94 research outputs found
Dilatonic Black Holes in Higher Curvature String Gravity
We give analytical arguments and demonstrate numerically the existence of
black hole solutions of the Effective Superstring Action in the presence
of Gauss-Bonnet quadratic curvature terms. The solutions possess non-trivial
dilaton hair. The hair, however, is of ``secondary" type", in the sense that
the dilaton charge is expressed in terms of the black hole mass. Our solutions
are not covered by the assumptions of existing proofs of the ``no-hair"
theorem. We also find some alternative solutions with singular metric
behaviour, but finite energy. The absence of naked singularities in this system
is pointed out.Comment: 22 pages, Latex file, 7 Latex figures already include
Do stringy corrections stabilize coloured black holes?
We consider hairy black hole solutions of Einstein-Yang-Mills-Dilaton theory,
coupled to a Gauss-Bonnet curvature term, and we study their stability under
small, spacetime-dependent perturbations. We demonstrate that the stringy
corrections do not remove the sphaleronic instabilities of the coloured black
holes with the number of unstable modes being equal to the number of nodes of
the background gauge function. In the gravitational sector, and in the limit of
an infinitely large horizon, the coloured black holes are also found to be
unstable. Similar behaviour is exhibited by the magnetically charged black
holes while the bulk of the neutral black holes are proven to be stable under
small, gauge-dependent perturbations. Finally, the electrically charged black
holes are found to be characterized only by the existence of a gravitational
sector of perturbations. As in the case of neutral black holes, we demonstrate
that for the bulk of electrically charged black holes no unstable modes arise
in this sector.Comment: 17 pages, Revtex, comments and a reference added, version to appear
in Physical Review
Echocardiographic phenomenon “RAC-sign” in the anomalous course of circumflex artery: a case report
Coronary artery (CA) anomalies are a group of congenital heart defects with a diverse clinical performance, from lifelong asymptomatic to severe consequences such as sudden cardiac death. In some cases, CA anomalies become an incidental finding during echocardiography. If there is a suspicion of CA anomaly, a radiographic investigation (computed tomography (CT) angiography or magnetic resonance imaging) should be performed to clarify the anatomy and indications for surgical correction.A case of diagnosing a tubular structure with hyperechoic walls in mitral valve projection during echocardiography is presented. The performed CT angiography confirmed the abnormal origin of circumflex artery from the right sinus of Valsalva with its retroaortic course. This echocardiographic sign is described in the English-language literature as Retroaortic Anomalous Coronary sign (RAC-sign)
Mixmaster Behavior in Inhomogeneous Cosmological Spacetimes
Numerical investigation of a class of inhomogeneous cosmological spacetimes
shows evidence that at a generic point in space the evolution toward the
initial singularity is asymptotically that of a spatially homogeneous spacetime
with Mixmaster behavior. This supports a long-standing conjecture due to
Belinskii et al. on the nature of the generic singularity in Einstein's
equations.Comment: 4 pages plus 4 figures. A sentence has been deleted. Accepted for
publication in PR
Dilatonic Black Holes with Gauss-Bonnet Term
We discuss black holes in an effective theory derived from a superstring
model, which includes a dilaton field, a gauge field and the Gauss-Bonnet term.
Assuming U(1) or SU(2) symmetry for the gauge field, we find four types of
spherically symmetric solutions, i.e., a neutral, an electrically charged, a
magnetically charged and a ``colored'' black hole, and discuss their
thermodynamical properties and fate via the Hawking evaporation process. For
neutral and electrically charged black holes, we find critical point and a
singular end point. Below the mass corresponding to the critical point,
nosolution exists, while the curvature on the horizon diverges and anaked
singularity appears at the singular point. A cusp structure in the mass-entropy
diagram is found at the critical point and black holes on the branch between
the critical and singular points become unstable. For magnetically charged and
``colored" black holes, the solution becomes singular just at the end point
with a finite mass. Because the black hole temperature is always finite even at
the critical point or the singular point, we may conclude that the evaporation
process will not be stopped even at the critical point or the singular point,
and the black hole will move to a dynamical evaporation phase or a naked
singularity will appear.Comment: 31pages, 11figures, LaTex styl
Gravitating monopole and its black hole solution in Brans-Dicke Theory
We find a self-gravitating monopole and its black hole solution in
Brans-Dicke theory. We mainly discuss the properties of these solutions in the
Einstein frame and compare the solutions with those in general relativity.Comment: 20 pages,revtex,26 figure
Aspects of hairy black holes in spontaneously-broken Einstein-Yang-Mills systems: Stability analysis and Entropy considerations
We analyze (3+1)-dimensional black-hole space-times in spontaneously broken
Yang-Mills gauge theories that have been recently presented as candidates for
an evasion of the scalar-no-hair theorem. Although we show that in principle
the conditions for the no-hair theorem do not apply to this case, however we
prove that the `spirit' of the theorem is not violated, in the sense that there
exist instabilities, in both the sphaleron and gravitational sectors. The
instability analysis of the sphaleron sector, which was expected to be unstable
for topological reasons, is performed by means of a variational method. As
shown, there exist modes in this sector that are unstable against linear
perturbations. Instabilities exist also in the gravitational sector. A method
for counting the gravitational unstable modes, which utilizes a
catastrophe-theoretic approach is presented. The r\^ole of the catastrophe
functional is played by the mass functional of the black hole. The Higgs vacuum
expectation value (v.e.v.) is used as a control parameter, having a critical
value beyond which instabilities are turned on. The (stable) Schwarzschild
solution is then understood from this point of view. The catastrophe-theory
appproach facilitates enormously a universal stability study of non-Abelian
black holes, which goes beyond linearized perturbations. Some elementary
entropy considerations are also presented...Comment: Latex file, 50 pages, 2 figures (included as PS files at the end:
plot1.ps, plot2.ps
No hair for spherical black holes: charged and nonminimally coupled scalar field with self--interaction
We prove three theorems in general relativity which rule out classical scalar
hair of static, spherically symmetric, possibly electrically charged black
holes. We first generalize Bekenstein's no--hair theorem for a multiplet of
minimally coupled real scalar fields with not necessarily quadratic action to
the case of a charged black hole. We then use a conformal map of the geometry
to convert the problem of a charged (or neutral) black hole with hair in the
form of a neutral self--interacting scalar field nonminimally coupled to
gravity to the preceding problem, thus establishing a no--hair theorem for the
cases with nonminimal coupling parameter or . The
proof also makes use of a causality requirement on the field configuration.
Finally, from the required behavior of the fields at the horizon and infinity
we exclude hair of a charged black hole in the form of a charged
self--interacting scalar field nonminimally coupled to gravity for any .Comment: 30 pages, RevTeX. Sec.IV corrected, simplified and shortened.
Corrections to Sec.IIA between Eqs. 2.7 and Eq.2.1. First two paragraphs of
Sec. VC new. To appear Phys. Rev. D, Oct. 15, 199
Numerical Approaches to Spacetime Singularities
This Living Review updates a previous version which its itself an update of a
review article. Numerical exploration of the properties of singularities could,
in principle, yield detailed understanding of their nature in physically
realistic cases. Examples of numerical investigations into the formation of
naked singularities, critical behavior in collapse, passage through the Cauchy
horizon, chaos of the Mixmaster singularity, and singularities in spatially
inhomogeneous cosmologies are discussed.Comment: 51 pages, 6 figures may be found in online version: Living Rev.
Relativity 2002-1 at www.livingreviews.or
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