arXiv subjects

G. G. L. Nashed

Publications and source records attributed to G. G. L. Nashed.

At least 19 recordsLinked to original sources

arXiv2026

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 $\mathrm{SO}(3)$ 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 $\ell\geq2$. 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 $K_Y\neq0$, 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. They do not, however, constitute a proof of mode stability, which requires the reduction of the complete quadratic action.

physics.gen-ph↗

arXiv2026

Late-Time Cosmological Tests of a Minisuperspace Generalized-Uncertainty-Principle Deformation

The investigation of possible quantum-gravity effects in cosmology is of considerable interest, especially when such effects can be tested against observations of the late-time expansion. In the present work we study a quadratic generalized-uncertainty-principle (GUP) deformation of the FLRW minisuperspace Poisson algebra. At first order in the deformation parameter, the construction modifies the Friedmann equation and hence the homogeneous expansion history. Since it does not determine a unique perturbation sector, we restrict the analysis to background observables, using 31 cosmic-chronometer measurements, the uncalibrated 1580-object Pantheon$+$ Hubble-flow sample, and the 13-component DESI DR2 baryon-acoustic-oscillation likelihood, while treating the sound horizon as a nuisance parameter. We compare the resulting cosmology with flat \LCDM{} and with the Chevallier--Polarski--Linder parametrization. For the baseline normalization we obtain $\betastar=-0.086^{+0.040}_{-0.032}$ at $68\%$ credibility and $-0.142<\betastar<0.005$ at $95\%$ credibility.

gr-qc↗

arXiv2026

Effective Constrained Scalar--Gauss--Bonnet Inflation Motivated by $f(R,\mathcal{G})$ Gravity

We develop an effective framework for inflation in a constrained scalar--Gauss--Bonnet theory motivated by a restricted sector of $f(R,\mathcal{G})$ gravity. Using unified parametrizations of the Hubble expansion rate and the Gauss--Bonnet coupling function within a generalized slow-roll formalism, we derive analytical expressions for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$, and study their dependence on the model parameters. We show that the Hubble parametrization mainly controls the scalar sector through the slow-roll parameter $ε_1$, while the Gauss--Bonnet-induced contribution $ε_4$ can significantly affect the scalar tilt and strongly suppress primordial tensor modes, naturally leading to very small values of $r$. A representative benchmark solution yields $n_s \simeq 0.958$ and $r \simeq 2.7 \times 10^{-4}$, marginally compatible with current Planck, ACT, and BICEP/Keck constraints. We further investigate the scalar perturbation structure of the exactly constrained theory, where the Lagrange-multiplier constraint forces the lapse perturbation to vanish and, together with the gravitational momentum constraint, implies $\dot{\mathcal{R}}=0$, eliminating the propagating scalar degree of freedom at linear order. This exact result clarifies that the generalized slow-roll treatment should be interpreted as an effective softly constrained description. We also discuss perturbative stability conditions, including the positivity of the relevant kinetic coefficients and propagation speeds. Our results demonstrate that the effective constrained scalar--Gauss--Bonnet framework provides a flexible and observationally viable description of inflation while clarifying the distinction between the exact constrained limit and its effective slow-roll realization.

physics.gen-ph↗

arXiv2026

Primordial Black Hole Formation in $f(R)=R+αR^2$ Gravity: Perturbative and Non-Perturbative Analysis

We present a complete analytic and semi-analytic study of gravitational collapse and primordial black hole (PBH) formation in the quadratic $f(R)$ model $f(R)=R+αR^2$. We first derive the perturbative expansion around General Relativity (GR), working to first order in the small parameter $α$. 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 $δ_c$, 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 $ϕ$ 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 $δ_c(k)$ for PBH formation near the end of inflation.

gr-qc↗

arXiv2026

Late-Time Cosmology and Structure Formation in Quadratic $f(Q)$ Gravity

We investigate the cosmological evolution associated with the quadratic symmetric teleparallel gravity framework, $ f(Q)=Q+αQ^{2}+β$ where the relation $Q\propto H^{2}$ generates an additional $H^{4}$ contribution to the Friedmann equation. Using the exact algebraic solution for $H(z)$, we reconstruct the effective dark-energy sector and compare the background evolution with $Λ$CDM using Type Ia supernovae, BAO, and cosmic-chronometer data. At the perturbative level, the model modifies the Poisson equation through a time-dependent effective gravitational coupling $G_{\textrm eff}(z)=G\big[1+\tfrac{2}{3}A E^{2}(z)\big]^{-1}$, where $A=18αH_{0}^{2}$. For $α>0$ this produces a weakened gravitational interaction, suppressing the linear growth factor $D(z)$, the growth rate $f(z)$, and the RSD observable $fσ_{8}(z)$. In the nonlinear regime, the reduced gravitational strength increases the spherical-collapse threshold and suppresses the halo mass function, leading to a lower predicted value of $S_{8}=σ_{8}\sqrt{Ω_{m}/0.3}$. Thus, the quadratic $f(Q)$ extension can reproduce mild deviations from $Λ$CDM at the background level while naturally alleviating the $S_{8}$ tension, offering a viable modified-gravity explanation for recent observational hints of dynamical dark energy.

physics.gen-ph↗

arXiv2026

Rotating traversable wormholes and particle dynamics in $f(R,T)$ gravity

Traversable wormholes are among the most interesting solutions of gravitational theories, but within General Relativity they generally require exotic matter violating the null energy condition. Modified gravity theories with matter-geometry coupling provide a promising framework in which wormhole geometries may instead be supported by effective gravitational contributions. Motivated by this possibility, we investigate rotating traversable wormholes in $f(R,T)$ gravity, where $R$ is the scalar curvature and $T$ is the trace of the energy-momentum tensor, within the slow-rotation approximation. We construct stationary and axisymmetric wormhole solutions supported by an anisotropic fluid and show that the obtained geometries are regular, asymptotically flat, horizonless, and satisfy the flare-out condition at the throat. A central result is that the matter sector satisfies both the null and strong energy conditions, indicating that traversable rotating wormholes can be supported without exotic matter. We further analyze particle motion, frame dragging, and non-geodesic effects arising from matter-geometry coupling, together with shadow deformation and gravitational lensing signatures induced by rotation. A preliminary stability analysis based on sound-speed conditions indicates the physical viability of the solutions. These results demonstrate that rotating wormholes in $f(R,T)$ gravity constitute physically consistent compact configurations with potentially observable astrophysical signatures.

gr-qc↗

arXiv2026

Photon Sphere and Shadow of a Perturbative Black Hole in $f(R,\mathcal{G})$ Gravity

We investigate the impact of higher-curvature corrections on black-hole observables within a perturbative $f(R, G)$ gravity framework. Working in a static, spherically symmetric spacetime, we construct leading-order deviations from the Schwarzschild solution by expanding the field equations in small coupling parameters associated with quadratic curvature invariants. The resulting metric corrections are obtained as asymptotic expansions and used to analyze null geodesics. We derive analytic expressions for the shift in the photon-sphere radius and show that higher-curvature terms modify the location of unstable photon orbits, with the Gauss--Bonnet sector producing a more significant contribution than mixed curvature terms. These modifications propagate to observable quantities, leading to corrections in the black-hole shadow radius. We identify the distinct roles of photon-sphere displacement and direct metric perturbations in determining the shadow size. We further discuss the implications of these corrections for strong gravitational lensing and quasinormal modes, highlighting the enhanced sensitivity of strong-field observables to higher-curvature effects. While the present analysis is based on an asymptotic perturbative treatment, our results provide a consistent framework for estimating leading-order deviations from general relativity and suggest that high-resolution observations, including very-long-baseline interferometry and gravitational-wave measurements, may offer constraints on modified gravity models.

gr-qc↗

arXiv2026

Light Deflection and Greybody Bound Around a BTZ-ModMax Black Hole in Plasma Medium

We study the deflection of light in a homogeneous plasma medium around a BTZ-ModMax black hole, focusing on the effects of the ModMax nonlinear electrodynamics parameter and the cosmological constant. Using the Gauss-Bonnet theorem applied to the corresponding optical geometry in plasma, we derive a modified expression for the deflection angle and examine how plasma dispersion alters the gravitational lensing behavior. The influence of the ModMax parameter in the presence of homogeneous plasma is compared with its vacuum counterpart, as well as with the charged and static BTZ black hole cases, revealing distinct signatures arising from nonlinear electrodynamics. This work highlights the combined impact of homogeneous plasma, spacetime curvature, and nonlinear field dynamics on light deflection in lower-dimensional black hole geometries. We further study the greybody factor and analyze how the presence of homogeneous plasma and the ModMax parameter modifies the energy emission spectrum of the black hole. Our results demonstrate that both plasma effects and nonlinear electrodynamics significantly influence the transmission probabilities and emission rates, providing deeper insight into wave propagation and observational signatures in lower-dimensional black hole geometries.

gr-qc↗

arXiv2026

3-dimensional charged black holes in $f({Q})$ gravity

We present new exact charged black hole solutions in (2+1) dimensions within the framework of $f({Q})$ gravity, where ${Q}$ denotes the non-metricity scalar. By considering a cubic $f({Q})$ 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 from the higher-order nature of the non-metricity 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 non-metricity corrections on orbital dynamics.

gr-qc↗

arXiv2026

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.

gr-qc↗

arXiv2026

A Minimal and Stable Vacuum Bounce in Exponential $f(R)$ Gravity

We investigate the realization of a nonsingular cosmological bounce in metric $f(R)$ gravity using a controlled exponential deformation of the Starobinsky $R^{2}$ model. Adopting a smooth Gaussian-type bouncing scale factor, we first demonstrate a no-go result showing that a positive-curvature vacuum bounce cannot be supported by the model $f(R)=R+αR^{2}(1-e^{-R/R_b})$ alone. We then show that a minimal extension obtained by introducing a constant term restores the bounce exactly, with the constant fixed algebraically by the bounce condition. A systematic parameter-space scan is performed to identify regions free of ghost and tachyonic instabilities. Working in the Einstein frame, we study the evolution of scalar and tensor perturbations across the bounce and show that both remain finite and well behaved. Our results establish a minimal, perturbatively stable realization of a vacuum bounce in $f(R)$ gravity that goes beyond background-level constructions.

gr-qc↗

arXiv2026

Black Hole Topologies and Geodesic Structures in Symmetric Teleparallel f(Q) Gravity

Black hole solutions are studied here within the symmetric teleparallel formulation of gravity, employing the $f(Q)$ model in which the gravitational dynamics are governed by the non-metricity scalar $Q$. We focus on static, circularly symmetric spacetimes in $(2+1)$-dimensions, analyzing both charged and uncharged cases. By adopting a power-law form for $f(Q)$, we derive exact black hole solutions and explore their thermodynamic and geometric properties. Curvature and non-metricity scalars reveal central singularities stronger than those in general relativity. we find that the horizon radii increase with the charge parameter while higher values of the non-metricity coefficient, $c_{4}$, or the cosmological constant $Λ$ tend to merge or eliminate horizons, reducing their total number and altering the near-origin structure of the spacetime. We perform a detailed topological analysis based on the Euler characteristic and examine the geodesic completeness of the spacetime. Our findings show that, depending on the presence of electric charge, the singularity may or may not be reachable by geodesics. The thermodynamic stability is confirmed via temperature, entropy, and heat capacity calculations. This study highlights the rich structure of $f(Q)$ gravity in lower-dimensional settings and offers new insights into the nature of singularities and black hole topologies in modified gravity theories.

gr-qc↗

arXiv2026

Holographic Dark Energy as a Source for Wormholes in Modified Gravity

Traversable wormhole solutions are explored in $f(\mathcal{R},\mathbb{T})$ gravity, a curvature--matter extension in which $\mathcal{R}$ is the Ricci scalar and $\mathbb{T}$ denotes the trace of the energy--momentum tensor. To generate explicit wormhole models, we prescribe holographic dark-energy densities based on entropy formalism proposed by Rényi, Moradpour, and Bekenstein--Hawking, namely \[ ρ_{\textit{R}} = \fracα{4α_1 r^4 c^2 κ}\ln\!\left(1+πα_1 r^2\right), \qquad ρ_{\textit{M}} = \fracα{4πr^2 c^2 κ\left(πα_1 r^2 + 1\right)}, \qquad ρ_{\textit{BH}} = \fracα{4 c^2 κr^2}, \] with $α$ and $β$ carrying dimensions of $L^{-2}$. The corresponding shape functions obtained from the field equations satisfy the standard throat and flare-out requirements for traversability. We then study how varying $α$ and $β$ affects (i) the balance of forces associated with equilibrium and (ii) the status of the energy conditions. In particular, the null energy condition is found to be violated, indicating that exotic matter (or an effective exotic sector) is required to support the wormhole geometry. The spatial structure of the solutions is further visualized through embedding surfaces.

gr-qc↗

arXiv2026

Stable black holes in lower dimensional $f(\mathbb{Q})$ non-metric gravity

We investigate exact charged and uncharged black hole solutions in a (2+1)-dimensional spacetime within the framework of quadratic form of $f(\mathbb{Q})$ symmetric teleparallel gravity, where $\mathbb{Q}$ is the non-metricity scalar. By adopting spherical symmetry and considering both vanishing and non-vanishing electromagnetic fields, we derive new classes of black hole solutions and analyze their geometric and physical properties. The study demonstrates that the inclusion of quadratic corrections in the gravitational Lagrangian significantly modifies the structure of solutions, producing deviations from the standard BTZ geometry. Invariants such as curvature and non-metricity scalars are calculated to classify the singularity structure and spacetime behavior. Thermodynamic quantities, including Hawking temperature, entropy, and heat capacity, are computed, showing consistency with the first law of black hole thermodynamics. Furthermore, we examine the geodesic motion of test particles and derive the effective potential to explore the stability of photon orbits. A notable outcome is the identification of weaker black hole singularities in comparison to General Relativity, attributed to the non-metricity corrections. The possibility of multi-horizon configurations is also explored. This study provides a comprehensive analysis of the gravitational, thermodynamic, and dynamical features of lower-dimensional black holes in $f(\mathbb{Q})$ gravity and highlights their distinct characteristics with respect to General Relativity.

gr-qc↗

arXiv2025

Constraining linear form of $f(\mathcal{R,G,T})$ gravity from astrophysical observations of the Pulsar U1724

In this work we examine the internal structure of compact stars within an extended gravitational framework described by the function $f(\mathcal{R},\mathcal{G},\mathcal{T})$. Throughout this work, the quantity $\mathcal{R}$ refers to the curvature scalar formed from the Ricci tensor. The term $\mathcal{G}$ denotes the Gauss--Bonnet curvature invariant, while $\mathcal{T}$ corresponds to the trace obtained by contracting the matter energy-momentum tensor. Our analysis is directed toward massive radio pulsars with masses above $1.8\,M_{\odot}$, which provide an exceptional testing ground for gravity under conditions inaccessible to laboratory experiments. Adopting the linear form $f(\mathcal{R},\mathcal{G},\mathcal{T})=\mathcal{R}+α\,\mathcal{G}+β\,\mathcal{T}$ where $α$ and $β$ are parameters of suitable dimensionality,\footnote{$α$ has dimensions of $[L^{2}]$ and $β$ carries units of $[N^{-1}]$.} we obtain an exact analytic solution for static anisotropic stellar matter in hydrostatic equilibrium. This solution allows all physical quantities to be expressed in terms of the dimensionless parameters $ α_{1}=α/R^{2},\qquad β_{1}=β/κ^{2}$ together with the compactness $C=2GM/(Rc^{2})$. We constraint the two parameters $α$ and $β$ by matching the model with the mass and radius of pulsar \textit{U1724} requires restricting these parameters to $α_{1}=\pm0.023$ and $β_{1}=\pm0.001$, where $κ^{2}=8πG/c^{4}$ is the standard Einstein coupling. The resulting stellar configuration satisfies the causal bound on the radial sound speed, $c_{s}^{2}<c^{2}/3$, distinguishing it from the corresponding behaviour in general relativity.

gr-qc↗

arXiv2025

Buchdahl limit of compact stars in presence of Weyl anomaly

We setup an anisotropic compact star model in presence of Weyl ``trace" anomaly. We derive an exact interior solution which determines the contribution of the vacuum trace anomaly. We introduce a dimentionless parameter, $β$, to characterize this contribution. Applying appropriate matching conditions with the exterior solution, we determine the model parameters in terms of the Weyl anomaly parameter $β$ and the compactness parameter, $C=\frac{2GM}{c^2 \mathcal{R}}$ where $M$ and $\mathcal{R}$ are the mass and the radius of the star. We investigate the parameter space $\{β, C\}$ and the corresponding modifications of Buchdahl limit on the maximum compactness. We use astrophysical observations of mass and radius of the pulsar PSR J0740+6620 to constrain the Weyl anomaly parameter $β$. Also, we investigate the Mass-Radius diagram with other observational constraints from NICER and LIGO/Virgo collaboration.

gr-qc↗

arXiv2025

Geodesics and Light Deflection in Schwarzschild-like Spacetime from Cosmology-Inspired Modified Gravity

We investigate cosmology-driven modifications to Schwarzschild-like black hole spacetimes and analyze their impact on photon propagation, gravitational lensing, and shadow observation. The gravitational deflection angle is computed using the Rindler-Ishak method, which incorporates finite-distance corrections and provides a consistent framework for non-asym-ptotically flat spacetimes. The effective potential for null geodesics exhibits a single unstable maximum corresponding to the photon sphere, and we study photon orbits classified according to the critical impact parameter into capture, escape, and unstable circular trajectories. Our analysis shows that the deflection angle decreases with increasing model parameter $(α)$, resulting in weaker light bending compared to the Schwarzschild case. In addition, we examine the angular diameter of the black hole shadow as measured by a static observer, highlighting its dependence on the cosmological modification parameters. These results suggest that high-precision astrometric and lensing observations can place meaningful constraints on cosmology-inspired modifications to gravity, thereby linking astrophysical black holes with cosmic expansion and offering a novel probe of gravitational physics in strong-field regimes.

gr-qc↗

arXiv2025

General form of $f(R)$ and $(2+1)$-dimensional charge/non-charge black hole solutions

We introduce novel black hole (BH) solutions, charge/non-charge, within the framework of $f(R)$ gravity, a theory that does not inherently include a cosmological constant, using equal/diffirent metric ansatzs. Remarkably, these solutions exhibit asymptotically Anti-de Sitter (AdS) or de Sitter (dS) behavior, depending on their parameter values. Unlike the BTZ solutions of General Relativity, which feature a causal singularity and constant scalar invariants, our solutions display strong spacetime singularities, as shown by their scalar invariants. We construct $f(R)$ functions that behave as polynomial functions, emphasizing the unique nature of these solutions. We demonstrate the stability of these solutions in two ways: first, by showing that their heat capacity is positive, which ensures thermodynamic stability; and second, by proving that the second derivative of $f(R)$ is positive, meeting the Ostrogradski criterion for dynamical stability. Furthermore, the solutions satisfy the first law of thermodynamics, confirming their consistency with fundamental thermodynamic principles.

gr-qc↗