951 research outputs found
Black Hole Mergers and Unstable Circular Orbits
We describe recent numerical simulations of the merger of a class of equal
mass, non-spinning, eccentric binary black hole systems in general relativity.
We show that with appropriate fine-tuning of the initial conditions to a region
of parameter space we denote the threshold of immediate merger, the binary
enters a phase of close interaction in a near-circular orbit, stays there for
an amount of time proportional to logarithmic distance from the threshold in
parameter space, then either separates or merges to form a single Kerr black
hole. To gain a better understanding of this phenomena we study an analogous
problem in the evolution of equatorial geodesics about a central Kerr black
hole. A similar threshold of capture exists for appropriate classes of initial
conditions, and tuning to threshold the geodesics approach one of the unstable
circular geodesics of the Kerr spacetime. Remarkably, with a natural mapping of
the parameters of the geodesic to that of the equal mass system, the scaling
exponent describing the whirl phase of each system turns out to be quite
similar. Armed with this lone piece of evidence that an approximate
correspondence might exist between near-threshold evolution of geodesics and
generic binary mergers, we illustrate how this information can be used to
estimate the cross section and energy emitted in the ultra relativistic black
hole scattering problem. This could eventually be of use in providing estimates
for the related problem of parton collisions at the Large Hadron Collider in
extra dimension scenarios where black holes are produced.Comment: 16 pages, 12 figures; updated to coincide with journal versio
The role of abuse in the development of irritable bowel syndrome: a comparative study: research
Irritable Bowel Syndrome (IBS) is defined as a chronic relapsing functional bowel disorder of unknown causes which is characterised by attacks of abdominal pain and change of bowel habit resulting in diarrhoea or constipation or both. The prevalence of IBS in the general population of Western countries is 14-24% of women and more commonly found in women than in men. Stress can exacerbate IBS and a significant association between IBS and sexual abuse and physical abuse in childhood and adulthood has been found in overseas studies. No such studies have however been conducted in South Africa. The role that abuse may play in the development of IBS forms the cornerstone of the present study. The participants consisted of 79 adult women aged over 20 years. They were grouped into four categories: abused women with IBS; abused women without IBS; non-abused women without IBS; and non-abused women with IBS. A statistical analysis was carried out to ascertain if there were any differences between the groups in terms of the sub-type of IBS, as well as the types of abuse suffered. The groups were also compared with regard to the levels of anger and anxiety experienced.
Prikkelbare Dermsindroom (PDS) word gedefinieer as 'n chroniese, herhalende, funksionele ingewandsversteuring wat gekenmerk word deur aanvalle van buikpyn en 'n verandering in ingewandsgewoontes, wat diarree of hardlywigheid, of beide, tot gevolg het. PDS kom voor by 14 - 24% van die vroulike populasie in Westerse lande en word meer by vroue as by mans aangetref. Volgens die verbandhoudende literatuur kan stres PDS vererger en 'n betekenisvolle verband is gevind tussen PDS en sowel seksuele as fisiese mishandeling in die kinderjare en volwassenheid. Die rol wat mishandeling kan speel in die ontwikkeling van PDS vorm die kern van die huidige studie. 'n Statistiese analise is gedoen om te bepaal of daar enige verskille tussen die groepe bestaan in terme van die subtipe van PDS wat voorkom, sowel as die tipes mishandeling wat ervaar is. Die groepe is ook vergelyk in terme van hulle vlakke van woede en angs.
Keywords: Functional Gastrointestinal disorders, Irritable Bowel Syndrome, Sexual abuse, Physical abuse and emotional abuse
(Health SA Gesondheid: interdisciplinary research journal: 2003 8(4): 88-98
An exact solution for 2+1 dimensional critical collapse
We find an exact solution in closed form for the critical collapse of a
scalar field with cosmological constant in 2+1 dimensions. This solution agrees
with the numerical simulation done by Pretorius and Choptuik of this system.Comment: 5 pages, 5 figures, Revtex. New comparison of analytic and numerical
solutions beyond the past light cone of the singularity added. Two new
references added. Error in equation (21) correcte
Learning about compact binary merger: the interplay between numerical relativity and gravitational-wave astronomy
Activities in data analysis and numerical simulation of gravitational waves
have to date largely proceeded independently. In this work we study how
waveforms obtained from numerical simulations could be effectively used within
the data analysis effort to search for gravitational waves from black hole
binaries. We propose measures to quantify the accuracy of numerical waveforms
for the purpose of data analysis and study how sensitive the analysis is to
errors in the waveforms. We estimate that ~100 templates (and ~10 simulations
with different mass ratios) are needed to detect waves from non-spinning binary
black holes with total masses in the range 100 Msun < M < 400 Msun using
initial LIGO. Of course, many more simulation runs will be needed to confirm
that the correct physics is captured in the numerical evolutions. From this
perspective, we also discuss sources of systematic errors in numerical waveform
extraction and provide order of magnitude estimates for the computational cost
of simulations that could be used to estimate the cost of parameter space
surveys. Finally, we discuss what information from near-future numerical
simulations of compact binary systems would be most useful for enhancing the
detectability of such events with contemporary gravitational wave detectors and
emphasize the role of numerical simulations for the interpretation of eventual
gravitational-wave observations.Comment: 19 pages, 12 figure
Simulation of Asymptotically AdS5 Spacetimes with a Generalized Harmonic Evolution Scheme
Motivated by the gauge/gravity duality, we introduce a numerical scheme based
on generalized harmonic evolution to solve the Einstein field equations on
asymptotically anti-de Sitter (AdS) spacetimes. We work in global AdS5, which
can be described by the (t,r,\chi,\theta,\phi) spherical coordinates adapted to
the R{\times}S3 boundary. We focus on solutions that preserve an SO(3) symmetry
that acts to rotate the 2-spheres parametrized by \theta,\phi. In the boundary
conformal field theory (CFT), the way in which this symmetry manifests itself
hinges on the way we choose to embed Minkowski space in R{\times}S3. We present
results from an ongoing study of prompt black hole formation via scalar field
collapse, and explore the subsequent quasi-normal ringdown. Beginning with
initial data characterized by highly distorted apparent horizon geometries, the
metrics quickly evolve, via quasi-normal ringdown, to equilibrium static black
hole solutions at late times. The lowest angular number quasi-normal modes are
consistent with the linear modes previously found in perturbative studies,
whereas the higher angular modes are a combination of linear modes and of
harmonics arising from non-linear mode-coupling. We extract the stress energy
tensor of the dual CFT on the boundary, and find that despite being highly
inhomogeneous initially, it nevertheless evolves from the outset in a manner
that is consistent with a thermalized N=4 SYM fluid. As a first step towards
closer contact with relativistic heavy ion collision physics, we map this
solution to a Minkowski piece of the R{\times}S3 boundary, and obtain a
corresponding fluid flow in Minkowski space
Generalized harmonic formulation in spherical symmetry
In this pedagogically structured article, we describe a generalized harmonic
formulation of the Einstein equations in spherical symmetry which is regular at
the origin. The generalized harmonic approach has attracted significant
attention in numerical relativity over the past few years, especially as
applied to the problem of binary inspiral and merger. A key issue when using
the technique is the choice of the gauge source functions, and recent work has
provided several prescriptions for gauge drivers designed to evolve these
functions in a controlled way. We numerically investigate the parameter spaces
of some of these drivers in the context of fully non-linear collapse of a real,
massless scalar field, and determine nearly optimal parameter settings for
specific situations. Surprisingly, we find that many of the drivers that
perform well in 3+1 calculations that use Cartesian coordinates, are
considerably less effective in spherical symmetry, where some of them are, in
fact, unstable.Comment: 47 pages, 15 figures. v2: Minor corrections, including 2 added
references; journal version
Scalar field collapse in three-dimensional AdS spacetime
We describe results of a numerical calculation of circularly symmetric scalar
field collapse in three spacetime dimensions with negative cosmological
constant. The procedure uses a double null formulation of the Einstein-scalar
equations. We see evidence of black hole formation on first implosion of a
scalar pulse if the initial pulse amplitude is greater than a critical
value . Sufficiently near criticality the apparent horizon radius
grows with pulse amplitude according to the formula .Comment: 10 pages, 1 figure; references added, to appear in CQG(L
Reducing orbital eccentricity in binary black hole simulations
Binary black hole simulations starting from quasi-circular (i.e., zero radial
velocity) initial data have orbits with small but non-zero orbital
eccentricities. In this paper the quasi-equilibrium initial-data method is
extended to allow non-zero radial velocities to be specified in binary black
hole initial data. New low-eccentricity initial data are obtained by adjusting
the orbital frequency and radial velocities to minimize the orbital
eccentricity, and the resulting ( orbit) evolutions are compared with
those of quasi-circular initial data. Evolutions of the quasi-circular data
clearly show eccentric orbits, with eccentricity that decays over time. The
precise decay rate depends on the definition of eccentricity; if defined in
terms of variations in the orbital frequency, the decay rate agrees well with
the prediction of Peters (1964). The gravitational waveforms, which contain
cycles in the dominant l=m=2 mode, are largely unaffected by the
eccentricity of the quasi-circular initial data. The overlap between the
dominant mode in the quasi-circular evolution and the same mode in the
low-eccentricity evolution is about 0.99.Comment: 27 pages, 9 figures; various minor clarifications; accepted to the
"New Frontiers" special issue of CQ
An Exact Solution for Static Scalar Fields Coupled to Gravity in -Dimensions
We obtain an exact solution for the Einstein's equations with cosmological
constant coupled to a scalar, static particle in static, "spherically"
symmetric background in 2+1 dimensions.Comment: 9 pages. Replaced by a revised versio
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