y propuestas para su conservación y utilización potencial.Meteorite impact structures are rare features in the planet. Due to the fact that these features have resulted of catastrophic events that caused mass extinctions and huge changes in the planet’s surface, they are extremely important for the understanding of the geological and biological evolution of the Earth. In Brazil there are eight impact craters recognized and accepted by the scientific community. The Geological Survey of Brazil has already identified four of these craters as geological sites. In spite of geoconservation is still a recent theme in the country, presently it is gaining an increasing recognition. Therefore, it is necessary to do a systematic characterization of impact structures, followed by a quantitative assessment of the values and risk of degradation using well-defined criteria and indicators. This work presents an on going project aiming the inventory of impact structures in Brazil and proposals for their potential use and conservation

Imperfect Imitation Can Enhance Cooperation

The promotion of cooperation on spatial lattices is an important issue in evolutionary game theory. This effect clearly depends on the update rule: it diminishes with stochastic imitative rules whereas it increases with unconditional imitation. To study the transition between both regimes, we propose a new evolutionary rule, which stochastically combines unconditional imitation with another imitative rule. We find that, surprinsingly, in many social dilemmas this rule yields higher cooperative levels than any of the two original ones. This nontrivial effect occurs because the basic rules induce a separation of timescales in the microscopic processes at cluster interfaces. The result is robust in the space of 2x2 symmetric games, on regular lattices and on scale-free networks.Comment: 4 pages, 4 figure

Analyzing library collections with starfield visualizations

This paper presents a qualitative and formative study of the uses of a starfield-based visualization interface for analysis of library collections. The evaluation process has produced feedback that suggests ways to significantly improve starfield interfaces and the interaction process to improve their learnability and usability. The study also gave us clear indication of additional potential uses of starfield visualizations that can be exploited by further functionality and interface development. We report on resulting implications for the design and use of starfield visualizations that will impact their graphical interface features, their use for managing data quality and their potential for various forms of visual data mining. Although the current implementation and analysis focuses on the collection of a physical library, the most important contributions of our work will be in digital libraries, in which volume, complexity and dynamism of collections are increasing dramatically and tools are needed for visualization and analysis