arXiv subjects

Kai Lin

Publications and source records attributed to Kai Lin.

At least 55 records · Page 3Linked to original sources

arXiv2023

MPAI-EEV: Standardization Efforts of Artificial Intelligence based End-to-End Video Coding

The rapid advancement of artificial intelligence (AI) technology has led to the prioritization of standardizing the processing, coding, and transmission of video using neural networks. To address this priority area, the Moving Picture, Audio, and Data Coding by Artificial Intelligence (MPAI) group is developing a suite of standards called MPAI-EEV for "end-to-end optimized neural video coding." The aim of this AI-based video standard project is to compress the number of bits required to represent high-fidelity video data by utilizing data-trained neural coding technologies. This approach is not constrained by how data coding has traditionally been applied in the context of a hybrid framework. This paper presents an overview of recent and ongoing standardization efforts in this area and highlights the key technologies and design philosophy of EEV. It also provides a comparison and report on some primary efforts such as the coding efficiency of the reference model. Additionally, it discusses emerging activities such as learned Unmanned-Aerial-Vehicles (UAVs) video coding which are currently planned, under development, or in the exploration phase. With a focus on UAV video signals, this paper addresses the current status of these preliminary efforts. It also indicates development timelines, summarizes the main technical details, and provides pointers to further points of reference. The exploration experiment shows that the EEV model performs better than the state-of-the-art video coding standard H.266/VVC in terms of perceptual evaluation metric.

cs.MM↗

arXiv2023

Echoes of axial gravitational perturbations in stars of uniform density

This work investigates the echoes in axial gravitational perturbations in compact objects. To this end, we propose an alternative scheme of the finite difference method implemented in two coordinate systems, where the initial conditions are placed on the axis of the tortoise coordinate with appropriate boundary conditions that fully respect the causality. The scheme is then employed to study the temporal profiles of the quasinormal oscillations in the Schwarzschild black hole and the uniform-density stars. When presented in a two-dimensional evolution profile, the resulting ringdown waveforms in the black hole metric are split into the reflected and transmitted waves as the initial perturbations evolve and collide with the peak of the effective potential. On the other hand, for compact stars, quasinormal oscillations might be characterized by echoes. Consistent with the causality arguments, the phenomenon is produced by the gravitational waves bouncing between the divergent potential at the star's center and the peak of the effective potential. The implications of the present study are also discussed.

gr-qc↗

arXiv2023

The Axial Gravitational Ringing of a Spherically Symmetric Black Hole Surrounded by Dark Matter Spike

Supermassive black holes at the center of each galaxy may be surrounded by dark matter. Such dark matter admits a spike structure and vanishes at a certain distance from the black hole. This dark matter will impact the spacetime near the black hole and the related ringing gravitational waves can show distinguished features of the black hole without dark matter. In the present work, we focus on the quasi-normal modes of the axial gravitational perturbation who dominate the ringdown process of the perturbed black holes surrounded by dark matter spikes. The relativistic modification results in less impact on QNMs. And the relative ringing frequency difference between the black holes with and without dark matter can be as large as $10^{-2}$. These features can be used in future gravitational wave detection about extremal mass ratio inspiral systems to probe the existence of dark matter around supermassive black holes.

gr-qc↗

arXiv2023

Quasinormal modes and echo effect of cylindrical anti-de Sitter black hole spacetime with a thin shell

This paper investigates the quasinormal mode (QNM) vibrations of a rotating cylindrical black hole (or black string) spacetime that is surrounded by a thin shell rotating synchronously with the black string's axis. The existence of the thin shell leads to a piecewise metric of the black hole spacetime beyond the horizon, which is divided into two stationary spacetime parts by the radius of the thin shell. As a result, the potential function $V(r)$ of the QNM equation is also discontinuous. To solve the QNM equation with the discontinuous potential function, we propose two methods, matrix method and generalized Horowitz-Hubeny Method. We find that the influence of the thin shell can reduce the QNM frequency of the black string while alleviating their amplitude decay rate. Our suggested method can be easily applied to other QNM calculations of black hole spacetime with discontinuous potential function, thus facilitating investigations into more intricate and realistic black hole spacetimes, such as those with accretion disks. Additionally, the finite difference method is employed to investigate the spacetime too. This analysis discloses a substantial gap in the potential function when the thin shell's mass and charge achieve sufficiently high values, resulting in the outer spacetime nearing gravitational collapse and extreme black hole scenarios. Within this gap, the QNM wave displays oscillations, producing an echo effect. Moreover, it is established that the closeness of the spacetime to the collapse threshold and charge extremality have positive correlation with the beat interval of this echo.

gr-qc↗

arXiv2023

On statistical fluctuations in collective flows

In relativistic heavy-ion collisions, event-by-event fluctuations are known to have non-trivial implications. Even though the probability distribution is geometrically isotropic for the initial conditions, the anisotropic $\varepsilon_n$ still differs from zero owing to the statistical fluctuations in the energy profile. On the other hand, the flow harmonics extracted from the hadron spectrum using the multi-particle correlators are inevitably subjected to non-vanishing variance due to the finite number of hadrons emitted in individual events. As one aims to extract information on the fluctuations in the initial conditions via flow harmonics and their fluctuations, finite multiplicity may play a role in interfering with such an effort. In this study, we explore the properties and impacts of such fluctuations in the initial and final states, which both notably appear to be statistical ones originating from the finite number of quanta of the underlying system. We elaborate on the properties of the initial-state eccentricities for the smooth and event-by-event fluctuating initial conditions and their distinct impacts on the resulting flow harmonics. Numerical simulations are performed. The possible implications of the present study are also addressed.

nucl-th↗

arXiv2023

High-order matrix method with delimited expansion domain

Motivated by the substantial instability of the fundamental and high-overtone quasinormal modes, recent developments regarding the notion of black hole pseudospectrum call for numerical results with unprecedented precision. This work generalizes and improves the matrix method for black hole quasinormal modes to higher orders, specifically aiming at a class of perturbations to the metric featured by discontinuity intimately associated with the quasinormal mode structural instability. The approach is based on the mock-Chebyshev grid, which guarantees its convergence in the degree of the interpolant. In practice, solving for black hole quasinormal modes is a formidable task. The presence of discontinuity poses a further difficulty so that many well-known approaches cannot be employed straightforwardly. Compared with other viable methods, the modified matrix method is competent in speed and accuracy. Therefore, the method serves as a helpful gadget for relevant studies.

gr-qc↗

arXiv2023

Echoes of Stars in tracking Rastall gravity

We construct a star with uniform density in frame of tracking Rastall gravity, in which the density of effective dark matter tracks the density of dark energy. To demonstrate stability of the star we analyse the axial gravitational perturbations of the star. We find that echo wavelets naturally appear in the axial gravitational waves when the dark matter plays a significant role in construction of the star. We show that the physical origin of this echo roots in the potential valley near the surface of star. This effect leads to a clue to detect Rastall dark energy from the ringdown phase of gravitational wave from binary stars enriched by dark matter.

gr-qc↗

arXiv2022

Dynamic generation or removal of a scalar hair

We study dynamic processes through which the scalar hair of black holes is generated or detached in a theory with a scalar field non-minimally coupled to Gauss-Bonnet and Ricci scalar invariants. We concentrate on the nonlinear temporal evolution of a far-from-equilibrium gravitational system. In our simulations, we choose the initial spacetime to be either a bald Schwarzschild or a scalarized spherically symmetric black hole. Succeeding continuous accretion of the scalar field onto the original black hole, the final fate of the system displays intriguing features, which depend on the initial configurations, strengths of the perturbation, and specific metric parameters. In addition to the scalarization process through which the bald black hole addresses scalar hair, we observe the dynamical descalarization, which removes scalar hair from an original hairy hole after continuous scalar field accretion. We examine the temporal evolution of the scalar field, the metrics, and the Misner-Sharp mass of the spacetime and exhibit rich phase structures through nonlinear dynamical processes.

gr-qc↗

arXiv2022

On the late-time tails of massive perturbations in spherically symmetric black holes

It was first pointed out by Koyama and Tomimatsu that, under reasonable assumptions, the asymptotic late-time tails of massive scalar perturbations in the far zone of spherically symmetric black hole spacetimes decays universally as $t^{-5/6}$. The late-time tail is furnished by the contribution from the branch cut of the frequency-domain Green's function, which is constructed in terms of two appropriate solutions of the corresponding homogeneous equation. The present study is focused on some particular forms of the in-going wave that were not explicitly considered in the original derivations but nonetheless have been taken into account in the literature by other authors. In this regard, we reassess the authors' arguments and provide a detailed complimentary analysis that covers a few specific aspects. For some particular cases, the tail is found to possess the form $t^{-1}$. We also discuss the possible implications of the present findings.

gr-qc↗

arXiv2022

Matrix method for perturbed black hole metric with discontinuity

Recent studies based on the notion of black hole pseudospectrum indicated substantial instability of the fundamental and high-overtone quasinormal modes. Besides its theoretical novelty, the details about the migration of the quasinormal mode spectrum due to specific perturbations may furnish valuable information on the properties of associated gravitational waves in a more realistic context. This work generalizes the matrix method for black hole quasinormal modes to cope with a specific class of perturbations to the metric featured by discontinuity, which is known to be intimately connected with the quasinormal mode structural instability. In practice, the presence of discontinuity poses a difficulty so that many well-known approaches for quasinormal modes cannot be straightforwardly applied. By comparing with other methods, we show that the modified matrix method is efficient, which can be used to solve for the low-lying modes with reasonable precision. Therefore, it might serve as an alternative gadget for relevant studies.

gr-qc↗

arXiv2022

Qusinormal oscillations and late-time tail of massless scalar perturbations of a magnetized black hole in Rastall gravity

In this paper, we study the quasinormal mode and late-time tail of charged massless scalar perturbations of a black hole in the generalized Rastall gravity. The black hole metric in question is spherically symmetric, accompanied by a power-Maxwell field surrounded by a quintessence fluid. It is shown that the massless scalar field, when {\it dressed up} with the magnetic field, acquires an effective mass which, in turn, significantly affects the properties of the resultant quasinormal oscillations and late-time tails. To be specific, the quasinormal frequencies become distorted and might even be unstable for particular spacetime configurations. Also, the exponent of the usual power-law tail is modified in accordance with the modification in the structure of the branch cut of the retarded Green's function. In particular, as the effective mass is generated dynamically due to the presence of the magnetic field, one may consider a process through which the field is gradually removed from the spacetime configuration. In this context, while the quasinormal oscillations converge to the case of massless perturbations, we argue that the properties of resultant late-time tails do not fall back to their massless counterpart. The relevant features are investigated by numerical and analytic approaches.

gr-qc↗

arXiv2022

Ellis drainhole solution in Einstein-Æther gravity and the axial gravitational quasinormal modes

In this work, the Ellis drainhole solution is derived in Einstein-Æther gravity, and subsequently, the axial quasinormal modes of the resulting drainhole are investigated. Owing to the presence of a minimally coupled scalar field with antiorthodox coupling polarity, the resultant metric solution is featured by a throat instead of a horizon, for which static æther solution becomes feasible. Moreover, the derived master equations for the axial gravitational perturbations consist of two coupled vector degrees of freedom. By utilizing the finite difference method, the temporal profiles of the quasinormal oscillations are evaluated, and, subsequently, the complex frequencies are extracted and compared against the specific values obtained by the WKB method when the coupling is turned off. Besides, the effect of the coupling on the low-lying quasinormal spectrum is explored, and its possible physical relevance is discussed.

gr-qc↗

arXiv2022

VRConvMF: Visual Recurrent Convolutional Matrix Factorization for Movie Recommendation

Sparsity of user-to-item rating data becomes one of challenging issues in the recommender systems, which severely deteriorates the recommendation performance. Fortunately, context-aware recommender systems can alleviate the sparsity problem by making use of some auxiliary information, such as the information of both the users and items. In particular, the visual information of items, such as the movie poster, can be considered as the supplement for item description documents, which helps to obtain more item features. In this paper, we focus on movie recommender system and propose a probabilistic matrix factorization based recommendation scheme called visual recurrent convolutional matrix factorization (VRConvMF), which utilizes the textual and multi-level visual features extracted from the descriptive texts and posters respectively. We implement the proposed VRConvMF and conduct extensive experiments on three commonly used real world datasets to validate its effectiveness. The experimental results illustrate that the proposed VRConvMF outperforms the existing schemes.

cs.IR↗

arXiv2022

Quasinormal modes in two-photon autocorrelation and the geometric-optics approximation

In this work, we study the black hole light echoes in terms of the two-photon autocorrelation and explore their connection with the quasinormal modes. It is shown that the above time-domain phenomenon can be analyzed by utilizing the well-known frequency-domain relations between the quasinormal modes and characteristic parameters of null geodesics. We found that the time-domain correlator, obtained by the inverse Fourier transform, naturally acquires the echo feature, which can be attributed to a collective effect of the asymptotic poles through a weighted summation of the squared modulus of the relevant Green's functions. Specifically, the contour integral leads to a summation taking over both the overtone index and angular momentum. Moreover, the dominant contributions to the light echoes are from those in the eikonal limit, consistent with the existing findings using the geometric-optics arguments. For the Schwarzschild black holes, we demonstrate the results numerically by considering a transient spherical light source. Also, for the Kerr spacetimes, we point out a potential difference between the resulting light echoes using the geometric-optics approach and those obtained by the black hole perturbation theory. Possible astrophysical implications of the present study are addressed.

gr-qc↗

arXiv2021

Complementary Fourier single-pixel imaging

Single-pixel imaging, with the advantages of a wide spectrum, beyond-visual-field imaging, and robustness to light scattering, has attracted increasing attention in recent years. Fourier single-pixel imaging (FSI) can reconstruct sharp images under sub-Nyquist sampling. However, the conventional FSI has difficulty with balancing the imaging quality and efficiency. To overcome this issue, we proposed a novel approach called complementary Fourier single-pixel imaging (CFSI) to reduce measurements while retaining its robustness. The complementary nature of Fourier patterns based on a four-step phase-shift algorithm is combined with the complementary nature of a digital micromirror device. CFSI only requires two phase-shifted patterns to obtain one Fourier spectral value. Four light intensity values are obtained by load the two patterns, and the spectral value is calculated through differential measurement, which has good robustness to noise. The proposed method is verified by simulations and experiments compared with FSI based on two-, three-, and four-step phase shift algorithms. CFSI performed better than the other methods under the condition that the best imaging quality of CFSI is not reached. The reported technique provides an alternative approach to realize real-time and high-quality imaging.

eess.IV↗

arXiv2021

Hawking radiation received at infinity in higher dimensional Reissner-Nordström black hole spacetimes

In this work, we investigate the Hawking radiation in higher dimensional Reissner-Nordström black holes as received by an observer, resides at infinity. The frequency-dependent transmission rates, which deform the thermal radiation emitted in the vicinity of the black hole horizon, are evaluated numerically. Apart from the case of four-dimensional spacetime, the calculations are extended to higher dimensional Reissner-Nordström metrics, and the results are found to be somewhat sensitive to the spacetime dimension. In general, it is observed that the transmission coefficients practically vanishes when the frequency of the emitted particle approaches zero. It increases with increasing frequency and eventually saturates to some value. For four-dimensional spacetime, the above result is shown to be mostly independent of the metric's parameter, neither of the orbital quantum number of the particle, once the location of the event horizon, $r_h$, and the product of the charges of the black hole and the particle $qQ$ are given. For higher-dimensional cases, on the other hand, the convergence becomes more slowly. Moreover, the difference between states with different orbital quantum numbers is found to be more significant. As the magnitude of the product of charges $qQ$ becomes more significant, the transmission coefficient exceeds one. In other words, the resultant spectral flux is amplified, which results in an accelerated process of black hole evaporation. The relation between the calculated outgoing transmission coefficient with existing results on the greybody factor is discussed.

gr-qc↗

arXiv2021

Quasinormal Modes for Dynamical Black Holes

Realistic black holes are usually dynamical, noticeable or sluggish. The Vaidya metric is a significant and tractable model for simulating a dynamical black hole. In this study, we consider scalar perturbations in a dynamical Vaidya black hole, and explore the quasinormal modes by employing the matrix method. We find the proper boundary conditions of the quasinormal modes from physical analysis in the background of a dynamical black hole for the first time. The results show that the eigenfrequencies become different at the apparent horizon and null infinity, because the physical interactions propagate with finite velocity in nature. Any variation of the hole does not affect the boundary condition at null infinity in a finite time. The quasinormal modes originated around the horizon would not immediately come down to, but slowly goes to the final state following the mass accretion process of the hole. The precision of the matrix method is quite compelling, which reveals more details of the eigenfrequencies of the quasinormal mode of perturbations in the Vaidya spacetime.

gr-qc↗

arXiv2020

A Control Strategy for Capacity Allocation of Hybrid Energy Storage System Based on Hierarchical Processing of Demand Power

Pursuing optimal power distribution in hybrid energy storage systems has always been the goal of researchers. Here, HESS is a combination of lithium battery and supercapacitor; this combination has been proven to effectively compensate for some of the deficiencies of lithium batteries as an energy system for electric vehicles. For example, the energy storage system with only lithium batteries cannot provide high power in a short time to meet the high acceleration performance of electric vehicles, and the excessive discharge current will cause the temperature of the battery pack to be too high, which will cause safety problems for the car. This paper proposes an intelligent energy management strategy combining fuzzy controller and improved Savitzky-Golay filter for real-time control. The simulation results show that compared with single ESS, the maximum current of the battery proposed by the strategy is reduced by 14.60%, and the usable cycle life of the battery is increased by 57.31% during the test driving cycle. Meanwhile, it explores various changes brought supercapacitor monomers in the same HESS, and predict the next supercapacitor will bring about 31.58% reduction of volume and mass.

eess.SY↗