Descripción: The geomagnetic field (B geo) sets a lower cutoff rigidity (R c) to the entry of cosmic particles to Earth which depends on the geomagnetic activity. From numerical simulations of the trajectory of a proton (performed with the MAGCOS code) in the B geo, we use backtracking to analyze particles arriving at the Auger Observatory location. We determine the asymptotic trajectories and the values of R c in different incidence directions. Simulations were done using several models of B geo that emulate different geomagnetic conditions. © 2012 International Astronomical Union.

Descripción: It is shown that Born-Infeld gravity -a high energy deformation of Einstein gravity-removes the singularities of a cosmic string. The respective vacuum solution results to be free of conical singularity and closed timelike curves. The space ends at a minimal circle where the curvature invariants vanish; but this circle cannot be reached in a finite proper time. © Published under licence by IOP Publishing Ltd.

Descripción: We put limits on the time variation of the electron mass in the early universe using observational primordial abundances of D, 4He, and 7Li, recent data from the cosmic microwave background, and the 2dFGRS power spectrum. Furthermore, we use these constraints together with other astronomical and geophysical bounds from the late universe to test the Barrow-Magueijo model for the variation in me. From our analysis we obtain -0.615 < Gω/c4 < -0.045 (3 σ interval), in disagreement with the result obtained in the original paper. © 2008. The American Astronomical Society. All rights reserved.

Descripción: Sudden Galactic Cosmic Ray (GCR) intensity decreases are related to the passage of Interplanetary Coronal Mass Ejections (ICMEs). These phenomena are also known as Forbush Decreases (FDs). The deepest FDs are associated with the passage of Magnetic Clouds (MCs). In this preliminary study we select "non-interacting" MCs associated with FDs observed from ground Neutron Monitors in the period 1996-2009, with the aim of reducing the complexity and the number of parameters involved in the GCR-MC interactions. We introduce a method to determine properties of the "ejecta component" of the FD. We analyze properties of the ejecta component in combination with properties of MCs. From the resulting selection of events, we find that those FDs containing ejecta components show stronger correlations with MC parameters than our total sample of events. © 2013 International Astronomical Union.

Descripción: We investigate a universe filled with interacting dark matter, holographic dark energy, and dark radiation for the spatially flat Friedmann-Robertson-Walker (FRW) spacetime. We use a linear interaction to reconstruct all the component energy densities in terms of the scale factor by directly solving the balance's equations along with the source equation. We apply the χ 2 method to the observational Hubble data for constraining the cosmic parameters, contrast with the Union 2 sample of supernovae, and analyze the amount of dark energy in the radiation era. It turns out that our model exhibits an excess of dark energy in the recombination era whereas the stringent bound Ωx(z≃1010) < 0.21 at big-bang nucleosynthesis is fulfilled. We find that the interaction provides a physical mechanism for alleviating the triple cosmic coincidence and this leads to Ωm0/Ωx0≃Ωr0/Ωx0≃O(1). © 2013 Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica.

Descripción: Aims.We calculate the bounds on the variation in the fine structure constant at the time of primordial nucleosynthesis and at the time of neutral hydrogen formation. We used these bounds and other bounds from the late universe to test the Bekenstein model.Methods.We modified the Kawano code, CAMB, and CosmoMC to include the possible variation in the fine structure constant. We used observational primordial abundances of D, He, and Li, recent data from the cosmic microwave background, and the 2dFGRS power spectrum, to obtain bounds on the variation in . We calculated a piecewise solution to the scalar field equation of the Bekenstein model in two different regimes: i) matter and radiation, ii) matter and cosmological constant. We match both solutions with the appropriate boundary conditions. We performed a statistical analysis, using the bounds obtained from the early universe and other bounds from the late universe to constrain the free parameters of the model.Results.Results are consistent with no variation in for the early universe. Limits on are inconsistent with the scale length of the theory being larger than the Planck scale.Conclusions.In order to fit all observational and experimental data, the assumption > implied in Bekenstein's model has to be relaxed. © 2008 ESO.

Descripción: We consider multiple k-essence sources and obtain the conditions their kinetic functions must satisfy so that purely kinetic k-essences lead to models with phantom barrier crossing. After that, we show that polynomial kinetic functions allow the integration of the dynamical equations determining the geometry and the k-fields. The models thus obtained have accelerated expansion at late cosmic time, representing universes ending at a finite time with a big rip singularity. In addition, these models begin to evolve from an initial singularity, so they have a finite time span. © 2006 Elsevier B.V. All rights reserved.

Descripción: We briefly review f(R) theories, both in the metric and Palatini formulations, their scalar-tensor representations and the chameleon mechanism that could explain the absence of perceptible consequences in the Solar System. We also review f(T) theories, a different approach to modified gravity consisting in a deformation of the teleparallel equivalent of General Relativity. We show some applications to cosmology and cosmic strings. As f(R)'s, f(T) theories are not exempted from additional degrees of freedom; we also discuss this still open issue. © 2012 American Institute of Physics.

Descripción: GPS radio occultation satellite data allowed to analyze in the last decade for the first time a large amount of atmospheric temperature profiles including both the troposphere and the stratosphere all over the globe. Wave amplitude enhancements have been systematically observed around tropopause levels, which are apparently due to artifacts generated by any digital filter used to isolate the wave components from these data. We present a new filtering method which can be equally applied to temperature or refractivity profiles. It was tested with synthetic temperature data based on NCEP reanalyes and observed wave climatologies and it was also statistically validated with GPS radio occultation profiles from the COSMIC mission. The suggested technique significantly reduces artificial enhancements around the tropopause, mainly at low latitudes, where a sharp lapse rate change usually exists. This represents an improvement in comparison to previous applications of standard filters. In addition it would allow the study of longer vertical wavelengths than previously done with other filtering procedures. © Author(s) 2011.

Descripción: We analyze a universe filled with interacting dark matter, a scalar field accommodated as dark radiation along with dark energy plus a decoupled radiation term within the framework of the spatially flat Friedmann-Robertson-Walker (FRW) spacetime. We work in a three-dimensional internal space spanned by the interaction vector and use a transversal interaction Qt for solving the source equation in order to find all the interacting component energy densities. We asymptotically reconstruct the scalar field and potential from an early radiation era to the late dominate dark energy one, passing through an intermediate epoch dominated by dark matter. We apply the χ2 method to the updated observational Hubble data for constraining the cosmic parameters, contrast with the Union 2 sample of supernovae, and analyze the amount of dark energy in the radiation era. It turns out that our model fulfills the severe bound of Ωφ{symbol}(z≃1100)<0.018 at 2σ level, is consistent with the recent analysis that includes cosmic microwave background anisotropy measurements from the Atacama Cosmology Telescope and the South Pole Telescope along with the future constraints achievable by Planck and CMBPol experiments, and satisfies the stringent bound Ωφ{symbol}(z≃1010)<0.04 at 2σ level in the big-bang nucleosynthesis epoch. © 2013 Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica.

Descripción: Using radio, X-ray, and optical observations, we present evidence for morphological changes due to efficient cosmic ray ion acceleration in the structure of the southeastern region of the supernova remnant SN 1006. SN 1006 has an apparent bipolar morphology in both the radio and high-energy X-ray synchrotron emission. In the optical, the shock front is clearly traced by a filament of Balmer emission in the southeast. This optical emission enables us to trace the location of the blast wave (BW) even in places where the synchrotron emission from relativistic electrons is either absent or too weak to detect. The contact discontinuity (CD) is traced using images in the low-energy X-rays (oxygen band), which we argue reveals the distribution of shocked ejecta. We interpret the azimuthal variations of the ratio of radii between the BW and CD plus the X-ray and radio synchrotron emission at the BW using CR-modified hydrodynamic models. We assumed different azimuthal profiles for the injection rate of particles into the acceleration process, magnetic field, and level of turbulence. We found that the observations are consistent with a model in which these parameters are all azimuthally varying, being largest in the brightest regions. © 2008. The American Astronomical Society. All rights reserved.

Descripción: We study a k-essence field evolving linearly with the cosmic time and the atypical k-essence model on a homogeneous and isotropic flat 3-brane. We show that the k-field is driven by an inverse quadratic polynomial potential. The solutions represent expanding, contracting or bouncing universes with a finite time span and some of them end in a big crunch or a big rip. Besides, by selecting the extended tachyonic kinetic functions we analyze the high and low energy limits of our model, obtaining the nearly power law solution. We introduce a tachyon field with negative energy density and show that the universe evolves between two singularities. © 2010 IOP Publishing Ltd.

Descripción: Meteorological phenomena are closely linked to the presence of water vapor. They mainly originate and develop in the troposphere, where almost all the atmospheric water is concentrated. The Global Positioning System radio occultation (GPS RO) technique provides vertical profiles of refractivity from which other properties such as temperature and water vapor can be derived. The GPS RO capability to reproduce global, synoptic, and regional climatological patterns over South America, which is a mostly oceanic continent, is tested. From FORMOSAT-3/COSMIC mission data (2006-2010), our previous knowledge regarding global and synoptic/regional patterns of temperature, equivalent potential temperature, specific humidity, and pressure is verified. Special cases such as baroclinic disturbances arriving at South American midlatitudes and storm events over a mountain region near the Andes are analyzed. The temporal evolution and the latitude-longitude distribution in several layers of the variables listed above are well described with this technique. Copyright 2012 by the American Geophysical Union.