12,164 research outputs found
Metric Solutions in Torsionless Gauge for Vacuum Conformal Gravity
In a recent paper we have established the form of the metric-torsional
conformal gravitational field equations, and in the present paper we study
their vacuum configurations; we will consider a specific situation that will
enable us to look for the torsionless limit: two types of special exact
solutions are found eventually. A discussion on general remarks will follow.Comment: 11 page
Static quantum corrections to the Schwarzschild spacetime
We study static quantum corrections of the Schwarzschild metric in the
Boulware vacuum state. Due to the absence of a complete analytic expression for
the full semiclassical Einstein equations we approach the problem by
considering the s-wave approximation and solve numerically the associated
backreaction equations. The solution, including quantum effects due to pure
vacuum polarization, is similar to the classical Schwarzschild solution up to
the vicinity of the classical horizon. However, the radial function has a
minimum at a time-like surface close to the location of the classical event
horizon. There the g_{00} component of the metric reaches a very small but
non-zero value. The analysis unravels how a curvature singularity emerges
beyond this bouncing point. We briefly discuss the physical consequences of
these results by extrapolating them to a dynamical collapsing scenario.Comment: 10 pages; Talk given at QG05, Cala Gonone (Italy), September 200
Semiclassical zero-temperature corrections to Schwarzschild spacetime and holography
Motivated by the quest for black holes in AdS braneworlds, and in particular
by the holographic conjecture relating 5D classical bulk solutions with 4D
quantum corrected ones, we numerically solve the semiclassical Einstein
equations (backreaction equations) with matter fields in the (zero temperature)
Boulware vacuum state. In the absence of an exact analytical expression for
in four dimensions we work within the s-wave approximation. Our
results show that the quantum corrected solution is very similar to
Schwarzschild till very close to the horizon, but then a bouncing surface for
the radial function appears which prevents the formation of an event horizon.
We also analyze the behavior of the geometry beyond the bounce, where a
curvature singularity arises. In the dual theory, this indicates that the
corresponding 5D static classical braneworld solution is not a black hole but
rather a naked singularity.Comment: 26 pages, 4 figures; revised version (title changed, conclusions
shortened), published as Phys. Rev. D73, 104023 (2006
A Discussion on Dirac Field Theory, No-Go Theorems and Renormalizability
We study Dirac field equations coupled to electrodynamics with metric and
torsion fields: we discuss how special spinorial solutions are incompatible
with torsion; eventually these results will be used to sketch a discussion on
the problem of renormalizability of point-like particles.Comment: 10 page
Semiclassical Gravity in the Far Field Limit of Stars, Black Holes, and Wormholes
Semiclassical gravity is investigated in a large class of asymptotically
flat, static, spherically symmetric spacetimes including those containing
static stars, black holes, and wormholes. Specifically the stress-energy
tensors of massless free spin 0 and spin 1/2 fields are computed to leading
order in the asymptotic regions of these spacetimes. This is done for spin 0
fields in Schwarzschild spacetime using a WKB approximation. It is done
numerically for the spin 1/2 field in Schwarzschild, extreme
Reissner-Nordstrom, and various wormhole spacetimes. And it is done by finding
analytic solutions to the leading order mode equations in a large class of
asymptotically flat static spherically symmetric spacetimes. Agreement is shown
between these various computational methods. It is found that for all of the
spacetimes considered, the energy density and pressure in the asymptotic region
are proportional to 1/r^5 to leading order. Furthermore, for the spin 1/2 field
and the conformally coupled scalar field, the stress-energy tensor depends only
on the leading order geometry in the far field limit. This is also true for the
minimally coupled scalar field for spacetimes containing either a static star
or a black hole, but not for spacetimes containing a wormhole.Comment: 43 pages, 2 figures. Reference added, minor changes, PRD versio
A modified theory of gravity with torsion and its applications to cosmology and particle physics
In this paper we consider the most general least-order derivative theory of
gravity in which not only curvature but also torsion is explicitly present in
the Lagrangian, and where all independent fields have their own coupling
constant: we will apply this theory to the case of ELKO fields, which is the
acronym of the German \textit{Eigenspinoren des LadungsKonjugationsOperators}
designating eigenspinors of the charge conjugation operator, and thus they are
a Majorana-like special type of spinors; and to the Dirac fields, the most
general type of spinors. We shall see that because torsion has a coupling
constant that is still undetermined, the ELKO and Dirac field equations are
endowed with self-interactions whose coupling constant is undetermined: we
discuss different applications according to the value of the coupling constants
and the different properties that consequently follow. We highlight that in
this approach, the ELKO and Dirac field's self-interactions depend on the
coupling constant as a parameter that may even make these non-linearities
manifest at subatomic scales.Comment: 21 page
From Multiview Image Curves to 3D Drawings
Reconstructing 3D scenes from multiple views has made impressive strides in
recent years, chiefly by correlating isolated feature points, intensity
patterns, or curvilinear structures. In the general setting - without
controlled acquisition, abundant texture, curves and surfaces following
specific models or limiting scene complexity - most methods produce unorganized
point clouds, meshes, or voxel representations, with some exceptions producing
unorganized clouds of 3D curve fragments. Ideally, many applications require
structured representations of curves, surfaces and their spatial relationships.
This paper presents a step in this direction by formulating an approach that
combines 2D image curves into a collection of 3D curves, with topological
connectivity between them represented as a 3D graph. This results in a 3D
drawing, which is complementary to surface representations in the same sense as
a 3D scaffold complements a tent taut over it. We evaluate our results against
truth on synthetic and real datasets.Comment: Expanded ECCV 2016 version with tweaked figures and including an
overview of the supplementary material available at
multiview-3d-drawing.sourceforge.ne
Modèle élasto-plastique constitutif pour les sols fins dans les zones de saturation partielle à totale
International audienceNear saturation, air bubbles and pockets can be trapped in the porous network of soils. The aim of this paper was to present a coupled model that takes into account the effect of this entrapped air on the poro-elasto-plastic behavior of the soil. The model takes into account the physical-mechanical interactions between different phases as well as the kinematics of each constituent (liquid water, dissolved air, gaseous air and solid grains). This new model was implemented in a FEM code. Some numerical simulations were performed to demonstrate its ability to reproduce a continuous transition of unsaturated to saturated states.Près de saturation, les bulles d'air et les poches d'air peuvent être emprisonnées dans le réseau poreux des sols. Le but de cet article est de présenter un modèle couplé qui tient compte de l'effet de cet air emprisonné sur le comportement poro-élasto-plastique du sol. Le modèle prend en compte les interactions physico-mécaniques entre les différentes phases ainsi que la cinématique de chaque constituant (eau liquide, air dissous, air gazeux et grains solides). Ce nouveau modèle a été implémenté dans un code d'éléments finis. Des simulations numériques ont été effectuées pour démontrer sa capacité à reproduire une transition continue d'un état non-saturé vers un état totalement saturé
Anxiety disorders and age-related changes in physiology
Background Anxiety disorders are leading contributors to the global disease burden, highly prevalent across the lifespan and associated with substantially increased morbidity and early mortality. Aims The aim of this study was to examine age-related changes across a wide range of physiological measures in middle-aged and older adults with a lifetime history of anxiety disorders compared with healthy controls. Method The UK Biobank study recruited >500 000 adults, aged 37-73, between 2006 and 2010. We used generalised additive models to estimate non-linear associations between age and hand-grip strength, cardiovascular function, body composition, lung function and heel bone mineral density in a case group and in a control group. Results The main data-set included 332 078 adults (mean age 56.37 years; 52.65% women). In both genders, individuals with anxiety disorders had a lower hand-grip strength and lower blood pressure, whereas their pulse rate and body composition measures were higher than in the healthy control group. Case-control group differences were larger when considering individuals with chronic and/or severe anxiety disorders, and differences in body composition were modulated by depression comorbidity status. Differences in age-related physiological changes between females in the anxiety disorder case group and healthy controls were most evident for blood pressure, pulse rate and body composition, whereas this was the case in males for hand-grip strength, blood pressure and body composition. Most differences in physiological measures between the case and control groups decreased with increasing age. Conclusions Findings in individuals with a lifetime history of anxiety disorders differed from a healthy control group across multiple physiological measures, with some evidence of case-control group differences by age. The differences observed varied by chronicity/severity and depression comorbidity
- …