665 research outputs found
On the Weyl Curvature Hypothesis
The Weyl curvature hypothesis of Penrose attempts to explain the high
homogeneity and isotropy, and the very low entropy of the early universe, by
conjecturing the vanishing of the Weyl tensor at the Big-Bang singularity.
In previous papers it has been proposed an equivalent form of Einstein's
equation, which extends it and remains valid at an important class of
singularities (including in particular the Schwarzschild, FLRW, and isotropic
singularities). Here it is shown that if the Big-Bang singularity is from this
class, it also satisfies the Weyl curvature hypothesis.
As an application, we study a very general example of cosmological models,
which generalizes the FLRW model by dropping the isotropy and homogeneity
constraints. This model also generalizes isotropic singularities, and a class
of singularities occurring in Bianchi cosmologies. We show that the Big-Bang
singularity of this model is of the type under consideration, and satisfies
therefore the Weyl curvature hypothesis.Comment: 10 pages, slides at
http://www.sciencedirect.com/science/article/pii/S000349161300171
Towards a classification of vacuum near-horizons geometries
We prove uniqueness of the near-horizon geometries arising from degenerate
Kerr black holes within the collection of nearby vacuum near-horizon
geometries.Comment: 16 pages, 3 figures; minor changes to match published versio
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.
Bianchi type II,III and V diagonal Einstein metrics re-visited
We present, for both minkowskian and euclidean signatures, short derivations
of the diagonal Einstein metrics for Bianchi type II, III and V. For the first
two cases we show the integrability of the geodesic flow while for the third
case a somewhat unusual bifurcation phenomenon takes place: for minkowskian
signature elliptic functions are essential in the metric while for euclidean
signature only elementary functions appear
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
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
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
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
- …