34 research outputs found
Colliding Wave Solutions in a Symmetric Non-metric Theory
A method is given to generate the non-linear interaction (collision) of
linearly polarized gravity coupled torsion waves in a non-metric theory.
Explicit examples are given in which strong mutual focussing of gravitational
waves containing impulsive and shock components coupled with torsion waves does
not result in a curvature singularity. However, the collision of purely torsion
waves displays a curvature singularity in the region of interaction.Comment: 16 pages, 1 ps figure, It will appear in Int. Jour. of Theor. Physic
Dilatonic interpolation between Reissner-Nordstrom and Bertotti-Robinson spacetimes with physical consequences
We give a general class of static, spherically symmetric, non-asymptotically
flat and asymptotically non-(anti) de Sitter black hole solutions in
Einstein-Maxwell-Dilaton (EMD) theory of gravity in 4-dimensions. In this
general study we couple a magnetic Maxwell field with a general dilaton
potential, while double Liouville-type potentials are coupled with the gravity.
We show that the dilatonic parameters play the key role in switching between
the Bertotti-Robinson and Reissner-Nordstr\"om spacetimes. We study the
stability of such black holes under a linear radial perturbation, and in this
sense we find exceptional cases that the EMD black holes are unstable. In
continuation we give a detailed study of the spin-weighted harmonics in
dilatonic Hawking radiation spectrum and compare our results with the
previously known ones. Finally, we investigate the status of resulting naked
singularities of our general solution when probed with quantum test particles.Comment: 27 pages, 4 figures, to appear in CQG
Colliding plane wave solution in F(R)=R^{N} gravity
We identify a region of F(R)=R^{N} gravity without external sources which is
isometric to the spacetime of colliding plane waves (CPW). From the derived
curvature sources, N (N>1) measures the strength (i.e. the charge) of the
source. The analogy renders construction and collision of plane waves in
F(R)=R^{N} gravity possible, as in the Einstein-Maxwell (EM) theory, simply
because R=0. A plane wave in this type of gravity is equivalent to a Weyl
curvature plus an electromagnetic energy-momentum-like term (i.e. 'source
without source'). For N=1 we recover naturally the plane waves (and their
collision) in Einstein's theory. Our aim is to find the effect of an expanding
universe by virtue of F(R)=R^{N} on the colliding gravitational plane waves of
Einstein.Comment: 9 pages, 2 figure
Morgan-Morgan-NUT disk space via the Ehlers transformation
Using the Ehlers transformation along with the gravitoelectromagnetic
approach to stationary spacetimes we start from the Morgan-Morgan disk
spacetime (without radial pressure) as the seed metric and find its
corresponding stationary spacetime. As expected from the Ehlers transformation
the stationary spacetime obtained suffers from a NUT-type singularity and the
new parameter introduced in the stationary case could be interpreted as the
gravitomagnetic monopole charge (or the NUT factor). As a consequence of this
singularity there are closed timelike curves (CTCs) in the singular region of
the spacetime. Some of the properties of this spacetime including its particle
velocity distribution, gravitational redshift, stability and energy conditions
are discussed.Comment: 18 pages, 5 figures, RevTex 4, replaced with the published versio
Stable Magnetic Universes Revisited
A regular class of static, cylindrically symmetric pure magnetic field
metrics is rederived in a different metric ansatz in all dimensions. Radial,
time dependent perturbations show that for dimensions d>3 such spacetimes are
stable at both near r\approx0 and large radius r\rightarrow\infty. In a
different gauge these stability analysis and similar results were known
beforehand. For d=3, however, simultaneous stability requirement at both, near
and far radial distances can not be reconciled for time - dependent
perturbations. Restricted, numerical geodesics for neutral particles reveal a
confinement around the center in the polar plane. Charged, time-like geodesics
for d=4 on the other hand are shown numerically to run toward infinity.Comment: 11 pages, 3figure
Solutions for f(R) gravity coupled with electromagnetic field
In the presence of external, linear / nonlinear electromagnetic fields we
integrate f(R) \sim R+2{\alpha}\surd(R+const.) gravity equations. In contrast
to their Einsteinian cousins the obtained black holes are non-asymptotically
flat with a deficit angle. In proper limits we obtain from our general solution
the global monopole solution in f(R) gravity. The scale symmetry breaking term
adopted as the nonlinear electromagnetic source adjusts the sign of the mass of
the resulting black hole to be physical.Comment: 7 pages no figure, final version for publication in European Physical
Journal
Null dust in canonical gravity
We present the Lagrangian and Hamiltonian framework which incorporates null
dust as a source into canonical gravity. Null dust is a generalized Lagrangian
system which is described by six Clebsch potentials of its four-velocity Pfaff
form. The Dirac--ADM decomposition splits these into three canonical
coordinates (the comoving coordinates of the dust) and their conjugate momenta
(appropriate projections of four-velocity). Unlike ordinary dust of massive
particles, null dust therefore has three rather than four degrees of freedom
per space point. These are evolved by a Hamiltonian which is a linear
combination of energy and momentum densities of the dust. The energy density is
the norm of the momentum density with respect to the spatial metric. The
coupling to geometry is achieved by adding these densities to the gravitational
super-Hamiltonian and supermomentum. This leads to appropriate Hamiltonian and
momentum constraints in the phase space of the system. The constraints can be
rewritten in two alternative forms in which they generate a true Lie algebra.
The Dirac constraint quantization of the system is formally accomplished by
imposing the new constraints as quantum operator restrictions on state
functionals. We compare the canonical schemes for null and ordinary dust and
emhasize their differences.Comment: 25 pages, REVTEX, no figure
Gradient models of the axion-photon coupling
We establish an extended version of the Einstein - Maxwell - axion model by
introducing into the Lagrangian cross-terms, which contain the gradient
four-vector of the pseudoscalar (axion) field in convolution with the Maxwell
tensor. The gradient model of the axion-photon coupling is applied to
cosmology: we analyze the Bianchi-I type Universe with an initial magnetic
field, electric field induced by the axion-photon interaction, cosmological
constant and dark matter, which is described in terms of the pseudoscalar
(axion) field. Analytical, qualitative and numerical results are presented in
detail for two distinguished epochs: first, for the early Universe with
magnetic field domination; second, for the stage of late-time accelerated
expansion.Comment: 26 pages, 5 figures, accepted for publication in The European
Physical Journal