2,491 research outputs found
Cauchy-perturbative matching and outer boundary conditions I: Methods and tests
We present a new method of extracting gravitational radiation from
three-dimensional numerical relativity codes and providing outer boundary
conditions. Our approach matches the solution of a Cauchy evolution of
Einstein's equations to a set of one-dimensional linear wave equations on a
curved background. We illustrate the mathematical properties of our approach
and discuss a numerical module we have constructed for this purpose. This
module implements the perturbative matching approach in connection with a
generic three-dimensional numerical relativity simulation. Tests of its
accuracy and second-order convergence are presented with analytic linear wave
data.Comment: 13 pages, 6 figures, RevTe
Cauchy-perturbative matching and outer boundary conditions: computational studies
We present results from a new technique which allows extraction of
gravitational radiation information from a generic three-dimensional numerical
relativity code and provides stable outer boundary conditions. In our approach
we match the solution of a Cauchy evolution of the nonlinear Einstein field
equations to a set of one-dimensional linear equations obtained through
perturbation techniques over a curved background. We discuss the validity of
this approach in the case of linear and mildly nonlinear gravitational waves
and show how a numerical module developed for this purpose is able to provide
an accurate and numerically convergent description of the gravitational wave
propagation and a stable numerical evolution.Comment: 20 pages, RevTe
Critical behavior at Mott-Anderson transition: a TMT-DMFT perspective
We present a detailed analysis of the critical behavior close to the
Mott-Anderson transition. Our findings are based on a combination of numerical
and analytical results obtained within the framework of Typical-Medium Theory
(TMT-DMFT) - the simplest extension of dynamical mean field theory (DMFT)
capable of incorporating Anderson localization effects. By making use of
previous scaling studies of Anderson impurity models close to the
metal-insulator transition, we solve this problem analytically and reveal the
dependence of the critical behavior on the particle-hole symmetry. Our main
result is that, for sufficiently strong disorder, the Mott-Anderson transition
is characterized by a precisely defined two-fluid behavior, in which only a
fraction of the electrons undergo a "site selective" Mott localization; the
rest become Anderson-localized quasiparticles.Comment: 4+ pages, 4 figures, v2: minor changes, accepted for publication in
Phys. Rev. Let
Nevirapine-Induced Heratotoxicity: Incidence, Risk Factors and Associated Mortality in a Primary Care ART Programme in South Africa
CROI 200
Guns and gender-based violence in South Africa
Background. The criminal use of firearms in South Africa is widespread and a major factor in the country having the thirdhighest homicide rate in the world. Violence is a common feature of South African society. A firearm in the home is a risk factor in intimate partner violence, but this has not been readily demonstrated in South Africa because of a lack of data. Methods. We drew on several South African studies including national homicide studies, intimate partner studies, studies with male participants and studies from the justice sector, to discuss the role of gun ownership in gender-based violence. Conclusion. Guns play a significant role in violence against women in South Africa, most notably in the killing of intimate partners. Although the overall homicide data suggest that death by shooting is decreasing, data for intimate partner violence are not readily available. We have no idea if the overall decrease in gunshot homicides applies to women in relationships, and therefore gun control should remain high on the legislative agenda
Waveform propagation in black hole spacetimes: evaluating the quality of numerical solutions
We compute the propagation and scattering of linear gravitational waves off a
Schwarzschild black hole using a numerical code which solves a generalization
of the Zerilli equation to a three dimensional cartesian coordinate system.
Since the solution to this problem is well understood it represents a very good
testbed for evaluating our ability to perform three dimensional computations of
gravitational waves in spacetimes in which a black hole event horizon is
present.Comment: 13 pages, RevTeX, to appear in Phys. Rev.
Spin Dynamics of a J1-J2-K Model for the Paramagnetic Phase of Iron Pnictides
We study the finite-temperature spin dynamics of the paramagnetic phase of
iron pnictides within an antiferromagnetic J_1-J_2 Heisenberg model on a square
lattice with a biquadratic coupling between the
nearest-neighbor spins. Our focus is on the paramagnetic phase in the parameter
regime of this J_1-J_2-K model where the ground state is a (\pi,0) collinear
antiferromagnet. We treat the biquadratic interaction via a
Hubbard-Stratonovich decomposition, and study the resulting effective
quadratic-coupling model using both modified spin wave and Schwinger boson
mean-field theories; the results for the spin dynamics derived from the two
methods are very similar. We show that the spectral weight of dynamical
structure factor S(q,\omega) is peaked at ellipses in the momentum space at low
excitation energies. With increasing energy, the elliptic features expand
towards the zone boundary, and gradually split into two parts, forming a
pattern around (\pi,\pi). Finally, the spectral weight is anisotropic, being
larger along the major axis of the ellipse than along its minor axis. These
characteristics of the dynamical structure factor are consistent with the
recent measurements of the inelastic neutron scattering spectra on BaFe_2As_2
and SrFe_2As_2.Comment: 13 pages, 11 figures, to be published in Phys. Rev.
Ill-posedness in the Einstein equations
It is shown that the formulation of the Einstein equations widely in use in
numerical relativity, namely, the standard ADM form, as well as some of its
variations (including the most recent conformally-decomposed version), suffers
from a certain but standard type of ill-posedness. Specifically, the norm of
the solution is not bounded by the norm of the initial data irrespective of the
data. A long-running numerical experiment is performed as well, showing that
the type of ill-posedness observed may not be serious in specific practical
applications, as is known from many numerical simulations.Comment: 13 pages, 3 figures, accepted for publication in Journal of
Mathematical Physics (to appear August 2000
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