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Ragab M. Gad

Publications and source records attributed to Ragab M. Gad.

At least 19 recordsLinked to original sources

Conformal Solutions of Static Plane Symmetric Cosmological Models in Cases of a Perfect Fluid and a Cosmic String Cloud

In this work, we obtained exact solutions of Einstein's field equations for plane symmetric cosmological models by assuming that thy admit conformal motion. The space-time geometry of these solutions is found to be nonsingular, non-vacuum and conformally flat. We have shown that in the case of a perfect fluid, these solutions have an energy-momentum tensor possessing dark energy with negative pressure and the energy equation of state is $ρ+p=0$. We have shown that a fluid has acceleration, rotation, shear-free, vanishing expansion, and rotation. In the case of a cosmic string cloud, we found that the tension density and particle density decrease as the fluid moves along the direction of the strings, then vanish at infinity. We shown that the exact conformal solution for a static plane symmetric model reduces to the the well-known anti-De Sitter space time. We obtained that the space-time under consideration admits a conformal vector field orthogonal to the four-velocity vector and does not admits a vector parallel to the four-velocity vector. Some physical and kinematic properties of the resulting models are also discussed.

gr-qc

On Axially Symmetric Space-Times Admitting Homothetic Vector Fields in Lyra's Geometry

This paper investigates axially symmetric space-times that admit a homothetic vector field based on Lyra's geometry. The cases when the displacement vector is a function of $t$ and when it is constant are studied. In the context of this geometry, we find and classify the solutions of the Einstein's field equations (EFE) for the space-time under consideration, which display a homothetic symmetry.

gr-qc

Homothetic Motion in a Bianchi Type-I Model in Lyra Geometry

In this paper we study a homothetic vector field of a Bianchi type-I model based on Lyra geometry. The cases when a displacement vector is function of $t$ and when it is constant are considered. In both two cases we investigate the equation of state. A comparison between the obtained results, using Lyra geometry, and that have obtained previously in the context of General Relativity, based on Riemannian geometry, will be given.

gr-qc

Harmonic and Wave Maps Coupled with Einstein's gravitation

In this paper we discuss the coupled dynamics, following from a suitable Lagrangian, of a harmonic or wave map $ϕ$ and Einstein's gravitation described by a metric $g$. The main results concern energy conditions for wave maps, harmonic maps from warped product manifolds, and wave maps from wave-like Lorentzian manifolds.

math-ph

Axially Symmetric Cosmological Mesonic Stiff Fluid Models in Lyra's Geometry

In this paper, we obtained a new class of axially symmetric cosmological mesonic stiff fluid models in the context of Lyra's geometry. Expressions for the energy, pressure and the massless scalar field are derived by considering the time dependent displacement field. We found that the mesonic scalar field depends on only $t$ coordinate. Some physical properties of the obtained models are discussed.

gr-qc

On Spherically Symmetric Non-Static Space-Times Admitting Homothetic Motions

Spherically symmetric solutions admitting a homothetic Killing vector field (HKVF) either orthogonal, $η_{\bot}$, or parallel,$η_{||}$, to the 4-velocity vector field, $u^a$, are studied. New self-similar solution of Einstein's field equation is found in the case when HKVF is in a general form. Some physical properties of the obtained solution are examined.

gr-qc

Geodesics and Geodesic Deviation in static Charged Black Holes

The radial motion along null geodesics in static charged black hole space-times, in particular, the Reissner-Nordström and stringy charged black holes are studied. We analyzed the properties of the effective potential. The circular photon orbits in these space-times are investigated. We found that the radius of circular photon orbits in both charged black holes are different and differ from that given in Schwarzschild space-time. We studied the physical effects of the gravitational field between two test particles in stringy charged black hole and compared the results with that given in Schwarzschild and Reissner-Nordström black holes.

math-ph

Møller's Energy in the Kantowski-Sachs Space-time

We present a counter example to paper \cite{P71} and show that the result obtained is correct for a class of metric but not general. We calculate the total energy of the Kantowski-Sachs space-time by using the energy-momentum definitions of Møller in the theory of general relativity and the tetrad theory of gravity.

gr-qc

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.

gr-qc

Møller Energy-Momentum Complex of a Static Axially Symmetric Vacuum Space-Time

The energy and momentum densities associated with the Weyl metric are calculated using Møller's energy-momentum complex. These results are compared with the results obtained by using the energy-momentum complexes of Einstein, Landau and Lifshitz, Papapetrou and Bergmann. We show that the aforementioned different prescriptions and that of Møller do not give the same energy density, while give the same momentum density.

gr-qc

Energy and Momentum Densities Associated with Solutions Exhibiting Directional Type Singularities

We obtain the energy and momentum densities of a general static axially symmetric vacuum space-time described by the Weyl metric, using Landau-Lifshitz and Bergmann-Thomson energy-momentum complexes. These two definitions of the energy-momentum complex do not provide the same energy density for the space-time under consideration, while give the same momentum density. We show that, in the case of Curzon metric which is a particular case of the Weyl metric, these two definitions give the same energy only when $R \to \infty$. Furthermore, we compare these results with those obtained using Einstein, Papapetrou and Møller energy momentum complexes.

gr-qc

Energy and Momentum densities of cosmological models, with equation of state $ρ=μ$, 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.

gr-qc

Energy and momentum associated with a Static Axially Symmetric Vacuum Space-Time

We use the Einstein and Papapetrou energy-momentum complexes to calculate the energy and momentum densities of Weyl metric as well as Curzon metric. We show that these two different definitions of energy-momentum complexes do not provide the same energy density for Weyl metric, although they give the same momentum density. We show that, in the case of Curzon metric, these two definitions give the same energy only when $R \to \infty$. Furthermore, we compare these results with those obtained using Landau and Lifshitz, Bergmann and Møller.

gr-qc

On the Singularities of Reissner-Nordström Space-Time

It is shown that if two Reissner-Nordström space-times, both with the same mass m and charge e, glued together in the singularities, then the light ray in black hole of the first space-time can go continuously through the singularity into black hole of the second. The behavior of tidal forces near the Reissner-Nordström space-time singularity is examined by considering what happens between two particles falling freely towards the singularity.

gr-qc

Energy Distribution of a Gödel-Type Space-Time

We calculate the energy and momentum distributions associated with a Gödel-type space-time, using the well-known energy-momentum complexes of Landau and Lifshitz and Møller. We show that the definitions of Landau and Lifshitz and Møller do not furnish a consistent result.

gr-qc

On The Geometrical and Physical Properties of Spherically Symmetric Non-Static Space-Times: Self-Similarity

An exact solution of the Einstein field equations is found under the assumption of spherically symmetry and the existence of one-parameter group of homothetic motions. This solution has a singularity at $r = 0$, and has non-vanishing expansion, acceleration and shear. Tidal forces in radial direction will not stretch an observer falling in this fluid. If the material is represented by perfect fluid, we can verify that the solution coincides with stiff matter case. In this case, the solution has zero expansion. Tidal forces in radial direction will not stretch an observer falling in this fluid and they not squeeze him in transverse directions.

gr-qc

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.

gr-qc