Gamal Gergess Lamee Nashed
Current Organizations / Institutes
The organizations/institutes from the scholar's latest article
Research Subjects
Gamal Gergess Lamee Nashed has a total of 54 co-authors. Here are the top co-authors by number of shared publications:
- Waleed El Hanafy
Centre for Theoretical Physics The British University in Egypt El Sherouk City P.O. Box 43 11837 Cairo Egypt
38 shared publications - Shin'Ichi Nojiri
High Energy Accelerator Research Organization (KEK)
24 shared publications - Kazuharu Bamba
Faculty of Symbiotic Systems Science, Fukushima University, Fukushima 960-1296, Japan
21 shared publications - Ali Eid
Department of Physics, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh, Kingdom of Saudi Arabia
13 shared publications - Salvatore Capozziello
Scuola Superiore Meridionale
13 shared publications - Adel Awad
Ain Shams University
8 shared publications
Latest Publications
- Open Access
- 9 November 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (11) , 1289
Odd-parity perturbations of geometrically regular black holes with hedgehog scalar hair
In this paper, we study linear perturbations of a geometrically regular black hole supported by a constrained $$\textrm{SO}(3)$$ scalar triplet in the hedgehog configuration. Since the individual background scalars depend explicitly on the angular coordinates, the decoupling of polar and axial perturbations cannot be concluded from the spherical form of the metric or the energy–momentum tensor alone. To settle this point, we introduce the polar and axial metric and triplet amplitudes simultaneously and derive the corresponding projections of the radiative linearized Einstein–matter equations for $$\ell \ge 2$$ . The polar equations contain only polar amplitudes, whereas the axial equations contain only axial amplitudes; hence, the two off-diagonal blocks vanish at linear order. The resulting odd-parity sector forms a closed Einstein-scalar system in which a tangent perturbation of the scalar triplet is coupled to the Regge–Wheeler metric amplitudes. We show that its scalar equation has a second-order hyperbolic principal part whenever $$K_Y\ne 0$$ , and that, for the nonlinear kinetic function supporting the background, $$K_Y>0$$ throughout the black-hole exterior. The coefficient of the direct metric-scalar mixing is largest at the event horizon and decreases as $$r^{-4}$$ at large distance, while the metric block in the vacuum limit reproduces the standard Regge–Wheeler equation and potential; the scalar kinetic coefficient simultaneously vanishes, so the full operator changes rank. These results establish linear parity decoupling and the consistency of the closed axial equations. The odd sector can furthermore be reduced to two coupled master variables, and the corresponding reduced quadratic action has positive kinetic and radial-gradient eigenvalues at every finite exterior radius for the regular branch with $$K_Y>0$$ . The scalar normalization tends to zero asymptotically, however, and positivity of the full coupled potential operator is not established; hence we do not claim a complete mode-stability theorem.- 9 November 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (11) , 1289
Open AccessOdd-parity perturbations of geometrically regular black holes with hedgehog scalar hair
In this paper, we study linear perturbations of a geometrically regular black hole supported by a constrained $$\textrm{SO}(3)$$ scalar triplet in the hedgehog configuration. Since the individual background scalars depend explicitly on the angular coordinates, the decoupling of polar and axial perturbations cannot be concluded from the spherical form of the metric or the energy–momentum tensor alone. To settle this point, we introduce the polar and axial metric and triplet amplitudes simultaneously and derive the corresponding projections of the radiative linearized Einstein–matter equations for $$\ell \ge 2$$ . The polar equations contain only polar amplitudes, whereas the axial equations contain only axial amplitudes; hence, the two off-diagonal blocks vanish at linear order. The resulting odd-parity sector forms a closed Einstein-scalar system in which a tangent perturbation of the scalar triplet is coupled to the Regge–Wheeler metric amplitudes. We show that its scalar equation has a second-order hyperbolic principal part whenever $$K_Y\ne 0$$ , and that, for the nonlinear kinetic function supporting the background, $$K_Y>0$$ throughout the black-hole exterior. The coefficient of the direct metric-scalar mixing is largest at the event horizon and decreases as $$r^{-4}$$ at large distance, while the metric block in the vacuum limit reproduces the standard Regge–Wheeler equation and potential; the scalar kinetic coefficient simultaneously vanishes, so the full operator changes rank. These results establish linear parity decoupling and the consistency of the closed axial equations. The odd sector can furthermore be reduced to two coupled master variables, and the corresponding reduced quadratic action has positive kinetic and radial-gradient eigenvalues at every finite exterior radius for the regular branch with $$K_Y>0$$ . The scalar normalization tends to zero asymptotically, however, and positivity of the full coupled potential operator is not established; hence we do not claim a complete mode-stability theorem. - Open Access
- 6 November 2026
- Published by Springer Nature in The European Physical Journal Plus
- Vol. 141 (11) , 1268
Charge dependence of cusp formation in a phenomenological KZ-type black-hole metric
We examine the equatorial capture shadow of a phenomenological charged Konoplya–Zhidenko-type metric with $$\Delta =r^2+a^2-2Mr+Q^2-\epsilon /r$$ . The charge term is introduced in Kerr–Newman form; no field-equation derivation is asserted. Using unstable spherical photon orbits that satisfy angular admissibility and the full radial-capture condition, we calculate contacts of projected orbit images. For the theoretical benchmark $$(a/M,\epsilon /M^3)=(2,0.5)$$ , the cusp-removal contact reaches the celestial axis at $$Q_{\textrm{off}}/M\simeq 1.3520$$ . A sampling of spin and deformation values shows how this contact charge varies within the ansatz. In the moderate-spin family $$(a/M,\epsilon /M^3)=(0.9,0.5)$$ , the shadow is smooth over $$0\le Q/M\le 0.6$$ , develops a cusp near $$Q/M=0.9518$$ , and loses it near $$Q/M=1.3313$$ . The cusp morphology therefore depends on the chosen parameter family; the values are theoretical controls, not observational constraints.- 6 November 2026
- Published by Springer Nature in The European Physical Journal Plus
- Vol. 141 (11) , 1268
Open AccessCharge dependence of cusp formation in a phenomenological KZ-type black-hole metric
We examine the equatorial capture shadow of a phenomenological charged Konoplya–Zhidenko-type metric with $$\Delta =r^2+a^2-2Mr+Q^2-\epsilon /r$$ . The charge term is introduced in Kerr–Newman form; no field-equation derivation is asserted. Using unstable spherical photon orbits that satisfy angular admissibility and the full radial-capture condition, we calculate contacts of projected orbit images. For the theoretical benchmark $$(a/M,\epsilon /M^3)=(2,0.5)$$ , the cusp-removal contact reaches the celestial axis at $$Q_{\textrm{off}}/M\simeq 1.3520$$ . A sampling of spin and deformation values shows how this contact charge varies within the ansatz. In the moderate-spin family $$(a/M,\epsilon /M^3)=(0.9,0.5)$$ , the shadow is smooth over $$0\le Q/M\le 0.6$$ , develops a cusp near $$Q/M=0.9518$$ , and loses it near $$Q/M=1.3313$$ . The cusp morphology therefore depends on the chosen parameter family; the values are theoretical controls, not observational constraints. - 28 July 2026
- Published by World Scientific Pub Co Pte Ltd in International Journal of Geometric Methods in Modern Physics
Modeling anisotropic compact stars in f(R,G,T) gravity through a singularity-free approach
We investigate compact stellar configurations in the framework of [Formula: see text] gravity, where [Formula: see text], [Formula: see text], and [Formula: see text] denote the Ricci scalar, Gauss–Bonnet invariant term, and trace of the energy–momentum tensor, respectively. Adopting a linear functional form [Formula: see text], we explore the combined influence of higher-order curvature corrections ([Formula: see text]) and matter–geometry coupling ([Formula: see text]). Assuming a static, spherically symmetric spacetime with anisotropic matter distribution, we obtain exact and regular solutions to the modified field equations using the Krori–Barua metric ansatz. Model parameters are constrained with observational inputs from the LMXB neutron star 4U 1608-52, characterized by mass [Formula: see text] and radius [Formula: see text]. We analyze three representative cases: negative [Formula: see text], null [Formula: see text] corresponding to General Relativity, and positive [Formula: see text] values of [Formula: see text]. The resulting solutions satisfy all standard physical requirements, including causality condition, energy conditions and stability criteria. An effective equation of state is constructed via a curve-fitting procedure by optimizing the central sound speed. Employing the Buchdahl compactness limit and a central density [Formula: see text], we estimate the maximum mass and radius. Notably, the model predicts a maximum mass up to [Formula: see text] for [Formula: see text], and may be utilized to study the properties of primary mass [Formula: see text] detected in the gravitational wave event GW230529_181500. By varying the central density, the model reproduces radii consistent with many observed pulsars and recent gravitational wave events. These results demonstrate that the proposed framework provides stable configurations and offers a consistent extension of General Relativity for probing the internal structure of compact stars.
- 28 July 2026
- Published by World Scientific Pub Co Pte Ltd in International Journal of Geometric Methods in Modern Physics
Modeling anisotropic compact stars in f(R,G,T) gravity through a singularity-free approach
We investigate compact stellar configurations in the framework of [Formula: see text] gravity, where [Formula: see text], [Formula: see text], and [Formula: see text] denote the Ricci scalar, Gauss–Bonnet invariant term, and trace of the energy–momentum tensor, respectively. Adopting a linear functional form [Formula: see text], we explore the combined influence of higher-order curvature corrections ([Formula: see text]) and matter–geometry coupling ([Formula: see text]). Assuming a static, spherically symmetric spacetime with anisotropic matter distribution, we obtain exact and regular solutions to the modified field equations using the Krori–Barua metric ansatz. Model parameters are constrained with observational inputs from the LMXB neutron star 4U 1608-52, characterized by mass [Formula: see text] and radius [Formula: see text]. We analyze three representative cases: negative [Formula: see text], null [Formula: see text] corresponding to General Relativity, and positive [Formula: see text] values of [Formula: see text]. The resulting solutions satisfy all standard physical requirements, including causality condition, energy conditions and stability criteria. An effective equation of state is constructed via a curve-fitting procedure by optimizing the central sound speed. Employing the Buchdahl compactness limit and a central density [Formula: see text], we estimate the maximum mass and radius. Notably, the model predicts a maximum mass up to [Formula: see text] for [Formula: see text], and may be utilized to study the properties of primary mass [Formula: see text] detected in the gravitational wave event GW230529_181500. By varying the central density, the model reproduces radii consistent with many observed pulsars and recent gravitational wave events. These results demonstrate that the proposed framework provides stable configurations and offers a consistent extension of General Relativity for probing the internal structure of compact stars.
- Open Access
- 7 June 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (6)
Primordial black hole formation in $f(R)=R+\alpha R^2$ gravity: perturbative and non-perturbative analysisWe present a complete analytic and semi-analytic study of gravitational collapse and primordial black hole (PBH) formation in the quadratic f ( R ) model $$f(R)=R+\alpha R^2$$ . We first derive the perturbative expansion around General Relativity (GR), working to first order in the small parameter $$\alpha $$ . For a collapsing flat FLRW dust interior we compute the explicit first-order corrections to the scale factor, the stellar radius, and the horizon formation time. We then use these results to infer the expected trend in the PBH formation threshold $$\delta _c$$ , rather than a direct quantitative determination. Within this perturbative framework, the quadratic correction modifies the dust collapse dynamics at first order, while the flat radiation-dominated background does not receive a nontrivial correction at the same order. As result, any modification of PBH formation in the radiation era must arise from nonlinear or non-perturbative effects. The perturbative analysis therefore provides qualitative insight into how curvature corrections influence collapse, particularly in high-curvature regimes. To access this regime we reformulate the theory in the Einstein frame, where the model becomes GR plus the scalaron field $$\phi $$ with the Starobinsky potential. We provide the complete ODE system governing both the cosmological background and the evolution of an overdense closed FLRW patch. This system can be numerically integrated to obtain the critical overdensity $$\delta _c(k)$$ for PBH formation near the end of inflation.- 7 June 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (6)
Open AccessPrimordial black hole formation in $f(R)=R+\alpha R^2$ gravity: perturbative and non-perturbative analysisWe present a complete analytic and semi-analytic study of gravitational collapse and primordial black hole (PBH) formation in the quadratic f ( R ) model $$f(R)=R+\alpha R^2$$ . We first derive the perturbative expansion around General Relativity (GR), working to first order in the small parameter $$\alpha $$ . For a collapsing flat FLRW dust interior we compute the explicit first-order corrections to the scale factor, the stellar radius, and the horizon formation time. We then use these results to infer the expected trend in the PBH formation threshold $$\delta _c$$ , rather than a direct quantitative determination. Within this perturbative framework, the quadratic correction modifies the dust collapse dynamics at first order, while the flat radiation-dominated background does not receive a nontrivial correction at the same order. As result, any modification of PBH formation in the radiation era must arise from nonlinear or non-perturbative effects. The perturbative analysis therefore provides qualitative insight into how curvature corrections influence collapse, particularly in high-curvature regimes. To access this regime we reformulate the theory in the Einstein frame, where the model becomes GR plus the scalaron field $$\phi $$ with the Starobinsky potential. We provide the complete ODE system governing both the cosmological background and the evolution of an overdense closed FLRW patch. This system can be numerically integrated to obtain the critical overdensity $$\delta _c(k)$$ for PBH formation near the end of inflation. - 30 May 2026
- Published by Elsevier in Annals of Physics
- Vol. 492
Effects of the equation of state on wormhole properties in f(Q,T) gravity
- 30 May 2026
- Published by Elsevier in Annals of Physics
- Vol. 492
Effects of the equation of state on wormhole properties in f(Q,T) gravity
- Open Access
- 23 May 2026
- Published by Elsevier in Physics Letters B
- Vol. 878
Late-time cosmology and structure formation in quadratic f(Q) gravity
- 23 May 2026
- Published by Elsevier in Physics Letters B
- Vol. 878
Open AccessLate-time cosmology and structure formation in quadratic f(Q) gravity
- 20 May 2026
- Published by Elsevier in Physics of the Dark Universe
- Vol. 52
Einstein–Gauss–Bonnet inflation and its consistency with ACT observations
- 20 May 2026
- Published by Elsevier in Physics of the Dark Universe
- Vol. 52
Einstein–Gauss–Bonnet inflation and its consistency with ACT observations
- Open Access
- 10 May 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (5) , 504
Analytic semiclassical backreaction of a Schwarzschild black hole in a finite cavity: horizon shift, temperature renormalization, and canonical stability in the Hartle–Hawking state
We construct an analytic model of static semiclassical backreaction for a Schwarzschild black hole in the Hartle–Hawking state enclosed within a finite spherical cavity. Using a minimal renormalized stress–energy tensor consistent with conservation, thermal asymptotics, and horizon regularity, we integrate the reduced semiclassical Einstein equations under Dirichlet boundary conditions at the cavity wall. This yields explicit expressions for the corrections to the mass function, redshift factor, horizon location, and surface gravity. We obtain a closed-form first-order correction to the Hawking temperature in terms of a dimensionless backreaction parameter and the cavity radius. The temperature shift decomposes into redshift renormalization, geometric horizon displacement, and a local energy-density contribution at the horizon. The perturbative expansion is controlled by a parameter of order $$M_P^2/M^2$$ , ensuring validity for macroscopic black holes. The near-horizon geometry retains its universal Rindler $$^{2}\times S^{2}$$ structure, indicating that semiclassical effects renormalize rather than modify the geometric origin of Hawking radiation.- 10 May 2026
- Published by Springer Nature in The European Physical Journal C
- Vol. 86 (5) , 504
Open AccessAnalytic semiclassical backreaction of a Schwarzschild black hole in a finite cavity: horizon shift, temperature renormalization, and canonical stability in the Hartle–Hawking state
We construct an analytic model of static semiclassical backreaction for a Schwarzschild black hole in the Hartle–Hawking state enclosed within a finite spherical cavity. Using a minimal renormalized stress–energy tensor consistent with conservation, thermal asymptotics, and horizon regularity, we integrate the reduced semiclassical Einstein equations under Dirichlet boundary conditions at the cavity wall. This yields explicit expressions for the corrections to the mass function, redshift factor, horizon location, and surface gravity. We obtain a closed-form first-order correction to the Hawking temperature in terms of a dimensionless backreaction parameter and the cavity radius. The temperature shift decomposes into redshift renormalization, geometric horizon displacement, and a local energy-density contribution at the horizon. The perturbative expansion is controlled by a parameter of order $$M_P^2/M^2$$ , ensuring validity for macroscopic black holes. The near-horizon geometry retains its universal Rindler $$^{2}\times S^{2}$$ structure, indicating that semiclassical effects renormalize rather than modify the geometric origin of Hawking radiation. - 8 May 2026
- Published by World Scientific Pub Co Pte Ltd in International Journal of Modern Physics D
- Vol. 35 (08)
Three-dimensional charged black holes in f(Q) gravity
In this paper, we present new exact charged black hole solutions in (2+1) dimensions within the framework of [ ] gravity, where Q denotes the nonmetricity scalar. By considering a cubic [ ] form, we derive classes of charged and uncharged spherically symmetric solutions, and we identify conditions under which these reduce to the well-known Banados–Teitelboim–Zanelli (BTZ) black hole. Notably, our analysis reveals a novel charged solution that is asymptotically anti-de Sitter (AdS) but cannot be continuously deformed into a GR counterpart, and thus arising purely form the higher-order nature of the nonmetricity corrections. Furthermore, we explore the geometrical properties of the solutions, demonstrating the existence of multiple horizons and a softer central singularity compared to GR. Additionally, we calculate various thermodynamic quantities, such as Hawking temperature, entropy and heat capacity, with results confirming thermal stability. Finally, a detailed study of the geodesic motion and the effective potentials unveils stable photon orbits, as well as the effect of cubic nonmetricity corrections on orbital dynamics.
- 8 May 2026
- Published by World Scientific Pub Co Pte Ltd in International Journal of Modern Physics D
- Vol. 35 (08)
Three-dimensional charged black holes in f(Q) gravity
In this paper, we present new exact charged black hole solutions in (2+1) dimensions within the framework of [ ] gravity, where Q denotes the nonmetricity scalar. By considering a cubic [ ] form, we derive classes of charged and uncharged spherically symmetric solutions, and we identify conditions under which these reduce to the well-known Banados–Teitelboim–Zanelli (BTZ) black hole. Notably, our analysis reveals a novel charged solution that is asymptotically anti-de Sitter (AdS) but cannot be continuously deformed into a GR counterpart, and thus arising purely form the higher-order nature of the nonmetricity corrections. Furthermore, we explore the geometrical properties of the solutions, demonstrating the existence of multiple horizons and a softer central singularity compared to GR. Additionally, we calculate various thermodynamic quantities, such as Hawking temperature, entropy and heat capacity, with results confirming thermal stability. Finally, a detailed study of the geodesic motion and the effective potentials unveils stable photon orbits, as well as the effect of cubic nonmetricity corrections on orbital dynamics.