arXiv subjects

Kelvin Ka-Ho Lam

Publications and source records attributed to Kelvin Ka-Ho Lam.

7 recordsLinked to original sources

arXiv2026

Living on the Edge of Effective Field Theory: Near-Extremal Black Holes in Quadratic Gravity

Near-extremal black holes can amplify higher-derivative corrections to general relativity, making them sharp probes of the perturbative control of gravitational effective field theory. We study this question in quadratic gravity with a dynamical scalar degree of freedom, focusing on dynamical Chern-Simons and scalar Gauss-Bonnet gravity. Using pseudospectral methods, we construct rapidly rotating exterior solutions at fixed surface gravity and fixed horizon angular velocity, and follow this controlled family toward extremality. In dynamical Chern-Simons gravity, the sequence approaches an asymptotically flat extremal exterior whose near-horizon limit agrees with the dynamical-Chern-Simons-deformed near-horizon extremal Kerr geometry and inherits its enhanced $SO(2,1)\times U(1)$ symmetry. In scalar Gauss-Bonnet gravity, the same limiting procedure behaves differently: the scalar develops an unavoidable logarithmic singularity at the horizon, the exterior metric develops a small boundary layer near the horizon, and the scalar-Gauss-Bonnet-deformed near-horizon extremal solution is not connected to a regular asymptotically flat exterior. We then use curvature invariants and tidal forces to diagnose the breakdown of the perturbative effective-field-theory expansion. We find that scalar curvature invariants can remain finite at the horizon even when tidal forces measured by infalling observers diverge as inverse powers of the surface gravity. These results separate distinct notions of effective-field-theory breakdown near extremality and provide a perturbative validity estimate that can be compared with current bounds on the dynamical Chern-Simons and scalar-Gauss-Bonnet couplings.

gr-qc↗

arXiv2026

Measuring a Black Hole's Area Immediately after Merger: A Direct-Wave Test of Hawking's Area Law

Black-hole area is the geometric variable behind horizon thermodynamics. We introduce a gravitational-wave method to infer a Kerr-equivalent horizon area from direct waves in the near-merger signal, before quasinormal ringing dominates at late times. Applied to GW250114, and interpreting the fitted direct-wave frequency and damping rate as horizon quantities, we find that analyses initiated $3$--$4.5M$ before the peak-amplitude time yield an area consistent with the Kerr remnant. This result gives a first area measurement using direct waves and a new near-merger test of Hawking's area law.

gr-qc↗

arXiv2026

Quadratic gravity corrections to scalar QNMs of rapidly rotating black holes

In an effective-field-theory framework for gravity, black-hole quasinormal mode spectra acquire corrections in quadratic-curvature, scalar-tensor extensions of general relativity. Previous calculations of such corrections were limited to moderate spins, since the corresponding background solutions relied on expansions in the spin parameter. Using recently constructed numerical black-hole solutions valid for large spin, we compute the leading-order deviations from general relativity in the scalar quasinormal mode spectrum of rotating black holes in scalar Gauss-Bonnet and dynamical Chern-Simons gravity. We solve the resulting perturbation equations with pseudo-spectral collocation methods, allowing us to determine the quasinormal-mode corrections for dimensionless spins up to $a/M=0.99$, with accuracy better than $\lesssim 10^{-3}$ for the $l=m=0$ mode and $\lesssim 10^{-6}$ for higher multipoles. For spins $a/M>0.9$, the corrections to certain modes can increase by orders of magnitude.

gr-qc↗

arXiv2026

Spinning Black Holes in Modified Gravity via Spectral Methods

Rapidly-rotating black-hole spacetimes outside general relativity are key to many tests of Einstein's theory. We here develop an efficient spectral method to represent such spacetimes analytically, in closed-form, and to high accuracy, in a large class of effective-field-theory extensions of general relativity. We exemplify this method by constructing, for the first time, closed-form and analytic representations of spinning black holes in scalar-Gauss-Bonnet, dynamical Chern-Simons, and axidilaton gravity to an accuracy better than $10^{-8}$ for all dimensionless spins below 0.99.

gr-qc↗

arXiv2026

Analytic and accurate approximate metrics for black holes with arbitrary rotation in beyond-Einstein gravity using spectral methods

A key obstacle for theory-specific tests of general relativity is the lack of accurate black-hole solutions in beyond-Einstein theories, especially for moderate to high spins. We address this by developing a general framework--based on spectral and pseudospectral methods--to obtain analytic, closed-form spacetimes representing stationary, axisymmetric black holes in effective-field-theory extensions of general relativity to leading order in the coupling constants. The approach models the spacetime (and extra fields) as a stationary, axisymmetric deformation of the Kerr metric in Boyer-Lindquist-like coordinates, expands metric deformations as a spectral series in radius and polar angle, and converts the resulting beyond-Einstein field equations into algebraic equations for the spectral coefficients. For any given spin, these equations are solved via standard linear-algebra methods; the coefficients are then fitted as functions of spin with non-linear functions, yielding fully analytic metrics for rotating black holes in beyond-Einstein theories. We apply this to quadratic gravity theories--dynamical Chern-Simons, scalar-Gauss-Bonnet, and axi-dilaton gravity--obtaining solutions valid for any spin, including near-extremal cases with errors below machine precision for $a \leq 0.9$ and $\lesssim 10^{-8}$ for $a \leq 0.999$. We show that existing slowly-rotating solutions break down at $a \sim (0.2, 0.6)$, depending on approximation order and chosen accuracy. We then use our metrics to compute observables, such as the surface gravity, horizon angular velocity, and the locations of the innermost circular orbit and the photon ring. The framework is general and applicable to other effective-field-theory extensions for black holes of any spin.

gr-qc↗

arXiv2025

Quasinormal mode frequencies and gravitational perturbations of spinning black holes in modified gravity through METRICS: The dynamical Chern-Simons gravity case

We present the first precise calculations of the gravitational quasinormal-mode (QNM) frequencies for spinning black holes with dimensionless angular momenta $J/M^2 := a \lesssim 0.75$ in dynamical Chern-Simons gravity. Using the \textit{Metric pErTuRbations wIth speCtral methodS} (METRICS) framework, we compute the QNM frequencies of both axial and polar metric perturbations, focusing on the $nl m = 022$, $033$, and $032$ modes. The METRICS frequencies for the 022 mode achieve numerical uncertainties $\lesssim 10^{-4}$ when $0 \leq a \leq 0.5$ and $\lesssim 10^{-3}$ for $0.5 \leq a \leq 0.75$, without decoupling or simplifying the linearized field equations. We also derive optimal fitting polynomials to enable efficient and accurate evaluations of the leading-order frequency shifts in these modes. The METRICS frequencies and fitting expressions are a robust and indispensable step toward enabling gravitational-wave ringdown tests of dynamical Chern-Simons gravity.

gr-qc↗

arXiv2025

I-Love-Q Relations of Fermion-Boson Stars

We investigate the properties of fermion-boson stars (FBSs), which can be viewed as neutron stars with a bosonic dark matter (DM) admixture. A challenge in studying the impact of DM on neutron stars is the absence of a universally accepted nuclear-matter equation of state (EOS), making it difficult to distinguish between the effects of DM and various EOS models. To address this issue, we extend the study of the I-Love-Q universal relations of neutron stars to FBSs with a nonrotating bosonic component by solving the Einstein-Klein-Gordon system. We study how DM parameters, such as the boson particle mass and self-interaction strength, would affect the structure of FBSs and explore the parameter space that leads to deviations from the I-Love-Q relations. The properties of FBSs and the level of deviations in general depend sensitively on the DM parameters. For boson particle mass within the range of $\mathcal{O}(10^{-10} \ \mathrm{eV})$, where the Compton wavelength is comparable to the Schwarzschild radius of a $1 M_\odot$ star, the deviation is up to about 5% level if the star contains a few percent of DM admixture. The deviation increases significantly with a higher amount of DM. We also find that the universal relations are still valid to within a 1% deviation level for boson particle mass $m_b \geq 26.8\times10^{-10} \ \mathrm{eV}$. This effectively sets an upper bound on the boson particle mass, beyond which it becomes not feasible to probe the properties of FBSs by investigating the I-Love-Q relation violations.

astro-ph.HE↗