17,977 research outputs found
On the stability of spacelike hypersurfaces
In this paper we study the strong stability of spacelike hypersurfaces with
constant -th mean curvature in Generalized Robertson-Walker spacetimes of
constant sectional curvature. In particular, we treat the case in which the
ambient spacetime is the de Sitter space
Influence of disordered porous media in the anomalous properties of a simple water model
The thermodynamic, dynamic and structural behavior of a water-like system
confined in a matrix is analyzed for increasing confining geometries. The
liquid is modeled by a two dimensional associating lattice gas model that
exhibits density and diffusion anomalies, in similarity to the anomalies
present in liquid water. The matrix is a triangular lattice in which fixed
obstacles impose restrictions to the occupation of the particles. We show that
obstacules shortens all lines, including the phase coexistence, the critical
and the anomalous lines. The inclusion of a very dense matrix not only suppress
the anomalies but also the liquid-liquid critical point
Kinematics of a Spacetime with an Infinite Cosmological Constant
A solution of the sourceless Einstein's equation with an infinite value for
the cosmological constant \Lambda is discussed by using Inonu-Wigner
contractions of the de Sitter groups and spaces. When \Lambda --> infinity,
spacetime becomes a four-dimensional cone, dual to Minkowski space by a
spacetime inversion. This inversion relates the four-cone vertex to the
infinity of Minkowski space, and the four-cone infinity to the Minkowski
light-cone. The non-relativistic limit c --> infinity is further considered,
the kinematical group in this case being a modified Galilei group in which the
space and time translations are replaced by the non-relativistic limits of the
corresponding proper conformal transformations. This group presents the same
abstract Lie algebra as the Galilei group and can be named the conformal
Galilei group. The results may be of interest to the early Universe Cosmology.Comment: RevTex, 7 pages, no figures. Presentation changes, including a new
Title. Version to appear in Found. Phys. Let
Cosmic microwave background constraints on the epoch of reionization
We use a compilation of cosmic microwave anisotropy data to constrain the
epoch of reionization in the Universe, as a function of cosmological
parameters. We consider spatially-flat cosmologies, varying the matter density
(the flatness being restored by a cosmological constant), the Hubble
parameter and the spectral index of the primordial power spectrum. Our
results are quoted both in terms of the maximum permitted optical depth to the
last-scattering surface, and in terms of the highest allowed reionization
redshift assuming instantaneous reionization. For critical-density models,
significantly-tilted power spectra are excluded as they cannot fit the current
data for any amount of reionization, and even scale-invariant models must have
an optical depth to last scattering of below 0.3. For the currently-favoured
low-density model with and a cosmological constant, the
earliest reionization permitted to occur is at around redshift 35, which
roughly coincides with the highest estimate in the literature. We provide
general fitting functions for the maximum permitted optical depth, as a
function of cosmological parameters. We do not consider the inclusion of tensor
perturbations, but if present they would strengthen the upper limits we quote.Comment: 9 pages LaTeX file with ten figures incorporated (uses mn.sty and
epsf). Corrects some equation typos, superseding published versio
Automatic Face Recognition System Based on Local Fourier-Bessel Features
We present an automatic face verification system inspired by known properties
of biological systems. In the proposed algorithm the whole image is converted
from the spatial to polar frequency domain by a Fourier-Bessel Transform (FBT).
Using the whole image is compared to the case where only face image regions
(local analysis) are considered. The resulting representations are embedded in
a dissimilarity space, where each image is represented by its distance to all
the other images, and a Pseudo-Fisher discriminator is built. Verification test
results on the FERET database showed that the local-based algorithm outperforms
the global-FBT version. The local-FBT algorithm performed as state-of-the-art
methods under different testing conditions, indicating that the proposed system
is highly robust for expression, age, and illumination variations. We also
evaluated the performance of the proposed system under strong occlusion
conditions and found that it is highly robust for up to 50% of face occlusion.
Finally, we automated completely the verification system by implementing face
and eye detection algorithms. Under this condition, the local approach was only
slightly superior to the global approach.Comment: 2005, Brazilian Symposium on Computer Graphics and Image Processing,
18 (SIBGRAPI
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