85 research outputs found
Constraints on non-minimally coupled curved space electrodynamics from astrophysical observations
We study interactions of electro-magnetic fields with the curvature tensor of
the form . Such
coupling terms though are invariant under general coordinate transformation and
CPT, however violate the Einstein equivalence principle. These couplings do not
cause any energy dependent dispersion of photons but they exhibit
birefringence. We put constraints on the coupling constant using
results from solar system radar ranging experiments and millisecond-pulsar
observations. We find that the most stringent constraint comes from pulsar
observations and is given by obtained from the
timing of binary pulsar PSR B1534+12.Comment: 9 pages latex, accepted in CQ
Self-gravitating clouds of generalized Chaplygin and modified anti-Chaplygin Gases
The Chaplygin gas has been proposed as a possible dark energy, dark matter
candidate. As a working fluid in a Friedmann-Robertson-Walker universe, it
exhibits early behavior reminiscent of dark matter, but at later times is more
akin to a cosmological constant. In any such universe, however, one can expect
local perturbations to form. Here we obtain the general equations for a
self-gravitating relativistic Chaplygin gas. We solve these equations and
obtain the mass-radius relationship for such structures, showing that only in
the phantom regime is the mass-radius relationship large enough to be a serious
candidate for highly compact massive objects at the galaxy core. In addition,
we study the cosmology of a modified anti-Chaplygin gas. A self-gravitating
cloud of this matter is an exact solution to Einstein's equations.Comment: 16 page
Ricci Collineations for type B warped space-times
We present the general structure of proper Ricci Collineations (RC) for type
B warped space-times. Within this framework, we give a detailed description of
the most general proper RC for spherically symmetric metrics. As examples,
static spherically symmetric and Friedmann-Robertson-Walker space-times are
considered.Comment: 18 pages, Latex, To appear in GR
Manter ou suspender a revacinação BCG em adolescentes
BCG revaccination was introduced in Brazil in the mid-90's but no study has evaluated vaccine coverage through direct assessment of the scar. BCG scars were assessed in 2,785 public school adolescents, aged 13 to 14 years, in Southeastern Brazil, between September 2001 and May 2002. The prevalence rate of revaccination was 64.3% (95% CI: 62.5-66.0). Despite the efforts to implement and sustain revaccination, continuing this practice should be reevaluated by public health authorities due to the lack of evidence on its effectiveness.A revacinação com BCG foi introduzida no Brasil em meados dos anos 90 e desde então não foram conduzidos estudos que avaliassem a cobertura vacinal alcançada por meio de investigação da cicatriz do BCG. Nesse sentido, foram estudados 2.785 adolescentes de 13-14 anos matriculados em escolas públicas da cidade de Belo Horizonte, Estado de Minas Gerais, entre setembro de 2001 e maio de 2002. A prevalência da revacinação foi de 64,3% (IC 95%: 62,5-66,0). Os esforços empreendidos para obter tais níveis de cobertura conflitam com a falta de evidências sobre sua efetividade e recomendam a reavaliação da continuidade da revacinação nos serviços de saúde brasileiros
On the back reaction of gravitational and particle emission and absorption from straight thick cosmic strings: A toy model
The emission and absorption of gravitational waves and massless particles of
an infinitely long straight cosmic string with finite thickness are studied. It
is shown in a general term that the back reaction of the emission and
absorption {\em always} makes the symmetry axis of the string singular. The
singularity is a scalar singularity and cannot be removed.Comment: To appear in Gen. Relativ. Gra
On certain relationships between cosmological observables in the Einstein-Cartan gravity
We show that in the Einstein-Cartan gravity it is possible to obtain a
relation between Hubble's expansion and the global rotation (vorticity) of the
Universe. Gravitational coupling can be reduced to dimensionless quantity of
order unity, fixing the scalar mass density and the resulting negative
cosmological constant at spacelike infinity. Current estimates of the expansion
and rotation (see also astro-ph/9703082) of the Universe favour the massive
spinning particles as candidate particles for cold and hot dark matter. Nodland
and Ralston vorticity (Phys. Rev. Lett. 78 (1997) 3043) overestimates the value
favoured by the Einstein-Cartan gravity for three orders of magnitude.Comment: 7 pages, LaTeX styl
Taub's plane-symmetric vacuum spacetime revisited
The gravitational properties of the {\em only} static plane-symmetric vacuum
solution of Einstein's field equations without cosmological term (Taub's
solution, for brevity) are presented: some already known properties (geodesics,
weak field limit and pertainment to the Schwarzschild family of spacetimes) are
reviewed in a physically much more transparent way, as well as new results
about its asymptotic structure, possible matchings and nature of the source are
furnished. The main results point to the fact that the solution must be
interpreted as representing the exterior gravitational field due to a {\em
negative} mass distribution, confirming previous statements to that effect in
the literature. Some analogies to Kasner's spatially homogeneous cosmological
model are also referred to.Comment: plain LaTex, four Postscript figure
- …