211 research outputs found
Negative Pressure and Naked Singularities in Spherical Gravitational Collapse
Assuming the weak energy condition, we study the nature of the non-central
shell-focussing singularity which can form in the gravitational collapse of a
spherical compact object in classical general relativity. We show that if the
radial pressure is positive, the singularity is covered by a horizon. For
negative radial pressures, the singularity will be covered if the ratio of
pressure to the density is greater than -1/3 and naked if this ratio is .Comment: 7 pages, LaTeX Fil
The need for dark matter in galaxies
Cooperstock and Tieu have proposed a model to account for galactic rotation
curves without invoking dark matter. I argue that no model of this type can
work
Energy and Momentum of a Class of Rotating Gravitational Waves
We calculate energy and momentum for a class of cylindrical rotating
gravitational waves using Einstein and Papapetrou's prescriptions. It is shown
that the results obtained are reduced to the special case of the cylindrical
gravitational waves already available in the literature.Comment: 11 pages, no figure, Late
Energy Distribution in Melvin's Magnetic Universe
We use the energy-momentum complexes of Landau and Lifshitz and Papapetrou to
obtain the energy distribution in Melvin's magnetic universe. For this
space-time we find that these definitions of energy give the same and
convincing results. The energy distribution obtained here is the same as we
obtained earlier for the same space-time using the energy-momentum complex of
Einstein. These results uphold the usefulness of the energy-momentum complexes.Comment: 8 pages, RevTex, no figure
Nonlinear Gravitational Waves: Their Form and Effects
A gravitational wave must be nonlinear to be able to transport its own
source, that is, energy and momentum. A physical gravitational wave, therefore,
cannot be represented by a solution to a linear wave equation. Relying on this
property, the second-order solution describing such physical waves is obtained.
The effects they produce on free particles are found to consist of nonlinear
oscillations along the direction of propagation.Comment: 15 pages, no figures. v2: presentation changes aiming at clarifying
the text; matches published versio
Energy and Momentum densities of cosmological models, with equation of state , in general relativity and teleparallel gravity
We calculated the energy and momentum densities of stiff fluid solutions,
using Einstein, Bergmann-Thomson and Landau-Lifshitz energy-momentum complexes,
in both general relativity and teleparallel gravity. In our analysis we get
different results comparing the aforementioned complexes with each other when
calculated in the same gravitational theory, either this is in general
relativity and teleparallel gravity. However, interestingly enough, each
complex's value is the same either in general relativity or teleparallel
gravity. Our results sustain that (i) general relativity or teleparallel
gravity are equivalent theories (ii) different energy-momentum complexes do not
provide the same energy and momentum densities neither in general relativity
nor in teleparallel gravity. In the context of the theory of teleparallel
gravity, the vector and axial-vector parts of the torsion are obtained. We show
that the axial-vector torsion vanishes for the space-time under study.Comment: 15 pages, no figures, Minor typos corrected; version to appear in
International Journal of Theoretical Physic
Energy Distribution of a Stationary Beam of Light
Aguirregabiria et al showed that Einstein, Landau and Lifshitz, Papapetrou,
and Weinberg energy-momentum complexes coincide for all Kerr-Schild metric.
Bringely used their general expression of the Kerr-Schild class and found
energy and momentum densities for the Bonnor metric. We obtain these results
without using Aguirregabiria et al results and verify that Bringley's results
are correct. This also supports Aguirregabiria et al results as well as
Cooperstock hypothesis. Further, we obtain the energy distribution of the
space-time under consideration.Comment: Latex, no figures [Admin note: substantial overlap with gr-qc/9910015
and hep-th/0308070
The gravitational interaction of light: from weak to strong fields
An explanation is proposed for the fact that pp-waves superpose linearly when
they propagate parallely, while they interact nonlinearly, scatter and form
singularities or Cauchy horizons if they are antiparallel. Parallel pp-waves do
interact, but a generalized gravitoelectric force is exactly cancelled by a
gravitomagnetic force. In an analogy, the interaction of light beams in
linearized general relativity is also revisited and clarified, a new result is
obtained for photon to photon attraction, and a conjecture is proved. Given
equal energy density in the beams, the light-to-light attraction is twice the
matter-to-light attraction and four times the matter-to-matter attraction.Comment: 17 pages, LaTeX, no figures. To appear in General Relativity and
Gravitatio
Energy and Momentum Distributions of Kantowski and Sachs Space-time
We use the Einstein, Bergmann-Thomson, Landau-Lifshitz and Papapetrou
energy-momentum complexes to calculate the energy and momentum distributions of
Kantowski and Sachs space-time. We show that the Einstein and Bergmann-Thomson
definitions furnish a consistent result for the energy distribution, but the
definition of Landau-Lifshitz do not agree with them. We show that a signature
switch should affect about everything including energy distribution in the case
of Einstein and Papapetrou prescriptions but not in Bergmann-Thomson and
Landau-Lifshitz prescriptions.Comment: 12 page
Galactic Dynamics via General Relativity: A Compilation and New Developments
We consider the consequences of applying general relativity to the
description of the dynamics of a galaxy, given the observed flattened rotation
curves. The galaxy is modeled as a stationary axially symmetric pressure-free
fluid. In spite of the weak gravitational field and the non-relativistic source
velocities, the mathematical system is still seen to be non-linear. It is shown
that the rotation curves for various galaxies as examples are consistent with
the mass density distributions of the visible matter within essentially
flattened disks. This obviates the need for a massive halo of exotic dark
matter. We determine that the mass density for the luminous threshold as
tracked in the radial direction is kgm for these
galaxies and conjecture that this will be the case for other galaxies yet to be
analyzed. We present a velocity dispersion test to determine the extent, if of
any significance, of matter that may lie beyond the visible/HI region. Various
comments and criticisms from colleagues are addressed.Comment: 35 pages, 13 figure
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