454 research outputs found
On non-existence of static vacuum black holes with degenerate components of the event horizon
We present a simple proof of the non-existence of degenerate components of
the event horizon in static, vacuum, regular, four-dimensional black hole
spacetimes. We discuss the generalisation to higher dimensions and the
inclusion of a cosmological constant.Comment: latex2e, 9 pages in A
Complete Calabi-Yau metrics from Kahler metrics in D=4
In the present work the local form of certain Calabi-Yau metrics possessing a
local Hamiltonian Killing vector is described in terms of a single non linear
equation. The main assumptions are that the complex -form is of the form
, where is preserved by the Killing
vector, and that the space of the orbits of the Killing vector is, for fixed
value of the momentum map coordinate, a complex 4-manifold, in such a way that
the complex structure of the 4-manifold is part of the complex structure of the
complex 3-fold. The link with the solution generating techniques of [26]-[28]
is made explicit and in particular an example with holonomy exactly SU(3) is
found by use of the linearization of [26], which was found in the context of D6
branes wrapping a holomorphic 1-fold in a hyperkahler manifold. But the main
improvement of the present method, unlike the ones presented in [26]-[28], does
not rely in an initial hyperkahler structure. Additionally the complications
when dealing with non linear operators over the curved hyperkahler space are
avoided by use of this method.Comment: Version accepted for publication in Phys.Rev.
Diagnosis of lung cancer – improving survival rates
Lung cancer is a major global health burden with high incidence rates but poor long-term survival. Currently, the majority of cases are diagnosed at an advanced stage when surgical resection is not feasible. Screening for lung cancer has been a major focus of research for the last 40 years. Despite this, there is still a lack of evidence to promote its use outside clinical trials. More recently, interest has focused on promoting earlier recognition of symptomatic disease among both the general public and primary care physicians in order to encourage more timely investigation and referral to secondary care. The hope is that this approach may increase the proportion of disease identified in the early tages, allowing more surgical resections and improved five-year survival rates. This article provides an overview of the current evidence base in terms of early diagnosis of lung cancer and provides some examples of innovations to promote this
On Israel-Wilson-Perjes black holes
We show, under certain conditions, that regular Israel-Wilson-Perj\'es black
holes necessarily belong to the Majumdar-Papapetrou family
Harmonic functions, central quadrics, and twistor theory
Solutions to the -dimensional Laplace equation which are constant on a
central quadric are found. The associated twistor description of the case
is used to characterise Gibbons-Hawking metrics with tri-holomorphic SL(2,
\C) symmetry.Comment: Final version. To appear in CQ
On quasi-local charges and Newman--Penrose type quantities in Yang--Mills theories
We generalize the notion of quasi-local charges, introduced by P. Tod for
Yang--Mills fields with unitary groups, to non-Abelian gauge theories with
arbitrary gauge group, and calculate its small sphere and large sphere limits
both at spatial and null infinity. We show that for semisimple gauge groups no
reasonable definition yield conserved total charges and Newman--Penrose (NP)
type quantities at null infinity in generic, radiative configurations. The
conditions of their conservation, both in terms of the field configurations and
the structure of the gauge group, are clarified. We also calculate the NP
quantities for stationary, asymptotic solutions of the field equations with
vanishing magnetic charges, and illustrate these by explicit solutions with
various gauge groups.Comment: 22 pages, typos corrected, appearing in Classical and Quantum Gravit
All electro--vacuum Majumdar--Papapetrou space--times with nonsingular black holes
We show that all Majumdar--Papapetrou electrovacuum space--times with a
non--empty black hole region and with a non--singular domain of outer
communications are the standard Majumdar--Papapetrou space--times.Comment: 9 pages, Late
Implementing an apparent-horizon finder in three dimensions
Locating apparent horizons is not only important for a complete understanding
of numerically generated spacetimes, but it may also be a crucial component of
the technique for evolving black-hole spacetimes accurately. A scheme proposed
by Libson et al., based on expanding the location of the apparent horizon in
terms of symmetric trace-free tensors, seems very promising for use with
three-dimensional numerical data sets. In this paper, we generalize this scheme
and perform a number of code tests to fully calibrate its behavior in
black-hole spacetimes similar to those we expect to encounter in solving the
binary black-hole coalescence problem. An important aspect of the
generalization is that we can compute the symmetric trace-free tensor expansion
to any order. This enables us to determine how far we must carry the expansion
to achieve results of a desired accuracy. To accomplish this generalization, we
describe a new and very convenient set of recurrence relations which apply to
symmetric trace-free tensors.Comment: 14 pages (RevTeX 3.0 with 3 figures
Quasi-Local Gravitational Energy
A dynamically preferred quasi-local definition of gravitational energy is
given in terms of the Hamiltonian of a `2+2' formulation of general relativity.
The energy is well-defined for any compact orientable spatial 2-surface, and
depends on the fundamental forms only. The energy is zero for any surface in
flat spacetime, and reduces to the Hawking mass in the absence of shear and
twist. For asymptotically flat spacetimes, the energy tends to the Bondi mass
at null infinity and the \ADM mass at spatial infinity, taking the limit along
a foliation parametrised by area radius. The energy is calculated for the
Schwarzschild, Reissner-Nordstr\"om and Robertson-Walker solutions, and for
plane waves and colliding plane waves. Energy inequalities are discussed, and
for static black holes the irreducible mass is obtained on the horizon.
Criteria for an adequate definition of quasi-local energy are discussed.Comment: 16 page
Regge Calculus in Teleparallel Gravity
In the context of the teleparallel equivalent of general relativity, the
Weitzenbock manifold is considered as the limit of a suitable sequence of
discrete lattices composed of an increasing number of smaller an smaller
simplices, where the interior of each simplex (Delaunay lattice) is assumed to
be flat. The link lengths between any pair of vertices serve as independent
variables, so that torsion turns out to be localized in the two dimensional
hypersurfaces (dislocation triangle, or hinge) of the lattice. Assuming that a
vector undergoes a dislocation in relation to its initial position as it is
parallel transported along the perimeter of the dual lattice (Voronoi polygon),
we obtain the discrete analogue of the teleparallel action, as well as the
corresponding simplicial vacuum field equations.Comment: Latex, 10 pages, 2 eps figures, to appear in Class. Quant. Gra
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