arXiv subjects

Zong-Kuan Guo

Publications and source records attributed to Zong-Kuan Guo.

At least 19 recordsLinked to original sources

arXiv2026

Quasinormal modes of Schwarzschild--AdS black holes with a near-horizon reflective surface

We investigate the scalar quasinormal mode (QNM) spectrum of Schwarzschild-AdS black holes with a partially reflective surface placed near the event horizon. The corresponding QNM problem is solved numerically using Chebyshev spectral collocation. The resulting spectra exhibit a characteristic cavity structure at sufficiently large real part, where the modes form approximately equally spaced sequences whose spacing and damping are controlled mainly by the cavity length and the surface reflectivity. For large AdS radius, an additional weakly damped quasibound-state branch appears due to trapping between the centrifugal barrier and the AdS potential wall. We show that these spectral features can be understood within a finite radial cavity picture. A WKB analysis provides a quantitative asymptotic description of the spectra which have high real parts. We find that changes in the near-horizon boundary condition can lead to substantial changes in the QNM spectrum, indicating the strong sensitivity of Schwarzschild--AdS QNMs to the effect of horizon correction.

gr-qc↗

arXiv2026

Inspiral waveforms of charged compact binaries and observational constraints

Electric charges carried by compact objects can affect binary dynamics and imprint characteristic signatures on gravitational-wave signals. We derive next-to-leading-order post-Newtonian frequency-domain waveforms for charged compact binaries in both the gravitational-quadrupole and electric-dipole dominated regimes, including electromagnetic corrections beyond leading-order electric-dipole radiation that reduce the degeneracy between the component charge-to-mass ratios. Using gravitational wave events from GWTC-5.0, we place constraints on the charge-to-mass ratio and mass of each component of the binary system.

gr-qc↗

arXiv2026

Symmetry preservation in black hole quasinormal mode spectra

We establish a general relation between symmetries of gravitational theories and black hole (BH) quasinormal mode (QNM) spectra. We show that a symmetry implies isospectrality when it induces a bijection between the corresponding QNM boundary value problems. However, conformally related BHs have been reported to exhibit both conformal factor dependent and independent QNM spectra. To resolve this issue, we develop a reduction scheme for higher order perturbation equations. Applied to pure Weyl gravity, it yields the complete axial spectrum of Schwarzschild, including Regge-Wheeler and spin-1 branches. Our results show that these seemingly contradictory conclusions result from differences in the perturbation dynamics or in the boundary conditions.

gr-qc↗

arXiv2026

Constraints on Buchdahl-Inspired Gravity from Future Pulsar Timing near Sgr A*

Future pulsar timing observations near Sgr~A* offer a unique probe of gravitational physics in the vicinity of a supermassive black hole. We forecast the ability of such measurements to constrain a Buchdahl-inspired $R^2$ gravity, parameterized by a single deviation parameter $ε$, using a timing framework that self-consistently integrates orbital dynamics with light-propagation delays and preserves the full timing solution across the observing span. Through Fisher-matrix forecasts for a representative pulsar, we systematically isolate how the precision on $ε$ depends on orbital geometry. We find that shorter orbital periods and higher eccentricities significantly enhance sensitivity, consistent with a substantial contribution from observations near periastron. As a benchmark comparison, we further consider a hypothetical pulsar on an S2-like orbit ($P_b=16~{\rm yr}$, $e=0.88$) and obtain a statistical sensitivity of $σ_ε\sim 10^{-4}$ within the adopted weak-field, static, and spherically symmetric timing model. This sensitivity is comparable to the natural order-of-magnitude truncation scale of the 1PN expansion and should not be interpreted as a complete forecast for the real Sgr~A* system. Under the adopted idealized assumptions, the characteristic statistical scale is several orders of magnitude below the current S2 95\% confidence interval half-width ($|ε|_{\rm S2}^{\rm 95\%}\approx 0.56$), though this comparison is heuristic given the differing confidence levels. These trends provide quantitative guidance for target selection and campaign design in future Galactic-center pulsar searches.

astro-ph.HE↗

arXiv2026

The (in)stability on total transmission modes with small bumps

Total transmission modes (TTMs) are a class of reflectionless solutions to black hole perturbation equations, closely related to quasinormal modes (QNMs), and can exhibit significant sensitivity to weak environmental perturbations. In this work, we investigate the spectrum (in)stability of TTMs of Tangherlini black holes by introducing a localized Pöschl-Teller bump perturbation into the effective potential, and employ a modified Chebyshev-Lobatto grid to improve the numerical accuracy of the localized perturbation. For $d=14$, $\ell=2$, and $s=2$, the purely imaginary TTM exhibits relatively strong spectrum stability, whereas the genuine complex TTMs undergo significant migrations even for small perturbations, consistent with the spectrum stability revealed by previous pseudospectrum analyses.

gr-qc↗

arXiv2026

Post-Newtonian dynamics of charged compact binaries

We investigate the dissipative dynamics of charged compact binaries in Einstein-Maxwell theory. By evaluating the mass and electric multipole moments, we compute the gravitational and electromagnetic fluxes {through next-to-leading order in the post-Newtonian expansion}. Using the flux-balance equations, we derive the evolution of the orbital angular frequency for quasi-circular inspirals. We further analyze circular orbit stability in charged black-hole binaries and quantify how the charge-to-mass ratios affect the inspiral dynamics.

gr-qc↗

arXiv2026

Parametric resonance amplification of gravitational waves in dynamical Chern-Simons gravity

Within the effective field theory of dynamical Chern-Simons (dCS) gravity, we study parametric resonance amplification of gravitational waves driven by an oscillating environmental field coupled to the dCS pseudoscalar. We find that the black hole potential barrier and external shell form a resonant cavity, producing a Mathieu instability whose optimal frequency is fixed by the cavity length. The instability shows a horizon leakage threshold, Floquet sidebands, and a delayed secondary burst in axial gravitational perturbations. This mechanism reveals that dCS corrections at ultraweak coupling can still accumulate via long term parametric amplification, leaving discernible signatures in gravitational wave signals.

gr-qc↗

arXiv2026

Astrophysical Graviton Squeezing Can Be Hidden in the Far-Field

While localized astrophysical sources can generate macroscopic graviton squeezing, their observable quantum signatures at far-field detectors remain unresolved. In this work, we investigate the propagation dynamics of the squeezed states using spatial quantum optics methods to evaluate correlation functions accessible to a local observer. Crucially, we reveal a severe kinematic conflict in same-cone measurements, which highly suppresses local quantum coherence. Consequently, these macroscopically squeezed states appear classically thermal to a single detector. Our results demonstrate that global squeezing does not guarantee local observability, and the measurable quantum signatures may be significantly weaker than what would be expected from the overall squeezing parameter of the state.

gr-qc↗

arXiv2026

Hubble tension: a short review of theoretical explanations

The $Λ$ cold dark matter model successfully describes a wide range of cosmological observations. However, the persistent discrepancy between the value of the Hubble constant inferred from cosmic microwave background (CMB) measurements within this model and that obtained from local distance-ladder determinations points to a significant inconsistency. This short review examines theoretical responses across the cosmological inference chain, from the gravitational field equations to the pre-recombination sound horizon and the late-time distance-redshift relation. It discusses early-time and late-time mechanisms, as well as modified gravity, as ways to alter the acoustic ruler, distances, structure growth, or gravitational response. Current proposals can reduce the nominal tension, but often at the cost of correlated shifts in CMB spectra, standard-ruler distances, lensing, structure growth, or calibrator information. Further progress requires unified likelihoods and multi-probe tests linking all key observables under the same model assumptions.

astro-ph.CO↗

arXiv2026

Nonminimally coupled quintessence with sign-switching interaction

We propose a new nonminimally coupled quintessence model to account for the late-time dark energy dynamics indicated by recent Dark Energy Spectroscopic Instrument (DESI) measurements. Within this framework, the quintessence density begins to decrease only when it starts to dominate the Universe, which naturally accounts for the late-time onset of dark energy weakening. The coupling also induces a sign change in the effective energy transfer between dark matter and dark energy during cosmic evolution. While the scalar field itself remains canonical and never crosses the phantom divide, the modified evolution of the dark matter density gives rise to an effective crossing behavior in the observationally inferred dark energy sector. Compared with both $Λ\mathrm{CDM}$ and $w_0w_a\mathrm{CDM}$ models, our model is favored more strongly by current cosmological data. This work may provide a promising avenue for understanding the observational late-time weakening of dark energy and the origin of its dynamics.

astro-ph.CO↗

arXiv2026

Low-finesse scattering and non-stationary dispersive dynamics of gravitational wave echoes

We study environmental echoes induced by a weak potential barrier outside a Schwarzschild black hole. In the low-finesse limit, the time domain response is governed by a sequence of transient wave packets formed by finite round-trip scattering, rather than steady state cavity modes. We establish quantitative criteria for the breakdown of the steady state resonance picture, dictated by frequency domain spectral aliasing and time domain truncation from the black hole power law tail. Based on non-stationary dispersive dynamics, we analytically derive the arrival time gliding, central frequency drift, and dispersion driven asymmetric tails of these echoes. Accordingly, we construct a five-parameter analytical template that approaches the theoretical maximum matching degree bounded by the exact transfer function for the first echo. Our results demonstrate that early low-finesse environmental echoes must be theoretically modeled as non-stationary transient scattering signals.

gr-qc↗

arXiv2026

Localized Steps toward ACT-Favored Inflation

Recent ACT measurements favor a scalar spectral index larger than the Planck value, posing a challenge to many single-field slow-roll inflation models. We show that a smooth, localized step in the inflaton potential can shift the predicted scalar spectral index and tensor-to-scalar ratio by displacing the field value at which the CMB pivot scale exits the horizon. This mechanism can move monomial and, in particular, plateau-like attractor models toward the ACT-favored region, whereas the induced shift remains insufficient in natural inflation. We derive semi-analytical expressions for the step-induced remapping and quantify the associated effective e-fold shift, finding that it can be comparable to, and in some cases exceed, the shift allowed by conservative reheating uncertainties.

hep-ph↗

arXiv2026

Extended parameterized spin expansion formalism for ringdown analysis with GW250114

Parameterized descriptions of black-hole quasinormal-mode spectra are essential for testing gravity with ringdown observations. We extend the Parameterized Spin Expansion Coefficients (ParSpec) formalism by simultaneously sampling the characteristic length scale $\tilde{\ell}$ and the scaling index $\tilde{p}$, rather than fixing $\tilde{p}$ to a theory-motivated integer and constraining $\ell$. Physically, this extension promotes the scaling of the spectral corrections coming from higher-curvature operators to an observable quantity. Methodologically, it enables us to investigate the enlarged ParSpec parameter space and identify the prior geometry induced by conditions on the effective coupling $γ$. We examine the robustness of the framework by using the $220$ and $220+221$ ringdown models over different start times with informative priors on mass and luminosity for GW250114, and further study a joint constraint with GW231123. We find that $\tilde{p}$ remains prior dominated and that the data show no evidence for deviations from general relativity (GR). Among the coupling prescriptions considered, $γ<1$ avoids an artificial correlation between $\tilde{p}$ and $\tilde{\ell}$. At the current signal-to-noise ratio, the results based on the Kullback-Leibler divergence show that the $220$-only model provides more informative constraints than the $220+221$ model. Higher-SNR ringdowns and hierarchical analyses of a larger event population will be required to break the $\tilde{p}-\tilde{\ell}$ degeneracy and directly probe the scaling structure of corrections to GR.

gr-qc↗

arXiv2026

Parity violating spectral dynamics of black holes in dynamical Chern-Simons gravity

We study how environmentally driven spectral instabilities of quasinormal modes respond to parity violating gravito-scalar coupling in black holes. Focusing on dynamical Chern-Simons gravity as a paradigm for parity violation, we perturb the Schwarzschild background with a localized potential bump. Our analysis reveals three distinctive phenomena absent in general relativity: 1) branch reconnections in the complex frequency plane, 2) a counterintuitive mode stabilization that delays overtaking transitions, and 3) scalar mode dominance emerging at intermediate coupling strengths. These frequency domain features show how comparatively weak static sector differences manifest as distinct dynamical signatures, thereby linking parity violating black hole perturbations with non-Hermitian spectral physics. Our results provide a frequency domain characterization of parity violating coupling and motivate future targeted ringdown studies of modified gravity.

gr-qc↗

arXiv2026

Waveform stability of black hole ringdown with stochastic horizon structure

We examine the robustness of black hole ringdown to stochastic horizon-scale structure within an effective field framework in a proof-of-principle Schwarzschild setup.Consistent with the understanding that the spectral instability of quasinormal modes does not necessarily imply observational breakdown, our results demonstrate that the macroscopic gravitational waveform remains robust. We identify the phase averaging mechanism as the physical origin of this stability, demonstrating that the spatial integration of the wave equation efficiently attenuates ultraviolet geometric details below the resolution limit of the probing wavelength. Building on the scaling law $\mathcal{M} \propto ε^2$ and the characteristic mismatch profile with respect to $L_c$, we propose a geometric selection rule for observability: a detectable signal imposes a strict dual constraint requiring both macroscopic spatial coherence ($L_c \sim M$) and classical-level intensity ($ε\gtrsim 10^{-4}$). This criterion quantitatively rules out the observability of incoherent, high-entropy quantum foam in the present static Schwarzschild model, suggesting that any significant ringdown deviation would instead serve as evidence for macroscopically coherent horizon structures.

gr-qc↗

arXiv2026

Fixing the Renormalization of Inflationary Loops via Ward Identities

Evaluating quantum loop corrections to curvature perturbations in non-attractor inflation presents theoretical ambiguities. A crucial aspect of this challenge lies in the unconstrained finite contributions in renormalization counterterms and regularization scheme dependence. In this work, we derive exact Ward identities via the path integral formalism based on the large gauge symmetry of the background-perturbation split. These identities are shown to impose strict, model-independent constraints on the renormalization procedure. Provided the ultraviolet completion respects this symmetry, the Ward identities non-perturbatively govern the infrared evolution of the power spectrum. This symmetry-based framework offers a systematic resolution to recent theoretical discrepancies concerning one-loop corrections in ultra-slow-roll inflation.

gr-qc↗

arXiv2026

Ward Identity Constraints on Loop Corrections in Non-Attractor Inflation

The conservation of super-horizon curvature perturbations in strongly interacting inflationary models, particularly in the presence of quantum-loop corrections, remains a topic of active debate. We found that this conservation is essentially a direct consequence of the symmetry in perturbation theory. We demonstrate that the associated Ward identity imposes strict non-perturbative constraints on the infrared power spectrum. This finding provides a rigorous, symmetry-based framework for understanding nonlinear quantum fluctuations in the primordial universe.

astro-ph.CO↗

arXiv2026

Resolving the Planck-DESI tension by nonminimally coupled quintessence

The Planck measurement of the cosmic microwave background (CMB) has established the $Λ$-cold-dark-matter ($Λ$CDM) model as the concordant model along with other observations. However, recent measurements of baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) have renewed the matter fraction $Ω_\mathrm{m}$ tension between Planck-$Λ$CDM and DESI-$Λ$CDM. Directly reconciling this CMB-BAO tension with a dynamical DE in Chevallier-Polarski-Linder (CPL) parametrization seems to imply a crossing of the equation-of-state (EOS) through $w=-1$ at low redshifts. In this paper, we resolve this $Ω_\mathrm{m}$ tension by allowing for the DM nonminimally coupled to gravity via a quintessence field. This non-minimal coupling is preferred over $3σ$ confidence level. Consequently, even though the usual effective EOS of the coupled quintessence apart from the standard CDM part never crosses but always is above $w=-1$, a misidentification with the $w_0w_a$CDM model would exactly fake such a crossing behavior, and the tensions on neutrino mass and growth rate in the $Λ$CDM model are also relieved in our model as a result of the resolved $Ω_\mathrm{m}$ tension.

astro-ph.CO↗