arXiv subjects

Meng-Yun Lai

Publications and source records attributed to Meng-Yun Lai.

At least 19 recordsLinked to original sources

arXiv2026

Critical Scalarization for a Self-Gravitating Bosonic Condensate

We establish a minimal nonrelativistic realization of scalarization in a self-gravitating bosonic condensate coupled to a scalar response field. A local effective-mass shift and nonlinear saturation generate a multibranch structure with two distinct transition routes. In the linearly stable regime, a finite perturbation drives a first-order transition with type-I logarithmic scaling near threshold. Beyond the linear onset, small perturbations grow tachyonically, and the response time controls both the growth and the subsequent breathing dynamics. These results identify a common mechanism for scalarization across relativistic compact objects and nonrelativistic condensates, and point toward laboratory analogues in coherent media.

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arXiv2026

Spontaneous scalarization of regular Hayward black holes in Einstein-nonlinear electromagnetic-scalar gravity

Regular Hayward black holes provide a useful setting for investigating scalarization in theories with nonminimally coupled matter sectors. Within the framework of Einstein-nonlinear electromagnetic-scalar gravity, we identify the tachyonic threshold that signals the bifurcation from the bald Hayward background and then obtain scalarized charged black holes for both quadratic $(1-αϕ^2)$ and exponential $(e^{-αϕ^2})$ couplings. These configurations form a discrete set of branches classified by the number of nodes in the scalar field. The branch with $n=0$ is the fundamental branch, whereas solutions with $n\geq 1$ are excited branches. By studying radial perturbations, we find that the fundamental branch is stable for both coupling choices, which makes it the most relevant branch for future phenomenological and observational studies.

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arXiv2026

Spin-Induced Nonlinear Scalarization of Kerr Black Holes in Einstein-scalar-Gauss-Bonnet Gravity

We investigate spin-induced scalarization of Kerr black holes in an Einstein-scalar-Gauss-Bonnet (EsGB) model that does not admit a linear tachyonic instability of the scalar-free solution. The scalarization mechanism is therefore genuinely nonlinear. We first analyze the decoupled scalar dynamics on fixed Kerr backgrounds and show that sufficiently rapid rotation modifies the Gauss-Bonnet invariant such that a negative near-horizon region develops near the poles. This region provides a geometric trapping mechanism for nonlinear scalar growth, which becomes effective above a threshold spin $χ=0.5$. We then construct stationary scalarized black hole solutions with full backreaction and determine their domain of existence. We find that the solutions occupy a finite low-mass high-spin wedge in the spin-mass plane. This is in contrast to spin-induced spontaneous scalarization, where the scalarized solutions form a narrow band. In this wedge, toward the high-spin end, the scalar hair becomes stronger, and the solutions approach a near-extremal regime, while toward the low-spin boundary, the scalar field is strongly suppressed and approaches a weak-hair limit as $χ\to 0.5$.

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arXiv2026

Thermodynamics and phase transitions of nonlinearly scalarized black holes in Einstein-scalar-Gauss-Bonnet theory

We investigate the thermodynamic properties of static nonlinearly scalarized black holes in Einstein-scalar-Gauss-Bonnet theory with polynomial coupling functions. Based on the scalarized solutions constructed previously, we compute thermodynamical quantities of these scalarized black holes. Moreover, we examine the first law of black hole thermodynamics and consider the phase transitions between Schwarzschild and scalarized black holes. It shows that a phase transition from Schwarzschild black hole to scalarized black hole is a first-order with non-zero latent heat.

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arXiv2026

Spin-charge induced scalarization of Kerr-Newman black holes in the Einstein-Maxwell-scalar theory with scalar potential

We investigate the spin-charge-induced scalarization of Kerr--Newman (KN) black holes in the Einstein--Maxwell-scalar (EMS) theory with a scalar potential and positive coupling parameter. In the linearized theory, there exists a bound of $0<a<a_0$ with onset spin $a_c$ for the negative region signaling instability by analyzing the effective scalar mass term in the $θ$-direction. Solving the $(2+1)$-dimensional evolution equation numerically, we find the region where the KN black hole becomes unstable, giving rise to scalarized KN black holes. The threshold curve for representing the boundary between stable and unstable KN black holes depends on charge $Q$, scalar mass $m_ϕ$, coupling parameter $α$, and spin parameter $a$ with upper bound $a^2\le M^2-Q^2$.

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arXiv2026

Why Barriola--Vilenkin Global Monopoles Cannot Rotate?

The Barriola--Vilenkin global monopoles are topological defects predicted by certain grand unified theories and have been extensively studied for their astrophysical and cosmological implications, including their distinctive spacetime geometry and characteristic gravitational lensing effects. Despite this interest, an exact solution for a global monopole remains elusive, with research largely confined to approximations of the static, spherically symmetric case. This paper addresses the fundamental question of whether a rotating global monopole can exist as a solution to the coupled Einstein-scalar field equations. We first prove that metrics generated by applying the Newman-Janis algorithm to the static monopole are inconsistent with the scalar field's equation of motion. Furthermore, we perform an asymptotic analysis for general static, axially symmetric spacetimes and establish that the only such solution that is regular at large distances is the spherically symmetric one. These results lead to the conclusion that the Barriola--Vilenkin global monopoles are incompatible with rotating spacetime within the framework of Einstein's general relativity.

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arXiv2026

Phase Structure of Scalarized Black Holes in Einstein-Scalar-Gauss-Bonnet Gravity

We revisit scalarized black holes in Einstein-scalar-Gauss-Bonnet gravity and analyze the thermodynamic phase transition between the Schwarzschild solution of general relativity and scalarized black holes. Restricting to spherically symmetric configurations, we investigate several classes of scalar-Gauss-Bonnet coupling functions. For the simplest quadratic coupling that triggers spontaneous scalarization, the scalarized solutions are thermodynamically disfavored and no phase transition occurs. For an exponential coupling, the phase structure depends strongly on the coupling parameter, allowing for the absence of a transition, a continuous second-order transition, or a discontinuous first-order transition. For couplings leading to purely nonlinear scalarization, we find either a first-order transition or no transition. These results reveal a rich phase structure of scalarized black holes controlled by the scalar-Gauss-Bonnet coupling.

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arXiv2026

Existence of nonlinearly scalarized black holes in Einstein-scalar-Gauss-Bonnet theory with polynomial couplings

Nonlinearly scalarized black holes are investigated in Einstein-scalar-Gauss-Bonnet (EsGB) theory with polynomial coupling functions $ζ(ϕ)$ satisfying $ζ''(0) = 0$, where $ζ'(ϕ) = 0$ features besides $ϕ=0$ solutions with constant $ϕ_{\rm s} \ne 0$. We determine the threshold amplitudes for Gaussian pulses, above which Schwarzschild black holes (SBHs) %become unstable and may transition to scalarized black holes for two coupling functions: $ζ(ϕ)=αϕ^4-βϕ^8$ and $ζ(ϕ)=αϕ^4-βϕ^6$. In contrast, for the quartic coupling function $ζ(ϕ)=αϕ^4$ SBHs are stable. Treating $ζ(ϕ)R_{GB}^2$ as an effective potential $V_\text{eff}$ provides an explanation for the ``plateau" and the divergence observed in the time evolution. We then construct the branches of nonlinearly scalarized black holes in the probe limit and with backreaction. While the pattern of the solution branches in the probe limit exhibits universal features, the presence of backreaction reveals a distinct dependence on the coupling strength $β$.

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arXiv2025

Phase transitions of boson stars in scalar-tensor theories

In scalar-tensor theories, compact objects may experience spontaneous scalarization. Recently, it was shown that matter-induced spontaneous scalarization of neutron stars is predominantly associated with a first-order phase transition. Here we consider matter-induced spontaneous scalarization of boson stars. Employing a repulsive quartic potential for the bosonic matter, we find only first-order phase transitions.

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arXiv2025

Finite-Distance Gravitational Lensing of a Global Monopole in a Schwarzschild-de Sitter Spacetime

We investigate the gravitational lensing of a Schwarzschild-de Sitter black hole with a global monopole at finite distances. In this asymptotically nonflat spacetime, the deflection angle of light is decomposed into two parts: the first derives from the orbit differential equation, and the second originates from the metric itself. By absorbing the cosmological constant into the effective impact parameter, we derive an analytical expression for the first part using elliptic integrals. Combined with the second part, we obtain a complete exact solution for the deflection angle in this context. Additionally, considering that the distances from the source to the observer are large, we derive expressions for the light deflection angle in both the weak and strong field limits. In both cases, we find that the deflection is enhanced by the presence of the global monopole, further supporting its potential role as an alternative to elusive dark matter.

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arXiv2024

Gravitational lensing effect of black holes in effective quantum gravity

In the present work, we investigate the gravitational lensing effects of two quantum-modified black hole models recently proposed in effective quantum gravity. The light deflection angles are calculated for both the weak-field and strong-field limits. Furthermore, using the data for the supermassive black holes SgrA* and M87*, we calculate the lensing observables in the strong-field limit. We find that the quantum parameter plays a role analogous to the electric charge in weak gravitational lensing. In the strong-field limit, in contrast, the effects of the quantum parameter on the deflection angle, the angular separation, and the relative magnification are opposite to those of the electric charge, the scalar charge, and the quantum parameters in some gravity theories. The results indicate the crucial difference between the classical black holes and the two quantum-modified black hole models that depend on the quantum correction, making them a valuable tool for distinguishing these black hole models.

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arXiv2024

Identifying doppelgange Black Holes through Shadow Images

Recently, an interesting \textit{doppelgänge} black hole solution is obtained in the string-inspired Euler-Heisenberg theory, where the black holes have the same radii but share different charges. We found, however, they possess different ISCOs and photon spheres, and hence affect their shadow images. In this work, we investigate the optical appearances, illuminated by an optically and geometrically thin disk, are investigated, of such black hole. One finds that doppelgänge black holes have different optical appearances. Even the horizon radii are the same, the size of shadows are not equal. Furthermore, we found that the large magnetic charge $Q_m$ black holes give rise to novel shadow images that the usual bright rings inside shadow are not clear, The optical appearances illuminated by spherically accretions are also examined, and it can also identify two doppelgänge black holes.

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arXiv2024

Scalarization of Kerr-Newman black holes in the Einstein-Chern-Simons-scalar theory

We investigate the tachyonic instability of Kerr-Newman (KN) black hole with a rotation parameter $a$ in the Einstein-Chern-Simons-scalar theory coupled with a quadratic massive scalar field. This instability analysis corresponds to exploring the onset of spontaneous scalarization for KN black holes. First, we find no $a$-bound for $α<0$ case by considering (1+1)-dimensional analytical method. A direct numerical method is adopted to explore (2+1)-dimensional time evolution of a massive scalar perturbation with positive and negative $α$ to obtain threshold curves numerically. We obtain threshold curves $α_{\rm th}(a)$ of tachyonic instability for positive $α$ without any $a$-bounds. We expect to find the same threshold curves $α_{\rm th}(a)$ of tachyonic instability for negative $α$ without any $a$-bound because its linearized scalar theory is invariant under the transformation of $α\to -α$ and $θ\to -θ$. In addition, it is found that the scalar mass term suppresses tachyonic instability of KN black holes.

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arXiv2023

Nonlinearly scalarized rotating black holes in Einstein-scalar-Gauss-Bonnet theory

In this paper, we discuss a fully nonlinear mechanism for the formation of scalarized rotating black holes in Einstein-scalar-Gauss-Bonnet gravity, where Kerr black holes are linearly stable, but unstable against nonlinear scalar perturbations. With the help of the pseudospectral method, we obtain a spectrum of nonlinearly scalarized rotating black hole solutions with multiple scalarized branches. Moreover, we investigate the thermodynamic properties of nonlinearly scalarized rotating black holes and find the phase transition between Kerr and these scalarized black holes.

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arXiv2023

Analytical approximations to charged black hole solutions in Einstein-Maxwell-Weyl gravity

The Homotopy Analysis Method (HAM) is a useful method to derive analytical approximate solutions of black holes in modified gravity theories. In this paper, we study the Einstein-Weyl gravity coupled with Maxwell field, and obtain analytical approximation solutions for charged black holes by using the HAM. It is found that the analytical approximate solutions are sufficiently accurate in the entire spacetime outside the black hole's event horizon, and also consistent with numerical ones for charged black holes in the Einstein-Maxwell-Weyl gravity.

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arXiv2023

Analytical approximate solutions for scalarized AdS black holes

The spontaneous scalarization of Schwarzscild-AdS is investigated in the Einstein-scalar-Gauss--Bonnet (ESGB) theory. Firstly, we construct scalarized AdS black holes numerically. Secondly, making use of the homotopy analysis method (HAM), we obtain analytical approximate solutions for scalarized AdS black holes in the ESGB theory. It is found that scalarized AdS black holes constructed numerically are consistent with analytical approximate solutions in the whole space.

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arXiv2022

Thermodynamic and tachyonic instability for asymptotically flat black holes

It was confirmed that the negative modes of the Euclidean section for asymptotically flat black holes reveal the thermodynamic instability of these black holes in the grand canonical ensemble (GCE). These include Schwarzschild, Reissner-Nordström, Kerr, and Kerr-Newman black holes. In this work, we develop the relation between thermodynamic instability in the GCE and tachyonic instability for asymptotically flat black holes, where the latter is the onset for obtaining black holes with scalar hair. This implies that the tachyonic instability of black holes when introducing scalar coupling to the Gauss-Bonnet term or Maxwell term reflects thermodynamic instability of these black holes in the GCE. We go on further to consider the Schwarzschild-AdS black hole.

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arXiv2022

Spin-Charge induced spontaneous scalarization of Kerr-Newman black holes

We investigate the tachyonic instability of Kerr-Newman (KN) black holes in the Einstein-Maxwell-scalar (EMS) theory with a positive coupling parameter $α$. This corresponds to exploring the onset of spontaneous scalarization for KN black holes. For this purpose, we use the hyperboloidal foliation method (HFM) to solve the linearized scalar equation numerically. We obtain a 3D graph [$\log_{10}α(a,Q)$] which indicates the onset surface for spontaneous scalarization of KN black holes in the EMS theory. We find that there is no lower bound on the rotational parameter $a$, but its upper bound is given by $M^2-Q^2 \ge a^2$. Also, we confirm that the high rotation enhances spontaneous scalarization of KN black holes in the EMS theory.

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