127 research outputs found
Negative Energy Densities in Extended Sources Generating Closed Timelike Curves in General Relativity with and without Torsion
Near a spinning point particle in (2+1)-dimensional gravity (or near an
infinitely thin, straight, spinning string in 3+1 dimensions) there is a region
of space-time with closed timelike curves. Exact solutions for extended sources
with apparently physically acceptable energy-momentum tensors, have produced
the same exterior space-time structure. Here it is pointed out that in the case
with torsion, closed timelike curves appear only for spin densities so high
that the spin energy density is higher than the net effective energy density.
In models without torsion, the presence of closed time-like curves is related
to a heat flow of unphysical magnitude. This corroborates earlier arguments
against the possibility of closed timelike curves in space-time geometries
generated by physical sources.Comment: (to be published in Phys. Rev. D), 5 pages, REVTEX 3.0, NORDITA 93/62
A (Sept. 10/Revised Nov. 1, 1993
Modification of the Coulomb potential from a Kaluza-Klein model with a Gauss-Bonnet term in the action
In four dimensions a Gauss-Bonnet term in the action corre- sponds to a total
derivative, and it does not contribute to the classical equations of motion.
For higher-dimensional geometries this term has the interesting property
(shared with other dimensionally continued Euler densities) that when the
action is varied with respect to the metric, it gives rise to a symmetric,
covariantly conserved tensor of rank two which is a function of the metric and
its first and second order derivatives. Here we review the unification of
General Relativity and electromagnetism in the classical five-dimen- sional,
restricted (with g_55 = 1) Kaluza-Klein model. Then we discuss the
modifications of the Einstein-Maxwell theory that results from adding the
Gauss-Bonnet term in the action. The resulting four-dimensional theory
describes a non-linear U(1) gauge theory non-minimally coupled to gravity. For
a point charge at rest, we find a perturbative solution for large distances
which gives a mass-dependent correction to the Coulomb potential. Near the
source we find a power-law solution which seems to cure the short-distance
divergency of the Coulomb potential. Possible ways to obtain an experimen- tal
upper limit to the coupling of the hypothetical Gauss- Bonnet term are also
considered.Comment: 27 pages, compressed and uuencoded postscript file with unpacking
instructions; major revision to section IV.D.2 on pages 15-16 ("Corrections
to the Coulomb potential at short distances") and to the figure on page 27,
revised unpacking instruction; to be published in The Annals of Physics (NY),
NORDITA 94/5
Experimental limits to the density of dark matter in the solar system
On the scales of galaxies and beyond there is evidence for unseen dark
matter. In this paper we find the experimental limits to the density of dark
matter bound in the solar system by studying its effect upon planetary motion.Comment: 9 pages, REVTeX, no figure
Supergravity domain walls
We review the status of domain walls in N=1 supergravity theories for both the vacuum domain walls as well as dilatonic domain walls. We concentrate on a systematic analysis of the nature of the space-time in such domain wall backgrounds and the special role that supersymmetry is playing in determining the nature of such configurations.We review the status of domain walls in supergravity theories for both the vacuum domain walls as well as dilatonic domain walls. We concentrate on a systematic analysis of the nature of the space-time in such domain wall backgrounds and the special role that supersymmetry is playing in determining the nature of such configurations
Time machines: the Principle of Self-Consistency as a consequence of the Principle of Minimal Action
We consider the action principle to derive the classical, non-relativistic
motion of a self-interacting particle in a 4-D Lorentzian spacetime containing
a wormhole and which allows the existence of closed time-like curves. For the
case of a `hard-sphere' self-interaction potential we show that the only
possible trajectories (for a particle with fixed initial and final positions
and which traverses the wormhole once) minimizing the classical action are
those which are globally self-consistent, and that the `Principle of
self-consistency' (originally introduced by Novikov) is thus a natural
consequence of the `Principle of minimal action.'Comment: 26 pages, plain latex; modified version includes extra constraint for
collinear collision case and other minor misprints correction
Charged black points in General Relativity coupled to the logarithmic gauge theory
The exact solution for a static spherically symmetric field outside a charged
point particle is found in a non-linear gauge theory with a logarithmic
Lagrangian. The electromagnetic self-mass is finite, and for a particular
relation between mass, charge, and the value of the non-linearity coupling
constant, , the electromagnetic contribution to the Schwarzschild mass
is equal to the total mass. If we also require that the singularity at the
origin be hidden behind a horizon, the mass is fixed to be slightly less than
the charge. This object is a {\em black point.}Comment: 7 pages, REVTeX, no figure
Volterra Distortions, Spinning Strings, and Cosmic Defects
Cosmic strings, as topological spacetime defects, show striking resemblance
to defects in solid continua: distortions, which can be classified into
disclinations and dislocations, are line-like defects characterized by a delta
function-valued curvature and torsion distribution giving rise to rotational
and translational holonomy. We exploit this analogy and investigate how
distortions can be adapted in a systematic manner from solid state systems to
Einstein-Cartan gravity. As distortions are efficiently described within the
framework of a SO(3) {\rlap{\supset}\times}} T(3) gauge theory of solid
continua with line defects, we are led in a straightforward way to a Poincar\'e
gauge approach to gravity which is a natural framework for introducing the
notion of distorted spacetimes. Constructing all ten possible distorted
spacetimes, we recover, inter alia, the well-known exterior spacetime of a
spin-polarized cosmic string as a special case of such a geometry. In a second
step, we search for matter distributions which, in Einstein-Cartan gravity, act
as sources of distorted spacetimes. The resulting solutions, appropriately
matched to the distorted vacua, are cylindrically symmetric and are interpreted
as spin-polarized cosmic strings and cosmic dislocations.Comment: 24 pages, LaTeX, 9 eps figures; remarks on energy conditions added,
discussion extended, version to be published in Class. Quantum Gra
Inverse Square Law of Gravitation in (2+1)-Dimensional Space-Time as a Consequence of Casimir Energy
The gravitational effect of vacuum polarization in space exterior to a
particle in (2+1)-dimensional Einstein theory is investigated. In the weak
field limit this gravitational field corresponds to an inverse square law of
gravitational attraction, even though the gravitational mass of the quantum
vacuum is negative. The paradox is resolved by considering a particle of finite
extension and taking into account the vacuum polarization in its interior.Comment: 10 pages, LaTeX, Report: UPR-0540-T, To appear in Physica Script
Dark matter and non-Newtonian gravity from General Relativity coupled to a fluid of strings
An exact solution of Einstein's field equations for a point mass surrounded
by a static, spherically symmetric fluid of strings is presented. The solution
is singular at the origin. Near the string cloud limit there is a
correction to Newton's force law. It is noted that at large distances and small
accelerations, this law coincides with the phenomenological force law invented
by Milgrom in order to explain the flat rotation curves of galaxies without
introducing dark matter. When interpreted in the context of a cosmological
model with a string fluid, the new solution naturally explains why the critical
acceleration of Milgrom is of the same order of magnitude as the Hubble
parameter.Comment: 12 pages, REVTeX, no figure
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