18,856 research outputs found
Charging Interacting Rotating Black Holes in Heterotic String Theory
We present a formulation of the stationary bosonic string sector of the whole
toroidally compactified effective field theory of the heterotic string as a
double Ernst system which, in the framework of General Relativity describes, in
particular, a pair of interacting spinning black holes; however, in the
framework of low--energy string theory the double Ernst system can be
particularly interpreted as the rotating field configuration of two interacting
sources of black hole type coupled to dilaton and Kalb--Ramond fields. We
clarify the rotating character of the --component of the
antisymmetric tensor field of Kalb--Ramond and discuss on its possible torsion
nature. We also recall the fact that the double Ernst system possesses a
discrete symmetry which is used to relate physically different string vacua.
Therefore we apply the normalized Harrison transformation (a charging symmetry
which acts on the target space of the low--energy heterotic string theory
preserving the asymptotics of the transformed fields and endowing them with
multiple electromagnetic charges) on a generic solution of the double Ernst
system and compute the generated field configurations for the 4D effective
field theory of the heterotic string. This transformation generates the
vector field content of the whole low--energy heterotic string
spectrum and gives rise to a pair of interacting rotating black holes endowed
with dilaton, Kalb--Ramond and multiple electromagnetic fields where the charge
vectors are orthogonal to each other.Comment: 15 pages in latex, revised versio
Is the cosmological dark sector better modeled by a generalized Chaplygin gas or by a scalar field?
Both scalar fields and (generalized) Chaplygin gases have been widely used
separately to characterize the dark sector of the Universe. Here we investigate
the cosmological background dynamics for a mixture of both these components and
quantify the fractional abundances that are admitted by observational data from
supernovae of type Ia and from the evolution of the Hubble rate. Moreover, we
study how the growth rate of (baryonic) matter perturbations is affected by the
dark-sector perturbations.Comment: 20 pages, 9 figures, substantially revised, section on matter
perturbations added, accepted for publication in EPJ
Cosmology with Ricci-type dark energy
We consider the dynamics of a cosmological substratum of pressureless matter
and holographic dark energy with a cutoff length proportional to the Ricci
scale. Stability requirements for the matter perturbations are shown to single
out a model with a fixed relation between the present matter fraction
and the present value of the equation-of-state
parameter of the dark energy. This model has the same number of free parameters
as the CDM model but it has no CDM limit. We discuss the
consistency between background observations and the mentioned
stability-guaranteeing parameter combination.Comment: 6 pages, 3 figures, submitted to the Proceedings of the CosmoSurII
conference, Valpara\'iso, Chile, 27 - 31 May 201
CaracterÃsticas Antropométricas en Atletas de Jiu Jitsu Brasilero de Alto Nivel: Rol del Estilo de Lucha
Indexación: Web of Science; Scielo.SUMMARY: Human performance efficiency and effectiveness in different sports depend to a large extent on the size, weight and proportion of the physique of the athlete. The aim of this study was to identify morphological characteristics of Brazilian Jiu-Jitsu (BJJ) athletes. The sample consisted of 25 highly trained male athletes who were classified according to their fighting style; guard fighter (GF) vs. pass fighter (PF). The athletes were assessed for somatotype, body composition and proportionality. For the whole group of athletes the somatotype was 2.23±0.68, 6.33±1.14, and 1.75±0.87 for endomorph, mesomorph and ectomorph components, respectively. Muscle and adipose tissue percentages were 52.34±2.15% and 19.30±2.51%, respectively. PF were significantly more mesomorph (p< 0.05) and less ectomorph (p< 0.05) than GF. Also, PF had significantly higher phantom Z score for bone mass vs. GF (0.51±0.57 vs. 0.01±0.54; p<0.05), and significantly lower muscle mass- bone mass ratio (4.55±0.31 vs. 4.77±0.56; p<0.05), height (1.71±0.06 vs. 1.77±0.07; p<0.05) and height weight ratio (40.58±1.11 vs. 41.84±1.22). Our results show that morphological characteristics are related to different fighting styles in BJJ athletes.RESUMEN: La eficiencia y efectividad del rendimiento humano en diferentes deportes depende en gran medida del tamaño, peso y proporción del fÃsico del atleta. El objetivo de este estudio fue identificar las caracterÃsticas morfológicas de atletas jiu-jitsu brasileros. La muestra consintió de 25 atletas varones altamente entrenados, quienes fueron clasificados de acuerdo a su estilo de lucha; guarderos (GF) vs pasadores (PF). Se evaluó en los atletas somatotipo, composición corporal y proporcionalidad. Para el grupo total de atletas el somatotipo fue 2,23±0,68, 6,33±1,14 y 1,75±0,78 para el endomorfismo, mesomorfismo y ectomorfismo, respectivamente. Los porcentajes de tejido muscular y adiposo fueron 52,34±2,15% and 19,30±2,51%, respectivamente. PF fueron significativamente las mesomorfos (p<0,05) y menos ectomorfos (p<0,05) que GF. Además PF tuvieron una significativamente alto Z score para la masa ósea (4,55±0,31 vs. 4,77±0,56; p<0,05), estatura (1,71±0,06 vs. 1,77±0,07; p<0,05) y relación altura peso (40,58±1.11 vs. 41,84±1.22). Nuestros resultados muestran que las caracterÃsticas morfológicas están relacionadas a diferentes estilos de lucha en atletas de BJJ.http://ref.scielo.org/bzhxy
The anatomy of urban social networks and its implications in the searchability problem
The appearance of large geolocated communication datasets has recently
increased our understanding of how social networks relate to their physical
space. However, many recurrently reported properties, such as the spatial
clustering of network communities, have not yet been systematically tested at
different scales. In this work we analyze the social network structure of over
25 million phone users from three countries at three different scales: country,
provinces and cities. We consistently find that this last urban scenario
presents significant differences to common knowledge about social networks.
First, the emergence of a giant component in the network seems to be controlled
by whether or not the network spans over the entire urban border, almost
independently of the population or geographic extension of the city. Second,
urban communities are much less geographically clustered than expected. These
two findings shed new light on the widely-studied searchability in
self-organized networks. By exhaustive simulation of decentralized search
strategies we conclude that urban networks are searchable not through
geographical proximity as their country-wide counterparts, but through an
homophily-driven community structure
Moments of inertia for solids of revolution and variational methods
We present some formulae for the moments of inertia of homogeneous solids of
revolution in terms of the functions that generate the solids. The development
of these expressions exploits the cylindrical symmetry of these objects, and
avoids the explicit use of multiple integration, providing an easy and
pedagogical approach. The explicit use of the functions that generate the solid
gives the possibility of writing the moment of inertia as a functional, which
in turn allows us to utilize the calculus of variations to obtain a new insight
into some properties of this fundamental quantity. In particular, minimization
of moments of inertia under certain restrictions is possible by using
variational methods.Comment: 6 pages, 6 figures, LaTeX2e. Two paragraphs added. Minor typos
corrected. Version to appear in European Journal of Physic
On holographic dark-energy models
Different holographic dark-energy models are studied from a unifying point of
view. We compare models for which the Hubble scale, the future event horizon or
a quantity proportional to the Ricci scale are taken as the infrared cutoff
length. We demonstrate that the mere definition of the holographic dark-energy
density generally implies an interaction with the dark-matter component. We
discuss the relation between the equation-of-state parameter and the energy
density ratio of both components for each of the choices, as well as the
possibility of non-interacting and scaling solutions. Parameter estimations for
all three cutoff options are performed with the help of a Bayesian statistical
analysis, using data from supernovae type Ia and the history of the Hubble
parameter. The CDM model is the clear winner of the analysis.
According to the Bayesian Information Criterion (), all holographic models
should be considered as ruled out, since the difference to the
corresponding CDM value is . According to the Akaike Information
Criterion (), however, we find for models with
Hubble-scale and Ricci-scale cutoffs, indicating, that they may still be
competitive. As we show for the example of the Ricci-scale case, also the use
of certain priors, reducing the number of free parameters to that of the
CDM model, may result in a competitive holographic model.Comment: 37 pages, 11 figures, 3 tables, statistical analysis improved,
accepted for publication in Phys.Rev.
On the stability of the shear-free condition
The evolution equation for the shear is reobtained for a spherically
symmetric anisotropic, viscous dissipative fluid distribution, which allows us
to investigate conditions for the stability of the shear-free condition. The
specific case of geodesic fluids is considered in detail, showing that the
shear-free condition, in this particular case, may be unstable, the departure
from the shear-free condition being controlled by the expansion scalar and a
single scalar function defined in terms of the anisotropy of the pressure, the
shear viscosity and the Weyl tensor or, alternatively, in terms of the
anisotropy of the pressure, the dissipative variables and the energy density
inhomogeneity.Comment: 19 pages Latex. To appear in Gen. Rel. Gra
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