Ghosts in asymmetric brane gravity and the decoupled stealth limit

We study the spectrum of gravitational perturbations around a vacuum de Sitter brane in a 5D asymmetric braneworld model, with induced curvature on the brane. This generalises the stealth acceleration model proposed by Charmousis, Gregory and Padilla (CGP) which realises the Cardassian cosmology in which power law cosmic acceleration can be driven by ordinary matter. Whenever the bulk has infinite volume we find that there is always a perturbative ghost propagating on the de Sitter brane, in contrast to the Minkowski brane case analysed by CGP. We discuss the implication of this ghost for the stealth acceleration model, and identify a limiting case where the ghost decouples as the de Sitter curvature vanishes.Comment: 21 page

A short review of "DGP Specteroscopy"

In this paper we provide a short review of the main results developed in hep-th/0604086. We focus on linearised vacuum perturbations about the self-accelerating branch of solutions in the DGP model. These are shown to contain a ghost in the spectrum for any value of the brane tension. We also comment on hep-th/0607099, where some counter arguments have been presented.Comment: Minor typos correcte

Synergistic Gravity and the Role of Resonances in GRS-Inspired Braneworlds

We consider 5D braneworld models of quasi-localized gravity in which 4D gravity is reproduced at intermediate scales while the extra dimension opens up at both the very short and the very long distances, where the geometry is flat. Our main interest is the interplay between the zero mode of these models, whenever a normalizable zero mode exists, and the effects of zero energy graviton resonant modes coming from the contributions of massive KK modes. We first consider a compactified version of the GRS model and find that quasi-localized gravity is characterized by a scale for which both the resonance and the zero mode have significant contribution to 4D gravity. Above this scale, gravity is primarily mediated by the zero mode, while the resonance gives only minor corrections. Next, we consider an asymmetric version of the standard non-compact GRS model, characterized by different cosmological constants on each AdS side. We show that a resonance is present but the asymmetry, through the form of the localizing potential, can weaken it, resulting in a shorter lifetime and, thus, in a shorter distance scale for 4D gravity. As a third model exhibiting quasi-localization, we consider a version of the GRS model in which the central positive tension brane has been replaced by a configuration of a scalar field propagating in the bulk.Comment: 18 pages, 3 figures, added 1 figure, revised version as published in Class. Quant. Gra

TeV Burst of Gamma-Ray Bursts and Ultra High Energy Cosmic Rays

Some recent experiments detecting very high energy (VHE) gamma-rays above 10-20 TeV independently reported VHE bursts for some of bright gamma-ray bursts (GRBs). If these signals are truly from GRBs, these GRBs must emit a much larger amount of energy as VHE gamma-rays than in the ordinary photon energy range of GRBs (keV-MeV). We show that such extreme phenomena can be reasonably explained by synchrotron radiation of protons accelerated to \sim 10^{20-21} eV, which has been predicted by Totani (1998a). Protons seem to carry about (m_p/m_e) times larger energy than electrons, and hence the total energy liberated by one GRB becomes as large as \sim 10^{56} (\Delta \Omega / 4 \pi) ergs. Therefore a strong beaming of GRB emission is highly likely. Extension of the VHE spectrum beyond 20 TeV gives a nearly model-independent lower limit of the Lorentz factor of GRBs, as \gamma \gtilde 500. Furthermore, our model gives the correct energy range and time variability of ordinary keV-MeV gamma-rays of GRBs by synchrotron radiation of electrons. Therefore the VHE bursts of GRBs strongly support the hypothesis that ultra high energy cosmic rays observed on the Earth are produced by GRBs.Comment: Final version to appear in ApJ Lett. Emphasizing that the extremely large energy required in this model is not theoretically impossible if GRB emission is strongly beamed. References update

Relaciones entre los aspectos cognitivos y emocionales de la enseñanza

Desde que Shulman (1986) propuso el concepto de conocimiento didáctico de contenido (CDC) se han realizado una gran cantidad de investigaciones al respecto; algunas referentes a las ideas o conceptos que se deben incluir en el CDC, otras referentes a las relaciones con la materia y otras sobre cómo el profesor desarrolla su CDC. Sin embargo, a pesar de que muchos autores han señalado la importancia de las emociones en la enseñanza, poco o nada se ha hecho para estudiar si el CDC tiene un lado emocional. En este trabajo se estudian las relaciones entre ambos aspectos de la enseñanza universitaria en química; para ello se parte del modelo de CDC propuesto por Magnusson et al. (1999) y se proponen tres nuevos componentes relacionados con las emociones del docente hacia la materia, los estudiantes y hacia su propia enseñanza

Determinants of food selection by bivalve larvae

Selection of food particles for consumption by larvae impacts nutritional gain needed for growth, development, and metamorphosis. Past work has suggested that molluscan larvae are capable of collecting food within a narrow size range. Recent studies have found evidence of size-independent food selection in molluscan larvae, but relatively little is known about the characteristics of particles that larvae preferentially capture. Therefore, we conducted experiments with the larvae of two mussels, Mytilus trossulus and Mytilus edulis, to determine whether they are selective feeders, and if so, whether we could determine the characteristics of particles selected as food. We fed larvae microalgae and polystyrene microspheres of different sizes, nutritional content, surface charge, and hydrophobicity. We found that for both species, there was no effect of size on particle selection for particles 2–8 μm, but, surprisingly, these two congeners preferentially captured particles with different characteristics. Larvae of M. trossulus preferentially captured particles that were more hydrophilic and had a more negativesurface charge, but there was no effect of nutritional content. The larvae of M. edulis showed a different pattern; they preferentially captured particles with low surface charge and greater food value, but hydrophobicity did not affect selection. Larvae of these two congeners are indeed selective in which particles they collect but appear to be using different rules for selection. More work is needed to determine whether there are any general patterns that govern particle selection for larvae and mechanisms that could produce the observed patterns. Such work is needed to help us to determine whether individual species use different rules or whether there are general patterns in the types of particles larvae select