arXiv subjects

Fu-Wen Shu

Publications and source records attributed to Fu-Wen Shu.

At least 19 recordsLinked to original sources

arXiv2026

Scalar-induced gravitational waves from inflation with symmetry breaking

We investigate scalar-induced gravitational waves (SIGWs) in an inflationary model with symmetry breaking, in which charged scalar fields are coupled to an isotropic triplet of Abelian gauge fields through a kinetic function. Such SIGWs can be enhanced to a detectable level when the mixing between the inflaton and gauge-field perturbations is sufficiently large. We find that the longitudinal mode and the charge-dependent mixing between perturbations become relevant only when the gauge-field excitation occurs sufficiently late during inflation. In this regime, the corresponding SIGWs are shifted to ultra-high frequencies, typically can around the GHz band. We show that the parameters characterizing the effects of the longitudinal mode and charge-dependent mixing affect the signal in qualitatively different ways. This provides characteristic signatures for distinguishing the neutral case from the charged one through the frequency profile of the stochastic gravitational-wave background.

astro-ph.CO↗

arXiv2026

Massive scalar fields in eccentric regime: Detectability and constraints from LISA observations of extreme mass-ratio inspirals

Extreme mass-ratio inspirals (EMRIs) are among the prime sources for future space-borne gravitational wave (GW) observatories and provide a useful setting for testing the presence of fundamental fields and possible deviations from general relativity (GR) in both strong and weak gravity regimes. In this work, we study the effect of a massive scalar field on eccentric equatorial EMRI dynamics around Kerr black holes. Considering that the inspiralling stellar-mass object carries a scalar charge and emits scalar radiation together with tensor GWs, we compute the relevant relativistic fluxes within the adiabatic treatment of the inspiral. With the solution of the scalar perturbation equation in the frequency domain, the resulting fluxes are presented through the Chebyshev interpolants in order to have the efficient inspiral evolution across the parameter space considered. We quantify the impact of scalar field mass and scalar charge on the orbital evolution and GW signal through phase shifts and waveform mismatches relative to both GR and the massless-scalar scenario. We find that massive scalar radiation can generate significant GW dephasing that increases with orbital eccentricity; however, the scalar flux is suppressed as the scalar field mass is becoming larger. Using a Fisher information matrix (FIM) analysis, we estimate the ability of Laser Interferometer Space Antenna (LISA) to measure or constrain the scalar charge and scalar field mass. Our results indicate that eccentric EMRIs can place meaningful constraints on massive scalar fields and provide a promising as well as important avenue for testing scalar-tensor extensions of gravity in the region of a strong gravitational field.

gr-qc↗

arXiv2026

Curvature-induced scalarization of charged AdS black holes

We investigate how a negative cosmological constant affects the Gauss-Bonnet (GB) scalarization in the Einstein-Maxwell-scalar-Gauss-Bonnet theory with a scalar coupling constant $\eta$ to GB term. We focus on the instability of Reissner-Nordstr\"om-AdS (RN-AdS) black holes under a scalar perturbation governed by an effective mass $\mu^2_{\text{eff}}$ sourced by the GB term. Unlike the asymptotically flat spacetime case, the onset of scalarization is not merely determined by $\mu^2_{\text{eff}} < 0$, but it is constrained by the Breitenlohner-Freedman (BF) bound. In case that the BF bound is violated ($\eta>2.25$ with $\Lambda=-0.5$), one may find AdS-tachyonic instability. We find that for $0<\eta<2.25$, the GB$^+$ scalarization may be performed through spontaneous scalarization, while for $\eta<0$ the GB$^-$ scalarization is found to give the single branch of scalarized AdS black holes. For the GB$^+$ scalarization in $\eta_{th}\le\eta<2.25$ with $\eta_{th}$ threshold instability, we obtain the single branch ($n=0$ fundamental branch) of scalarized AdS black holes, in contrast to the infinite branches in asymptotically flat spacetime. A bulk fixed-charge thermodynamic analysis is performed thoroughly for GB$^\pm$ scalarizations.

gr-qc↗

arXiv2026

Bound-State Resonances of Schwarzschild-de Sitter Black Holes: Analytic Treatment

Inspired by Mashhoon's framework connecting black hole quasi-normal modes (QNMs) to bound-state resonances in inverted potentials, V$\ddot{\text{o}}$lkel's recent numerical analysis of asymptotically flat Schwarzschild black holes revealed a counterintuitive phenomenon: highly excited bound states rapidly delocalize, become extremely weakly bound, and exhibit wavefunctions highly sensitive to far-field perturbations. To analytically explain this phenomenon and extend the investigation to Schwarzschild-de Sitter (SdS) black holes, we derive the characteristic equation for excited bound-state resonances in SdS spacetime and obtain compact closed-form analytical expressions for their resonance energies. In the $\Lambda\rightarrow 0$ limit, our SdS-derived spectrum aligns perfectly with recent results for Schwarzschild black holes. We analytically demonstrate that the rapid and infinite delocalization of highly excited resonances is a universal feature of asymptotically flat Schwarzschild systems. More significantly, we prove that SdS black holes support only a finite number of bound-state resonance levels -- in sharp contrast to the infinite spectrum of the asymptotically flat case. This finiteness implies an upper bound on the oscillatory domain of the resonance eigenfunctions in SdS geometries, thereby preventing infinite delocalization and offering a fundamental distinction in the resonance structure of black holes in different asymptotic backgrounds. Surprisingly, we also find that delocalized half-bound states exist in SdS black holes when the $\Lambda$ takes specific discrete values. This is a unique feature of SdS black holes and is absent in asymptotically flat Schwarzschild black holes. We also reveal the deep connection between half-bound states and the number of bound-state resonance energy levels.

gr-qc↗

arXiv2026

Self-resonance preheating in deformed attractor models: oscillon formation and evolution

It is well known that, in potentials that are quadratic near the minimum but shallower away, such as small $\alpha$ ($\ll M_P^2$) attractors, the inflaton condensate fragments into localized compact objects known as oscillons during self-resonance preheating. In this work we investigate the self-resonance in deformed $\alpha$-attractor T-model with a Gaussian feature near the minimum, distant from inflation's end. Linear analysis reveals altered resonance bands and deformed Floquet charts dependent on feature parameters. In fully nonlinear lattice simulations, we find that the gradient energy transfer is largely independent of the potential feature parameter $h$. In contrast, after resonance terminates, the subsequent evolution of gradient energy becomes strongly dependent on $h$. Statistical analysis reveals that models with the potential feature produce larger number of smaller oscillons, with a reduced energy stored in these objects, increasingly suppressed as the magnitude of $h$ grows. By tracking the total energy and the gradient energy contained in oscillons, we find that in models with nonzero $h$ oscillons are systematically shorter-lived, with this effect strengthening for larger $h$. The gravitational wave emission is dominated by the resonance stage and is strongly suppressed once oscillons form. Potential features leave the low-frequency spectrum largely unchanged but significantly modify the high-frequency tail. Although a complete reheating description requires external couplings and higher-resolution simulations, clear qualitative differences of cosmic expansion history already emerge within our simulated time window. These results highlight the important role of potential features in shaping reheating dynamics and their cosmological implications, and provide a deeper understanding of preheating dynamics and the properties of oscillons.

astro-ph.CO↗

arXiv2025

Constraint on massive vector field with extreme-mass-ratio inspirals around a slowly rotating black hole

We study the influence of a massive vector (Proca) field on the energy fluxes from extreme-mass-ratio inspirals (EMRIs) around a slowly rotating Kerr black hole. The secondary compact object, carrying a Proca hair, emits additional dipolar radiation that alters total energy flux relative to general relativity (GR). These modifications induce a secular drift in the orbital evolution of circular geodesic orbits, leading to measurable dephasing in the resulting EMRIs waveforms. By evaluating waveform mismatches between the Einstein-Proca framework and its GR counterpart, we show that the Laser Interferometer Space Antenna (LISA) can distinguish the signatures of a light Proca field when black hole rotation is included. Furthermore, using a Fisher information matrix analysis, we forecast LISA's capability to place stringent constraints on the Proca mass with EMRIs signal from slowly rotating Kerr black holes. For representative EMRIs configurations, we find that LISA can detect or constrain Proca masses down to $\mu_v\sim 10^{-20}$eV, with typical fractional uncertainties at the level of tens percent, depending on the black-hole spin.

gr-qc↗

arXiv2025

Eccentric extreme-mass-ratio inspirals: a new window into ultra-light vector fields

Space-based gravitational-wave detectors, such as the Laser Interferometer Space Antenna (LISA), provide a platform to probe new fundamental fields through extreme-mass-ratio inspirals (EMRIs), where a compact secondary object carrying scalar or vector charges inspirals into a massive primary. In a theory-agnostic framework, we compute the ultra-light vector and gravitational radiation emitted by eccentric EMRIs and determine the corresponding inspiral trajectories. We evaluate the impact of a massive vector (Proca) field on EMRIs waveform through dephasing and mismatches with predictions by general relativity. Using a Fisher information matrix analysis, we further assess LISA's capability to constrain the Proca mass from future EMRIs observations. We find that orbital eccentricity can improve estimation accuracy of parameters, making the vector mass $\mu$ become detectable for the case of $\mu=0.02$ . Correlation analysis further reveals strong positive dependencies between the Proca mass and intrinsic source parameters, indicating that improved measurement of these parameters directly tightens constraints on vector mass. These results demonstrate that high-eccentricity EMRIs observed by LISA offer a powerful channel to detect or constrain massive vector-field extensions of GR in the strong-field regime.

gr-qc↗

arXiv2025

Wavelets for power spectral density estimation of gravitational wave data

Power spectral density (PSD) estimation is a critical step in gravitational wave (GW) detectors data analysis. The Welch method is a typical non-parametric spectral estimation approach that estimates the PSD of stationary noise by averaging periodograms of several time segments, or by taking the median of periodograms to adapt to non-stationary noise. In this work, we propose a wavelet-based approach for fast PSD estimation of both stationary and non-stationary noise. For stationary noise, we apply wavelet smoothing to the periodogram, avoiding the segmentation step in the Welch method, and enabling PSD estimates with high frequency resolution and low variance. The wavelet smoothing PSD outperforms Welch PSD in matched filtering and parameter estimation. For non-stationary noise, we estimate the PSD by taking the median of wavelet packet coefficients in each frequency bin, which offers greater robustness than the traditional median periodogram method. This work introduces a new PSD estimation approach for GW data analysis and expands the application of wavelet methods in this field.

gr-qc↗

arXiv2025

Constraints on Lorentz Invariance Violation from GRB 221009A Using the DisCan Method

Lorentz symmetry is a cornerstone of modern physics, and testing its validity remains a critical endeavor. In this work, we analyze the photon time-of-flight and time-shift data from LHAASO observations of Gamma-Ray Burst GRB 221009A to search for signatures of Lorentz violation. We employed the DisCan (dispersion cancellation) method with various information entropies as cost functions, designating the results obtained with Shannon entropy as our representative outcome. This choice is attributed to the parameter-free statistical properties of Shannon entropy, which has demonstrated remarkable stability as we continually refine and enhance our methodology. In the absence of more detailed data and physical context, it provides more stable and reliable results. We constrain the energy scale associated with Lorentz invariance violation. Our results yield 95\% confidence level lower limits of $E_{\text{QG},1} > 5.4 \times 10^{19} \, \text{GeV}$ (subluminal) and $E_{\text{QG},1} > 2.7 \times 10^{19} \, \text{GeV}$ (superluminal) for the linear case ($n$=1), and $E_{\text{QG},2} > 10.0 \times 10^{12} \, \text{GeV}$ (subluminal) and $E_{\text{QG},2} > 2.4 \times 10^{12} \, \text{GeV}$ (superluminal) for the quadratic case ($n$=2). Subsequently, we incorporated WCDA photons and the Knuth binning method to further optimize and complement our approach, while also performing filter using information entropies. Furthermore, we demonstrate that employing different information entropy measures as cost functions does not alter the order of magnitude of these constraints.

astro-ph.HE↗

arXiv2025

Primordial black holes with anisotropic hair

A mechanism for generating anisotropic enhancements of the curvature perturbation through a vector field is proposed. We find that when the mixing between the inflaton perturbation and the vector-field perturbation is sufficiently strong in the anisotropic inflation, the power spectrum becomes dominated by anisotropic constant modes. This suggests that statistical anisotropy in primordial black hole (PBH) formation may be inevitable if inflation undergoes an anisotropic inflationary phase. Our findings offer a novel approach to probe vector fields during inflation and to test the cosmic no-hair conjecture.

astro-ph.CO↗

arXiv2025

Entanglement induced by quantum gravity in an infinite square well

In this work, we examine the entanglement dynamics of two massive particles confined within their respective infinite square potential wells induced by gravity. Assuming that each particle is initially in the ground state of its infinite square well, we investigate the formation of entanglement between the particles as a result of their gravitational interaction during their adiabatic movement towards each other. Our results reveal that, under suitable parameters, entanglement swiftly emerges. Compared to previous schemes, our approach significantly reduces the particle mass, greatly lowering the experimental threshold.

quant-ph↗

arXiv2024

Entanglement island and Page curve for one-sided charged black hole

In this paper, we extend the method of calculating the entanglement entropy of Hawking radiation of black holes using the "in" vacuum state, which describes one-sided asymptotically flat neutral black hole formed by gravitational collapse, to dynamic charged black holes. We explore the influence of charge on the position of the boundary of island $\partial I$ and the Page time. Due to their distinct geometric structures, we discuss non-extremal and extremal charged black holes separately. In non-extremal cases, the emergence of island saves the bound of entropy at late times, and the entanglement entropy of Hawking radiation satisfies the Page curve. Moreover, we also find that the position of the boundary of island $\partial I$ depends on the position of the cutoff surface (observers), differing from the behavior in eternal charged black holes. In extremal black holes, when the island exists, the entanglement entropy is approximately equal to the Bekenstein-Hawking entropy, while the entanglement entropy becomes ill-defined when island is absent. Our analysis underscores how different geometric configurations significantly influence the behavior of entropy.

gr-qc↗

arXiv2024

Quantum Gravity Induced Entanglement of Masses With Extra Dimensions

It is believed that gravity can be considered as a quantum coherent mediator. In this study, we propose a plan to test the existence of extra dimensions using the Quantum Gravity Induced Entanglement of Masses (QGEM) experiment. This experiment involves two freely falling test masses passing through a Stern-Gerlach-like device. We investigate the entanglement witness between these masses within the framework of the Randall-Sundrum II model (RS-II). Our findings indicate that the system reaches entanglement more rapidly in the presence of extra dimensions, particularly when the radius of the extra dimension is large.

gr-qc↗

arXiv2023

Gravity-induced entanglement between two massive microscopic particles in curved spacetime: II.Friedmann- Lema\^itre-Robertson-Walker universe

In our previous work, we have explored quantum gravity induced entanglement of mass (QGEM) in curved spacetime, observing entanglement formation between particles moving along geodesics in a Schwarzschild spacetime background. We find that long interaction time induces entanglement, even for particles with microscopic mass, addressing decoherence concerns. In this work, we build upon our previous work by extending our investigation to a time-dependent spacetime. Specifically, we explore the entanglement induced by the mutual gravitation of massive particles in the Friedmann-Lema\^itre-Robertson-Walker(FLRW) universe. Through calculations of the phase variation and the QGEM spectrum, our proposed scheme offers a potential method for observing the formation of entanglement caused by the quantum gravity of massive particles as they propagate in the FLRW universe. Consequently, our research provides fresh insights into the field of entanglement in cosmology.

gr-qc↗

arXiv2023

Gravity-induced entanglement between two massive microscopic particles in curved spacetime: I.The Schwarzschild background

The experiment involving the entanglement of two massive particles through gravitational fields has been devised to discern the quantum attributes of gravity. In this paper, we present a scheme to extend this experiment's applicability to more generalized curved spacetimes, with the objective of validating universal quantum gravity within broader contexts. Specifically, we direct our attention towards the quantum gravity induced entanglement of mass (QGEM) in astrophysical phenomena, such as particles traversing the interstellar medium. Notably, we ascertain that the gravitational field within curved spacetime can induce observable entanglement between particle pairs in both scenarios, even when dealing with particles significantly smaller than mesoscopic masses. Furthermore, we obtain the characteristic spectra of QGEM across diverse scenarios, shedding light on potential future experimental examinations. This approach not only establishes a more pronounced and extensive manifestation of the quantum influences of gravity compared to the original scheme but also opens avenues for prospective astronomical experiments. These experiments, aligned with our postulates, hold immense advantages and implications for the detection of quantum gravity and can be envisioned for future design.

gr-qc↗

arXiv2023

Primordial black holes from an inflationary potential valley

Primordial black holes (PBHs) could be formed if large perturbations are generated on small scales in inflation. We study a toy inflation model with a local minimum. The curvature perturbations are enhanced when the inflaton passes through the local minimum, with more efficient amplification rate than that of quasi-inflection point inflation, leading to the production of PBHs on small scales. The PBHs could comprise the total dark matter in the mass window $10^{16}$--$10^{20}$g.

astro-ph.CO↗

arXiv2023

Inflation with shallow dip and primordial black holes

Primordial black holes may arise through ultra slow-roll inflation. In this work we study a toy model of ultra slow-roll inflation with a shallow dip. The ultra slow-roll stage enhances the curvature perturbations and thus the primordial scalar power spectrum. We analyze the features of the power spectrum numerically and analytically, and then give a rough estimate of the lower and upper bound of the enhancement. These large perturbations also produce second order gravitational waves, which are in the scope of future observations.

astro-ph.CO↗

arXiv2023

A Note on Entanglement Entropy for Primary Fermion Fields in JT Gravity

In this paper we analyse and discuss 2D Jackiw-Teitelboim (JT) gravity coupled to primary fermion fields in asymptotically anti-de Sitter (AdS) spacetimes. We get a particular solution of the massless Dirac field outside the extremal black hole horizon and find the solution for the dilaton in JT gravity. Two dimensional JT gravity spacetime is conformally flat, we calculate the two point correlators of primary fermion fields under the Weyl transformations. The key point of this work is to present a standard technique which is called resolvent rather than CFT methods. We redefine the fields in terms of the conformal factor as the fermion fields, and we use the resolvent technique to derive the renormalized entanglement entropy for massless Dirac fields in JT gravity.

hep-th↗