A Rotating Charged Black Hole Solution in f(R) Gravity

In the context of f(R) theories of gravity, we address the problem of finding a rotating charged black hole solution in the case of constant curvature. The new metric is obtained by solving the field equations and we show that the behavior of it is typical of a rotating charged source. In addition, we analyze the thermodynamics of the new black hole. The results ensures that the thermodynamical properties in f(R) gravities are qualitatively similar to those of standard General Relativity.Comment: 9 pages, no figure

A f(R) gravity without cosmological constant

In this work we consider the possibility of describing the current evolution of the universe, without the introduction of any cosmological constant or dark energy (DE), by modifying the Einstein-Hilbert (EH) action. In the context of the f(R) gravities within the metric formalism, we show that it is possible to find an action without cosmological constant which exactly reproduces the behavior of the EH action with cosmological constant. In addition the f(R) action is analytical at the origin having Minkowski and Schwarzschild as vacuum solutions. The found f(R) action is highly non-trivial and must be written in terms of hypergeometric functions but, in spite of looking somewhat artificial, it shows that the cosmological constant, or more generally the DE, is not a logical necessity.Comment: 5 pages, no figure

Towards a new quantization of Dirac's monopole

There are several mathematical and physical reasons why Dirac's quantization must hold. How far one can go without it remains an open problem. The present work outlines a few steps in this direction.Comment: To appear in Proceedings of "IV Taller de la Division de Gravitacion y Fisica Matematica". Misprints corrected, references and acknowledgments adde

Sparse inversion of Stokes profiles. I. Two-dimensional Milne-Eddington inversions

Inversion codes are numerical tools used for the inference of physical properties from the observations. Despite their success, the quality of current spectropolarimetric observations and those expected in the near future presents a challenge to current inversion codes. The pixel-by-pixel strategy of inverting spectropolarimetric data that we currently utilize needs to be surpassed and improved. The inverted physical parameters have to take into account the spatial correlation that is present in the data and that contains valuable physical information. We utilize the concept of sparsity or compressibility to develop an new generation of inversion codes for the Stokes parameters. The inversion code uses numerical optimization techniques based on the idea of proximal algorithms to impose sparsity. In so doing, we allow for the first time to exploit the presence of spatial correlation on the maps of physical parameters. Sparsity also regularizes the solution by reducing the number of unknowns. We compare the results of the new inversion code with pixel-by-pixel inversions, demonstrating the increase in robustness of the solution. We also show how the method can easily compensate for the effect of the telescope point spread function, producing solutions with an enhanced contrast.Comment: 13 pages, 8 figures, accepted for publication in A&

Primary Productivity of Coastal Marshes in Mississippi

The annual net primary production of nine types of marsh communities common in Mis­sissippi Gulf Coast estuaries were studied by means of the Harvest Method. Production values ranged from 600 g m-2 yr-1 for a Sagittaria lancifolia marsh to 2330 g m-2 yr-1 for a Phragmites communis marsh. Primary productivity values for the other marsh types are Juncus roemerianus - 1697 g m-2 yr-1, Scirpus robustus - 1056 g m-2 yr-1, Spartina cynosuroides - 2190 g m-2 yr-1, Spartina patens - 1922 g m-2 yr-1, Spartina altemijlora tall form - 1964 g m-2 yr-1, S. alternijlora short form - 1089 g m-2 yr-1, and Distichlis spicata - 1484 g m-2 yr-1. Annual net productivity in Mississippi marshes are generally slightly higher than those reported for the Atlantic marshes

Some cosmological and astrophysical aspects of modified gravity theories

This thesis will try to contribute to the understanding of open issues in cosmology by considering f(R) and brane-world theories. In Chapter 1, we shall summarize the main features of f(R) gravities in the metric formalism and we shall introduce both the notion of brane excitations, the branons, and the brane-skyrmions. We shall finish the chapter by providing some insight about the possibility of mini black holes detection in the LHC as a signature for the validity of these modified gravity theories. The Chapter 2 will deal with f(R) theories able to mimic Einstein-Hilbert plus cosmological constant solutions and f(R) theories will be shown to be able to mimic the cosmological evolution generated by any perfect fluid with constant equation of state. The Chapter 3 will be devoted to the computation of cosmological perturbations for f(R) theories. Special attention will be paid here to obtain a completely general differential equation for the evolution of perturbations and its particularization for the so-called sub-Hubble scales will be explicitly shown. In the Chapter 4 we shall focus on the study of black holes in f(R) gravities in an arbitrary number of dimensions. With this purpose we shall study constant curvature solutions for f(R) theories as well as perturbative solutions around the standard SAdS geometry. An important part of this chapter will be then devoted to the thermodynamics of SAdS black holes in f(R) theories. In the Chapter 5 we will thoroughly study brane-skyrmions. In this context, the recent claim of detection of an unexpected feature in the CMB, referred to as the cold spot, will be explained as a topological defect on the brane in complete agreement with those calculations in the literature that tried to explain that cold spot as a texture of a NLSM. Main conclusions are summarized all together in Chapter 6 .Comment: PhD. thesis, April 2010, Complutense University of Madrid. 129 pages. Preface, five chapters and conclusions. Two appendice

Changes in One Teacher\u27s Proportional Reasoning Instruction after Participating in a CGI Professional Development Workshop

This study examined how one teacher used research-based knowledge of how adolescents think about proportions. Observations, interviews, document collection, and a workshop intervention were utilized. The design of the workshop was inspired by the cognitively guided instruction studies and its purpose was to explore the research findings on adolescents\u27 thinking about proportions. An individual case study was created to describe the teacher\u27s instruction related to proportion concepts, rationales for instructional decisions, beliefs, and changes in all of these areas after participating in the workshop intervention. The case presented here shows positive changes in the teacher\u27s instruction and beliefs after the workshop