arXiv subjects

G. G. L. Nashed

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

At least 37 records · Page 2Linked to original sources

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.

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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.

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arXiv2025

An innovative black hole solution and thermodynamic properties in higher-order curvature gravity with a scalar field

A spherically symmetric black hole solution defined by the gravitational mass is explored in higher-order curvature gravity associated with a scalar field. It is demonstrated that the singularities of the curvature invariants will be far weaker around the central region of the black hole than those in general relativity, owing to the effect originating from higher-order curvatures with the coupling to a dynamical scalar field. Furthermore, thermodynamic properties and the Davies-type phase transition of the black hole are investigated for Barrow entropy with a quantum effect of gravitation. It is found that both the quasi-local energy and Gibbs free energy are positive, and the black hole can hence be stable on the smooth horizon surface. In addition, by analyzing the geodesic deviation, the stability conditions of the black hole are explicitly shown.

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arXiv2025

Stable soliton dark matter wormhole in non-minimally coupled $f({\cal Q},{\cal T})$ gravity

We show that non-minimal coupling between matter and geometry can indeed help in constructing stable, traversable, wormholes (WHs) without requiring exotic matter under certain conditions. In models like $f({\cal Q},{\cal T})={\cal Q}+β{\cal T}$ gravity, where ${\cal Q}$ is the non-metricity scalar, and ${\cal T}$ is the trace of the energy-momentum tensor, the coupling between matter and geometry introduces additional degrees of freedom in terms of the parameter $β$. These can mimic the effects of exotic matter or even replace it entirely under specific parameter choice. The analysis involves deriving WH shape functions based on two dark matter (DM) density profiles: a solitonic core at the center of DM halos, and the outer halo follows the universal Navarro-Frenk-White (NFW) density profile of cold DM (CDM). The wormhole solutions derived in these models satisfy important geometric conditions like: Flaring-out condition (necessary for traversability) and asymptotic flatness condition. For large positive coupling parameter, the null energy condition (NEC) can be satisfied at the wormhole throat, meaning exotic matter is not needed, while the wormhole is no longer Lorentzian and the flaring-out condition is broken. However, for large negative coupling parameter, the NEC can be satisfied, allowing for healthy wormholes without exotic matter, provided the coupling strength remains within certain bounds. In the latter case, the NEC is broken only effectively. We investigate the stability of the obtained wormhole solutions by virtue of a modified version of Tolman-Oppenheimer-Volkoff (TOV) equation, which includes a new force due to matter-geometry non-minimal, showing that these wormholes can be dynamically stable.

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arXiv2025

Four-scalar model and spherically symmetric solution in $f({\cal T})$ theory

In this work, we investigate the construction of spherically symmetric solutions within the framework of modified teleparallel gravity, focusing in particular on $f({\cal T})$ theory, where ${\cal T}$ represents the torsion scalar. Conventional mimetic gravity and its extension with two scalar fields are unable to reproduce general spherically symmetric geometries in $f({\cal T})$ gravity, since consistency requires either a constant torsion scalar or a linear form of $f({\cal T})$, which corresponds to the teleparallel equivalent of General Relativity (TEGR). To overcome this restriction, we introduce a four-scalar field formulation that generalizes the two-scalar model and enables the realization of arbitrary spherically symmetric spacetimes. We further demonstrate that the model avoids ghost degrees of freedom by imposing appropriate constraints through Lagrange multipliers. As an application of the formalism, we examine a specific spherically symmetric metric and determine the corresponding four scalar fields, analyzing their properties. In the presence of a quadratic extension of the action, ${\cal T} + \tfracα{2}{\cal T}^2$, our approach successfully reconstructs physically viable solutions free from ghost instabilities. These findings highlight the potential of scalar-field-based constructions for generating consistent spacetime geometries in modified gravity theories, thereby extending beyond the standard formulations.

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arXiv2025

Spherically symmetric solution in quadratic non-metricity gravity

We explicitly find an exact spherically symmetric solution in quadratic non-metricity gravity. We show that the quadratic term acts as a cosmological constant. This solution contradicts all the claims in the literature that there is no spherically symmetric solution for higher-order non-metricity gravity. Moreover, we demonstrate that for the charged field equations, the solution can be identical to the non-charged case. This is because the off-diagonal components of the field equation do not feel the effect of the charge.

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arXiv2025

Properties of compact objects in quadratic non-metricity gravity

Astrophysical compact objects are studied in the context of quadratic non-metricity gravity. The solutions to the gravitational field equations, which include fluid components, are analyzed to investigate the density and pressure properties of radio pulsars. It is explicitly demonstrated that the theoretically stable models are consistent with astronomical data, due to the geometric features of the quadratic component. Furthermore, it is shown that, in contrast to the compactness limits of black holes in general relativity, the core density can significantly exceed the density at which nuclear saturation occurs, and the surface density can also surpass the value of nuclear saturation. Additionally, it is found that the radial sound speed remains below the conformal upper bound for sound velocity established by perturbative quantum chromodynamics.

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arXiv2025

Stellar isotropic model in the symmetric teleparallel equivalent of general relativity theory

Recently, the theory of symmetric teleparallel equivalent of general relativity (STEGR) has gained much interest in the cosmology and astrophysics community. Within this theory, we discuss the method of deriving a stellar isotropic model. In this respect, we implement the equations of motion of STEGR theory to a spacetime that is symmetric in a spherical manner, resulting in a set of nonlinear differential equations with more unknowns than equations. To solve this issue, we assume a special form of $g_{tt}$, and suppose a null value of the anisotropy to obtain the form of $g_{rr}$. We then investigate the possibility of obtaining an isotropic stellar model consistent with observational data. To test the stability of our model, we apply the adiabatic index and the Tolman-Oppenheimer-Volkoff equation. Furthermore, we examine our model using different observed values of radii and masses of pulsars, showing that all of them fit in a consistent way.

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arXiv2025

General form of the function $f(\mathbb{Q})$ using cylindrically static spacetime

We find an exact static solution in four dimensions to the field equations of the $f(\mathbb{Q})$ gravity by using a cylindrically static spacetime with two different ansatz, $ν(r)$ and $μ(r)$. This solution is derived without imposing any conditions on $f(\mathbb{Q})$. The black hole solution involves four constants: $c_1$, $c_2$, $c_3$, and $c_4$. Among these, $c_1$ is linked to the cosmological constant, $c_2$ to the black hole's mass, while $c_3$ and $c_4$ are responsible for the deviation of the solution from the linear form of $f(\mathbb{Q})$. We demonstrate how the analytical function $f(\mathbb{Q})$ relies on $c_3$. When $c_3$ is zero, $f(\mathbb{Q})$ becomes a constant function, leading to the non-metricity case. We investigate the singularity of this solution and show that the Kretschmann invariant has a much milder singularity compared to the non-metricity case. We produce a black hole that rotates with non-vanishing values of $\mathbb{Q}$ and $f(\mathbb{Q})$ by using a coordinate transformation. Then, we analyze the laws of thermodynamics to determine the physical characteristics of this black hole solution and demonstrate that it is locally thermodynamically stable.

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arXiv2025

$f(Q)$ gravitational theory and its structure via red-shift

We present a novel approach for reconstructing the $f(Q)$ gravitational theory using parameterizations of the deceleration parameter or alternative options. This method enables the development of modified gravity scenarios that align with cosmological observations. We analyze two deceleration parameter models and one effective equation of state model from the literature. By leveraging the matter density parameter's asymptotic behaviour, we further refine the viable parameter space for these models. One tested model demonstrates that even slight modifications can yield viable cosmic scenarios, where the dark energy (DE) sector becomes dynamic and is entirely described instead of using a cosmological constant, we use modified gravity. These scenarios are qualitatively distinct yet quantitatively consistent with $Λ$CDM.

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arXiv2025

Special $N$-dimensional charged anti-de-Sitter black holes in $f(\mathbb{Q})$ gravitational theory

Due to the absence of spherically symmetric black hole solutions in $f(\mathbb{Q})$ because of the constraint derived from its field equations, which yields either $\mathbb{Q}=0 $ or $f_{\mathbb{Q} \mathbb{Q}}=0 $ \cite{Heisenberg:2023lru,Maurya:2023muz}. We are going to introduce a tours solutions for charged anti-de-Sitter black holes in $N$-dimensions within the framework of the quadratic form of $f(\mathbb{Q})$ gravity, where the coincident gauge condition is applied \cite{Heisenberg:2023lru}. Here, $f(\mathbb{Q})=\mathbb{Q}+\frac{1}2α\mathbb{Q}^2-2Λ$, and the condition $N \geq 4$ is satisfied. These black hole solutions exhibit flat or cylindrical horizons as their distinctive features. An intriguing aspect of these black hole solutions lies in the coexistence of electric monopole and quadrupole components within the potential field, which are indivisible and exhibit interconnected momenta. This sets them apart from the majority of known charged solutions in the linear form of the non-metricity theory and its extensions. Moreover, the curvature singularities in these solutions are less severe compared to those found in known charged black hole solutions within the characteristic can be demonstrated by computing certain invariants of the curvature and non-metricity tensors. Finally, we calculate thermodynamic parameters, including entropy, Hawking temperature, and Gibbs free energy. These thermodynamic computations affirm the stability of our model.

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arXiv2025

Maxwell-$f(Q)$ theory

Exploring the four-dimensional AdS black hole is crucial within the framework of the AdS/CFT correspondence. In this research, considering the charged scenario, we investigate the four-dimensional stationary and rotating AdS solutions in the framework of the $f(Q)$ gravitational theory. Our emphasis is on the power-law ansatz, which is consistent with observations and is deemed the most viable. Because this solution does not have an uncharged version or relate to general relativity, it falls into a new category, which derives its features from changes in non-metricity and incorporates the Maxwell domain. We analyze the singularities of such a solution, computing all the quantities of different curvature and non-metricity invariants. Our results indicate the presence of a central singularity, albeit with a softer nature compared to standard non-metricity or Einstein general relativity, attributed to the influence of the effect of $f(Q)$. We examine several physical characteristics of black holes from a thermodynamics perspective and demonstrate the existence of an outer event horizon in addition to the inner Cauchy horizons. However, under the conditions of a sufficiently large electric charge, a naked singularity emerges. Finally, we derive a class of rotating black hole in four-dimensional $f(Q)$ gravity that are asymptotically anti-de Sitter charged.

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arXiv2024

Time dependent black holes and gravitational wave in Einstein-Gauss-Bonnet theory with two scalar fields

This paper explores time-varying black holes within the framework of the Einstein-Gauss-Bonnet theory with two scalar fields, examining the propagation of gravitational waves (GW). In reconstructed models, ghosts frequently emerge but can be eliminated by applying certain constraints. We investigate the behavior of high-frequency gravitational waves by examining the effects of varying Gauss-Bonnet coupling during their propagation. The speed of transmission is altered by the coupling in the creation of black holes. The speed of gravitational waves varies as they enter a black hole compared to when they exit.

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arXiv2024

Structure, maximum mass, and stability of compact stars in f(Q,T) gravity

Physically based changes to general relativity (GR) often predict significant differences in how spacetime behaves near massive neutron stars. One of these modifications is represented by $f(\mathcal{Q}, { \mathcal{T}})$, with $\mathcal{Q}$ being the non-metricity and ${ \mathit{T}}$ representing the energy-momentum tensor trace. This theory is viewed as a neutral expansion of GR. Neutron stars weighing more than 1.8 times the mass of the Sun, when observed as radio pulsars, provide valuable opportunities to test fundamental physics under extreme conditions that are rare in the observable universe and cannot be replicated in experiments conducted on land. We derive an exact solution through utilizing the form $f(\mathcal{Q}, { \mathcal{T}})=\mathcal{Q}+ψ{ \mathcal{T}}$, where $ψ$ represents a dimensional expression. We elucidate that all physical quantities within the star can be expressed using the dimensional parameter $ψ$ and the compactness, which is defined as $C=\frac{ 2GM}{Rc^2}$. We set $ψ$ to a maximum value of $ψ_1=\fracψ{κ^2}=-0.04$ in the negative range, based on observational constraints related to radius and mass of the pulsar ${\textit SAX J1748.9-2021}$. Here, ${\mathrm κ^2}$ represents the coupling constant of Einstein, defined as ${\mathrm κ^2=\frac{8πG}{c^4}}$. Unlike in GR, the solution we derived results in a stable compact object without violating the conjectured sound speed condition $c_s^2\leq\frac{c^2}3$.It is crucial to mention that no equations of state were assumed in this investigation. Nevertheless, our model fits nicely with linear form. Generally, when $ψ$ is negative, the theory predicts a star with a slightly larger size than GR for the same mass. The difference in predicted size between the theory with a negative $ψ$ and GR for the same mass is attributed to an additional force.

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arXiv2024

Gravitational collapse, primordial black hole, and gravitational wave in Einstein--Gauss-Bonnet theory with two scalar fields

In this paper, we investigate the gravitational collapse to form the black hole in the acceleratingly expanding universe in the frame of Einstein--Gauss-Bonnet theory having two scalar fields and we study the propagation of the gravitational wave (GW). This may describe the creation of the primordial black holes in the early stages of the universe and the impact of their creation on the propagation of primordial gravitational waves. The collapsing spacetime can be obtained by using the formulation of the ``reconstruction'', that is, we find a model that realises the desired or given geometry. In the reconstructed models, ghosts often appear, which could be eliminated by imposing constraints. We show that the standard cosmological solutions or self-gravitating objects such as a planet, the Sun, various types of stars, etc., in Einstein's gravity, are also solutions in this model. Using the dynamical value of the Gauss-Bonnet coupling, the propagation of the high-frequency GW is investigated. The propagating speed changes due to the coupling during the period of the black hole formation. The speed of the GW propagation going into the black hole is different from that of the wave going out.

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arXiv2024

General geometry realized by four-scalar model and application to $f(Q)$ gravity

In this paper, we propose a model including four scalar fields coupled with general gravity theories, which is a generalization of the two-scalar model proposed in Phys. Rev. D \textbf{103} (2021) no.4, 044055, where it has been shown that any given spherically symmetric static/time-dependent spacetime can be realized by using the two-scalar model. We show that by using the four-scalar model, we can construct a model that realizes any given spacetime as a solution even if the spacetime does not have a spherical symmetry or any other symmetry. We also show that by imposing constraints on the scalar fields by using the Lagrange multiplier fields, the scalar fields become non-dynamical and they do not propagate. This tells that there does not appear any sound which is usually generated by the density fluctuation of the fluid. In this sense, the model with the constraints is a most general extension of the mimetic theory in JHEP \textbf{11} (2013), 135, where there appears an effective dark matter. The dark matter is non-dynamical and it does not collapse even under gravitational force. Our model can be regarded as a general extension of any kind of fluid besides dark matter. We may consider the case that the potential of the scalar fields vanishes and the model becomes a non-linear $σ$ model. Then our formulation gives a mapping from the geometry of the spacetime to the geometry of the target space of the non-linear $σ$ model via gravity theory although the physical meaning has not been clear. We also consider the application of the model to $f(Q)$ gravity theory, which is based on a non-metricity tensor and $Q$ is a scalar quantity constructed from the non-metricity tensor. ...

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arXiv2024

Charged solution with equal metric ansatz in Gauss-Bonnet theory coupled to scalar field

In the course of this research, we employ the Gauss-Bonnet equation of motion alongside the scalar field and potential to acquire a fresh solution for a spherically symmetrical charged black hole. Specifically, we derive this black hole solution by employing a metric potential where the components are equal, that is, $g_{tt}=g_{rr}$. In our research, we achieve several accomplishments, including fixing the characteristics of the scalar field, and the Gauss-Bonnet term. We thoroughly examine the physical properties associated with such black hole and show that we have supplementary terms when compared to the Reissner-Nordström black hole solution. These additional terms are of the order $O(\frac{1}{r^6})$ and $O(\frac{1}{r^9})$. The presence of these supplementary terms can be attributed to the impact of the scalar function denoted as $ξ$. Such expressions play a crucial role in generating the multi-horizon black hole solution. The presence of these extra terms facilitates the derivation of a modified first law of thermodynamics and the corresponding Smarr relation.

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arXiv2024

Revisiting Flat Rotation Curves in Chern-Simons Modified Gravity

We revisit slow rotating black hole (BH) solutions in Chern-Simons modified gravity (CSMG) by considering perturbative solution about Schwarzschild BH. In particular, the case when nondynamical CSMG with noncanonical CS scalar is considered. We provide a new solution different from the previously obtained one \cite{Konno:2007ze} which we refer to as KMT model. The present solution accounts for frame dragging effect which includes not only radial dependence as in the KMT. Nevertheless, it reduces to KMT as a particular case. We show that the tidal gravitational force (Kretschmann invariant) associated to the present solution contains a term of order $1/r^3$ additional to Schwarzschild but absent from directional divergence, unlike KMT model which diverges along the axis of symmetry. We derive the corresponding circular velocity of a massive test particle in which the KMT velocity is recovered in addition to an extra term $\propto r$. We investigate possible constraints on KMT and the present solutions from the observed rotation curve of UGC11455 galaxy as an example. We show that perturbation solutions cannot physically explain the flattening of galactic rotation curves.

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