622 research outputs found
Polixeni Papapetrou
Artist, Polixeni Papapetrou, discusses her photographic work
Self-accelerated Universe
It is widely believed that the large redshifts for distant supernovae are
explained by the vacuum energy dominance, or, in other words, by the
cosmological constant in Einstein's equations, which is responsible for the
anti-gravitation effect. A tacit assumption is that particles move along a
geodesic for the background metric. This is in the same spirit as the consensus
regarding the uniform Galilean motion of a free electron. However, there is a
runaway solution to the Lorentz--Dirac equation governing the behavior of a
radiating electron, in addition to the Galilean solution. Likewise, a runaway
solution to the entire system of equations, both gravitation and matter
equations of motion including, may provide an alternative explanation for the
accelerated expansion of the Universe, without recourse to the hypothetic
cosmological constant.Comment: 11 pages; Talk at the 9th Adriatic Meeting, Dubrovnic, Croatia, 4-14
September, 2003, Minor improvement, references added; to appear in ``Progress
in General Relativity and Quantum Cosmology Research'', Nova Science
Publisher
Tensor-Scalar Torsion
A theory of gravity with torsion is examined in which the torsion tensor is
constructed from the exterior derivative of an antisymmetric rank two potential
plus the dual of the gradient of a scalar field. Field equations for the theory
are derived by demanding that the action be stationary under variations with
respect to the metric, the antisymmetric potential, and the scalar field. A
material action is introduced and the equations of motion are derived. The
correct conservation law for rotational angular momentum plus spin is observed
to hold in this theory.Comment: 10 pages, LaTeX, Mod. Phys. Lett. A accepte
A note on the entropy of charged multi - black - holes
Majumdar--Papapetrou multi--black-hole solutions of the Einstein--Maxwell
equations are considered in four and higher dimensions. The Euclidean action
with boundary conditions appropriate to the canonical ensemble is shown to lead
to zero entropy.Comment: LaTeX, 8 page
A Slowly Rotating Charged Black Hole in Five Dimensions
Black hole solutions in higher dimensional Einstein and Einstein-Maxwell
gravity have been discussed by Tangherlini as well as Myers and Perry a long
time ago. These solutions are the generalizations of the familiar
Schwarzschild, Reissner-Nordstrom and Kerr solutions of four-dimensional
general relativity. However, higher dimensional generalization of the
Kerr-Newman solution in four dimensions has not been found yet. As a first step
in this direction I shall report on a new solution of the Einstein-Maxwell
system of equations that describes an electrically charged and slowly rotating
black hole in five dimensions.Comment: Talk given at GR17: 17th International Conference on General
Relativity and Gravitation, Dublin, 18-24 Jule,2004 ; Corrected typos, minor
changes, new formula adde
On asymptotically flat solutions of Einstein's equations periodic in time I. Vacuum and electrovacuum solutions
By an argument similar to that of Gibbons and Stewart, but in a different
coordinate system and less restrictive gauge, we show that any
weakly-asymptotically-simple, analytic vacuum or electrovacuum solutions of the
Einstein equations which are periodic in time are necessarily stationary.Comment: 25 pages, 2 figures, published in Class. Quant. Grav
A Note on trapped Surfaces in the Vaidya Solution
The Vaidya solution describes the gravitational collapse of a finite shell of
incoherent radiation falling into flat spacetime and giving rise to a
Schwarzschild black hole. There has been a question whether closed trapped
surfaces can extend into the flat region (whereas closed outer trapped surfaces
certainly can). For the special case of self-similar collapse we show that the
answer is yes, if and only if the mass function rises fast enough.Comment: 14 pages, 4 figures; minor polish added to version
Energy Distribution of a Charged Regular Black Hole
We calculate the energy distribution of a charged regular black hole by using
the energy-momentum complexes of Einstein and M{\o}ller.Comment: 6 pages, no figure
Stationary perturbations and infinitesimal rotations of static Einstein-Yang-Mills configurations with bosonic matter
Using the Kaluza-Klein structure of stationary spacetimes, a framework for
analyzing stationary perturbations of static Einstein-Yang-Mills configurations
with bosonic matter fields is presented. It is shown that the perturbations
giving rise to non-vanishing ADM angular momentum are governed by a
self-adjoint system of equations for a set of gauge invariant scalar
amplitudes. The method is illustrated for SU(2) gauge fields, coupled to a
Higgs doublet or a Higgs triplet. It is argued that slowly rotating black holes
arise generically in self-gravitating non-Abelian gauge theories with bosonic
matter, whereas, in general, soliton solutions do not have rotating
counterparts.Comment: 8 pages, revtex, no figure
Gravitational magnetic monopoles and Majumdar-Papapetrou stars
A large amount of work has been dedicated to studying general relativity
coupled to non-Abelian Yang-Mills type theories. It has been shown that the
magnetic monopole, a solution of the Yang-Mills-Higgs equations can be coupled
to gravitation. For a low Higgs mass there are regular solutions, and for a
sufficiently massive monopole the system develops an extremal magnetic
Reissner-Nordstrom quasi-horizon. These solutions, called quasi-black holes,
although non-singular, are arbitrarily close to having a horizon. However, at
the critical value the quasi-black hole turns into a degenerate spacetime. On
the other hand, for a high Higgs mass, a sufficiently massive monopole develops
also a quasi-black hole, but it turns into an extremal true horizon, with
matter fields outside. One can also put a small Schwarzschild black hole inside
the magnetic monopole, an example of a non-Abelian black hole. Surprisingly,
Majumdar-Papapetrou systems, Abelian systems constructed from extremal dust,
also show a resembling behavior. Previously, we have reported that one can find
Majumdar-Papapetrou solutions which can be arbitrarily close of being a black
hole, displaying quasi-black hole behavior. With the aim of better
understanding the similarities between gravitational monopoles and
Majumdar-Papapetrou systems, we study a system composed of two extremal
electrically charged spherical shells (or stars, generically) in the
Einstein--Maxwell--Majumdar-Papapetrou theory. We review the gravitational
properties of the monopoles, and compare with the properties of the double
extremal electric shell system. These quasi-black holes can help in the
understanding of true black holes, and can give insight into the nature of the
entropy of black holes in the form of entanglement.Comment: 38 pages,9 Figures, minor change
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