Showing 1–44 of 44 results for author: Li, D

Modified Teukolsky formalism: Null testing and numerical benchmarking

Authors: Fawzi Aly, Mahmoud A. Mansour, Luis Lehner, Dejan Stojkovic, Dongjun Li, Pratik Wagle

Abstract: Next-generation gravitational-wave detectors will make black-hole ringdown an increasingly sensitive probe of small departures from General Relativity in the strong-field regime. This motivates obtaining high-precision predictions of gravitational effective field theory, as spectral shifts can be quite small. Here we perform a focused stress test of the modified-Teukolsky framework by designing tw… ▽ More Next-generation gravitational-wave detectors will make black-hole ringdown an increasingly sensitive probe of small departures from General Relativity in the strong-field regime. This motivates obtaining high-precision predictions of gravitational effective field theory, as spectral shifts can be quite small. Here we perform a focused stress test of the modified-Teukolsky framework by designing two null diagnostics. First, we consider an action with redundant operators that must produce zero first-order vacuum QNM shifts. Second, we exploit a Ricci-flat identity relating two physical cubic Riemann to test such a relation is satisfied by the ringdown spectra obtained. We compute the shifts using two independent numerical approaches: the eigenvalue-perturbation and generalized continued-fraction (Leaver-type) methods. Both null tests are passed across multiple multipoles and overtones, and the control-operator results agree in magnitude with the benchmark values reported in Ref. [1]. These validations support using the framework for obtaining accurate predictions for robust strong-field tests, with straightforward extensions to rotating backgrounds and coupling with matter fields. △ Less

Submitted 10 March, 2026; v1 submitted 2 March, 2026; originally announced March 2026.

Comments: 24 pages,6 figures

Evolving extreme mass-ratio inspirals in a perturbed Schwarzschild spacetime

Authors: Michael LaHaye, Colin Weller, Dongjun Li, Patrick Bourg, Yanbei Chen, Huan Yang

Abstract: In this work, we develop the modified Teukolsky formalism that describes the GW radiation from a point mass orbiting around a perturbed Schwarzschild BH. This perturbation of the background spacetime induces a secular change in the orbital phase of the point mass. In turn, this causes a modification in the GW flux, which can be used to probe the background spacetime. We explicitly apply this forma… ▽ More In this work, we develop the modified Teukolsky formalism that describes the GW radiation from a point mass orbiting around a perturbed Schwarzschild BH. This perturbation of the background spacetime induces a secular change in the orbital phase of the point mass. In turn, this causes a modification in the GW flux, which can be used to probe the background spacetime. We explicitly apply this formalism to a bumpy Schwarzschild spacetime as a proof of principle. The results pave the way for the description of EMRIs in generic perturbed Kerr spacetime in future developments. △ Less

Submitted 17 October, 2025; originally announced October 2025.

Comments: 31 pages, 1 figure

Signals from Fermionic inflationary cosmology with Yukawa interaction

Authors: Lin-Hong Sui, Dan Li, Jia-Ze Sun, Xi-Bin Li

Abstract: We investigate an inflationary model wherein the Dirac field $ψ$ is directly coupled to a scalar inflaton $φ$ via a Yukawa interaction $gφ\barψψ$ and examine the resulting observational implications. Within the slow-roll approximation, we derive analytical solutions of the Dirac equations during inflation. The analytical result on the fermion pair density $\langle n\rangle$ indicates that the Yuka… ▽ More We investigate an inflationary model wherein the Dirac field $ψ$ is directly coupled to a scalar inflaton $φ$ via a Yukawa interaction $gφ\barψψ$ and examine the resulting observational implications. Within the slow-roll approximation, we derive analytical solutions of the Dirac equations during inflation. The analytical result on the fermion pair density $\langle n\rangle$ indicates that the Yukawa interaction strength $g$ is to characterize the degree of non-adiabaticity. For large value of the dimensionless effective mass $\tilde m=(m+gφ)/H$, i.e. $\tilde m\gtrsim 1$, the tensor-to-scalar ratio $r$ is suppressed by a factor of approximately $1/(1+2.95π^2g^2)$. This condition is also characterized by a significant backreaction. Conversely, if $\tilde m \ll 1$, the value of $r$ remains consistent with that observed in standard cold inflation. Our analysis is performed under the assumption of the highest inflationary energy scales compatible with current observational constraints. △ Less

Submitted 17 October, 2025; originally announced October 2025.

OCTOPUS: A Versatile, User-Friendly, and Extensible Public Code for General-Relativistic Ray-Tracing in Spherically Symmetric and Static Spacetimes

Authors: Shiyang Hu, Shijie Tan, Dan Li, Lina Zhang, Chen Deng, Wenfu Cao

Abstract: This paper presents OCTOPUS, a relativistic ray-tracing algorithm developed within a Fortran-based, OpenMP-accelerated framework, designed for asymptotically flat, spherically symmetric curved spacetimes. The code efficiently and accurately computes key relativistic features -- including the black hole event horizon, photon rings, critical curves, and innermost stable circular orbits -- and simula… ▽ More This paper presents OCTOPUS, a relativistic ray-tracing algorithm developed within a Fortran-based, OpenMP-accelerated framework, designed for asymptotically flat, spherically symmetric curved spacetimes. The code efficiently and accurately computes key relativistic features -- including the black hole event horizon, photon rings, critical curves, and innermost stable circular orbits -- and simulates black hole shadows, redshift factor distributions, accretion disk images, toroidal images, as well as gravitational lensing, light curves, and gravitational radiation from hot-spots. OCTOPUS provides an automated, modular solution for qualitative studies of black hole observables and multi-messenger correlations between electromagnetic and gravitational signals in curved spacetime. Its implementation requires only the metric potential and its first-, second-, and third-order radial derivatives as input, ensuring low user barriers while remaining highly extensible and adaptable. Using a Schwarzschild black hole surrounded by a Dehnen-type dark matter halo, we thoroughly validate the algorithm's precision, efficiency, and functionality, and investigate how dark matter halo parameters affect observational signatures. Our results demonstrate that increasing the scale and density of the dark matter halo strengthens the spacetime's gravitational field, an effect clearly reflected in black hole images and supported by hot-spot light curve signatures. A future version of OCTOPUS, with expanded capabilities for axisymmetric spacetimes, is planned for release. △ Less

Submitted 14 October, 2025; originally announced October 2025.

Comments: 49 pages, 24 figures, comments are welcome

Quasibound states of massless spin particles in Schwarzschild equivalent mediums

Authors: Li-Qin Mi, Dandan Li, Zhong-Heng Li

Abstract: We show that, in Schwarzschild equivalent mediums, the massless spin particles obey the same dynamical equation, from which we obtain remarkably simple formulae for the frequencies of the quasibound states. We find that the quasibound frequencies of different bosons can be identical at the same quantum number $l$, and the same is true of different fermions, but a quasibound frequency for bosons ca… ▽ More We show that, in Schwarzschild equivalent mediums, the massless spin particles obey the same dynamical equation, from which we obtain remarkably simple formulae for the frequencies of the quasibound states. We find that the quasibound frequencies of different bosons can be identical at the same quantum number $l$, and the same is true of different fermions, but a quasibound frequency for bosons can never equal a quasibound frequency for fermions. These results mean that, in Schwarzschild equivalent mediums with the quasibound-state boundary conditions, characteristics of electromagnetic waves are the same as those for all the massless bosonic waves, thereby allowing electromagnetic waves to simulate gravitational waves. Our predictions can be tested in future experiments, building upon the successful preparation of Schwarzschild equivalent mediums. △ Less

Submitted 22 September, 2025; originally announced September 2025.

Comments: 22 pages, 2 figures

Journal ref: Commun. Theor. Phys. 78 (2026) 025402

Light Curves of Chaotic Charged Hot-Spots in Curved Spacetime: Opening an Observational Window to Chaos

Authors: Shiyang Hu, Dan Li, Chen Deng

Abstract: The observed scarcity of chaotic phenomena in astronomy contrasts sharply with their theoretical significance, primarily due to the absence of a robust framework for detecting chaos. In this study, we numerically simulate the light curves of hot-spots in Kerr spacetime under the influence of an external asymptotically uniform electromagnetic field. Our results reveal a clear distinction between th… ▽ More The observed scarcity of chaotic phenomena in astronomy contrasts sharply with their theoretical significance, primarily due to the absence of a robust framework for detecting chaos. In this study, we numerically simulate the light curves of hot-spots in Kerr spacetime under the influence of an external asymptotically uniform electromagnetic field. Our results reveal a clear distinction between the light curves of chaotic and regular hot-spots, particularly in their power spectra: the latter display isolated, sharp peaks, while the former exhibit broad, continuous peaks of low amplitude. These findings highlight the potential of using light curves as a probe for chaotic orbits in curved spacetime. △ Less

Submitted 24 August, 2025; originally announced August 2025.

Comments: 6 pages, 3 figures

Influence of the external electromagnetic field on the properties of the Novikov-Thorne accretion disk in Kerr spacetime

Authors: Shiyang Hu, Yuxiang Zuo, Dan Li, Chen Deng

Abstract: The Novikov-Thorne accretion disk model is widely employed in astrophysics, yet computing its blackbody spectrum theoretically requires analytical expressions for the orbital parameters -- specific energy, angular momentum, and angular velocity -- of the constituent timelike particles, a task extremely challenging in non-integrable curved spacetimes. In this work, we numerically obtain these orbit… ▽ More The Novikov-Thorne accretion disk model is widely employed in astrophysics, yet computing its blackbody spectrum theoretically requires analytical expressions for the orbital parameters -- specific energy, angular momentum, and angular velocity -- of the constituent timelike particles, a task extremely challenging in non-integrable curved spacetimes. In this work, we numerically obtain these orbital parameters for quasi-Keplerian motion in Kerr spacetime with an asymptotically uniform magnetic field using iterative, finite-difference, and interpolation methods, enabling simulations of the disk's energy flux density, temperature, and blackbody spectra across diverse spin parameters, observational inclinations, and magnetic field strengths. We demonstrate that when the magnetic field aligns with the black hole's angular momentum, the disk's radiation positively correlates with field strength, while spectral analysis for our specific black hole mass and accretion rate reveals a conservative detectable threshold of $1.0638 \times 10^{-9}$ T for ambient magnetic fields. This study not only extends the Novikov-Thorne model to non-integrable axisymmetric spacetimes but also establishes the first direct relationship between external magnetic fields and disk properties, providing critical theoretical support for future magnetic environment studies through disk radiation observations. △ Less

Submitted 19 July, 2025; originally announced July 2025.

Comments: 21 pages, 4 figures

Extreme mass-ratio inspiral within an ultralight scalar cloud I. Scalar radiation

Authors: Dongjun Li, Colin Weller, Patrick Bourg, Michael LaHaye, Nicolás Yunes, Huan Yang

Abstract: In this work, we study the dynamics of an extreme mass-ratio inspiral (EMRI) embedded within a scalar cloud populated around the massive black hole. This cloud may be generated through the black hole superradiant process if the wavelength of the scalar particle is comparable to the size of the massive black hole. The EMRI motion perturbs the cloud, producing scalar radiation towards infinity and i… ▽ More In this work, we study the dynamics of an extreme mass-ratio inspiral (EMRI) embedded within a scalar cloud populated around the massive black hole. This cloud may be generated through the black hole superradiant process if the wavelength of the scalar particle is comparable to the size of the massive black hole. The EMRI motion perturbs the cloud, producing scalar radiation towards infinity and into the black hole horizon. In addition, the backreaction of the scalar radiation onto the orbit modifies the motion of the EMRI and induces an observable gravitational-wave phase shift for a range of system parameters. We quantify the scalar flux and the induced phase shift, as one of the examples of exactly-solvable, environmental effects of EMRIs. △ Less

Submitted 7 July, 2025; v1 submitted 2 July, 2025; originally announced July 2025.

Comments: 21 pages, 13 figures. Minor modifications made

Probing the Strong Gravity Region of Black Holes with eXTP

Authors: Qingcui Bu, Cosimo Bambi, Lijun Gou, Yanjun Xu, Phil Uttley, Alessandra De Rosa, Andrea Santangelo, Silvia Zane, Hua Feng, Shuang-Nan Zhang, Chichuan Jin, Haiwu Pan, Xinwen Shu, Francesco Ursini, Yanan Wang, Jianfeng Wu, Bei You, Yefei Yuan, Wenda Zhang, Stefano Bianchi, Lixin Dai, Tiziana Di Salvo, Michal Dovciak, Yuan Feng, Hengxiao Guo , et al. (20 additional authors not shown)

Abstract: We present the novel capabilities of the enhanced X-ray Timing and Polarimetry (eXTP) mission to study the strong gravity region around stellar-mass black holes in X-ray binary systems and supermassive black holes in active galactic nuclei. eXTP can combine X-ray spectral, timing, and polarimetric techniques to study the accretion process near black holes, measure black hole masses and spins, and… ▽ More We present the novel capabilities of the enhanced X-ray Timing and Polarimetry (eXTP) mission to study the strong gravity region around stellar-mass black holes in X-ray binary systems and supermassive black holes in active galactic nuclei. eXTP can combine X-ray spectral, timing, and polarimetric techniques to study the accretion process near black holes, measure black hole masses and spins, and test Einstein's theory of General Relativity in the strong field regime. We show how eXTP can improve the current measurements of black holes of existing X-ray missions and we discuss the scientific questions that can be addressed. △ Less

Submitted 8 September, 2025; v1 submitted 9 June, 2025; originally announced June 2025.

Comments: accepted for publication in the SCIENCE CHINA Physics, Mechanics & Astronomy

The ringdown of a black hole surrounded by a thin shell of matter

Authors: Andrew Laeuger, Colin Weller, Dongjun Li, Yanbei Chen

Abstract: Recent studies have shown that far-field perturbations to the curvature potential of a black hole spacetime may destabilize its quasinormal mode (QNM) spectrum while only mildly affecting time-domain ringdown signals. In this work, we study the QNM spectrum and ringdown behavior of a Schwarzschild black hole with a far-field perturbation to its physical environment -- a thin matter shell with fini… ▽ More Recent studies have shown that far-field perturbations to the curvature potential of a black hole spacetime may destabilize its quasinormal mode (QNM) spectrum while only mildly affecting time-domain ringdown signals. In this work, we study the QNM spectrum and ringdown behavior of a Schwarzschild black hole with a far-field perturbation to its physical environment -- a thin matter shell with finite surface tension. After accounting for the dynamics of the interaction between GWs and the shell, we find that the fundamental mode can migrate perturbatively or be destabilized by the appearance of new modes with no analogue in the vacuum case, much like studies of ``bumps" in the curvature potential. However, unlike these previous works, we find that the coupling between metric perturbations and oscillations of the shell also sources weakly-damped QNMs which are exclusive to the polar sector. We then study whether the analysis tools of least-squares QNM fits and the full and rational ringdown filters can clearly identify the signatures of the shell in representative ringdown waveforms. We conclude that ringdown at sufficiently early times is insensitive to the shell; weakly-damped QNMs (in the polar sector) and echoes, which may enable the analysis methods considered here to infer the presence of a shell, only appear at late times and are generally weak. △ Less

Submitted 19 September, 2025; v1 submitted 30 May, 2025; originally announced June 2025.

Comments: 26 pages, 12 figures. Submitted to Physical Review D

Shadows of three black holes in static equilibrium configuration

Authors: Dan Li, Yuxiang Zuo, Shiyang Hu, Chen Deng, Yu Wang, Wenfu Cao

Abstract: In this paper, we employ a ray-tracing algorithm to simulate the shadows of three equal-mass black holes in static equilibrium across a wide parameter space. We find that the shadows consist of a larger primary shadow and several distorted, eyebrow-like secondary shadows. The boundaries of these profiles exhibit self-similar fractal structures, which can be attributed to the photon chaotic scatter… ▽ More In this paper, we employ a ray-tracing algorithm to simulate the shadows of three equal-mass black holes in static equilibrium across a wide parameter space. We find that the shadows consist of a larger primary shadow and several distorted, eyebrow-like secondary shadows. The boundaries of these profiles exhibit self-similar fractal structures, which can be attributed to the photon chaotic scattering. In certain parameter spaces, we also observe the ring-like shadows, with the ring diameter associated with the spacing of black holes. Furthermore, when the black holes approach each other sufficiently, their shadows can merge into a standard disk, suggesting a shadow degeneracy between closely arranged triple black holes and a single massive, spherically symmetric black hole. The shadow features of the triple black holes revealed in this study have potential implications for analyzing the shadow formation mechanisms, as well as the gravitational lensing during the merger and inspiral of black holes. △ Less

Submitted 18 August, 2025; v1 submitted 5 April, 2025; originally announced April 2025.

Comments: 15 pages, 9 figures, accepted for publication in EPJC

Perturbations of spinning black holes in dynamical Chern-Simons gravity: Slow rotation quasinormal modes

Authors: Dongjun Li, Pratik Wagle, Yanbei Chen, Nicolás Yunes

Abstract: Gravitational waves offer new ways to test general relativity (GR) in the strong-field regime, including tests involving the ringdown phase of binary black hole mergers, characterized by oscillating and quickly decaying quasinormal modes (QNMs). Recent advances have extended QNM calculations to several theories beyond GR through the development of the modified Teukolsky formalism, including higher… ▽ More Gravitational waves offer new ways to test general relativity (GR) in the strong-field regime, including tests involving the ringdown phase of binary black hole mergers, characterized by oscillating and quickly decaying quasinormal modes (QNMs). Recent advances have extended QNM calculations to several theories beyond GR through the development of the modified Teukolsky formalism, including higher derivative gravity and dynamical Chern-Simons (dCS) gravity. Using the modified Teukolsky formalism, we previously derived radial second-order differential equations governing curvature and scalar field perturbations in dCS gravity at leading order in spin. In this work, we compute the QNM frequency shifts for slowly rotating black holes in dCS gravity from these modified Teukolsky equations, and (1) show that the radial equations for Weyl scalars $Ψ_{0,4}$ can be separated into even- and odd-parity parts, confirming that the scalar field couples only to the odd-parity sector; (2) extend the eigenvalue perturbation method to coupled fields; (3) compute the QNM spectrum, obtaining consistent results across independent calculations using different radiation gauges; (4) calculate the overtones in the QNM spectra for the first time in dCS gravity; (5) show that our findings align with previous metric perturbation studies and mark the first QNM spectrum calculation in a non-minimally coupled scalar-tensor theory via the modified Teukolsky formalism. This work lays the foundation for studying fast-rotating black holes in dCS gravity, advancing black hole spectroscopy in beyond-GR contexts. △ Less

Submitted 19 March, 2025; originally announced March 2025.

Comments: 35 pages, 7 figures, 5 tables

Novel inner shadows of the Kerr black hole with a tilted thin accretion disk

Authors: Shiyang Hu, Dan Li, Chen Deng

Abstract: The inner shadow of a black hole, as a projection of the event horizon, is regarded as a potential tool for testing gravitational theories and constraining system parameters. Whether this holds in the case of a tilted accretion disk warrants further investigation. In this paper, we employ a ray-tracing algorithm to simulate images of the Kerr black hole illuminated by a tilted thin accretion disk,… ▽ More The inner shadow of a black hole, as a projection of the event horizon, is regarded as a potential tool for testing gravitational theories and constraining system parameters. Whether this holds in the case of a tilted accretion disk warrants further investigation. In this paper, we employ a ray-tracing algorithm to simulate images of the Kerr black hole illuminated by a tilted thin accretion disk, with particular attention to the relationship between the inner shadow and system parameters. Our findings reveal that in the case of an equatorial accretion disk, the Kerr black hole exhibits a minimum inner shadow size of $S_{\textrm{min}} = 13.075$ M$^{2}$, where M denotes the black hole mass. This minimum is achieved when the viewing angle is $0^{\circ}$ and the spin parameter approaches $1$. However, with a non-zero disk tilt, the inner shadow exhibits novel configurations -- taking on petal, crescent, or eyebrow shapes -- significantly smaller than $S_{\textrm{min}}$ across various parameter spaces. This indicates that the inner shadow is highly sensitive to the accretion environment, suggesting that caution is needed when using it as a diagnostic tool for black holes. Notably, an observed inner shadow smaller than $S_{\textrm{min}}$ would either indicate the presence of a tilted accretion disk or support the viability of modified gravity. Moreover, in certain parameter spaces, we identify the emergence of a dual-shadow structure, which could also serve as a probe for the tilted accretion disk. △ Less

Submitted 20 May, 2025; v1 submitted 12 October, 2024; originally announced October 2024.

Comments: 22 pages, 11 figures, accepted for publication in JCAP

Observational features of deformed Schwarzschild black holes illuminated by an anisotropic accretion disk

Authors: Dan Li, Shiyang Hu, Chen Deng, Xin Wu

Abstract: The projection effect of an anisotropic accretion disk causes its electromagnetic radiation to depend on the emission angle. Although this dependency has the potential to influence the observational characteristics of black holes, it has not received sufficient attention. In this paper, we employ a relativistic ray-tracing algorithm to numerically simulate 86 GHz and 230 GHz images of deformed Sch… ▽ More The projection effect of an anisotropic accretion disk causes its electromagnetic radiation to depend on the emission angle. Although this dependency has the potential to influence the observational characteristics of black holes, it has not received sufficient attention. In this paper, we employ a relativistic ray-tracing algorithm to numerically simulate 86 GHz and 230 GHz images of deformed Schwarzschild black holes illuminated by an equatorial anisotropic accretion disk, aiming to reveal the observational signatures of the target black hole and the impact of the projection effect on the images. The study demonstrates that while the introduction of the projection effect does not alter the profiles of the black hole's inner shadow and critical curve, it significantly suppresses the specific intensity of light rays, particularly in direct emission, thereby reducing image brightness. The extent of this reduction depends on both the observation inclination and frequency. This phenomenon aids in the extraction of geometric information from higher-order subrings in the image. Furthermore, we find that increasing the deformation parameter enhances the brightness of the deformed Schwarzschild black hole image, accompanied by a reduction in the size of the critical curve and inner shadow. This relationship establishes a connection between the intrinsic properties of deformed Schwarzschild black holes and their observational characteristics, providing a reliable tool for testing the no-hair theorem and gravitational theories. Specifically, we propose a novel method for constraining parameters based on the silhouette of the inner shadow, which holds promise for extension to any spherically symmetric black hole in other gravitational theories. △ Less

Submitted 7 September, 2024; v1 submitted 3 September, 2024; originally announced September 2024.

Comments: 30 pages, 14 figures

SLOCC and LU classification of black holes with eight electric and magnetic charges

Authors: Dafa Li, Maggie Cheng, Xiangrong Li, Shuwang Li

Abstract: In \cite{Linde}, Kallosh and Linde discussed the SLOCC classification of black holes. However, the criteria for the SLOCC classification of black holes have not been given. In addition, the LU classification of black holes has not been studied in the past. In this paper we will consider both SLOCC and LU classification of the STU black holes with four integer electric charges $q_{i} $ and four int… ▽ More In \cite{Linde}, Kallosh and Linde discussed the SLOCC classification of black holes. However, the criteria for the SLOCC classification of black holes have not been given. In addition, the LU classification of black holes has not been studied in the past. In this paper we will consider both SLOCC and LU classification of the STU black holes with four integer electric charges $q_{i} $ and four integer magnetic charges $p^{i}$, $i=0,1,2,3$. Two STU black holes with eight charges are considered SLOCC (LU) equivalent if and only if their corresponding states of three qubits are SLOCC (LU) equivalent. Under this definition, we give criteria for the classification of the eight-charge STU black holes under SLOCC and under LU, respectively. We will study the classification of the black holes via the classification of SLOCC and LU entanglement of three qubits. We then identify a set of black holes corresponding to the state W of three qubits, which is of interest since it has the maximal average von Neumann entropy of entanglement. Via von Neumann entanglement entropy, we partition the STU black holes corresponding to pure states of GHZ SLOCC class into five families under LU. △ Less

Submitted 15 August, 2024; originally announced August 2024.

Journal ref: Int J theor phys 63, issue 6, 144 (2024)

Spectroscopy of bumpy BHs: non-rotating case

Authors: Colin Weller, Dongjun Li, Yanbei Chen

Abstract: Recent detections of gravitational waves have made black hole quasinormal modes a powerful tool in testing predictions of general relativity. Understanding the spectrum of these quasinormal modes in a broad class of theories beyond general relativity and a variety of astrophysical environments around black holes remains vital. In this work, we study the quasinormal mode spectrum of parametrized de… ▽ More Recent detections of gravitational waves have made black hole quasinormal modes a powerful tool in testing predictions of general relativity. Understanding the spectrum of these quasinormal modes in a broad class of theories beyond general relativity and a variety of astrophysical environments around black holes remains vital. In this work, we study the quasinormal mode spectrum of parametrized deformations of a non-rotating black hole in the vacuum. Following Vigeland and Hughes, we model these parametrized deformations as axisymmetric multipole moments in the Weyl coordinates with amplitudes much less than the amplitude of the Schwarzschild potential. These tiny bumps in the black hole geometry satisfy the linearized vacuum Einstein equations and are asymptotically flat. We use the recently developed modified Teukolsky formalism to derive one decoupled differential equation for the radiative Weyl scalar $Ψ_0$. We then use the eigenvalue perturbation method to compute the quasinormal mode frequency shifts of both even- and odd-parity modes with $\ell=2,3$ and up to the overtone number $n=2$ for the Weyl multipoles with $\ell_W=2,3$. Our calculation provides an avenue to directly connect the multipole moments of a modified black hole spacetime to the QNM frequency shifts in a parametric way. △ Less

Submitted 31 May, 2024; originally announced May 2024.

Comments: 18 pages, 5 figures, 4 tables

Stringent Tests of Lorentz Invariance Violation from LHAASO Observations of GRB 221009A

Authors: The LHAASO Collaboration, Zhen Cao, F. Aharonian, Axikegu, Y. X. Bai, Y. W. Bao, D. Bastieri, X. J. Bi, Y. J. Bi, W. Bian, A. V. Bukevich, Q. Cao, W. Y. Cao, Zhe Cao, J. Chang, J. F. Chang, A. M. Chen, E. S. Chen, H. X. Chen, Liang Chen, Lin Chen, Long Chen, M. J. Chen, M. L. Chen, Q. H. Chen , et al. (261 additional authors not shown)

Abstract: On October 9, 2022, the Large High Altitude Air Shower Observatory (LHAASO) reported the observation of the very early TeV afterglow of the brightest-of-all-time GRB 221009A, recording the highest photon statistics in the TeV band ever from a gamma-ray burst. We use this unique observation to place stringent constraints on an energy dependence of the speed of light in vacuum, a manifestation of Lo… ▽ More On October 9, 2022, the Large High Altitude Air Shower Observatory (LHAASO) reported the observation of the very early TeV afterglow of the brightest-of-all-time GRB 221009A, recording the highest photon statistics in the TeV band ever from a gamma-ray burst. We use this unique observation to place stringent constraints on an energy dependence of the speed of light in vacuum, a manifestation of Lorentz invariance violation (LIV) predicted by some quantum gravity (QG) theories. Our results show that the 95% confidence level lower limits on the QG energy scales are $E_{\mathrm{QG},1}>10$ times of the Planck energy $E_\mathrm{Pl}$ for the linear, and $E_{\mathrm{QG},2}>6\times10^{-8}E_\mathrm{Pl}$ for the quadratic LIV effects, respectively. Our limits on the quadratic LIV case improve previous best bounds by factors of 5--7. △ Less

Submitted 13 February, 2026; v1 submitted 8 February, 2024; originally announced February 2024.

Comments: 10pages, 7 figures

Perturbations of spinning black holes in dynamical Chern-Simons gravity I. Slow rotation equations

Authors: Pratik Wagle, Dongjun Li, Yanbei Chen, Nicolas Yunes

Abstract: The detection of gravitational waves resulting by the LIGO-Virgo-Kagra collaboration has inaugurated a new era in gravitational physics, providing an opportunity to test general relativity and its modifications in the strong gravity regime. One such test involves the study of the ringdown phase of gravitational waves from binary black-hole coalescence, which can be decomposed into a superposition… ▽ More The detection of gravitational waves resulting by the LIGO-Virgo-Kagra collaboration has inaugurated a new era in gravitational physics, providing an opportunity to test general relativity and its modifications in the strong gravity regime. One such test involves the study of the ringdown phase of gravitational waves from binary black-hole coalescence, which can be decomposed into a superposition of quasinormal modes. In general relativity, the spectra of quasinormal modes depend on the mass, spin, and charge of the final black hole, but they can be influenced by additional properties of the black hole, as well as corrections to general relativity. In this work, we employ the modified Teukolsky formalism developed in a previous study to investigate perturbations of slowly rotating black holes in a modified theory known as dynamical Chern-Simons gravity. Specifically, we derive the master equations for the $Ψ_0$ and $Ψ_4$ Weyl scalar perturbations that characterize the radiative part of gravitational perturbations, as well as for the scalar field perturbations. We employ metric reconstruction techniques to obtain explicit expressions for all relevant quantities. Finally, by leveraging the properties of spin-weighted spheroidal harmonics to eliminate the angular dependence from the evolution equations, we derive two, radial, second-order, ordinary differential equations for $Ψ_0$ and $Ψ_4$, respectively. These equations are coupled to another radial, second-order, ordinary differential equation for the scalar field perturbations. This work is the first attempt to derive a master equation for black holes in dynamical Chern-Simons gravity using curvature perturbations. The master equations can be numerically integrated to obtain the quasinormal mode spectrum of slowly rotating black holes in this theory, making progress in the study of ringdown in dynamical Chern-Simons gravity. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Comments: 40 pages, 2 figures and 1 table

Isospectrality breaking in the Teukolsky formalism

Authors: Dongjun Li, Asad Hussain, Pratik Wagle, Yanbei Chen, Nicolás Yunes, Aaron Zimmerman

Abstract: General relativity, though the most successful theory of gravity, has been continuously modified to resolve its incompatibility with quantum mechanics and explain the origin of dark energy or dark matter. One way to test these modified gravity theories is to study the gravitational waves emitted during the ringdown of binary mergers, which consist of quasinormal modes. In several modified gravity… ▽ More General relativity, though the most successful theory of gravity, has been continuously modified to resolve its incompatibility with quantum mechanics and explain the origin of dark energy or dark matter. One way to test these modified gravity theories is to study the gravitational waves emitted during the ringdown of binary mergers, which consist of quasinormal modes. In several modified gravity theories, the even- and odd-parity gravitational perturbations of non-rotating and slowly rotating black holes have different quasinormal mode frequencies, breaking the isospectrality of general relativity. For black holes with arbitrary spin in modified gravity, there were no avenues to compute quasinormal modes except numerical relativity, until recent extensions of the Teukolsky formalism. In this work, we describe how to use the modified Teukolsky formalism to study isospectrality breaking in modified gravity. We first introduce how definite-parity modes are defined through combinations of Weyl scalars in general relativity, and then, we extend this definition to modified gravity. We then use the eigenvalue perturbation method to show how the degeneracy in quasinormal mode frequencies of different parity is broken in modified gravity. To demonstrate our analysis, we also apply it to some specific modified gravity theories. Our work lays the foundation for studying isospectrality breaking of quasinormal modes in modified gravity for black holes with arbitrary spin. △ Less

Submitted 9 October, 2023; originally announced October 2023.

Comments: 29 pages

Report number: UTWI-14-2023

Influences of tilted thin accretion disks on the observational appearance of hairy black holes in Horndeski gravity

Authors: Shiyang Hu, Dan Li, Chen Deng, Xin Wu, Enwei Liang

Abstract: Research on the observational appearance of black holes, both in general relativity and modified gravity, has been in full swing since the Event Horizon Telescope Collaboration announced photos of M87$^{*}$ and Sagittarius A$^{*}$. Nevertheless, limited attention has been given to the impact of tilted accretion disks on black hole images. This paper investigates the $230$ GHz images of non-rotatin… ▽ More Research on the observational appearance of black holes, both in general relativity and modified gravity, has been in full swing since the Event Horizon Telescope Collaboration announced photos of M87$^{*}$ and Sagittarius A$^{*}$. Nevertheless, limited attention has been given to the impact of tilted accretion disks on black hole images. This paper investigates the $230$ GHz images of non-rotating hairy black holes illuminated by tilted, thin accretion disks in Horndeski gravity with the aid of a ray tracing method. The results indicate that reducing the scalar hair parameter effectively diminishes image luminosity and extends both the critical curve and the inner shadow. This trend facilitates the differentiation between hairy black holes and Schwarzschild black holes, especially in certain parameter spaces where the current Event Horizon Telescope array is capable of capturing such variations. Furthermore, we observe that the inclination of the tilted accretion disk can mimic the observation angle, consequently affecting image brightness and the morphology of the inner shadow. In specific parameter spaces, alterations in the tilt or position of the accretion disk can lead to a drift in the light spot within the images of hairy black holes. This finding may establish a potential correlation between the precession of the tilted accretion disk and image features. Additionally, through an examination of images depicting hairy black holes surrounded by two thin accretion disks, we report the obscuring effect of the accretion environment on the inner shadow of the black hole. △ Less

Submitted 14 March, 2024; v1 submitted 19 September, 2023; originally announced September 2023.

Comments: 32 pages, 17 figures, accepted for publication in JCAP

Cosmology with fast radio bursts in the era of SKA

Authors: Ji-Guo Zhang, Ze-Wei Zhao, Yichao Li, Jing-Fei Zhang, Di Li, Xin Zhang

Abstract: We present a forecast of the cosmological parameter estimation using fast radio bursts (FRBs) from the upcoming Square Kilometre Array (SKA), focusing on the issues of dark energy, the Hubble constant, and baryon density. We simulate $10^5$ and $10^6$ localized FRBs from a 10-year SKA observation, and find that: (i) using $10^6$ FRB data alone can tightly constrain dark-energy equation of state pa… ▽ More We present a forecast of the cosmological parameter estimation using fast radio bursts (FRBs) from the upcoming Square Kilometre Array (SKA), focusing on the issues of dark energy, the Hubble constant, and baryon density. We simulate $10^5$ and $10^6$ localized FRBs from a 10-year SKA observation, and find that: (i) using $10^6$ FRB data alone can tightly constrain dark-energy equation of state parameters better than CMB+BAO+SNe, providing an independent cosmological probe to explore dark energy; (ii) combining the FRB data with gravitational-wave standard siren data from 10-year observation with the Einstein Telescope, the Hubble constant can be constrained to a sub-percent level, serving as a powerful low-redshift probe; (iii) using $10^6$ FRB data can constrain the baryon density $Ω_{\rm b}h$ to a precision of $\sim 0.1\%$. Our results indicate that SKA-era FRBs will provide precise cosmological measurements to shed light on both dark energy and the missing baryon problem, and help resolve the Hubble tension. △ Less

Submitted 4 September, 2023; v1 submitted 4 July, 2023; originally announced July 2023.

Comments: 17 pages, 7 figures; accepted for publication in Science China-Physics, Mechanics & Astronomy

Journal ref: Sci. China-Phys. Mech. Astron. 66, 120412 (2023)

Quantum Gravity Background in Next-Generation Gravitational Wave Detectors

Authors: Mathew W. Bub, Yanbei Chen, Yufeng Du, Dongjun Li, Yiwen Zhang, Kathryn M. Zurek

Abstract: We study the effects of geontropic vacuum fluctuations in quantum gravity on next-generation terrestrial gravitational wave detectors. If the VZ effect proposed in Ref. [1], as modeled in Refs. [2, 3], appears in the upcoming GQuEST experiment, we show that it will be a large background for astrophysical gravitational wave searches in observatories like Cosmic Explorer and the Einstein Telescope. We study the effects of geontropic vacuum fluctuations in quantum gravity on next-generation terrestrial gravitational wave detectors. If the VZ effect proposed in Ref. [1], as modeled in Refs. [2, 3], appears in the upcoming GQuEST experiment, we show that it will be a large background for astrophysical gravitational wave searches in observatories like Cosmic Explorer and the Einstein Telescope. △ Less

Submitted 18 May, 2023; originally announced May 2023.

Comments: 19 pages, 9 figures

Report number: CALT-TH-2023-012

Massless Entanglement Islands in Cone Holography

Authors: Dongqi Li, Rong-Xin Miao

Abstract: It is controversial whether entanglement islands can exist in massless gravity theories. Recently, it is found that the massless entanglement island appears in wedge holography with DGP gravity on the branes. In this paper, we generalize the discussions to the codim-n holography named cone holography. For simplicity, we focus on the case with a codim-2 E brane and a codim-1 Q brane. We discuss the… ▽ More It is controversial whether entanglement islands can exist in massless gravity theories. Recently, it is found that the massless entanglement island appears in wedge holography with DGP gravity on the branes. In this paper, we generalize the discussions to the codim-n holography named cone holography. For simplicity, we focus on the case with a codim-2 E brane and a codim-1 Q brane. We discuss the effective action, mass spectrum and holographic entanglement entropy for cone holography with DGP terms. We verify that there is massless gravity on the branes, and recover non-trivial entanglement islands and Page curves. Besides, we work out the parameter space which allows entanglement islands and Page curves. Compared with wedge holography, there are several new features. First, one can not add DGP gravity on the codim-2 E brane. That is because the energy density has to be a constant on codim-2 branes for Einstein gravity in bulk. Second, the Hartman-Maldacena surface ends only on the codim-1 Q brane. Third, the Hartman-Maldacena surface can be defined only in a finite time. We notice that this unusual situation also appears in AdS/dCFT and even in AdS/CFT. Fortunately, it does not affect the Page curve since it happens after Page time. Our results provide more support that the entanglement island is consistent with massless gravity theories. △ Less

Submitted 13 June, 2023; v1 submitted 20 March, 2023; originally announced March 2023.

Comments: 32 pages, 16 figures, minor revision published in JHEP

Journal ref: JHEP06(2023)056

Interferometer Response to Geontropic Fluctuations

Authors: Dongjun Li, Vincent S. H. Lee, Yanbei Chen, Kathryn M. Zurek

Abstract: We model vacuum fluctuations in quantum gravity with a scalar field, characterized by a high occupation number, coupled to the metric. The occupation number of the scalar is given by a thermal density matrix, whose form is motivated by fluctuations in the vacuum energy, which have been shown to be conformal near a light-sheet horizon. For the experimental measurement of interest in an interferomet… ▽ More We model vacuum fluctuations in quantum gravity with a scalar field, characterized by a high occupation number, coupled to the metric. The occupation number of the scalar is given by a thermal density matrix, whose form is motivated by fluctuations in the vacuum energy, which have been shown to be conformal near a light-sheet horizon. For the experimental measurement of interest in an interferometer, the size of the energy fluctuations is fixed by the area of a surface bounding the volume of spacetime being interrogated by an interferometer. We compute the interferometer response to these "geontropic" scalar-metric fluctuations, and apply our results to current and future interferometer measurements, such as LIGO and the proposed GQuEST experiment. △ Less

Submitted 15 September, 2022; originally announced September 2022.

Comments: 17 pages, 6 figures

Report number: CALT-TH-2022-033

Island on codimension-two branes in AdS/dCFT

Authors: Peng-Ju Hu, Dongqi Li, Rong-Xin Miao

Abstract: The previous studies of the island and double holography mainly focus on codimension-one branes. This paper explores the island on the codimension-two brane in AdS/dCFT. The codimension-two brane is closely related to conical singularity, which is very different from the codimension-one brane. We analyze the mass spectrum of gravitons on the codimension-two brane and find that the larger the brane… ▽ More The previous studies of the island and double holography mainly focus on codimension-one branes. This paper explores the island on the codimension-two brane in AdS/dCFT. The codimension-two brane is closely related to conical singularity, which is very different from the codimension-one brane. We analyze the mass spectrum of gravitons on the codimension-two brane and find that the larger the brane tension is, the smaller the gravitational mass is. The massless mode is forbidden by either the boundary or normalization conditions. We prove that the first massive gravitational mode is located on the codimension-two brane; the larger the tension, the better the localization. It is similar to the case of codimension-one brane and builds an excellent physical foundation for the study of black hole evolution on codimension-two branes. We find that the Page curve of eternal black holes can be recovered due to the island ending on the codimension-two brane. The new feature is that the extremal surface passing the horizon cannot be defined after some finite time in the no-island phase. Fortunately, this unusual situation does not affect the Page curve since it happens after Page time. △ Less

Submitted 2 November, 2022; v1 submitted 25 August, 2022; originally announced August 2022.

Comments: 46 pages, 20 figures, reference added, minor revision published in JHEP

Journal ref: JHEP11(2022)008

Perturbations of spinning black holes beyond General Relativity: Modified Teukolsky equation

Authors: Dongjun Li, Pratik Wagle, Yanbei Chen, Nicolás Yunes

Abstract: The detection of gravitational waves from compact binary mergers by the LIGO/Virgo collaboration has, for the first time, allowed for tests of relativistic gravity in the strong, dynamical and nonlinear regime. Outside Einstein's relativity, spinning black holes may be different from their general relativistic counterparts, and their merger may then lead to a modified ringdown. We study the latter… ▽ More The detection of gravitational waves from compact binary mergers by the LIGO/Virgo collaboration has, for the first time, allowed for tests of relativistic gravity in the strong, dynamical and nonlinear regime. Outside Einstein's relativity, spinning black holes may be different from their general relativistic counterparts, and their merger may then lead to a modified ringdown. We study the latter and, for the first time, derive a modified Teukolsky equation, i.e., a set of linear, decoupled differential equations that describe dynamical perturbations of non-Kerr black holes for the radiative Newman-Penrose scalars $Ψ_0$ and $Ψ_4$. We first focus on non-Ricci-flat, Petrov type D black hole backgrounds in modified gravity, and derive the modified Teukolsky equation through direct decoupling and through a new approach, proposed by Chandrasekhar, that uses certain gauge conditions. We then extend this analysis to non-Ricci-flat, Petrov type I black hole backgrounds in modified gravity, assuming they can be treated as a linear perturbation of Petrov type D, black hole backgrounds in GR by generalizing Chandrasekhar's approach, and derive the decoupled modified Teukolsky equation. Our work lays the foundation to study the gravitational waves emitted in the ringdown phase of black hole coalescence in modified gravity for black holes of any spin. Our work can also be extended to compute gravitational waves emitted by extreme mass-ratio binary inspirals in modified gravity. △ Less

Submitted 25 May, 2023; v1 submitted 21 June, 2022; originally announced June 2022.

Comments: 33 pages, 2 figures, 1 table

Snowmass 2021 CMB-S4 White Paper

Authors: Kevork Abazajian, Arwa Abdulghafour, Graeme E. Addison, Peter Adshead, Zeeshan Ahmed, Marco Ajello, Daniel Akerib, Steven W. Allen, David Alonso, Marcelo Alvarez, Mustafa A. Amin, Mandana Amiri, Adam Anderson, Behzad Ansarinejad, Melanie Archipley, Kam S. Arnold, Matt Ashby, Han Aung, Carlo Baccigalupi, Carina Baker, Abhishek Bakshi, Debbie Bard, Denis Barkats, Darcy Barron, Peter S. Barry , et al. (331 additional authors not shown)

Abstract: This Snowmass 2021 White Paper describes the Cosmic Microwave Background Stage 4 project CMB-S4, which is designed to cross critical thresholds in our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. We provide an overview of the science case, the technical design, and project plan. This Snowmass 2021 White Paper describes the Cosmic Microwave Background Stage 4 project CMB-S4, which is designed to cross critical thresholds in our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. We provide an overview of the science case, the technical design, and project plan. △ Less

Submitted 15 March, 2022; originally announced March 2022.

Comments: Contribution to Snowmass 2021. arXiv admin note: substantial text overlap with arXiv:1908.01062, arXiv:1907.04473

AdS/BCFT and Island for curvature-squared gravity

Authors: Qi-Lin Hu, Dongqi Li, Rong-Xin Miao, Yu-Qian Zeng

Abstract: In this paper, we investigate AdS/BCFT for curvature-squared gravity. To warm up, we start with Gauss-Bonnet gravity. We derive the one point function of stress tensor and show that the central charge related to the norm of displacement operator is positive for the couplings obeying causality constraints. Furthermore, by imposing the null energy condition on the end-of-the-world brane, we prove th… ▽ More In this paper, we investigate AdS/BCFT for curvature-squared gravity. To warm up, we start with Gauss-Bonnet gravity. We derive the one point function of stress tensor and show that the central charge related to the norm of displacement operator is positive for the couplings obeying causality constraints. Furthermore, by imposing the null energy condition on the end-of-the-world brane, we prove the holographic g-theorem for Gauss-Bonnet gravity. This corrects a wrong point of view in the literature, which claims that the holographic g-theorem is violated for Gauss-Bonnet gravity. As a by-product, we obtain the boundary entropy and A-type boundary central charges in general dimensions. We also study AdS/BCFT for general curvature-squared gravity. We find that it is too restrictive for the shape of the brane and the dual BCFT is trivial if one imposes Neumann boundary conditions for all of the gravitational modes. Instead, we propose to impose Dirichlet boundary condition for the massive graviton, while imposing Neumann boundary condition for the massless graviton. In this way, we obtain non-trivial shape dependence of stress tensor and well-defined central charges. In particular, the holographic g-theorem is satisfied by general curvature-squared gravity. Finally, we discuss the island and show that the Page curve can be recovered for Gauss-Bonnet gravity. Interestingly, there are zeroth-order phase transitions for the Page curve within one range of couplings obeying causality constraints. Generalizing the discussions to holographic entanglement entropy and holographic complexity in AdS/CFT, we get new constraints for the Gauss-Bonnet coupling, which is stronger than the causality constraint. △ Less

Submitted 18 August, 2022; v1 submitted 7 February, 2022; originally announced February 2022.

Comments: 49 pages, 29 figures, revision accepted for publication in JHEP, main improvements: prove that our g-function can recover the universal term of boundary entropy in general dimensions; add a toy model to explain the novel zeroth-order phase transition of the Page curve analytically

De Sitter braneworld and gravitational waves

Authors: Dong-Yu Li, Zhao-Xiang Wu, Hao Hu, Bao-Min Gu

Abstract: We study the braneworld theory constructed by multi scalar fields. The model contains a smooth and infinitely large extra dimension, allowing the background fields propagating in it. We give a de Sitter solution for the four-dimensional cosmology as a good approximation to the early universe inflation. We show that the graviton has a localizable massless mode, and a series of continuous massive mo… ▽ More We study the braneworld theory constructed by multi scalar fields. The model contains a smooth and infinitely large extra dimension, allowing the background fields propagating in it. We give a de Sitter solution for the four-dimensional cosmology as a good approximation to the early universe inflation. We show that the graviton has a localizable massless mode, and a series of continuous massive modes, separated by a mass gap. There could be a normalizable massive mode, depending on the background solution. The gravitational waves of massless mode evolve the same as the four dimensional theory, while that of the massive modes evolve greatly different from the massless mode. △ Less

Submitted 8 November, 2021; originally announced November 2021.

Comments: 8 pages, 4 figures, accepted by Canadian Journal of Physics

Simulating magnetized neutron stars with discontinuous Galerkin methods

Authors: Nils Deppe, François Hébert, Lawrence E. Kidder, William Throwe, Isha Anantpurkar, Cristóbal Armaza, Gabriel S. Bonilla, Michael Boyle, Himanshu Chaudhary, Matthew D. Duez, Nils L. Vu, Francois Foucart, Matthew Giesler, Jason S. Guo, Yoonsoo Kim, Prayush Kumar, Isaac Legred, Dongjun Li, Geoffrey Lovelace, Sizheng Ma, Alexandra Macedo, Denyz Melchor, Marlo Morales, Jordan Moxon, Kyle C. Nelli , et al. (11 additional authors not shown)

Abstract: Discontinuous Galerkin methods are popular because they can achieve high order where the solution is smooth, because they can capture shocks while needing only nearest-neighbor communication, and because they are relatively easy to formulate on complex meshes. We perform a detailed comparison of various limiting strategies presented in the literature applied to the equations of general relativisti… ▽ More Discontinuous Galerkin methods are popular because they can achieve high order where the solution is smooth, because they can capture shocks while needing only nearest-neighbor communication, and because they are relatively easy to formulate on complex meshes. We perform a detailed comparison of various limiting strategies presented in the literature applied to the equations of general relativistic magnetohydrodynamics. We compare the standard minmod/$ΛΠ^N$ limiter, the hierarchical limiter of Krivodonova, the simple WENO limiter, the HWENO limiter, and a discontinuous Galerkin-finite-difference hybrid method. The ultimate goal is to understand what limiting strategies are able to robustly simulate magnetized TOV stars without any fine-tuning of parameters. Among the limiters explored here, the only limiting strategy we can endorse is a discontinuous Galerkin-finite-difference hybrid method. △ Less

Submitted 28 June, 2022; v1 submitted 24 September, 2021; originally announced September 2021.

Comments: matches published version. 21 pages, 12 figures. Added KH instability, TOV runs with classical limiters. arXiv admin note: text overlap with arXiv:2109.11645

The Gravitational-Wave Physics II: Progress

Authors: Ligong Bian, Rong-Gen Cai, Shuo Cao, Zhoujian Cao, He Gao, Zong-Kuan Guo, Kejia Lee, Di Li, Jing Liu, Youjun Lu, Shi Pi, Jian-Min Wang, Shao-Jiang Wang, Yan Wang, Tao Yang, Xing-Yu Yang, Shenghua Yu, Xin Zhang

Abstract: It has been a half-decade since the first direct detection of gravitational waves, which signifies the coming of the era of the gravitational-wave astronomy and gravitational-wave cosmology. The increasing number of the detected gravitational-wave events has revealed the promising capability of constraining various aspects of cosmology, astronomy, and gravity. Due to the limited space in this revi… ▽ More It has been a half-decade since the first direct detection of gravitational waves, which signifies the coming of the era of the gravitational-wave astronomy and gravitational-wave cosmology. The increasing number of the detected gravitational-wave events has revealed the promising capability of constraining various aspects of cosmology, astronomy, and gravity. Due to the limited space in this review article, we will briefly summarize the recent progress over the past five years, but with a special focus on some of our own work for the Key Project ``Physics associated with the gravitational waves'' supported by the National Natural Science Foundation of China. In particular, (1) we have presented the mechanism of the gravitational-wave production during some physical processes of the early Universe, such as inflation, preheating and phase transition, and the cosmological implications of gravitational-wave measurements; (2) we have put constraints on the neutron star maximum mass according to GW170817 observations; (3) we have developed a numerical relativity algorithm based on the finite element method and a waveform model for the binary black hole coalescence along an eccentric orbit. △ Less

Submitted 3 November, 2021; v1 submitted 17 June, 2021; originally announced June 2021.

Comments: v1, 93 pages, 18 figures, prepared as a status review of project report for Sci. China Phys. Mech. Astron.; v2, 95 pages, minor revision, references added, accepted for publication in Sci. China Phys. Mech. Astron; v3, to match the published version

Journal ref: Sci. China Phys. Mech. Astron. 64, 120401 (2021)

A shell of Bosons in Spherically Symmetric spacetimes

Authors: Duo Li, Bin Wu, Zhen-Ming Xu, Wen-Li Yang

Abstract: The thermodynamic properties of a shell of bosons with the inner surface locating at Planck length away from the horizon of Schwarzschild black holes by using statistical mechanics are studied. The covariant partition function of bosons is obtained, from which the Bose-Einstein condensation of bosons is found at a non-zero temperature in the curved spacetimes. As a special case of bosons, we analy… ▽ More The thermodynamic properties of a shell of bosons with the inner surface locating at Planck length away from the horizon of Schwarzschild black holes by using statistical mechanics are studied. The covariant partition function of bosons is obtained, from which the Bose-Einstein condensation of bosons is found at a non-zero temperature in the curved spacetimes. As a special case of bosons, we analyze the entropy of photon gas near the horizon of the Schwarzschild black hole, which shows an area dependence similar to the Bekenstein-Hawking entropy. The results may offer new perspectives on the study of black hole thermodynamics. All these are extended to the $D+1$ dimensional spherically symmetric static spacetimes. △ Less

Submitted 28 August, 2021; v1 submitted 16 June, 2021; originally announced June 2021.

Comments: 16 pages;v2:published version

Journal ref: Phys.Lett.B 820(2021) 136588

The equations of motion for consistency of a post-Newtonian Lagrangian formulation

Authors: Dan Li, Yu Wang, Chen Deng, Xin Wu

Abstract: Equations of motion for a general relativistic post-Newtonian Lagrangian approach mainly refer to acceleration equations, i.e. differential equations of velocities. They are directly from the Euler-Lagrangian equations, and usually have higher-order terms truncated when they remain at the same post-Newtonian order of the Lagrangian. In this sense, they are incoherent equations of the Lagrangian an… ▽ More Equations of motion for a general relativistic post-Newtonian Lagrangian approach mainly refer to acceleration equations, i.e. differential equations of velocities. They are directly from the Euler-Lagrangian equations, and usually have higher-order terms truncated when they remain at the same post-Newtonian order of the Lagrangian. In this sense, they are incoherent equations of the Lagrangian and approximately conserve constants of motion in this system. In this paper, we show that the Euler-Lagrangian equations can also yield the equations of motion for consistency of the Lagrangian in the general case. The coherent equations are the \emph{differential} equations of generalized momenta rather than those of the velocities, and have no terms truncated. The velocities are not integration variables, but they can be solved from the \emph{algebraic} equations of the generalized momenta with an iterative method. Taking weak relativistic fields in the Solar System and strong relativistic fields of compact objects as examples, we numerically evaluate the accuracies of the constants of motion in the two sets of equations of motion. It is confirmed that these accuracies well satisfy the theoretical need if the chosen integrator can provide a high enough precision. The differences in the dynamical behavior of order and chaos between the two sets of equations are also compared. Unlike the incoherent post-Newtonian Lagrangian equations of motion, the coherent ones can theoretically, strictly conserve all integrals in some post-Newtonian Lagrangian problems, and therefore are worth recommending. △ Less

Submitted 27 May, 2019; originally announced May 2019.

Comments: 14 pages, 8 figures

Anomalous Current Due to Weyl Anomaly for Conformal Field Theory

Authors: Jiang-Jin Zheng, Dongqi Li, Yu-Qian Zeng, Rong-Xin Miao

Abstract: Recently it is found that Weyl anomaly leads to new anomalous currents in an external electromagnetic field in the curved spacetime. For simplicity, the initial works mainly focus on weak gravitational fields and the anomalous current is obtained for conformally flat spaces with small scale factors. In this paper, we generalize the results to the case with arbitrary scale factors. Firstly, we deri… ▽ More Recently it is found that Weyl anomaly leads to new anomalous currents in an external electromagnetic field in the curved spacetime. For simplicity, the initial works mainly focus on weak gravitational fields and the anomalous current is obtained for conformally flat spaces with small scale factors. In this paper, we generalize the results to the case with arbitrary scale factors. Firstly, we derive the transformation law of current under Weyl transformation, from which one can read off the anomalous current in general conformally flat spaces. Secondly, by using the Weyl transformation of currents we provide a new derivation of the Weyl-anomaly-induced current near the boundary. Thus we have obtained the two kinds of anomalous currents in the literature from a unified formula. Finally, we extend the discussions to n-form fields and find similar anomalous currents. △ Less

Submitted 18 August, 2019; v1 submitted 15 April, 2019; originally announced April 2019.

Comments: 18 pages, 0 figure, match the published version in PLB

Chaotic motion of neutral and charged particles in the magnetized Ernst-Schwarzschild spacetime

Authors: Dan Li, Xin Wu

Abstract: Neutral test particles around a Schwarzschild black hole immersed in an external uniform magnetic field have no interactions of electromagnetic forces, but their motions can be chaotic. This chaotic behavior is induced owing to the gravitational effect of the magnetic field leading to the nonintegrability of the magnetized Ernst-Schwarzschild spacetime geometry. In fact, chaos is strengthened typi… ▽ More Neutral test particles around a Schwarzschild black hole immersed in an external uniform magnetic field have no interactions of electromagnetic forces, but their motions can be chaotic. This chaotic behavior is induced owing to the gravitational effect of the magnetic field leading to the nonintegrability of the magnetized Ernst-Schwarzschild spacetime geometry. In fact, chaos is strengthened typically with an increase of the energy or the magnetic field under appropriate circumstances. When these test particles have charges, the electromagnetic forces are included. As a result, the electromagnetic forces have an effect on strengthening or weakening the extent of chaos caused by the gravitational effect of the magnetic field. △ Less

Submitted 6 March, 2018; originally announced March 2018.

Comments: 11 pages, 9 figures,

Electromagnetic counterparts of high-frequency gravitational waves having additional polarization states: distinguishing and probing tensor-mode, vector-mode and scalar-mode gravitons

Authors: Fang-Yu Li, Hao Wen, Zhen-Yun Fang, Di Li, Tong-Jie Zhang

Abstract: GWs from extra dimensions, very early universe, and some high-energy astrophysical process, might have at most six polarizations: plus- and cross-type (tensor-mode gravitons), x-, y-type (vector-mode), and b-, l-type (scalar-mode). Peak or partial peak regions of some of such GWs are just distributed in GHz or higher frequency band, which would be optimal band for electromagnetic(EM) response. In… ▽ More GWs from extra dimensions, very early universe, and some high-energy astrophysical process, might have at most six polarizations: plus- and cross-type (tensor-mode gravitons), x-, y-type (vector-mode), and b-, l-type (scalar-mode). Peak or partial peak regions of some of such GWs are just distributed in GHz or higher frequency band, which would be optimal band for electromagnetic(EM) response. In this paper we investigate EM response to such high-frequency GWs(HFGWs) having additional polarizations. For the first time we address:(1)concrete forms of analytic solutions for perturbed EM fields caused by HFGWs having all six possible polarizations in background stable EM fields; (2)perturbed EM signals of HFGWs with additional polarizations in three-dimensional-synchro-resonance-system(3DSR system) and in galactic-extragalactic background EM fields. These perturbative EM fields are actually EM counterparts of HFGWs, and such results provide a novel way to simultaneously distinguish and display all possible six polarizations. It is also shown: (i)In EM response, pure cross-, x-type and pure y-type polarizations can independently generate perturbative photon fluxes(PPFs, signals), while plus-, b- and l-type polarizations produce PPFs in different combination states. (ii) All such six polarizations have separability and detectability. (iii)In EM response to HFGWs from extra-dimensions, distinguishing and displaying different polarizations would be quite possible due to their very high frequencies, large energy densities and special properties of spectrum. (iv)Detection band(10^8 to 10^12 Hz or higher) of PPFs by 3DSR and observation range(7*10^7 to 3*10^9 Hz) of PPFs by FAST (Five-hundred-meter-Aperture-Spherical Telescope, China), have a certain overlapping property, so their coincidence experiments will have high complementarity. △ Less

Submitted 21 December, 2021; v1 submitted 3 December, 2017; originally announced December 2017.

Comments: 27 pages, 16 figures

Journal ref: Eur. Phys. J. C (2020) 80:879

Oscillating scalar fields in extended quintessence

Authors: Dan Li, Shi Pi, Robert J. Scherrer

Abstract: We study a rapidly-oscillating scalar field with potential $V(φ) = k|φ|^n$ nonminally coupled to the Ricci scalar $R$ via a term of the form $(1- 8 πG_0 ξφ^2) R$ in the action. In the weak coupling limit, we calculate the effect of the nonminimal coupling on the time-averaged equation of state parameter $γ= (p + ρ)/ρ$. The change in $\langle γ\rangle$ is always negative for $n \ge 2$ and always po… ▽ More We study a rapidly-oscillating scalar field with potential $V(φ) = k|φ|^n$ nonminally coupled to the Ricci scalar $R$ via a term of the form $(1- 8 πG_0 ξφ^2) R$ in the action. In the weak coupling limit, we calculate the effect of the nonminimal coupling on the time-averaged equation of state parameter $γ= (p + ρ)/ρ$. The change in $\langle γ\rangle$ is always negative for $n \ge 2$ and always positive for $n < 0.71$ (which includes the case where the oscillating scalar field could serve as dark energy), while it can be either positive or negative for intermediate values of $n$. Constraints on the time-variation of $G$ force this change to be infinitesimally small at the present time whenever the scalar field dominates the expansion, but constraints in the early universe are not as stringent. The rapid oscillation induced in $G$ also produces an additional contribution to the Friedman equation that behaves like an effective energy density with a stiff equation of state, but we show that, under reasonable assumptions, this effective energy density is always smaller than the density of the scalar field itself. △ Less

Submitted 3 October, 2017; originally announced October 2017.

Comments: 14 pages

Report number: YITP-17-103

Journal ref: Phys. Rev. D 97, 023530 (2018)

Conformal Bootstrap in the Regge Limit

Authors: Daliang Li, David Meltzer, David Poland

Abstract: We analytically solve the conformal bootstrap equations in the Regge limit for large N conformal field theories. For theories with a parametrically large gap, the amplitude is dominated by spin-2 exchanges and we show how the crossing equations naturally lead to the construction of AdS exchange Witten diagrams. We also show how this is encoded in the anomalous dimensions of double-trace operators… ▽ More We analytically solve the conformal bootstrap equations in the Regge limit for large N conformal field theories. For theories with a parametrically large gap, the amplitude is dominated by spin-2 exchanges and we show how the crossing equations naturally lead to the construction of AdS exchange Witten diagrams. We also show how this is encoded in the anomalous dimensions of double-trace operators of large spin and large twist. We use the chaos bound to prove that the anomalous dimensions are negative. Extending these results to correlators containing two scalars and two conserved currents, we show how to reproduce the CEMZ constraint that the three-point function between two currents and one stress tensor only contains the structure given by Einstein-Maxwell theory in AdS, up to small corrections. Finally, we consider the case where operators of unbounded spin contribute to the Regge amplitude, whose net effect is captured by summing the leading Regge trajectory. We compute the resulting anomalous dimensions and corrections to OPE coefficients in the crossed channel and use the chaos bound to show that both are negative. △ Less

Submitted 11 October, 2017; v1 submitted 9 May, 2017; originally announced May 2017.

Comments: 40 pages, 1 figure; V2: Small corrections and clarifications

Parametric Solution of a Small-Large Black Hole Coexistence Curve

Authors: Shanshan Li, Dan-Dan Li, Li-Qin Mi, Zhong-Heng Li

Abstract: We consider the first-order phase transition of a charged anti-de Sitter black hole, and find that the equation of state with the conditions of the two coexisting phases, leads to the two coupled equations about the thermodynamic volumes of small black hole and large black hole. By solving the equations, it is found that each reduced volume is only a function of the parameter $ω$ . All properties… ▽ More We consider the first-order phase transition of a charged anti-de Sitter black hole, and find that the equation of state with the conditions of the two coexisting phases, leads to the two coupled equations about the thermodynamic volumes of small black hole and large black hole. By solving the equations, it is found that each reduced volume is only a function of the parameter $ω$ . All properties of the coexistence curve can be studied from the two volume functions. In particular, each thermodynamic quantity is described by a piecewise analytic function. The demarcation point is located at $ω_{d}=12(2\sqrt{3}-3)$. The thermodynamic function but not its derivative, is continuous at the point. This property is completely different from that of the ven der Waals fluid. Moreover, the thermodynamic behaviors as $ω\rightarrow0$ are discussed. From which one can easily obtain some critical exponents and amplitudes for small-large black hole phase transitions. △ Less

Submitted 19 January, 2017; originally announced January 2017.

Classifying the behavior of noncanonical quintessence

Authors: Dan Li, Robert J. Scherrer

Abstract: We derive general conditions for the existence of stable scaling solutions for the evolution of noncanonical quintessence, with a Lagrangian of the form $\mathcal{L}(X,φ)=X^α-V(φ)$, for power-law and exponential potentials when the expansion is dominated by a background barotropic fluid. Our results suggest that in most cases, noncanonical quintessence with such potentials does not yield interesti… ▽ More We derive general conditions for the existence of stable scaling solutions for the evolution of noncanonical quintessence, with a Lagrangian of the form $\mathcal{L}(X,φ)=X^α-V(φ)$, for power-law and exponential potentials when the expansion is dominated by a background barotropic fluid. Our results suggest that in most cases, noncanonical quintessence with such potentials does not yield interesting models for the observed dark energy. When the scaling solution is not an attractor, there is a wide range of model parameters for which the evolution asymptotically resembles a zero-potential solution with equation of state parameter $w = 1/(2α-1)$, and oscillatory solutions are also possible for positive power-law potentials; we derive the conditions on the model parameters which produce both types of behavior. We investigate thawing noncanonical models with a nearly-flat potential and derive approximate expressions for the evolution of $w(a)$. These forms for $w(a)$ differ in a characteristic way from the corresponding expressions for canonical quintessence. △ Less

Submitted 13 April, 2016; v1 submitted 15 February, 2016; originally announced February 2016.

Comments: 6 pages, 1 figure, minor clarifications and corrections

Journal ref: Phys. Rev. D 93, 083509 (2016)

Note on stability of new hyperbolic AdS black holes and phase transitions in Rényi entropies

Authors: Zhen Fang, Song He, Danning Li

Abstract: We construct a series of new hyperbolic black hole solutions in Einstein-Scalar system and we apply holographic approach to investigate the spherical Rényi entropy in various deformations of dual conformal field theories (CFTs). Especially, we introduce various powers of scalars in the scalar potentials for massive and massless scalar. These scalar potentials correspond to deformation of dual CFTs… ▽ More We construct a series of new hyperbolic black hole solutions in Einstein-Scalar system and we apply holographic approach to investigate the spherical Rényi entropy in various deformations of dual conformal field theories (CFTs). Especially, we introduce various powers of scalars in the scalar potentials for massive and massless scalar. These scalar potentials correspond to deformation of dual CFTs. Then we solve asymptotically hyperbolic AdS black hole solutions numerically. We map the instabilities of these black hole solutions to phase transitions of field theory in terms of CHM mapping between hyperbolic hairy AdS black hole and spherical Rényi entropy in dual field theories. Based on these solutions, we study the temperature dependent condensation of dual operator of massive and massless scalar respectively. These condensations show that there might exist phase transitions in dual deformed CFTs. We also compare free energy between asymptotically hyperbolic AdS black hole solutions and hyperbolic AdS Schwarz (AdS-SW) black hole to test phase transitions. In order to confirm the existence of phase transitions, we turn on linear in-homogenous perturbation to test stability of these hyperbolic hairy AdS black holes. In this paper, we show how potential parameters affect the stability of hyperbolic black holes in several specific examples. For general values of potential parameters, it needs further studies to see how the transition happens. Finally, we comment on these instabilities associated with spherical Rényi entropy in dual deformed CFTs. △ Less

Submitted 13 July, 2017; v1 submitted 21 January, 2016; originally announced January 2016.

Comments: 40 Pages, 18 figures. Title changed, References updated, Structure reorganized, Typos and Mistakes corrected. Accepted by NPB

Dynamical holographic QCD model: resembling renormalization group from ultraviolet to infrared

Authors: Danning Li, Mei Huang

Abstract: Resembling the renormalization group from ultraviolet to infrared, we construct a dynamical holographic model in the graviton-dilaton-scalar framework, where the dilaton background field $Φ$ and scalar field $X$ are responsible for the gluodynamics and chiral dynamics, respectively. At the UV boundary, the dilaton field is dual to the dimension-4 gluon operator, and the scalar field is dual to the… ▽ More Resembling the renormalization group from ultraviolet to infrared, we construct a dynamical holographic model in the graviton-dilaton-scalar framework, where the dilaton background field $Φ$ and scalar field $X$ are responsible for the gluodynamics and chiral dynamics, respectively. At the UV boundary, the dilaton field is dual to the dimension-4 gluon operator, and the scalar field is dual to the dimension-3 quark-antiquark operator. The metric structure at IR is automatically deformed by the nonperturbative gluon condensation and chiral condensation in the vacuum. The produced scalar glueball spectra in the graviton-dilaton framework agree well with lattice data, and the light-flavor meson spectra generated in the graviton-dilaton-scalar framework are in well agreement with experimental data. Both the chiral symmetry breaking and linear confinement are realized in this dynamical holographic QCD model. The necessary condition for the existence of linear quark potential is discussed, and the pion form factor is also investigated in the dynamical holographic QCD model. △ Less

Submitted 4 November, 2013; originally announced November 2013.

Comments: 8 figures, Contribution for the Proceedings of the Karl Schwarzschild Meeting 2013 (Frankfurt, July 22-26 2013)

Entanglement Temperature in Non-conformal Cases

Authors: Song He, Danning Li, Jun-Bao Wu

Abstract: Potential reconstruction can be used to find various analytical asymptotical AdS solutions in Einstein dilation system generally. We have generated two simple solutions without physical singularity called zero temperature solutions. We also proposed a numerical way to obtain black hole solution in Einstein dilaton system with special dilaton potential. By using this method, we obtain the correspon… ▽ More Potential reconstruction can be used to find various analytical asymptotical AdS solutions in Einstein dilation system generally. We have generated two simple solutions without physical singularity called zero temperature solutions. We also proposed a numerical way to obtain black hole solution in Einstein dilaton system with special dilaton potential. By using this method, we obtain the corresponding black hole solutions numerically and investigate the thermal stability of the black hole by comparing the free energy of thermal gas and the corresponding black hole. In two groups of non-conformal gravity solutions obtained in this paper, we find that the two thermal gas solutions are more unstable than black hole solutions respectively. Finally, we consider black hole solutions as a thermal state of zero temperature solutions to check that the first thermal dynamical law exists in entanglement system from holographic point of view. △ Less

Submitted 29 October, 2013; v1 submitted 4 August, 2013; originally announced August 2013.

Comments: 39 pages, 14 figures, version accepted by JHEP

Journal ref: JHEP10(2013)142

Measuring Neutron Star Mass and Radius with Three Mass-Radius Relations

Authors: C. M. Zhang, H. X. Yin, Y. Kojima, H. K. Chang, R. X. Xu, X. D. Li, B. Zhang, B. Kiziltan

Abstract: We propose to determine the mass and the radius of a neutron star (NS) using three measurable mass-radius relationships, namely the ``apparent'' radius inferred from neutron star thermal emission, the gravitational redshift inferred from the absorption lines, as well as the averaged stellar mass density inferred from the orbital Keplerian frequency derived from the kilohertz quasi periodic oscil… ▽ More We propose to determine the mass and the radius of a neutron star (NS) using three measurable mass-radius relationships, namely the ``apparent'' radius inferred from neutron star thermal emission, the gravitational redshift inferred from the absorption lines, as well as the averaged stellar mass density inferred from the orbital Keplerian frequency derived from the kilohertz quasi periodic oscillation (kHz QPO) data. We apply the method to constrain the NS mass and the radius of the X-ray sources, 1E 1207.4-5209, Aql X-1 and EXO 0748-676. △ Less

Submitted 21 November, 2006; originally announced November 2006.

Comments: 8 pages, 3 figures, accepted by MNRAS

Journal ref: Mon.Not.Roy.Astron.Soc.374:232-236,2007