1,485 research outputs found
Extending Sibgatullin's ansatz for the Ernst potential to generate a richer family of axially symmetric solutions of Einstein's equations
The scope of this talk is to present some preliminary results on an effort,
currently in progress, to generate an exact solution of Einstein's equation,
suitable for describing spacetime around a rotating compact object.
Specifically, the form of the Ernst potential on the symmetry axis and its
connection with the multipole moments is discussed thoroughly. The way to
calculate the multipole moments of spacetime directly from the value of the
Ernst potential on the symmetry axis is presented. Finally, a mixed ansatz is
formed for the Ernst potential including parameters additional to the ones
dictated by Sibgatullin. Thus, we believe that this talk can also serve as a
comment on choosing the appropriate ansatz for the Ernst potential.Comment: Talk given in the 11th Conference on Recent Developments in Gravity,
2-5 June 2004, Lesbos, Greec
Scale hierarchy in Horava-Lifshitz gravity: a strong constraint from synchrotron radiation in the Crab nebula
Horava-Lifshitz gravity models contain higher order operators suppressed by a
characteristic scale, which is required to be parametrically smaller than the
Planck scale. We show that recomputed synchrotron radiation constraints from
the Crab nebula suffice to exclude the possibility that this scale is of the
same order of magnitude as the Lorentz breaking scale in the matter sector.
This highlights the need for a mechanism that suppresses the percolation of
Lorentz violation in the matter sector and is effective for higher order
operators as well.Comment: 4 page, 2 figures; v2: minor changes to match published versio
Corrections and Comments on the Multipole Moments of Axisymmetric Electrovacuum Spacetimes
Following the method of Hoenselaers and Perj\'{e}s we present a new corrected
and dimensionally consistent set of multipole gravitational and electromagnetic
moments for stationary axisymmetric spacetimes. Furthermore, we use our results
to compute the multipole moments, both gravitational and electromagnetic, of a
Kerr-Newman black hole.Comment: This is a revised and corrected versio
A comment on "The Cauchy problem of f(R)- gravity", Class. Quantum Grav., 24, 5667 (2007), arXiv:0709.4414
A critical comment on [N. Lanahan--Tremblay and V. Faraoni, 2007, {\it Class.
Quantum Grav.}, {\bf 24}, 5667, arXiv:0709.4414] is given discussing the
well-formulation of the Chauchy problem for -gravity in metric-affine
theories.Comment: 3 page
Reply to `A comment on `The Cauchy problem of f(R) gravity''
We reply to a comment by Capozziello and Vignolo about the Cauchy problem of
Palatini f(R) gravity.Comment: 3 pages, late
Matching of analytical and numerical solutions for neutron stars of arbitrary rotation
We demonstrate the results of an attempt to match the two-soliton analytical
solution with the numerically produced solutions of the Einstein field
equations, that describe the spacetime exterior of rotating neutron stars, for
arbitrary rotation. The matching procedure is performed by equating the first
four multipole moments of the analytical solution to the multipole moments of
the numerical one. We then argue that in order to check the effectiveness of
the matching of the analytical with the numerical solution we should compare
the metric components, the radius of the innermost stable circular orbit
(), the rotation frequency and the
epicyclic frequencies . Finally we present some
results of the comparison.Comment: Contribution at the 13th Conference on Recent Developments in Gravity
(NEB XIII), corrected typo in of eq. 5 of the published versio
A comparative study of survival models for breast cancer prognostication based on microarray data: does a single gene beat them all?
Motivation: Survival prediction of breast cancer (BC) patients independently of treatment, also known as prognostication, is a complex task since clinically similar breast tumors, in addition to be molecularly heterogeneous, may exhibit different clinical outcomes. In recent years, the analysis of gene expression profiles by means of sophisticated data mining tools emerged as a promising technology to bring additional insights into BC biology and to improve the quality of prognostication. The aim of this work is to assess quantitatively the accuracy of prediction obtained with state-of-the-art data analysis techniques for BC microarray data through an independent and thorough framework
Covariant conservation of energy momentum in modified gravities
An explicit proof of the vanishing of the covariant divergence of the
energy-momentum tensor in modified theories of gravity is presented. The
gravitational action is written in arbitrary dimensions and allowed to depend
nonlinearly on the curvature scalar and its couplings with a scalar field. Also
the case of a function of the curvature scalar multiplying a matter Lagrangian
is considered. The proof is given both in the metric and in the first-order
formalism, i.e. under the Palatini variational principle. It is found that the
covariant conservation of energy-momentum is built-in to the field equations.
This crucial result, called the generalized Bianchi identity, can also be
deduced directly from the covariance of the extended gravitational action.
Furthermore, we demonstrate that in all of these cases, the freely falling
world lines are determined by the field equations alone and turn out to be the
geodesics associated with the metric compatible connection. The independent
connection in the Palatini formulation of these generalized theories does not
have a similar direct physical interpretation. However, in the conformal
Einstein frame a certain bi-metricity emerges into the structure of these
theories. In the light of our interpretation of the independent connection as
an auxiliary variable we can also reconsider some criticisms of the Palatini
formulation originally raised by Buchdahl.Comment: 8 pages. v2: more discussio
Destroying black holes with test bodies
If a black hole can accrete a body whose spin or charge would send the black
hole parameters over the extremal limit, then a naked singularity would
presumably form, in violation of the cosmic censorship conjecture. We review
some previous results on testing cosmic censorship in this way using the test
body approximation, focusing mostly on the case of neutral black holes. Under
certain conditions a black hole can indeed be over-spun or over-charged in this
approximation, hence radiative and self-force effects must be taken into
account to further test cosmic censorship.Comment: Contribution to the proceedings of the First Mediterranean Conference
on Classical and Quantum Gravity (talk given by T. P. S.). Summarizes the
results of Phys. Rev. Lett. 103, 141101 (2009), arXiv:0907.4146 [gr-qc] and
considers further example
Faithful transformation of quasi-isotropic to Weyl-Papapetrou coordinates: A prerequisite to compare metrics
We demonstrate how one should transform correctly quasi-isotropic coordinates
to Weyl-Papapetrou coordinates in order to compare the metric around a rotating
star that has been constructed numerically in the former coordinates with an
axially symmetric stationary metric that is given through an analytical form in
the latter coordinates. Since a stationary metric associated with an isolated
object that is built numerically partly refers to a non-vacuum solution
(interior of the star) the transformation of its coordinates to Weyl-Papapetrou
coordinates, which are usually used to describe vacuum axisymmetric and
stationary solutions of Einstein equations, is not straightforward in the
non-vacuum region. If this point is \textit{not} taken into consideration, one
may end up to erroneous conclusions about how well a specific analytical metric
matches the metric around the star, due to fallacious coordinate
transformations.Comment: 18 pages, 2 figure
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