25 research outputs found
Anti de Sitter 5D black hole solutions with a self-interacting bulk scalar field: a potential reconstruction approach
We construct asymptotically AdS black hole solutions, with a self-interacting
bulk scalar field, in the context of 5D general relativity. As the observable
universe is characterized by spatial flatness, we focus to solutions where the
horizon of the black hole, and subsequently all 3D hypersurfaces for fixed
radial coordinate,have zero spatial curvature. We examine two cases for the
black hole scalar hair: a) an exponential decaying scalar field profile and b)
an inverse power scalar field profile. The scalar black hole solutions we
present in this paper, are characterized by four functions f(r), a(r), phi(r)
and V(phi(r)). Only the functions phi(r) and a(r) are determined analytically,
while the functions f(r) and V(phi(r)) are expressed semi-analytically, by
integral formulas in terms of a(r). We present our numerical results in figures
and we study in detail the characteristic properties of our solutions. We also
note that the potential we obtain has a non-convex form in agreement with the
corresponding "no hair theorem" for AdS space-times.Comment: 15 pages, 7 figure
Raychaudhuri's equation and aspects of relativistic charged collapse
We use the Raychaudhuri equation to probe certain aspects related to the
gravitational collapse of a charged medium. The aim is to identify the stresses
the Maxwell field exerts on the fluid and discuss their potential implications.
Particular attention is given to those stresses that resist contraction. After
looking at the general case, we consider the two opposite limits of poor and
high electrical conductivity. In the former there are electric fields but no
currents, while in the latter the situation is reversed. When the conductivity
is low, we find that the main agents acting against the collapse are the
Coulomb forces triggered by the presence of an excess charge. At the ideal
Magnetohydrodynamic (MHD) limit, on the other hand, the strongest resistance
seems to come from the tension of the magnetic forcelines. In either case, we
discuss whether and how the aforementioned resisting stresses may halt the
contraction and provide a set of conditions making this likely to happen.Comment: Revised version, to appear in PR
Finsler Branes and Quantum Gravity Phenomenology with Lorentz Symmetry Violations
A consistent theory of quantum gravity (QG) at Planck scale almost sure
contains manifestations of Lorentz local symmetry violations (LV) which may be
detected at observable scales. This can be effectively described and classified
by models with nonlinear dispersions and related Finsler metrics and
fundamental geometric objects (nonlinear and linear connections) depending on
velocity/ momentum variables. We prove that the trapping brane mechanism
provides an accurate description of gravitational and matter field phenomena
with LV over a wide range of distance scales and recovering in a systematic way
the general relativity (GR) and local Lorentz symmetries. In contrast to the
models with extra spacetime dimensions, the Einstein-Finsler type gravity
theories are positively with nontrivial nonlinear connection structure,
nonholonomic constraints and torsion induced by generic off-diagonal
coefficients of metrics, and determined by fundamental QG and/or LV effects.Comment: latex2e, 11pt, 34 pages, the version accepted to Class. Quant. Gra
Weak Gravitational Field in Finsler-Randers Space and Raychaudhuri Equation
The linearized form of the metric of a Finsler - Randers space is studied in
relation to the equations of motion, the deviation of geodesics and the
generalized Raychaudhuri equation are given for a weak gravitational field.
This equation is also derived in the framework of a tangent bundle. By using
Cartan or Berwald-like connections we get some types "gravito -
electromagnetic" curvature. In addition we investigate the conditions under
which a definite Lagrangian in a Randers space leads to Einstein field
equations under the presence of electromagnetic field. Finally, some
applications of the weak field in a generalized Finsler spacetime for
gravitational waves are given.Comment: 22 pages, matches version published in GER
The General Very Special Relativity in Finsler Cosmology
General Very Special Relativity (GVSR) is the curved space-time of Very
Special Relativity (VSR) proposed by Cohen and Glashow. The geometry of GVSR
possesses a line element of Finsler Geometry proposed by Bogoslovsky. We
calculate the Einstein field equations and derive a modified FRW cosmology, for
an osculating Riemannian space. The Friedman equation of motion leads to an
explanation of the cosmological acceleration in terms of an alternative
non-Lorentz invariant theory. A first order approach for a primordial spurionic
vector field introduced into the metric, gives back an estimation of the energy
evolution and inflationComment: 14 pages- accepted to Physical Review
Friedmann Robertson-Walker model in generalised metric space-time with weak anisotropy
A generalized model of space-time is given, taking into consideration the
anisotropic structure of fields which are depended on the position and the
direction (velocity).In this framework a generalized FRW-metric the
Raychaudhouri and Friedmann equations are studied.A long range vector field of
cosmological origin is considered in relation to the physical geometry of
space-time in which Cartan connection has a fundamental role.The generalised
Friedmann equations are produced including anisotropic terms.The variation of
anisotropy is expressed in terms of the Cartan torsion tensor of the
Finslerian space-time.A possible estimation of the anisotropic parameter
can be achieved with the aid of the de-Sitter model of the empty flat universe
with weak anisotropy. Finally a physical generalisation for the model of
inflation is also studied.Comment: 21 pages- to appear in GR
Stringy Space-Time Foam and High-Energy Cosmic Photons
In this review, I discuss briefly stringent tests of Lorentz-violating
quantum space-time foam models inspired from String/Brane theories, provided by
studies of high energy Photons from intense celestial sources, such as Active
Galactic Nuclei or Gamma Ray Bursts. The theoretical models predict
modifications to the radiation dispersion relations, which are quadratically
suppressed by the string mass scale, and time delays in the arrival times of
photons (assumed to be emitted more or less simultaneously from the source),
which are proportional to the photon energy, so that the more energetic photons
arrive later. Although the astrophysics at the source of these energetic
photons is still not understood, and such non simultaneous arrivals, that have
been observed recently, might well be due to non simultaneous emission as a
result of conventional physics effects, nevertheless, rather surprisingly, the
observed time delays can also fit excellently the stringy space-time foam
scenarios, provided the space-time defect foam is inhomogeneous. The key
features of the model, that allow it to evade a plethora of astrophysical
constraints on Lorentz violation, in sharp contrast to other field-theoretic
Lorentz-violating models of quantum gravity, are: (i) transparency of the foam
to electrons and in general charged matter, (ii) absence of birefringence
effects and (iii) a breakdown of the local effective lagrangian formalism.Comment: 26 pages Latex, 4 figures, uses special macros. Keynote Lecture in
the International Conference "Recent Developments in Gravity" (NEB14),
Ioannina (Greece) June 8-11 201