2,326 research outputs found
Dual geometries and spacetime singularities
The notion of geometrical duality is discussed in the context of both
Brans-Dicke theory and general relativity. It is shown that, in some particular
solutions, the spacetime singularities that arise in usual Riemannian general
relativity may be avoided in its dual representation (Weyl-type general
relativity). This dual representation provides a singularity-free picture of
the World that is physicaly equivalent to the canonical general relativistic
one.Comment: 11 pages, LaTeX, no figures, version accepted for publication in PR
Gravitational memory of natural wormholes
A traversable wormhole solution of general scalar-tensor field equations is
presented. We have shown, after a numerical analysis for the behavior of the
scalar field of Brans-Dicke theory, that the solution is completely
singularity--free. Furthermore, the analysis of more general scalar field
dependent coupling constants indicates that the gravitational memory phenomenon
may play an important role in the fate of natural wormholes.Comment: 14 pages revtex, 1 ps figur
Usefulness of regional right ventricular and right atrial strain for prediction of early and late right ventricular failure following a left ventricular assist device implant: A machine learning approach
Background: Identifying candidates for left ventricular assist device surgery at risk of right ventricular failure remains difficult. The aim was to identify the most accurate predictors of right ventricular failure among clinical, biological, and imaging markers, assessed by agreement of different supervised machine learning algorithms. Methods: Seventy-four patients, referred to HeartWare left ventricular assist device since 2010 in two Italian centers, were recruited. Biomarkers, right ventricular standard, and strain echocardiography, as well as cath-lab measures, were compared among patients who did not develop right ventricular failure (N = 56), those with acute–right ventricular failure (N = 8, 11%) or chronic–right ventricular failure (N = 10, 14%). Logistic regression, penalized logistic regression, linear support vector machines, and naïve Bayes algorithms with leave-one-out validation were used to evaluate the efficiency of any combination of three collected variables in an “all-subsets” approach. Results: Michigan risk score combined with central venous pressure assessed invasively and apical longitudinal systolic strain of the right ventricular–free wall were the most significant predictors of acute–right ventricular failure (maximum receiver operating characteristic–area under the curve = 0.95, 95% confidence interval = 0.91–1.00, by the naïve Bayes), while the right ventricular–free wall systolic strain of the middle segment, right atrial strain (QRS-synced), and tricuspid annular plane systolic excursion were the most significant predictors of Chronic-RVF (receiver operating characteristic–area under the curve = 0.97, 95% confidence interval = 0.91–1.00, according to naïve Bayes). Conclusion: Apical right ventricular strain as well as right atrial strain provides complementary information, both critical to predict acute–right ventricular failure and chronic–right ventricular failure, respectively
Embeddings in Spacetimes Sourced by Scalar Fields
The extension of the Campbell-Magaard embedding theorem to general relativity
with minimally-coupled scalar fields is formulated and proven. The result is
applied to the case of a self-interacting scalar field for which new embeddings
are found, and to Brans-Dicke theory. The relationship between Campbell-Magaard
theorem and the general relativity, Cauchy and initial value problems is
outlined.Comment: RevTEX (11 pages)/ To appear in the Journal of Mathematical Physic
Can electro-magnetic field, anisotropic source and varying be sufficient to produce wormhole spacetime ?
It is well known that solutions of general relativity which allow for
traversable wormholes require the existence of exotic matter (matter that
violates weak or null energy conditions [WEC or NEC]). In this article, we
provide a class of exact solution for Einstein-Maxwell field equations
describing wormholes assuming the erstwhile cosmological term to be
space variable, viz., .
The source considered here not only a matter entirely but a sum of matters
i.e. anisotropic matter distribution, electromagnetic field and cosmological
constant whose effective parts obey all energy conditions out side the wormhole
throat. Here violation of energy conditions can be compensated by varying
cosmological constant. The important feature of this article is that one can
get wormhole structure, at least theoretically, comprising with physically
acceptable matters.Comment: Some changes have been mad
Classical Euclidean wormhole solutions in Palatini cosmology
We study the classical Euclidean wormholes in the context of extended
theories of gravity. With no loss of generality, we use the dynamical
equivalence between gravity and scalar-tensor theories to
construct a point-like Lagrangian in the flat FRW space time. We first show the
dynamical equivalence between Palatini gravity and the
Brans-Dicke theory with self-interacting potential, and then show the dynamical
equivalence between the Brans-Dicke theory with self-interacting potential and
the minimally coupled O'Hanlon theory. We show the existence of new Euclidean
wormhole solutions for this O'Hanlon theory and, for an special case, find out
the corresponding form of having wormhole solution. For small
values of the Ricci scalar, this is in agreement with the
wormhole solution obtained for higher order gravity theory .Comment: 11 page
Brans-Dicke wormholes in the Jordan and Einstein frames
We examine the possibility of static wormhole solutions in the vacuum
Brans-Dicke theory both in the original (Jordan) frame and in the conformally
rescaled (Einstein) frame. It turns out that, in the former frame, wormholes
exist only in a very narrow interval of the coupling parameter, viz.,
-3/2<omega<-4/3. It is shown that these wormholes are not traversable in
practice. In the latter frame, wormhole solutions do not exist at all unless
energy conditions are violated by hand.Comment: Minor errors corrected, uploaded for the benefit of the researcher
On a class of stable, traversable Lorentzian wormholes in classical general relativity
It is known that Lorentzian wormholes must be threaded by matter that
violates the null energy condition. We phenomenologically characterize such
exotic matter by a general class of microscopic scalar field Lagrangians and
formulate the necessary conditions that the existence of Lorentzian wormholes
imposes on them. Under rather general assumptions, these conditions turn out to
be strongly restrictive. The most simple Lagrangian that satisfies all of them
describes a minimally coupled massless scalar field with a reversed sign
kinetic term. Exact, non-singular, spherically symmetric solutions of
Einstein's equations sourced by such a field indeed describe traversable
wormhole geometries. These wormholes are characterized by two parameters: their
mass and charge. Among them, the zero mass ones are particularly simple,
allowing us to analytically prove their stability under arbitrary space-time
dependent perturbations. We extend our arguments to non-zero mass solutions and
conclude that at least a non-zero measure set of these solutions is stable.Comment: 23 pages, 4 figures, uses RevTeX4. v2: Changes to accommodate added
references. Statement about masses of the wormhole correcte
A quantum-like description of the planetary systems
The Titius-Bode law for planetary distances is reviewed. A model describing
the basic features of this rule in the "quantum-like" language of a wave
equation is proposed. Some considerations about the 't Hooft idea on the
quantum behaviour of deterministic systems with dissipation are discussed.Comment: LaTex file, 17 pages, no figures. Version published in Foundations of
Physics, August 200
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