Comment on "Quantum back-reaction through the Bohmian particle"

In this Comment I point out some limitations of the proposal of Prezhdo and Brooksby for coupling quantum and classical degrees of freedom (Phys.Rev.Lett.86(2001)3215) if it is pushed too far.Comment: 1 page, REVTEX, no figure

Efectos del incremento de la temperatura y las quemas sobre la mesofauna (Artrópodos) del suelo y la descomposición de hojarasca en un ecosistema de páramo en el Norte del Ecuador

Paramos are high altitude tropical ecosystems in the Andes that has extreme geographical and climatic conditions, which promotes high rates of endemism and great accumulation of organic matter. Paramos are of great importance for the valleys in the Andean slopes because they allow the water regulation. These features make of the Paramo a sensitive ecosystem to climate changes and human-caused disturbances (burning). In this context our study aims to analyze the possible effects of the interaction between burning and increased temperature on soil invertebrates and litter decomposition. Our data show clear changes on the diversity and abundance of mesofauna; however decomposition decreased as the temperature increased. The data on invertebrates follows the pattern of the intermediate disturbance hypothesis; this can be caused because when the dominant species abundance decreases allows new species to occupy a resource. This study indicates that the interrelationships between the elements of the ecosystem are narrow and when a factor is altered it can change ecological processes. We recommend new studies to determine the impacts that Paramo is facing at the time to have a better understanding of how climate change could affect it.Los páramos son ecosistemas altoandinos que están caracterizados por sus condiciones geográficas y climáticas extremas, las cuales permiten altas de endemismo y acumulación de materia orgánica. El páramo es un ecosistema de importancia porque regula el agua que desciende hacia los valles aledaños a los Andes. Estas características hacen del páramo un ecosistema sensible frente a los efectos del cambio climático y antropogénicos. En este contexto nuestro estudio pretende analizar la interacción entre los efectos del aumento de temperatura y la quema sobre la mesofauna del suelo y la descomposición de hojarasca. Nuestros datos muestran claros cambios sobre la diversidad y abundancia de la mesofauna, sin embargo la descomposición disminuyó cuando aumentó la temperatura. Los datos siguen el patrón de la Hipótesis de disturbio intermedio, esto puede ser causado porque cuando la especie dominante disminuye en tamaña permite a otras especies ocupar un recurso antes escaso. Este estudio nos indica que las interrelaciones entre los elementos del ecosistema son estrechas y cuando se altera un factor puede cambiar los procesos ecológicos. Recomendamos realizar estudios que determinen los efectos a los que el páramo ya se enfrenta para tener un mejor entendimiento de cómo el cambio climático podría afectarlo

Bounds on the mass and abundance of dark compact objects and black holes in dwarf spheroidal galaxy halos

We establish new dynamical constraints on the mass and abundance of compact objects in the halo of dwarf spheroidal galaxies. In order to preserve kinematically cold the second peak of the Ursa Minor dwarf spheroidal (UMi dSph) against gravitational scattering, we place upper limits on the density of compact objects as a function of their assumed mass. The mass of the dark matter constituents cannot be larger than 1000 solar masses at a halo density in UMi's core of 0.35 solar masses/pc^3. This constraint rules out a scenario in which dark halo cores are formed by two-body relaxation processes. Our bounds on the fraction of dark matter in compact objects with masses >3000 solar masses improve those based on dynamical arguments in the Galactic halo. In particular, objects with masses $\sim 10^{5}$ solar masses can comprise no more than a halo mass fraction $\sim 0.01$. Better determinations of the velocity dispersion of old overdense regions in dSphs may result in more stringent constraints on the mass of halo objects. For illustration, if the preliminary value of 0.5 km/s for the secondary peak of UMi is confirmed, compact objects with masses above $\sim 100$ solar masses could be excluded from comprising all its dark matter halo.Comment: 6 pages, 2 figures, accepted for publication in ApJ Letter

Cosmological Effects of Nonlinear Electrodynamics

It will be shown that a given realization of nonlinear electrodynamics, used as source of Einstein's equations, generates a cosmological model with interesting features, namely a phase of current cosmic acceleration, and the absence of an initial singularity, thus pointing to a way to solve two important problems in cosmology

Einstein-Maxwell-scalar black holes: the hot, the cold and the bald

The phenomenon of spontaneous scalarisation of charged black holes (BHs) has recently motivated studies of various Einstein-Maxwell-scalar models. Within these models, different classes of BH solutions are possible, depending on the non-minimal coupling function $f(\phi)$, between the scalar field and the Maxwell invariant. Here we consider the class wherein both the (bald) electrovacuum Reissner-Nordstr\"om (RN) BH and new scalarised BHs co-exist, and the former are never unstable against scalar perturbations. In particular we examine the model, within this subclass, with a quartic coupling function: $f(\Phi) = 1+\alpha \Phi ^4$. The domain of existence of the scalarised BHs, for fixed $\alpha$, is composed of two branches. The first branch (cold scalarised BHs) is continuously connected to the extremal RN BH. The second branch (hot scalarised BHs) connects to the first one at the minimum value of the charge to mass ratio and it includes overcharged BHs. We then assess the perturbative stability of the scalarised solutions, focusing on spherical perturbations. On the one hand, cold scalarised BHs are shown to be unstable by explicitly computing growing modes. The instability is quenched at both endpoints of the first branch. On the other hand, hot scalarised BHs are shown to be stable by using the S-deformation method. Thus, in the spherical sector this model possesses two stable BH local ground states (RN and hot scalarised). We point out that the branch structure of BHs in this model parallels the one of BHs in five dimensional vacuum gravity, with [Myer-Perry BHs, fat rings, thin rings] playing the role of [RN, cold scalarised, hot scalarised] BHs.Comment: 12 pages, 6 figure

Odd Parity Light Baryon Resonances

We use a consistent SU(6) extension of the meson-baryon chiral Lagrangian within a coupled channel unitary approach in order to calculate the T-matrix for meson-baryon scattering in s-wave. The building blocks of the scheme are the pion and nucleon octets, the rho nonet and the Delta decuplet. We identify poles in this unitary T-matrix and interpret them as resonances. We study here the non exotic sectors with strangeness S=0,-1,-2,-3 and spin J=1/2, 3/2 and 5/2. Many of the poles generated can be associated with known N, Delta, Sigma, Lambda and Xi resonances with negative parity. We show that most of the low-lying three and four star odd parity baryon resonances with spin 1/2 and 3/2 can be related to multiplets of the spin-flavor symmetry group SU(6). This study allows us to predict the spin-parity of the Xi(1620), Xi(1690), Xi(1950), Xi(2250), Omega(2250) and Omega(2380) resonances, which have not been determined experimentally yet.Comment: New appendix and references adde