130 research outputs found
Gravitational wave recoils in non-axisymmetric Robinson-Trautman spacetimes
We examine the gravitational wave recoil waves and the associated net kick
velocities in non-axisymmetric Robinson-Trautman spacetimes. We use
characteristic initial data for the dynamics corresponding to non-head-on
collisions of black holes. We make a parameter study of the kick distributions,
corresponding to an extended range of the incidence angle in the
initial data. For the range of examined () the kick distributions as a function of the symmetric mass
parameter satisfy a law obtained from an empirical modification of the
Fitchett law, with a parameter that accounts for the non-zero net
gravitational momentum wave fluxes for the equal mass case. The law fits
accurately the kick distributions for the range of examined, with a
rms normalized error of the order of . For the equal mass case the
nonzero net gravitational wave momentum flux increases as increases,
up to beyond which it decreases. The maximum net
kick velocity is about for for the boost parameter considered.
For the distribution is a monotonous function of
. The angular patterns of the gravitational waves emitted are examined.
Our analysis includes the two polarization modes present in wave zone
curvature.Comment: 10 pages, 5 figures. arXiv admin note: substantial text overlap with
arXiv:1403.4581, arXiv:1202.1271, arXiv:1111.122
Chaos and a Resonance Mechanism for Structure Formation in Inflationary Models
We exhibit a resonance mechanism of amplification of density perturbations in
inflationary mo-dels, using a minimal set of ingredients (an effective
cosmological constant, a scalar field minimally coupled to the gravitational
field and matter), common to most models in the literature of inflation. This
mechanism is based on the structure of homoclinic cylinders, emanating from an
unstable periodic orbit in the neighborhood of a saddle-center critical point,
present in the phase space of the model. The cylindrical structure induces
oscillatory motions of the scales of the universe whenever the orbit visits the
neighborhood of the saddle-center, before the universe enters a period of
exponential expansion. The oscillations of the scale functions produce, by a
resonance mechanism, the amplification of a selected wave number spectrum of
density perturbations, and can explain the hierarchy of scales observed in the
actual universe. The transversal crossings of the homoclinic cylinders induce
chaos in the dynamics of the model, a fact intimately connected to the
resonance mechanism occuring immediately before the exit to inflation.Comment: 4 pages. This essay received an Honorable Mention from the Gravity
Research Foundation, 1998-Ed. To appear in Mod. Phys. Lett.
Longevidade da solução de Verhöeff
As fibras elásticas representam cerca de 5% do tecido conjuntivo. São constituídas por elastina e microfibrilhas sendo responsáveis pelas propriedades de flexibilidade e retracção elástica dos tecidos. A Técnica de Verhoeff é o método mais comum para demonstrar a presença de fibras elásticas, proporcionando bons resultados. Esta técnica baseia-se num método regressivo, sendo preparada apenas aquando da sua utilização. No entanto, Bancroft e Gamble indicam que resultados satisfatórios têm sido obtidos com a utilização da solução até 48 horas. Estabeleceu-se como objectivo geral comparar a solução de Verhoeff extemporânea e a solução de Verhoeff não extemporânea, verificando a posteriori se ambas permitem obter resultados satisfatórios similares. Definindo-se especificamente: determinar a influência do tempo de repouso da solução de Verhoeff não extemporânea na coloração das fibras elásticas de cortes histológicos de pele, pulmão e artéria de origem suína; analisar e comparar microscopicamente as diferenças entre a solução extemporânea de Verhoeff (lâminas controlo) e o conjunto de diferentes tempos de repouso dessa mesma solução; aferir a longevidade da solução de Verhoeff, indicar o tempo de repouso de solução de Verhoeff até ao qual esta é passível de utilização
In silico scrutiny of genes revealing phylogenetic congruence with clinical prevalence or tropism properties of Chlamydia trachomatis strains
Microbes possess a multiplicity of virulence factors that confer them the ability to specifically
infect distinct biological niches. Contrary to what is known for other bacteria, for the obligate intracellular
human pathogen Chlamydia trachomatis, the knowledge of the molecular basis underlying serovars’ tissue
specificity is scarce. We examined all ~900 genes to evaluate the association between individual phylogenies
and cell-appetence or ecological success of C. trachomatis strains. Only ~1% of the genes presented a tree
topology showing the segregation of all three disease groups (ocular, urogenital, and lymphatic) into three wellsupported
clades. Approximately 28% of the genes, which include the majority of the genes encoding putative
type III secretion system effectors and Inc proteins, present a phylogenetic tree where only lymphogranuloma
venereum strains form a clade. Similarly, an exclusive phylogenetic segregation of the most prevalent genital
serovars was observed for 61 proteins. Curiously, these serovars are phylogenetically cosegregated with the
lymphogranuloma venereum serovars for ~20% of the genes. Some clade-specific pseudogenes were identified
(novel findings include the conserved hypothetical protein CT037 and the predicted a-hemolysin CT473),
suggesting their putative expendability for the infection of particular niches. Approximately 3.5% of the genes
revealed a significant overrepresentation of nonsynonymous mutations, and the majority encode proteins that
directly interact with the host. Overall, this in silico scrutiny of genes whose phylogeny is congruent with clinical
prevalence or tissue specificity of C. trachomatis strains may constitute an important database of putative targets
for future functional studies to evaluate their biological role in chlamydial infections.This work was supported by a grant, ERA-PTG/0004/2010, from
Fundação para a Ciência e a Tecnologia (FCT) (to J.P.G.), in the frame
of ERA-NET PathoGenoMics. A.N. is recipient of a FCT post-doctoral
fellowship (SFRH/BPD/75295/2010), V.B. and R.F. are recipients of
Ph.D. fellowships (SFRH/BD/68527/2010 and SFRH/BD/68532/2010,
respectively) from FCT, and V.D. is a recipient of fellowship on behalf
of the grant ERA-PTG/0004/2010
The low dimensional dynamical system approach in General Relativity: an example
In this paper we explore one of the most important features of the Galerkin
method, which is to achieve high accuracy with a relatively modest
computational effort, in the dynamics of Robinson-Trautman spacetimes.Comment: 7 pages, 5 figure
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