arXiv subjects

Zong-Kuan Guo

Publications and source records attributed to Zong-Kuan Guo.

At least 37 records · Page 2Linked to original sources

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

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↗

arXiv2026

Axion Inflation from Heavy-Fermion One-Loop Effects

We derive a one-loop effective description of axion inflation by integrating out a heavy Dirac fermion with an inflaton-dependent complex mass undergoing a smooth localized threshold transition. The threshold induces correlated corrections to the inflaton and gauge sectors, including a Coleman-Weinberg term, a vacuum-polarization correction, and an anomaly-induced Chern-Simons coupling. Together, these effects transiently enhance and localize gauge-field production, generating a chiral stochastic gravitational-wave background in the deci-hertz band within the projected sensitivities of BBO and DECIGO, while remaining below representative primordial-black-hole bounds.

hep-ph↗

arXiv2026

The quasinormal modes of the rotating quantum corrected black holes

The quasinormal modes (QNMs) of a rotating quantum corrected black hole (RQCBH) are studied by employing the hyperboloidal framework for the scalar perturbation. This framework is used to cast the QNMs spectra problem into a two-dimensional eigenvalue problem, then the spectra are calculated by imposing the two-dimensional pseudo-spectral method. Based on the resulting scalar spectra, a parameter estimation pipeline for this RQCBH model with gravitational wave data is constructed by using \texttt{pyRing} in the ringdown phase. We use informative priors in our inference that incorporates the mass and spin distributions predicted by the inspiral-merger phase as the prior distributions for the ringdown analysis. Notably, since the waveform model beyond Kerr black hole in $\texttt{pyRing}$ is designed for the tensor perturbation, the inferred posterior distributions should be interpreted as a methodological investigation rather than as physical constraints from observations. The methodological results show that the use of informative priors consistently yields a tighter posterior on the quantum correction parameter compared to analyses without such priors, and the spin inferred from the RQCBH model begins to be significant and differs from that of the Kerr model. This opens a promising avenue for testing quantum-gravity-induced deviations using gravitational-wave spectroscopy.

gr-qc↗

arXiv2026

Waveform stability for the piecewise step approximation of Regge-Wheeler potential

By interpreting the difference between the original Regge-Wheeler potential and its piecewise step approximation as perturbative effects induced by the external environment of the black hole, we investigate the stability of Schwarzschild black hole time-domain waveforms. In this work, we employ the Green's function method under the assumption of an observer at spatial infinity to obtain the waveform. For two cases in which the initial Gauss bump near the event horizon or at spatial infinity, we derive analytic expressions for the corresponding waveforms. Our results demonstrate that the waveform is indeed insensitive to tiny modifications of the effective potential, thereby confirming its stability. More importantly, we find that broader initial bumps imprint the influence of small environmental modifications more clearly on the waveform, which may provide theoretical guidance for probing the exterior environment of black holes.

gr-qc↗

arXiv2026

Double-layered vacuum bubbles and cosmological phase transitions

We investigate the evolution and formation of double-layered vacuum bubbles during cosmological phase transitions with multiple vacua. We employ lattice simulations to show that flyover transitions can produce double-layered vacuum bubbles by overcoming successive potential barriers, thereby suggesting a novel bubble vacuum configuration in cosmological phase transitions. The evolution of these bubbles, including wall acceleration, collisions, and the formation of trapped regions, is explored through numerical simulations. Our results show that the dynamics of double-layered bubbles differ significantly from standard single-wall bubbles, with implications for cosmological observables such as gravitational wave production and baryogenesis.

astro-ph.CO↗

arXiv2026

Distinguishing gravity theories with networks of space-based gravitational-wave detectors

We propose a method for separating and detecting the non-tensor modes of stochastic gravitational-wave backgrounds (SGWBs) using networks of space-based gravitational-wave detectors. We consider four distinct data-reconstruction schemes for the co-inclination and anti-inclination orbital configurations of the LISA-Taiji network. We find that the co-inclination configuration offers its advantages over the anti-inclination one and can achieve signal-to-noise ratios up to 17.3 for the vector modes and 10.4 for the scalar modes with the energy density spectrum as $Ω_{GW}^p(f)=10^{-12}$. Our method can be used to measure beyond-general-relativity polarization modes of SGWBs at mHz frequency band, opening a new avenue for testing alternative gravity theories.

gr-qc↗

arXiv2026

Fluctuation-Induced Friction in Bubble-Wall Dynamics of Cosmological First-Order Phase Transitions

We study bubble-wall dynamics in cosmological first-order phase transitions in a two-scalar-field model, where the wall is formed by $ϕ$ and an additional real scalar $s$ couples through a portal interaction. We evolve the coupled classical field equations on the lattice and demonstrate that for an initial Bose--Einstein distribution of $s$ fluctuations at the nucleation temperature $T_n$, the resulting patchy background intermittently modulates the local driving pressure on the wall. The wall therefore undergoes alternating episodes of acceleration and deceleration and approaches a quasi-stationary propagation regime with a smaller time-averaged speed than in the decoupled limit. We further identify three familiar propagation profiles -- deflagration, detonation, and hybrid -- distinguished by where the dynamical $s$-sector energy density is concentrated relative to the wall. These effects can impact gravitational wave and baryogenesis predictions.

hep-ph↗

arXiv2026

Dynamical backreaction of a mass-acquiring scalar field on first-order phase transitions

Phase transitions in the early Universe give rise to effective masses for massless fields in the symmetry-broken phase. We perform lattice simulations to study the dynamical impact of a mass-acquiring spectator field on the evolution of first-order phase transitions and the associated gravitational-wave production, while keeping the effective potential responsible for bubble nucleation fixed. In addition to the well-known friction effects, we identify a novel effect that significantly enhances the strength of first-order phase transitions. In contrast to the general scenario, although the effective potential governs the tunneling rate, the amplitude of the $χ$ field is strongly suppressed inside the true vacuum bubble, resulting in a faster bubble expansion than predicted by the effective potential alone. The amplitude of the mass-acquiring field is highly suppressed in the true vacuum bubbles, resulting in additional release of vacuum energy that concentrate on the bubble walls. We further develop an analytical framework that not only explains our numerical results but can also be used to improve the estimation of gravitational-wave signals in related phase-transition scenarios.

astro-ph.CO↗

arXiv2026

Q-balls from thermal balls during a first-order phase transition: a numerical study

We numerically study the Q-ball formation triggered by a cosmological first-order phase transition within the Friedberg-Lee-Sirlin model. By performing lattice simulations, we track the nonequilibrium dynamics throughout the transition, providing a precise description of the Q-ball formation mechanism and the resulting mass spectrum. Collapsing false-vacuum regions first form thermal balls, which subsequently cool via dissipative interactions and stabilize into long-lived Q-balls with nonzero spin. We observe a large population of low-mass Q-balls, as well as rare, massive Q-balls that are several times larger than the analytical prediction. The final Q-ball population exhibits a broad mass spectrum spanning over two orders of magnitude, characterized by an exponential tail of number density at large masses. The simulations suggest that the Q-ball abundance is approximately $50\%$ higher than predicted by analytical estimates, adjusting the result in the context of Q-balls as dark matter candidates.

astro-ph.CO↗

arXiv2026

Suppression of scalar perturbations due to a heavy axion

A fast-rolling axion can transfer its kinetic energy to gauge fields via the Chern-Simons coupling, leading to copious production of gauge quanta during inflation. The amplified gauge fields act as a source for both scalar and tensor perturbations. In this work, we propose a mechanism for suppressing scalar perturbations while sourcing strong tensor perturbations. We present an implementation of such a mechanism, demonstrating that sourced tensor perturbations are expected to be detected by upcoming next-generation CMB experiments.

hep-ph↗

arXiv2025

Measuring Cosmological Redshift Using Gravitational Waves from Compact Binaries with Mass Transfer

The mass transfer process is prevalent during the inspiral phase of compact binary systems. Detection of gravitational waves from the inspiral phase of binaries with white dwarfs will allow us to measure the mass transfer rate. Mass transfer effects provide additional contributions to the phase of gravitational waves, which can break the degeneracy between binary masses and redshift. Based on the analytic mass transfer rate to the first order post-Newtonian evolution of orbital angular frequency, we use the Fisher matrix to forecast the ability of DECIGO to measure the redshift of compact binaries with mass transfer. We conclude that for compact binary systems containing white dwarfs, the redshift can be determined to an accuracy of $10\%$ for $z=0.01$ with a $SNR\thicksim 30$.

gr-qc↗

arXiv2025

Ring formation from black hole superradiance through repeated particle production on bound orbits

Ultralight bosonic fields around a rotating black hole can extract energy and angular momentum through the superradiant instability and form a dense cloud. We investigate the scenario involving two scalar fields, $ϕ$ and $χ$, with a coupling term $\frac{1}{2}λϕχ^2$, which is motivated by the multiple-axion framework. The ultralight scalar $ϕ$ forms a cloud that efficiently produces $χ$ particles nonperturbatively via parametric resonance, with a large mass hierarchy, $μ_χ\gg μ_ϕ$. Rather than escaping the system as investigated by previous studies, these $χ$ particles remain bound, orbiting the black hole. Moreover, the particle production occurs primarily at the peak of the cloud's profile, allowing $χ$ particles in quasicircular orbits to pass repeatedly through resonant regions, leading to cumulative amplification. This selective process naturally forms a dense ring of $χ$ particles, with a mass ratio to the cloud fixed by $(μ_ϕ/μ_χ)^2$. Our findings reveal a novel mechanism for generating bound-state particles via parametric resonance, which also impacts the evolution of the cloud. This process can be probed through its imprint on binary dynamics and its gravitational-wave signatures.

gr-qc↗