Dynamical Evolution of an Unstable Gravastar with Zero Mass

Using the conventional gravastar model, that is, an object constituted by two components where one of them is a massive infinitely thin shell and the other one is a de Sitter interior spacetime, we physically interpret a solution characterized by a zero Schwarzschild mass. No stable gravastar is formed and it collapses without forming an event horizon, originating what we call a massive non-gravitational object. The most surprise here is that the collapse occurs with an exterior de Sitter vacuum spacetime. This creates an object which does not interact gravitationally with an outside test particle and it may evolve to a point-like topological defect.Comment: 8 pages, 10 figures, to appear in Astrophysics and Space Scienc

Dressing a Naked Singularity: an Example

Considering the evolution of a perfect fluid with self-similarity of the second kind, we have found that an initial naked singularity can be trapped by an event horizon due to collapsing matter. The fluid moves along time-like geodesics with a self-similar parameter $\alpha = -3$. Since the metric obtained is not asymptotically flat, we match the spacetime of the fluid with a Schwarzschild spacetime. All the energy conditions are fulfilled until the naked singularity.Comment: 14 pages, 1 figure. This version corrects an error in the calculus of the pressure and in the conclusion

Tuning the electrical conductivity of nanotube-encapsulated metallocene wires

We analyze a new family of carbon nanotube-based molecular wires, formed by encapsulating metallocene molecules inside the nanotubes. Our simulations, that are based on a combination of non-equilibrium Green function techniques and density functional theory, indicate that these wires can be engineered to exhibit desirable magnetotransport effects for use in spintronics devices. The proposed structures should also be resilient to room-temperature fluctuations, and are expected to have a high yield.Comment: 4 pages, 6 figures. Accepted in Physical Review Letter

Sex, drugs, bugs, and age: Rational selection of empirical therapy for outpatient urinary tract infection in an era of extensive antimicrobial resistance

AbstractBackgroundOptimal empirical therapy of urinary tract infection requires accurate knowledge of local susceptibility patterns, which may vary with organism and patient characteristics.MethodsAmong 9,798 consecutive, non-duplicate, community-source urine isolates from ambulatory patients ≥ 13 years old, from clinical laboratory and an academic medical center in Curitiba, Brazil (May 1st to December 1st, 2009), susceptibility data for ampicillin, nitrofurantoin, trimethoprim-sulfamethoxazole, gentamicin, fluoroquinolones, and ceftriaxone/cefotaxime were compared with organism and patient gender and age.ResultsThe female-to-male ratio decreased with age, from 28.1 (among 20–29 year-olds) to 3.3 (among > 80 year-olds). Overall, susceptibility prevalence varied widely by drug class, from unacceptably low levels (53.5% and 61.1%: ampicillin and trimethoprimsulfamethoxazole) to acceptable but suboptimal levels (81.2% to 91.7%: fluoroquinolones, ceftriaxone, nitrofurantoin, and gentamicin). E. coli isolates exhibited higher susceptibility rates than other isolates, from 3–4% higher (fluoroquinolones, gentamicin) to ≥ 30% (nitrofurantoin, ceftriaxone). Males exhibited lower susceptibility rates than females. Within each gender, susceptibility declined with increasing age. For females, only nitrofurantoin and gentamicin were suitable for empirical therapy (≥ 80% susceptibility) across all age cohorts; fluoroquinolones were suitable only through age 60, and ceftriaxone only through age 80. For males, only gentamicin yielded ≥ 80% susceptibility in any age cohort.ConclusionFew suitable empirical treatment options for community-source urinary tract infection were identified for women aged over 60 years or males of any age. Empirical therapy recommendations must consider the patient's demographic characteristics. Site-specific, age and gender-stratified susceptibility surveillance involving all uropathogens is needed

Gravitational Collapse of Self-Similar and Shear-free Fluid with Heat Flow

A class of solutions to Einstein field equations is studied, which represents gravitational collapse of thick spherical shells made of self-similar and shear-free fluid with heat flow. It is shown that such shells satisfy all the energy conditions, and the corresponding collapse always forms naked singularities.Comment: 34 pages, 9 figures, late

Gravitational Collapse of Massless Scalar Field with Negative Cosmological Constant in (2+1) Dimensions

The 2+1-dimensional geodesic circularly symmetric solutions of Einstein-massless-scalar field equations with negative cosmological constant are found and their local and global properties are studied. It is found that one of them represents gravitational collapse where black holes are always formed.Comment: no figure

Perturbed Self-Similar Massless Scalar Field in the Spacetimes with Circular Symmetry in 2+1 Gravity

We present in this work the study of the linear perturbations of the 2+1-dimensional circularly symmetric solution, obtained in a previous work, with kinematic self-similarity of the second kind. We have obtained an exact solution for the perturbation equations and the possible perturbation modes. We have shown that the background solution is a stable solution.Comment: no figure

Response of Spiking Neurons to Correlated Inputs

The effect of a temporally correlated afferent current on the firing rate of a leaky integrate-and-fire (LIF) neuron is studied. This current is characterized in terms of rates, auto and cross-correlations, and correlation time scale $\tau_c$ of excitatory and inhibitory inputs. The output rate $\nu_{out}$ is calculated in the Fokker-Planck (FP) formalism in the limit of both small and large $\tau_c$ compared to the membrane time constant $\tau$ of the neuron. By simulations we check the analytical results, provide an interpolation valid for all $\tau_c$ and study the neuron's response to rapid changes in the correlation magnitude.Comment: 4 pages, 3 figure

Universal properties of correlation transfer in integrate-and-fire neurons

One of the fundamental characteristics of a nonlinear system is how it transfers correlations in its inputs to correlations in its outputs. This is particularly important in the nervous system, where correlations between spiking neurons are prominent. Using linear response and asymptotic methods for pairs of unconnected integrate-and-fire (IF) neurons receiving white noise inputs, we show that this correlation transfer depends on the output spike firing rate in a strong, stereotyped manner, and is, surprisingly, almost independent of the interspike variance. For cells receiving heterogeneous inputs, we further show that correlation increases with the geometric mean spiking rate in the same stereotyped manner, greatly extending the generality of this relationship. We present an immediate consequence of this relationship for population coding via tuning curves

Collapsing Perfect Fluid in Higher Dimensional Spherical Spacetimes

The general metric for N-dimensional spherically symmetric and conformally flat spacetimes is given, and all the homogeneous and isotropic solutions for a perfect fluid with the equation of state $p = \alpha \rho$ are found. These solutions are then used to model the gravitational collapse of a compact ball. It is found that when the collapse has continuous self-similarity, the formation of black holes always starts with zero mass, and when the collapse has no such a symmetry, the formation of black holes always starts with a mass gap.Comment: Class. Quantum Grav. 17 (2000) 2589-259