arXiv subjects

Kok-Geng Lim

Publications and source records attributed to Kok-Geng Lim.

11 recordsLinked to original sources

arXiv2026

The Analytical Solutions of Dyonic Black Holes in Einstein-Euler-Heisenberg Theory

Recently, we constructed analytical purely electric and purely magnetic black-hole solutions in Einstein--Euler--Heisenberg theory, while the corresponding dyonic solutions were obtained numerically \cite{Luo:2026srx}. Motivated by the recent analytical construction of a dyonic black hole at the special coupling locus $b=a/2$ \cite{Ahmed:2026ufj}, we revisit the general dyonic sector. We show that exact analytical dyonic black-hole solutions can be constructed for nonlinear couplings satisfying $2b>a$, without imposing the restriction $b=a/2$. The mass function is expressed in closed form in terms of the Lauricella hypergeometric function. In particular, our construction yields an exact dyonic solution for the Euler--Heisenberg coupling $b=7a/4$. The solution of Ref.~\cite{Ahmed:2026ufj} is recovered in the limiting case $2b\rightarrow a$.

gr-qc↗

arXiv2026

Purely Electric, Magnetic, and Dyonic Black Holes in Einstein-Euler-Heisenberg Theory

We investigate static, spherically symmetric charged black holes in Einstein gravity coupled to Euler--Heisenberg (EH) nonlinear electrodynamics, including purely electric, purely magnetic, and dyonic configurations. Rather than adopting the Hamiltonian formulation based on the auxiliary electromagnetic invariant $\mathcal{P}$, we work directly with the physical electromagnetic invariant $\mathcal{F}$ in the Einstein-Euler--Heisenberg Lagrangian, thereby describing all charged configurations without introducing auxiliary variables. Within this approach, we derive an exact analytical solution for the purely electric case, recover the purely magnetic solution directly from the field equations, and construct the dyonic solutions numerically. We systematically study the horizon structure, causal properties, and thermodynamics of these solutions. While the purely electric branch exhibits the familiar Reissner--Nordstr\"om horizon structure, the purely magnetic branch naturally admits a novel three-horizon configuration consisting of one event horizon and two inner horizons. The dyonic solutions continuously interpolate between the electric and magnetic limits and exhibit either one- or three-horizon configurations, depending on the magnetic-to-electric charge ratio and the EH coupling. We further show that the EH nonlinear interaction significantly modifies the horizon structure and thermodynamic properties of charged black holes while leaving the central curvature singularity unresolved. These results demonstrate that EH nonlinear electrodynamics gives rise to qualitatively new causal structures beyond Einstein--Maxwell theory.

gr-qc↗

arXiv2026

The Interior of the Scalar Hairy Black Hole with Inverted Higgs Potential

We investigate the interior structure of asymptotically flat hairy black holes (HBHs) arising in the Einstein-Klein-Gordon theory with nonpositive-definite scalar potentials, where nontrivial scalar hair exists at the event horizon. While exterior properties, including shadow imaging for HBHs supported by an inverted Higgs-like potential have been extensively investigated, their interior structure remains largely unexplored. In many gravitational theories, backreaction of classical fields can significantly eliminate the Cauchy horizon, which is known to be highly unstable due to the mass inflation effect, raising important questions regarding the validity of the Strong Cosmic Censorship conjecture. These considerations motivate us to examine the interior structure of HBHs by numerically integrating the field equations inward from the outer horizon. We find that the scalar field and the metric functions increase monotonically inside the horizon and diverge as $r \rightarrow 0$. The Ricci and Kretschmann scalars also diverge at $r=0$, confirming the presence of a genuine curvature singularity. No additional root of the metric function is observed, indicating the absence of a Cauchy horizon in the electrically neutral HBHs considered here. Furthermore, the weak energy condition is violated throughout the interior region, and the degree of violation becomes more pronounced as the scalar field at the horizon increases. These results provide new insight into the global structure of HBHs and their implications for cosmic censorship.

gr-qc↗

arXiv2025

Shadow of the Scalar Hairy Black Hole with Inverted Higgs Potential

We study the imaging of a hairy black hole (HBH) in the Einstein-Klein-Gordon theory, where Einstein gravity is minimally coupled to a scalar potential $V(ϕ)=-Λϕ^4 + μϕ^2$ with $Λ$ and $μ$ are constants. As a consequence, a nontrivial scalar field at the event horizon $ϕ_H$ allows the HBH to evade the no-hair theorem, bifurcate from the Schwarzschild black hole by acquiring some new properties, which can affect the shadow of the HBH received by a distant observer. The framework of ray-tracing is adopted to investigate the optical appearance of the HBH, thus the trajectories of light rays around the HBH can be classified into three emissions: direct, lensed and photon ring. Employing three models of optically and geometrically thin accretion disk, we compare the differences between the Schwarzschild black hole and HBH with same horizon radius in a specific model, and find that the size of the shadow and accretion disk increases as $ϕ_H$ increases, but the brightness of the rings remain nearly unaffected, this implies our HBH can potentially mimic the Schwarzschild black hole if we vary the horizon radius of the HBH. Finally, we also constraint the parameter $Λ$ from the observations of supermassive black holes in the galactic center of M87 and Sgr A$^{*}$, which could offer new insights for imaging of black holes and astrophysical observations.

gr-qc↗

arXiv2024

Gravitating Scalarons with Inverted Higgs Potential

Previously, a class of regular and asymptotically flat gravitating scalar solitons (scalarons) has been constructed in the Einstein--Klein--Gordon (EKG) theory by adopting a phantom field with Higgs-like potential where the kinetic term has the wrong sign and the scalaron possesses the negative Arnowitt--Deser--Misner (ADM) mass as a consequence. In this paper, we demonstrate that the use of the phantom field can be avoided by inverting the Higgs-like potential in the EKG system when the kinetic term has a proper sign, such that the corresponding gravitating scalaron can possess the positive ADM mass. We systematically study the basic properties of the gravitating scalaron, such as the ADM mass, the energy conditions, the geodesics of test particles, etc. Moreover, we find that it can be smoothly connected to the counterpart hairy black hole solutions from our recent work in the small horizon limit.

gr-qc↗

arXiv2023

Scalar Hairy Black Holes with Inverted Mexican Hat Potential

We numerically construct the asymptotically flat solutions of hairy black holes supported by a symmetric inverted Mexican hat potential with a local minimum and two degenerate global maxima of a real scalar field that contains a quartic self-interaction term. The solutions of hairy black holes emerge from the Schwarzschild black hole when the non-trivial scalar field exists outside the event horizon. Therefore, we perform a comprehensive study on the properties of the hairy black holes such as the area of horizon, the Hawking temperature, the innermost stable circular orbit, the photon sphere, etc. We also numerically study their linear stability in the mode analysis, hence finding that they are unstable against the linear perturbation.

gr-qc↗

arXiv2022

Non-Abelian Wormholes Threaded by a Yang-Mills-Higgs Field beyond the BPS Limit

We construct numerically the symmetric non-Abelian wormholes which are supported by a phantom field in the Einstein-Yang-Mills-Higgs theory beyond Bogomol'nyi-Prasad-Sommerfield (BPS) limit where the Higgs self-interaction constant $λ$ is non-vanishing. Analogous to the BPS limit, the probe limit is the Yang-Mills-Higgs field in the background of the Ellis wormhole when the gravity is switched off. In the presence of gravity, the wormhole solutions possess the Yang-Mills-Higgs hair where families of hairy wormholes solutions emerge from the Ellis wormhole when the gravitational coupling constant increases. In contrast to the BPS limit, the properties of wormholes change drastically when the gravitational strength approaches a critical value for a fixed $λ$. The hairy wormholes possess two types of double throat configurations. The first type is wormholes that develop the double throat when the gravitational strength almost approaches the critical value for lower $λ$, whereas the second type is wormholes that exhibit the double throat for a certain range of gravitational strength for higher $λ$. These two types of double throat configurations can coexist for a certain range of $λ$ where it is a transition process for first type double throat disappears gradually and the second type double throat becomes dominant.

gr-qc↗

arXiv2021

Non--Abelian Wormholes Threaded by Yang--Mills--Higgs Field in the BPS Limit

In the Bogomol'nyi-Prasad-Sommerfield (BPS) limit of Einstein-Yang-Mills-Higgs theory, we construct numerically a non-Abelian wormhole supported by a phantom field. The probe limit is the Yang-Mills-Higgs field in the background of Ellis wormhole when the gravity is switched off. The wormhole solutions possess the Yang-Mills-Higgs hair in the presence of gravity; Thus a branch of hairy wormhole solutions emerge from the Ellis wormhole when the gravitational coupling constant increases. We find that the mass of wormholes and scalar charge of phantom field increase monotonically when the gravitational coupling constant increases. The wormhole spacetime possesses double throat configuration when the gravitational strength exceeds a critical value. Surprisingly, the wormholes satisfy the null energy condition in the large gravitational strength. We also briefly discuss the redshift factor.

gr-qc↗

arXiv2012

Monopole-Antimonopole Pair Dyons with Critical Electric Charges

In this paper, we investigate some electrically charged magnetic solutions of the SU(2) Yang-Mills-Higgs field theory in the net zero topological charge sector. We only examine the case when the Higgs field vanishes at two points along the z-axis and when the Higgs field vanishes along a ring with the z-axis as its symmetry axis. We study the possible electric charges the dyons can carry in relation to the electric-magnetic charge separations and calculate for the finite total energy and magnet dipole moment of these dyons. These stationary dyon solutions do not satisfy the first order Bogomol'nyi equations and are non BPS solutions. They are axially symmetric saddle point solutions and are characterized by the electric charge parameter, $-1<η< 1$, which determines the net electric charges of these dyons. These dyon solutions are solved numerically when the magnetic charges are $n$ = 1, 2, 3, 4, and 5; and when the strength of the Higgs field potential is non vanishing with $λ=1$. When $λ=1$, we found that the net electric charge approaches a finite critical value as $η$ approaches $\pm 1$. Hence the electromagnetic charge separation, total energy, and magnet dipole moment of the dyon also approach finite critical value.

hep-th↗

arXiv2010

Monopole-Antimonopole Pair Dyons

Monopole-antimonopole pair (MAP) with both electric and magnetic charges are presented. The MAP possess opposite magnetic charges but they carry the same electric charges. These stationary MAP dyon solutions possess finite energy but they do not satisfy the first order Bogomol'nyi equations and are not BPS solutions. They are axially symmetric solutions and are characterized by a parameter, $-1\leqη\leq 1$ which determines the net electric charges of these MAP dyons. These dyon solutions are solved numerically when the magnetic charges of the dipoles are $n=\pm 1, \pm 2$ and when the strength of the Higgs field potential $λ=0, 1$. When $λ=0$, the time component of the gauge field potential is parallel to the Higgs field in isospin space and the MAP separation distance, total energy and net electric charge increase exponentially fast to infinity when $η$ approaches $\pm 1$. However when $λ=1$, all these three quantities approach a finite critical value as $η$ approaches $\pm 1$.

hep-th↗

arXiv2010

Generalized Jacobi Elliptic One-Monopole - Type A

We present new classical generalized one-monopole solution of the SU(2) Yang-Mills-Higgs theory with the Higgs field in the adjoint representation. We show that this generalized solution with $θ$-winding number $m=1$ and $ϕ$-winding number $n=1$ is an axially symmetric Jacobi elliptic generalization of the 't Hooft-Polyakov one-monopole. We construct this axially symmetric one-monopole solution by generalizing the large distance asymptotic solution of the 't Hooft-Polyakov one-monopole to the Jacobi elliptic functions and solving the second order equations of motion numerically when the Higgs potential is vanishing and non vanishing. These solutions are regular non-BPS finite energy solutions.

hep-th↗