Strong curvature singularities in quasispherical asymptotically de Sitter dust collapse

We study the occurrence, visibility, and curvature strength of singularities in dust-containing Szekeres spacetimes (which possess no Killing vectors) with a positive cosmological constant. We find that such singularities can be locally naked, Tipler strong, and develop from a non-zero-measure set of regular initial data. When examined along timelike geodesics, the singularity's curvature strength is found to be independent of the initial data.Comment: 16 pages, LaTeX, uses IOP package, 2 eps figures; accepted for publication in Class. Quantum Gra

Distance learning perceptions from higher education students — The case of Portugal

This research study examines the attitudes of Portuguese higher education students regarding compulsory digital and distance learning university courses during the second semester of the academic year 2019–2020 during the COVID-19 pandemic. The methodology was quantitative, being the undergraduate and postgraduate students surveyed to find their perceptions about distance and online education in Portugal. The findings of the study highlighted the relationship between distance and online learning. The key concern of the respondents is related to the formal and contextual dimensions of the online class regime. The values examined, taken as a whole, allow us to conclude that with this teaching regime, in terms of awareness, there is acceptance and benefit. The sense of ambiguity in which this transformation took place, as well as the climate surrounding this phase, are worth noting. The teaching and evaluation methodologies used have been embraced and show a very wide range of choices on the part of the teaching teams and the students’ various interests, just as in the teaching regime of the classroom. The fact that students feel the need for face-to-face classes, however, is of great importance for practical and laboratory classes. This reality, which is a challenge to face in the future, is hard to overcome.info:eu-repo/semantics/publishedVersio

Gluon saturation and the Froissart bound: a simple approach

At very high energies we expect that the hadronic cross sections satisfy the Froissart bound, which is a well-established property of the strong interactions. In this energy regime we also expect the formation of the Color Glass Condensate, characterized by gluon saturation and a typical momentum scale: the saturation scale $Q_s$. In this paper we show that if a saturation window exists between the nonperturbative and perturbative regimes of Quantum Chromodynamics (QCD), the total cross sections satisfy the Froissart bound. Furthermore, we show that our approach allows us to describe the high energy experimental data on $pp/p\bar{p}$ total cross sections.Comment: 6 pages, 5 figures. Includes additional figures, discussion and reference

A note on the cylindrical collapse of counter-rotating dust

We find analytical solutions describing the collapse of an infinitely long cylindrical shell of counter-rotating dust. We show that--for the classes of solutions discussed herein--from regular initial data a curvature singularity inevitably develops, and no apparent horizons form, thus in accord with the spirit of the hoop conjecture.Comment: 8 pages, LaTeX, ijmpd macros (included), 1 eps figure; accepted for publication in Int. J. Mod. Phys.

Equilibrium fluctuations for the totally asymmetric zero-range process

We consider the one-dimensional Totally Asymmetric Zero-Range process evolving on $\mathbb{Z}$ and starting from the Geometric product measure $\mu_\rho$. On the hyperbolic time scale the temporal evolution of the density fluctuation field is deterministic, in the sense that the limit field at time $t$ is a translation of the initial one. We consider the system in a reference frame moving at this velocity and we show that the limit density fluctuation field does not evolve in time until $N^{4/3}$, which implies the current across a characteristic to vanish on this longer time scale.The author wants to express her gratitude to "Fundacao para a Ciencia e Tecnologia" for the grant /SFRH/BPD/39991/2007, to CMAT from University of Minho for support and to "Fundacao Calouste Gulbenkian" for the Prize: "Estimulo a investigacao" of the research project "Hydrodynamic limit of particle systems"

Why, when, and how fast innovations are adopted

When the full stock of a new product is quickly sold in a few days or weeks, one has the impression that new technologies develop and conquer the market in a very easy way. This may be true for some new technologies, for example the cell phone, but not for others, like the blue-ray. Novelty, usefulness, advertising, price, and fashion are the driving forces behind the adoption of a new product. But, what are the key factors that lead to adopt a new technology? In this paper we propose and investigate a simple model for the adoption of an innovation which depends mainly on three elements: the appeal of the novelty, the inertia or resistance to adopt it, and the interaction with other agents. Social interactions are taken into account in two ways: by imitation and by differentiation, i.e., some agents will be inclined to adopt an innovation if many people do the same, but other will act in the opposite direction, trying to differentiate from the "herd". We determine the conditions for a successful implantation of the new technology, by considering the strength of advertising and the effect of social interactions. We find a balance between the advertising and the number of anti-herding agents that may block the adoption of a new product. We also compare the effect of social interactions, when agents take into account the behavior of the whole society or just a part of it. In a nutshell, the present model reproduces qualitatively the available data on adoption of innovation.Comment: 11 pages, 13 figures (with subfigures), full paper (EPJB 2012) on innovation adoption mode

Quantum spinor field in the FRW universe with a constant electromagnetic background

The article is a natural continuation of our paper {\em Quantum scalar field in FRW Universe with constant electromagnetic background}, Int. J. Mod. Phys. {\bf A12}, 4837 (1997). We generalize the latter consideration to the case of massive spinor field, which is placed in FRW Universe of special type with a constant electromagnetic field. To this end special sets of exact solutions of Dirac equation in the background under consideration are constructed and classified. Using these solutions representations for out-in, in-in, and out-out spinor Green functions are explicitly constructed as proper-time integrals over the corresponding contours in complex proper-time plane. The vacuum-to-vacuum transition amplitude and number of created particles are found and vacuum instability is discussed. The mean values of the current and energy-momentum tensor are evaluated, and different approximations for them are presented. The back reaction related to particle creation and to the polarization of the unstable vacuum is estimated in different regimes.Comment: 36 pages, LaTex fil

Ca2+-H+ antiport activity in synaptic vesicles isolated from sheep brain cortex

Synaptic vesicles isolated from sheep brain cortex exhibit an ATP-dependent Ca2+ accumulation that is inhibited by the protonophore uncoupler carbonyl cyanide m-chorophenylhydrazone (CCCP) and completely released by the Ca2+ ionophore ionomycin. This transport activity was sensitive to the V-type ATPase inhibitor, bafilomycin, but not to the P-type ATPase inhibitor, vanadate. We also observed that the proton gradient, established across the synaptic vesicle membranes in the presence of ATP, is partially dissipated by the addition of Ca2+ (100-860 [mu]M) in correlation to an increase of ATP hydrolysis by the H+-pumping ATPase. In contrast, the activity of the H+-ATPase, measured under uncoupling conditions (presence of CCCP), appears to be unaltered by the calcium ion. The Ca2+-induced H+ release visualized by fluorescence quenching of acridine orange correlates well with the Ca2+ uptake determined isotopically. These results indicate that synaptic vesicles accumulate Ca2+, via a low affinity Ca2+-H+ antiport system energized by the protonmotive force originated from the H+-pumping ATPase activity.http://www.sciencedirect.com/science/article/B6T0G-3STG33F-M/1/23212cde5694bae6fc9101190baa18e