7,975 research outputs found
Some Considerations on Discrete Quantum Gravity
Recent results in Local Regge Calculus are confronted with Spin Foam
Formalism. Introducing Barrett-Crane Quantization in Local Regge Calculus makes
it possible to associate a unique Spin with an hinge , fulfilling
one of the requirements of Spin Foam definition. It is shown that inter-twiner
terms of Spin Foam can follow from the closure constraint in Local Regge
Calculus.
Dedicated to Beppe Marmo for his 65th BirthdayComment: 7 pages, FunInGeo Conference proceedings, Ischia-Italy, 08-12 June
2011; accepted for publication in the International Journal of Geometric
Methods in Modern Physic
From Local Regge Calculus towards Spin Foam Formalism?
We introduce the basic elements of SO(n)-local theory of Regge Calculus. A
first order formalism, in the sense of Palatini, is defined on the metric-dual
Voronoi complex of a simplicial complex. The Quantum Measure exhibits an
expansion, in four dimensions, in characters of irreducible representation of
SO(4) which has close resemblance and differences as well with the Spin Foam
Formalism. The coupling with fermionic matter is easily introduced which could
have consequences for the Spin Foam Formalism and Loop Quantum Gravity.Comment: 14 pages, 1 figures, proceeding of the Albert Einstein's
International Conference 18-22 July 200
Evaluation of the strain-line patterns in a human left ventricle: A simulation study
The aim of this paper is to emphasise the role of the primary strain-line patterns in a human left ventricle (LV) within the complex system that is the heart. In particular, a protocol is proposed for the measurement of the principal strain lines (PSL) in the walls of the LV; this protocol is tested by means of a computational model which resembles a human LV. When the analysis is focused on the epicardial surface, PSL can be used to derive information on the directions of muscle fibres during the entire cardiac cycle, not only the systolic phase. © 2013 Taylor & Francis
Jet-driven and jet-less fireballs from compact binary mergers
During a compact binary merger involving at least one neutron star, a small
fraction of the gravitational energy could be liberated in such a way to
accelerate a small fraction (~ 10^-6) of the neutron star mass in an isotropic
or quasi-isotropic way. In presence of certain conditions, a pair-loaded
fireball can form, which undergoes accelerated expansion reaching relativistic
velocities. As in the standard fireball scenario, internal energy is partly
transformed into kinetic energy. At the photospheric radius, the internal
radiation can escape, giving rise to a pulse that lasts for a time equal to the
delay time since the merger. The subsequent interaction with the interstellar
medium can then convert part of the remaining kinetic energy back into
radiation in a weak isotropic afterglow at all wavelengths. This scenario does
not require the presence of a jet: the associated isotropic prompt and
afterglow emission should be visible for all NS-NS and BH-NS mergers within 90
Mpc, independent of their inclination. The prompt emission is similar to that
expected from an off-axis jet, either structured or much slower than usually
assumed (Gamma ~ 10), or from the jet cocoon. The predicted afterglow emission
properties can discriminate among these scenarios.Comment: 5 pages, 1 figure, revised version submitted to MNRAS Letter
String duality transformations in gravity from Noether symmetry approach
We select gravity models that undergo scale factor duality
transformations. As a starting point, we consider the tree-level effective
gravitational action of bosonic String Theory coupled with the dilaton field.
This theory inherits the Busher's duality of its parent String Theory. Using
conformal transformations of the metric tensor, it is possible to map the
tree-level dilaton-graviton string effective action into gravity,
relating the dilaton field to the Ricci scalar curvature. Furthermore, the
duality can be framed under the standard of Noether symmetries and exact
cosmological solutions are derived. Using suitable changes of variables, the
string-based Lagrangians are shown in cases where the duality
transformation becomes a parity inversion.Comment: v1: 13 pages; v2: minor rephrasings, published versio
Contraceptive confidence and timing of first birth in Moldova: an event history analysis of retrospective data
Objectives: To test the contraceptive confidence hypothesis in a modern context. The hypothesis is that women using effective or modern contraceptive methods have increased contraceptive confidence and hence a shorter interval between marriage and first birth than users of ineffective or traditional methods. We extend the hypothesis to incorporate the role of abortion, arguing that it acts as a substitute for contraception in the study context.Setting: Moldova, a country in South-East Europe. Moldova exhibits high use of traditional contraceptive methods and abortion compared with other European countries.Participants: Data are from a secondary analysis of the 2005 Moldovan Demographic and Health Survey, a nationally representative sample survey. 5377 unmarried women were selected.Primary and secondary outcome measures: The outcome measure was the interval between marriage and first birth. This was modelled using a piecewise-constant hazard regression, with abortion and contraceptive method types as primary variables along with relevant sociodemographic controls.Results: Women with high contraceptive confidence (modern method users) have a higher cumulative hazard of first birth 36?months following marriage (0.88 (0.87 to 0.89)) compared with women with low contraceptive confidence (traditional method users, cumulative hazard: 0.85 (0.84 to 0.85)). This is consistent with the contraceptive confidence hypothesis. There is a higher cumulative hazard of first birth among women with low (0.80 (0.79 to 0.80)) and moderate abortion propensities (0.76 (0.75 to 0.77)) than women with no abortion propensity (0.73 (0.72 to 0.74)) 24?months after marriage.Conclusions: Effective contraceptive use tends to increase contraceptive confidence and is associated with a shorter interval between marriage and first birth. Increased use of abortion also tends to increase contraceptive confidence and shorten birth duration, although this effect is non-linear—women with a very high use of abortion tend to have lengthy intervals between marriage and first birth
Interpreting GRB170817A as a giant flare from a jet-less double neutron-star merger
We show that the delay between GRB170817A and GW170817 is incompatible with
de-beamed emission from an off-axis relativistic jet. The prompt emission and
the subsequent radio and X-ray observations can instead be interpreted within a
giant-flare-like scenario, being the result of a relativistic outflow driven by
the ultra-strong magnetic field produced by magnetohydrodynamic amplification
during the merger of the progenitor double neutron-star binary. Within such
picture, the data indicate that the outflow must be endowed with a steep
velocity profile, with a relatively fast tail extending to Gamma~8. Since the
conditions for the launch of such an outflow are quite general, and the
presence of a velocity profile is a natural expectation of the acceleration
process, most neutron star binary mergers should feature this quasi-isotropic,
hard X-ray emission component, that can be a powerful guide to the discovery of
additional kilonovae associated to relatively nearby gravitational wave events.Comment: 6 pages, 2 figures, accepted by Astronomy and Astrophysic
Taming the Non Abelian Born-Infeld Action
We show how to reduce the non abelian Born-Infeld action describing the
interaction of two D-particles to the sum of elliptic integrals depending on
simple kinematic invariants. This representation gives explicitly all alpha'
corrections to D-particle dynamics. The alpha' corrections induce a
stabilization of the classical trajectories such as the ``eikonal'' which are
unstable within the Yang-Mills approximation.Comment: 17 pages, Latex, 6 figure
Stochastic Properties of Static Friction
The onset of frictional motion is mediated by rupture-like slip fronts, which
nucleate locally and propagate eventually along the entire interface causing
global sliding. The static friction coefficient is a macroscopic measure of the
applied force at this particular instant when the frictional interface loses
stability. However, experimental studies are known to present important scatter
in the measurement of static friction; the origin of which remains unexplained.
Here, we study the nucleation of local slip at interfaces with slip-weakening
friction of random strength and analyze the resulting variability in the
measured global strength. Using numerical simulations that solve the
elastodynamic equations, we observe that multiple slip patches nucleate
simultaneously, many of which are stable and grow only slowly, but one reaches
a critical length and starts propagating dynamically. We show that a
theoretical criterion based on a static equilibrium solution predicts
quantitatively well the onset of frictional sliding. We develop a Monte-Carlo
model by adapting the theoretical criterion and pre-computing modal convolution
terms, which enables us to run efficiently a large number of samples and to
study variability in global strength distribution caused by the stochastic
properties of local frictional strength. The results demonstrate that an
increasing spatial correlation length on the interface, representing geometric
imperfections and roughness, causes lower global static friction. Conversely,
smaller correlation length increases the macroscopic strength while its
variability decreases. We further show that randomness in local friction
properties is insufficient for the existence of systematic precursory slip
events. Random or systematic non-uniformity in the driving force, such as
potential energy or stress drop, is required for arrested slip fronts. Our
model and observations..
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