arXiv subjects

Rustam Ibadov

Publications and source records attributed to Rustam Ibadov.

16 recordsLinked to original sources

arXiv2026

Nonlinear electrodynamics and stability of spherically symmetric space-times in scalar-tensor gravity

We study linear perturbations of static, spherically symmetric solutions of scalar-tensor theories (STT) of gravity from the Bergmann-Wagoner-Nordtvedt class, sourced by nonlinear electrodynamics (NED). We obtain a general expression for the effective potential $V_{\rm eff}$ governing the perturbation dynamics for theories with arbitrary scalar-electromagnetic interaction of the form $L(\psi, F)$, where $\psi$ is a scalar field and $F = F_{\mu\nu} F^{\mu\nu}$ the electromagnetic invariant. This consideration includes, in particular, arbitrary scalar self-interaction potentials and scalar fields that can be phantom in some regions of space-time (the so-called trapped ghosts). Only radial (monopole) perturbations are considered here as the most likely ones to cause an instability. It is shown, in particular, that if NED has a correct Maxwell weak field limit, the zero charge limit of $V_{\rm eff}$ does not contain any trace of NED, and the perturbation dynamics is the same as for vacuum STT solutions. The previously obtained stability results for STT-Maxwell solutions are shown to be extended without change to STT-NED solutions with equal electric and magnetic charges, implying $F =0$.

gr-qc↗

arXiv2026

Regular spherically symmetric solutions in General Relativity and scalar-tensor gravity coupled to nonlinear electrodynamics and their stability

We investigate the geometric, dynamical, and thermodynamic properties of a novel class of regular black holes in scalar-tensor gravity non-minimally coupled to nonlinear electrodynamics (NED). By incorporating a purely magnetic NED source within a scalar-tensor framework, we circumvent the classical ``no-go'' theorems. These theorems, strictly formulated for scalar-free Einstein-NED systems, prohibit regular configurations for purely electric fields due to the fundamental requirement of recovering the linear Maxwell limit at spatial infinity. The geometric analysis demonstrates the complete resolution of the central Penrose singularity, replacing it with a regular, globally bounded vacuum core ($|\rho_c| < \infty$). While purely electromagnetic regular black holes canonically possess a de Sitter center, the deep potential well of the scalar field in our model structurally alters this standard geometry, yielding a locally Anti-de Sitter (AdS-like) regime characterized by a strictly negative central energy density ($\rho_c < 0$). To assess the physical viability of these configurations, we analyze their dynamical stability against odd-parity (axial) linear gravitational perturbations. The derived Regge-Wheeler-like effective potential is strictly positive and convex outside the event horizon. Numerical time-domain integration, independently corroborated by the semi-analytical WKB approximation, confirms the total absence of exponentially growing modes, revealing a stable quasi-normal ringing phase followed by an exponential decay. Furthermore, our thermodynamic analysis of the mass-radius relation directly indicates that the semi-classical Hawking evaporation must terminate at the extremal limit ($M = M_{min}$), leaving behind a massive thermodynamic remnant, thereby providing a theoretical framework toward resolving the black hole information loss paradox.

gr-qc↗

arXiv2025

On the stability of exceptional Brans-Dicke wormholes

In our previous papers we have analyzed the stability of vacuum and electrovacuum static, spherically symmetric space-times in the framework of the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories (STT) of gravity. In the present paper, we continue this study by examining the stability of exceptional solutions of the Brans-Dicke theory with the coupling constant $\omega =0$ that were not covered in the previous studies. Such solutions describe neutral or charged wormholes and involve a conformal continuation: the standard conformal transformation maps the whole Einstein-frame manifold ${\mathbb M}_E$ to only a part of the Jordan-frame manifold ${\mathbb M}_J$, which has to be continued beyond the emerging regular boundary S, and the new region maps to another manifold ${\mathbb M}_{E-}$. The metric in ${\mathbb M}_J$ is symmetric with respect to S only if the charge $q$ is zero. Our stability study concerns radial (monopole) perturbations, and it is shown that the wormhole is stable if $q \ne 0$ and unstable only in the symmetric case $q=0$

gr-qc↗

arXiv2024

On the stability of electrovacuum space-times in scalar-tensor gravity

We study the behavior of static, spherically symmetric solutions to the field equations of scalar-tensor theories (STT) of gravity belonging to the Bergmann-Wagoner-Nordtvedt class, in the presence of an electric and/or magnetic charge. This class of theories includes the Brans-Dicke, Barker and Schwinger STT as well as nonminimally coupled scalar fields with an arbitrary parameter $\xi$. The study is restricted to canonical (nonphantom) versions of the theories and scalar fields without a self-interaction potential. Only radial (monopole) perturbations are considered as the most likely ones to cause an instability. The static background solutions contain naked singularities, but we formulate the boundary conditions in such a way that would preserve their meaning if a singularity is smoothed, for example, due to quantum gravity effects. These boundary conditions look more physical than those used by other authors. Since the solutions of all STT under study are related by conformal transformations, the stability problem for all of them reduces to the same wave equation, but the boundary conditions for perturbations (and sometimes the boundaries themselves) are different in different STT, which affects the stability results. The stability or instability conclusions are obtained for different branches of solutions in the theories under consideration and are presented in a table form.

gr-qc↗

arXiv2023

On the stability of spherically symmetric space-times in scalar-tensor gravity

We study the linear stability of vacuum static, spherically symmetric solutions to the gravitational field equations of the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories (STT) of gravity, restricting ourselves to nonphantom theories, massless scalar fields and configurations with positive Schwarzschild mass. We consider only small radial (monopole) perturbations as the ones most likely to cause an instability. The problem reduces to the same Schroedinger-like master equation as is known for perturbations of Fisher's solution of general relativity (GR), but the corresponding boundary conditions that affect the final result of the study depend on the choice of the STT and a particular solution within it. The stability or instability conclusions are obtained for the Brans-Dicke, Barker and Schwinger STT as well as for GR nonminimally coupled to a scalar field with an arbitrary parameter $\xi$.

gr-qc↗

arXiv2022

Arbitrary static, spherically symmetric space-times as solutions of scalar-tensor gravity

It is shown that an arbitrary static, spherically symmetric metric can be presented as an exact solution of a scalar-tensor theory (STT) of gravity with certain nonminimal coupling function $f(\phi)$ and potential $U(\phi)$. The scalar field in this representation can change its nature from canonical to phantom on certain coordinate spheres. This representation, however, is valid in general not in the full range of the radial coordinate but only piecewise. Two examples of STT representations are discussed: for the Reissner-Nordstr\"om metric and for the Simpson-Visser regularization of the Schwarzschild metric (the so-called black bounce space-time).

gr-qc↗

arXiv2021

Wormhole solutions with NUT charge in higher curvature theories

We present wormholes with a Newman-Unti-Tamburino (NUT) charge that arise in certain higher curvature theories, where a scalar field is coupled to a higher curvature invariant. For the invariants we employ i) a Gauss-Bonnet term and ii) a Chern-Simons term, which then act as source terms for the scalar field. We map out the domain of existence of wormhole solutions by varying the coupling parameter and the scalar charge for a set of fixed values of the NUT charge. The domain of existence for a given NUT charge is then delimited by the set of scalarized nutty black holes, a set of wormhole solutions with a degenerate throat and a set of singular solutions.

gr-qc↗

arXiv2020

Scalarized nutty wormholes

We construct scalarized wormholes with a NUT charge in higher curvature theories. We consider both Einstein-scalar-Gauss-Bonnet and Einstein-scalar-Chern-Simons theories, following a recent paper by Brihaye et al. [1], where spontaneously scalarised Schwarzschild-NUT solutions were studied. By varying the coupling parameter and the scalar charge we determine the domain of existence of the scalarized nutty wormholes, and their dependence on the NUT charge. In the Gauss-Bonnet case the known set of scalarized wormholes [2] is reached in the limit of vanishing NUT charge. In the Chern-Simons case, however, the limit is peculiar, since with vanishing NUT charge the coupling constant diverges. We focus on scalarized nutty wormholes with a single throat and study their properties. All these scalarized nutty wormholes feature a critical polar angle, beyond which closed timelike curves are present.

gr-qc↗

arXiv2020

Wormholes in Einstein-scalar-Gauss-Bonnet theories with a scalar self-interaction potential

We construct wormholes in Einstein-scalar-Gauss-Bonnet theories with a potential for the scalar field that includes a mass term and self-interaction terms. By varying the Gauss-Bonnet coupling constant we delimit the domain of existence of wormholes in these theories. The presence of the self-interaction enlarges the domain of existence significantly. There arise wormholes with a single throat and wormholes with an equator and a double throat. We determine the physical properties of these wormholes including their mass, their size and their geometry.

gr-qc↗

arXiv2013

Hairy Wormholes and Bartnik-McKinnon Solutions

We consider Lorentzian wormholes supported by a phantom field and threaded by non-trivial Yang-Mills fields, which may be regarded as hair on the Ellis wormhole. Like the Bartnik-McKinnon solutions and their associated hairy black holes, these hairy wormholes form infinite sequences, labeled by the node number $k$ of their gauge field function. We discuss the throat geometry of these wormholes, showing that odd-$k$ solutions may exhibit a double-throat, and evaluate their global charges. We analyze the limiting behavior exhibited by wormhole solutions as the gravitational coupling becomes large. The even-$k$ solutions approach smoothly the Bartnik-McKinnon solutions with $k/2$ nodes, while the odd-$k$ solutions develop a singular behavior at the throat in the limit of large coupling. In the limit of large $k$, on the other hand, an embedded Abelian wormhole solution is approached, when the throat is large. For smaller throats the extremal Reissner-Nordstr\"om solution plays a fundamental role in the limit.

gr-qc↗

arXiv2010

Properties of Charged Rotating Electroweak Sphaleron-Antisphaleron Systems

We perform a systematic study of stationary sphaleron-antisphaleron systems of Weinberg-Salam theory at the physical value of the weak mixing angle. These systems include rotating sphaleron-antisphaleron pairs, chains and vortex rings. We show that the angular momentum of these solutions is proportional to their electric charge. We study the dependence of their energy and magnetic moment on their angular momentum. We also investigate the influence of their angular momentum on their local properties, in particular on their energy density and on the node structure of their Higgs field configuration. Furthermore, we discuss the equilibrium condition for these solutions.

hep-th↗

arXiv2010

Rotating Electroweak Sphaleron-Antisphaleron Systems

At finite weak mixing angle the sphaleron solution of Weinberg-Salam theory can be endowed with angular momentum proportional to the electric charge. Here we show, that this holds also for sphaleron-antisphaleron systems such as pairs, chains and vortex rings. We also address the equilibrium conditions for these solutions.

hep-th↗

arXiv2008

Gravitating Sphaleron-Antisphaleron Systems

We present new classical solutions of Einstein-Yang-Mills-Higgs theory, representing gravitating sphaleron-antisphaleron pair, chain and vortex ring solutions. In these static axially symmetric solutions, the Higgs field vanishes on isolated points on the symmetry axis, or on rings centered around the symmetry axis. We compare these solutions to gravitating monopole-antimonopole systems, associating monopole-antimonopole pairs with sphalerons.

gr-qc↗

arXiv2007

Gravitating Dyons with Large Electric Charge

We consider non-Abelian dyons in Einstein-Yang-Mills-Higgs theory. The dyons are spherically symmetric with unit magnetic charge. For large values of the electric charge the dyons approach limiting solutions, related to the Penney solutions of Einstein-Maxwell-scalar theory.

gr-qc↗

arXiv2005

New Black Hole Solutions with Axial Symmetry in Einstein-Yang-Mills Theory

We construct new black hole solutions in Einstein-Yang-Mills theory. They are static, axially symmetric and asymptotically flat. They are characterized by their horizon radius and a pair of integers (k,n), where k is related to the polar angle and n to the azimuthal angle. The known spherically and axially symmetric EYM black holes have k=1. For k>1, pairs of new black hole solutions appear above a minimal value of n, that increases with k. Emerging from globally regular solutions, they form two branches, which merge and end at a maximal value of the horizon radius. The difference of their mass and their horizon mass equals the mass of the corresponding regular solution, as expected from the isolated horizon framework.

gr-qc↗

arXiv2004

New Regular Solutions with Axial Symmetry in Einstein-Yang-Mills Theory

We construct new regular solutions in Einstein-Yang-Mills theory. They are static, axially symmetric and asymptotically flat. They are characterized by a pair of integers (k,n), where k is related to the polar angle and $n$ to the azimuthal angle. The known spherically and axially symmetric EYM solutions have k=1. For k>1 new solutions arise, which form two branches. They exist above a minimal value of n, that increases with k. The solutions on the lower mass branch are related to certain solutions of Einstein-Yang-Mills-Higgs theory, where the nodes of the Higgs field form rings.

gr-qc↗