arXiv subjects
Publications and source records attributed to Gamal G. L. Nashed.
We give an exact solution to the gravitational field in the new general relativity. The solution creates Bonnor spacetime. This spacetime describes the gravitational field of a stationary beam of light. The energy and momentum of this solution is calculated using the energy-momentum complex given by Møller in (1978) within the framework of the Weitzenb{\rm $\ddot{o}$}ck spacetime.
Using a nonlinear electrodynamics coupled to teleparallel theory of gravity, three regular charged spherically symmetric solutions are obtained. The nonlinear theory reduces to the Maxwell one in the weak limit and the solutions correspond to charged spacetimes. The third solution contains an arbitrary function from which we can generate the other two solutions. The metric associated with these spacetimes is the same, i.e., a regular charged static spherically symmetric black hole. In calculating the energy content of the third solution using the gravitational energy-momentum given by Møller, within the framework of the teleparallel geometry, we find that the resulting form depends on the arbitrary function. Using the regularized expression of the gravitational energy-momentum we get the value of energy.
An exact charged axially symmetric solution of the coupled gravitational and electromagnetic fields in the teleparallel equivalent of Einstein theory is derived. It is characterized by three parameters ``$ $the gravitational mass $M$, the charge parameter $Q$ and the rotation parameter $a$" and its associated metric gives Kerr-Newman spacetime. The parallel vector field and the electromagnetic vector potential are axially symmetric. We then, calculate the total energy using the gravitational energy-momentum. The energy is found to be shared by its interior as well as exterior. Switching off the charge parameter we find that no energy is shared by the exterior of the Kerr-Newman black hole.
We give three different spherically symmetric spacetimes for the coupled gravitational and electromagnetic fields with charged source in the tetrad theory of gravitation. One of these, contains an arbitrary function and generates the others. These spacetimes give the Reissner Nordstr$\ddot{o}$m metric black hole. We then, calculated the energy associated with these spacetimes using the superpotential method. We find that unless the time-space components of the tetrad field go to zero faster than ${1/\sqrt{r}}$ at infinity, one gets different results for the energy.
An exact solution has an axial symmetry is obtained in the teleparallel theory of gravitation. The associated metric has the structure function G(xi)=1-xi^2-2mA(xi)^3. The cubic nature of the structure function can make calculations cumbersome. Using a coordinate transformation we get a tetrad that its associated metric has the structure function in a factorisable form. This new form has the advantage that its roots are now trivial to write down. The singularities of the obtained tetrad are studied. Using another coordinate transformation we get a tetrad that its associated metric gives the Schwarzschild spacetime. Calculate the energy content of this tetrad we get a meaningless result!
The field equations of a special class of teleparallel theory of gravitation and electromagnetic fields have been applied to tetrad space having cylindrical symmetry with four unknown functions of radial coordinate $r$ and azimuth angle $θ$. The vacuum stress-energy momentum tensor with one assumption concerning its specific form generates one non-trivial exact analytic solution. This solution is characterized by a constant magnetic field parameter $B_0$. If $B_0=0$ then, the solution will reduces to the flat spacetime. The energy content is calculated using the superpotential given in the framework of teleparallel geometry. The energy contained in a sphere is found to be different from the pervious results.
Charged axially symmetric solution of the coupled gravitational and electromagnetic fields in the tetrad theory of gravitation is derived. The metric associated with this solution is an axially symmetric metric which is characterized by three parameters ``$ $the gravitational mass $M$, the charge parameter $Q$ and the rotation parameter $a$". The parallel vector fields and the electromagnetic vector potential are axially symmetric. We calculate the total exterior energy. The energy-momentum complex given by Møller in the framework of the Weitzenb$\ddot{o}$ck geometry ``$ ${\it characterized by vanishing the curvature tensor constructed from the connection of this geometry}" has been used. This energy-momentum complex is considered as a better definition for calculation of energy and momentum than those of general relativity theory. The energy contained in a sphere is found to be consistent with pervious results which is shared by its interior and exterior. Switching off the charge parameter, one finds that no energy is shared by the exterior of the charged axially symmetric solution. The components of the momentum density are also calculated and used to evaluate the angular momentum distribution. We found no angular momentum contributes to the exterior of the charged axially symmetric solution if zero charge parameter is used.
Using the energy momentum complex given by Møller in 1978 based on the absolute parallelism, the energy distribution in Kerr spacetime is evaluated. The energy with this spacetime is found to be the same as it was earlier evaluated using different definitions mainly based on the metric tensor.
The singularity of the black hole solutions obtained before in Møller's theory are studied. It is found that although the two solutions reproduce the same associated metric the asymptotic behavior of the scalars of torsion tensor and basic vector are quite different. The stability of the associated metric of those solutions which is spherically symmetric non singular black hole is studied using the equations of geodesic deviation. The condition for the stability is obtained. From this condition the stability of the Schwarzschild solution and di Sitter solution can be obtained.
We find the most general spherically symmetric non singular black hole solution in a special class of teleparallel theory of gravitation. If $r$ is large enough, the general solution coincides with the Schwarzschild solution. Whereas, if $r$ is small, the general solution behaves in a manner similar to that of de Sitter solution. Otherwise it describes a spherically symmetric black hole singularity free everywhere. Moreover, the energy associated with the general solution is calculated using the superpotential given by Møller 1978.
Using an axial parallel vector field we obtain two exact solutions of a vacuum gravitational field equations. One of the exact solutions gives the Schwarzschild metric while the other gives the Kerr metric. The parallel vector field of the Kerr solution have an axial symmetry. The exact solution of the Kerr metric contains two constants of integration, one being the gravitational mass of the source and the other constant $h$ is related to the angular momentum of the rotating source, when the spin density ${S_{i j}}^μ$ of the gravitational source satisfies $\partial_μ{S_{i j}}^μ=0$. The singularity of the Kerr solution is studied.
The field equations of the new general relativity (NGR) have been applied to an absolute parallelism space having three unknown functions of the radial coordinate. The field equations have been solved using two different schemes. In the first scheme, we used the conventional procedure used in orthodox general relativity. In the second scheme, we examined the effect of signature change. The latter scheme gives a solution which is different from the Schwarzschild one. In both methods we find solution of the field equations under the same constraint imposed on the parameters of the theory. We also calculated the energy associated with the solutions in the two cases using the superpotential method. We found that the energetic content of one of the solutions is different from that of the other. A comparison between the two solutions obtained in the present work and a third one obtained by Hayashi and Shirafuji (1979) shows that the change of the signature may give rise to new physics.
Starting from a spherically symmetric tetrad with three unknown functions of the radial coordinate, a general solution of Møller's field equations in case of spherical symmetry nonsingular black hole is derived. The previously obtained solutions are verified as special cases of the general solution. The general solution is characterized by an arbitrary function and two constants of integration. The general solution gives no more than the spherically symmetric nonsingular black hole solution. The energy content of the general solution depends on the asymptotic behavior of the arbitrary function, and is different from the standard one.
The self-gravitating instability of a compressible-inviscid fluid cylinder immersed into a self-gravitating tenuous medium of negligible motion is developed. The stability criterion is derived based on the linear perturbation technique. Some previous reported works are recovered. The effect of different factors on the fluid cylinder instability is discussed. The compressibility has a tendency for a stabilizing the model in particular as the sound speed $a$ is very large in the stable domains $1.0233928\leq x<\infty$ but comparatively small in the unstable domains $0<x<1.0233928$ where $x=k R_{_{0}}$ is the dimensionless longitudinal wavenumber with $k$ is the axial wavenumber and $R_{_{0}}$ is the radius of the cylinder. In the absence of the compressibility factor the unstable domain is found to be $0<x<1.0678$.
The field equations of a special class of tetrad theory of gravitation have been applied to tetrad space having three unknown functions of radial coordinate. The spherically symmetric vacuum stress-energy momentum tensor with one assumption concerning its specific form generates two non-trivial different exact analytic solutions for these field equations. For large $r$, the exact analytic solutions coincide with the Schwarzschild solution, while for small $r$, they behave in a manner similar to the de Sitter solution and describe a spherically symmetric black hole singularity free everywhere. The solutions obtained give rise to two different tetrad structures, but having the same metric, i.e., a static spherically symmetric nonsingular black hole metric. We then, calculated the energy associated with these two exact analytic solutions using the superpotential method. We find that unless the time-space components of the tetrad go to zero faster than $ \displaystyle{1 \over \sqrt{r}}$ at infinity, the two solutions give different results. This fact implies that the time-space components of the tetrad must vanish faster than $\displaystyle{1 \over \sqrt{r}}$ at infinity.
We study the energy and momentum of an isolated system in the tetrad theory of gravitation, starting from the most general Lagrangian quadratic in torsion, which involves four unknown parameters. When applied to the static spherically symmetric case, the parallel vector fields take a diagonal form, and the field equation has an exact solution. We analyze the linearized field equation in vacuum at distances far from the isolated system without assuming any symmetry property of the system. The linearized equation is a set of coupled equations for a symmetric and skew-symmetric tensor fields, but it is possible to solve it up to $O(1/r)$ for the stationary case. It is found that the general solution contains two constants, one being the gravitational mass of the source and the other a constant vector ${\grave B_α}$. The total energy is calculated from this solution and is found to be equal to the gravitational mass of the source. We also calculate the spatial momentum and find that its value coincides with the constant vector ${\grave B_α}$. The linearized field equation in vacuum, which is valid at distances far from the source, does not give any information about whether the constant vector ${\grave B_α}$ is vanishing or not. For a weakly gravitating source for which the field is weak everywhere, we find that the constant vector ${\grave B_α}$ vanishes.