arXiv subjects

Xiao-Jun Yue

Publications and source records attributed to Xiao-Jun Yue.

13 recordsLinked to original sources

arXiv2026

Near-extremal asymptotics and strong cosmic censorship for black holes immersed in a Chaplygin-like dark fluid

We investigate strong cosmic censorship (SCC) for massless scalar perturbations of electrically neutral black holes immersed in a Chaplygin-like dark fluid (CDF). The matter distribution produces an asymptotically de Sitter exterior while supporting an inner Cauchy horizon. We derive a closed-form parametrization of the extremal and Nariai horizon boundaries. This analytic control yields explicit near-extremal asymptotics for the horizon splitting and Cauchy-horizon surface gravity, together with an analytic expression for the leading near-extremal scalar quasinormal spectrum. The fundamental near-extremal damping rate approaches the Cauchy-horizon surface gravity, whereas higher angular multipoles retain explicit dependence on the CDF extremal geometry. Combining these analytic results with global quasinormal-mode calculations, we determine the spectral gap and assess the Christodoulou formulation of SCC throughout the three-horizon domain. Potential SCC violation is confined to a narrow region near extremality. As the effective cosmological scale increases, the lower boundary of this region is controlled successively by de Sitter, photon-sphere, and de Sitter modes, while its normalized width varies nonmonotonically.

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arXiv2026

Deviations from Kerr: Polar critical curves and photon rings in a class of separable spacetimes

Black-hole critical curves and photon rings provide complementary probes of strong-field geometry, motivating a unified analytic description of these observables beyond Kerr and a direct comparison with the Kerr predictions. We develop such a description for polar observers in a radial class of stationary, axisymmetric spacetimes with separable null geodesics. The spacetime dependence is organized through natural optical combinations selected by the null dynamics and a Kerr-relative representation suited to weak deviations from Kerr. We derive compact exact expressions for the polar critical radius, Lyapunov exponent, time delay, and rotation parameter, together with general first-order formulas that can be applied once the deformation functions of a specific metric are supplied. The weak-deviation analysis shows that the combined polar observables probe only a finite set of local deformation values and radial derivatives at the critical orbit, rather than the full radial profiles.

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arXiv2026

Optical Appearance of the Kerr-Bertotti-Robinson Black Hole with a Magnetically Driven Synchrotron Emissivity Model

We investigate the optical appearance of a Kerr-Bertotti-Robinson (Kerr-BR) black hole illuminated by a geometrically and optically thin accretion disk. Instead of using a phenomenological power-law emissivity, we adopt a magnetically driven synchrotron emissivity proxy coupled to the local electromagnetic environment. With a backward ray-tracing framework, we examine the effects of the spin $a$, magnetic parameter $B$, and observer inclination $θ_O$ on the ray-classification maps, redshift distributions, and specific-intensity images. We show that the ISCO position is modified by both $a$ and $B$, and that rapidly rotating prograde configurations can develop an additional model-dependent inner cutoff when the magnetically dominated approximation underlying the emissivity prescription ceases to be applicable. High-resolution one-dimensional intensity profiles further separate the direct image, the $n=1$ lensing-ring contribution, and the higher-order $n\geq 2$ photon-ring subimages, while quantifying the Doppler-induced brightness asymmetry. Retrograde disks exhibit a wider emission-depleted central region because of the outwardly shifted ISCO, making the higher-order lensed components more clearly distinguishable from the direct emission. These results show that the disk inner boundary and the magnetic-field-dependent emissivity can substantially influence the observable appearance of Kerr-BR black holes.

astro-ph.HE↗

arXiv2026

Cosmological Constraints on 4D Einstein-Gauss-Bonnet Gravity and Kaniadakis Holographic Dark Energy: Implications for Black Hole Shadows

The direct imaging of black holes by the Event Horizon Telescope (EHT) enables strong-field tests of gravity. We study the cosmological evolution and the black-hole shadow radius in 4D Einstein-Gauss-Bonnet (EGB) gravity coupled to Kaniadakis holographic dark energy (KHDE), adopting the future event horizon as the infrared cutoff. Using Cosmic Chronometers, Pantheon+ Type Ia supernovae, and DESI BAO data, we constrain the model with a Markov Chain Monte Carlo analysis. The best-fit values favor a phantom-like equation of state driven by Kaniadakis entropy ($c\simeq 1.18$, $β\simeq 2.26$), but $β$ remains weakly constrained ($β=2.26^{+0.11}_{-2.20}$), consistent with the standard holographic limit $β\to0$ at $1σ$. The EGB coupling is constrained to $α\simeq -0.004$, also consistent with General Relativity ($α=0$) at $1σ$. Guided by the posterior, we define five representative scenarios to probe the dynamical phase space. We find that the accretion history is highly sensitive to the thermodynamic sector: standard holographic cases yield monotonic evolution, whereas phantom-divide crossing leads to non-monotonic behavior in both the black hole mass and the vacuum shadow radius. Including a dispersive plasma medium, refraction dominates over intrinsic mass growth and induces an overall shrinkage of the observable shadow at high redshift; nevertheless, a residual intrinsic deviation of $\sim6\%$ (for our conservative accretion setup) persists at $z\simeq2$ relative to the $Λ$CDM prediction. These results indicate that, despite environmental dominance, precision population analyses of black hole shadows may help disentangle subtle dynamical dark-energy imprints from the standard cosmological paradigm.

astro-ph.CO↗

arXiv2026

Gravitational-wave imprints of Kerr--Bertotti--Robinson black holes: frequency blue-shift and waveform dephasing

We investigate the orbital dynamics and gravitational wave signatures of neutral Extreme Mass Ratio Inspirals (EMRIs) in the spacetime of a Kerr black hole immersed in an asymptotically uniform magnetic field, described by the exact Kerr-Bertotti-Robinson (Kerr-BR) solution~\cite{Podolsky:2025tle}. Unlike the widely used Kerr-Melvin metric, the Kerr-BR solution is of algebraic type D, allowing for a rigorous analysis of geodesics and possessing a clear asymptotic structure. By analyzing the Innermost Stable Circular Orbit (ISCO), we confirm that the external magnetic field consistently pushes the ISCO to larger radii. However, contrary to Newtonian intuition, this radial expansion is accompanied by a systematic magnetically induced hardening of the spectrum, where the ISCO frequency is blue-shifted relative to the vacuum case. Notably, in the strong-field regime, we identify a non-monotonic frequency evolution, where the orbital frequency initially decreases before rising rapidly near the horizon, fundamentally altering the chirp character. We further demonstrate that retrograde orbits are significantly more sensitive to magnetic fields than prograde orbits, leading to frequency crossover phenomena where magnetic effects can invert the usual spin-frequency hierarchy. Finally, employing a semi-analytic adiabatic evolution scheme, we quantify the dephasing accumulated during the final year of inspiral. Our results demonstrate that space-borne detectors like LISA can distinguish magnetic environments from vacuum spacetimes for field strengths as low as $B \sim 10^{-4}$, suggesting that environmental magnetic fields could introduce systematic biases in parameter estimation if not properly modeled.

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arXiv2025

Black holes immersed in modified Chaplygin-like dark fluid and cloud of strings: shadows, quasinomal modes and greybody factors

We present a unified investigation of black hole shadows, quasinormal modes (QNMs), and greybody factors (GBFs) for a static, spherically symmetric black hole within a composite environment of a modified Chaplygin-like dark fluid (MCDF) and a cloud of strings (CoS). We examine the structure of critical photon orbits and the corresponding optical appearance under spherical accretion. Using the Wentzel-Kramers-Brillouin (WKB) approximation, we compute the quasinormal frequencies and greybody spectra, and explore their correspondence with the black hole shadows in the eikonal limit. A systematic parameter study demonstrates that the CoS intensity has the primary influence on the shadows, QNMs and GBFs, while the MCDF parameters introduce more complex but characterizable modifications to each. Our results demonstrate that these environmental components imprint distinct yet interrelated signatures on key observables, offering specific predictions for probing exotic black hole environments.

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arXiv2025

Black holes immersed in modified Chaplygin-like dark fluid and cloud of strings: geodesics, shadows, and images

This study investigates a black hole surrounded by a cloud of strings and a cosmological dark fluid characterized by a modified Chaplygin-like equation of state (MCDF), $p=Aρ-B/ρ^β$. We analyze its geodesic structure, shadow, and optical appearance. Analysis of the effective potential and epicyclic frequencies reveals that the existence of innermost/outermost stable circular orbits (ISCOs/OSCOs) for timelike particles is controlled by the parameters of the MCDF and the cloud of strings. The behavior of orbital conserved quantities and the Keplerian frequency are also examined. By equating the influence of the MCDF on the spacetime metric at spatial infinity with that of a cosmological constant, we constrain the MCDF parameters using the observed shadow radii of Sgr A* and M87*. We investigate the effects of the cloud of strings and MCDF on the black hole's shadows and optical images, assuming various thin disk accretion profiles. Using the method developed by Wald and collaborators, light trajectories are classified by their impact parameters into direct emission, the lensing ring, and the photon ring. The presence of OSCOs can lead to the existence of outer edges in the direct emission and lensing ring images. Observed brightness primarily originates from direct emission, with a minor contribution from the lensing ring, while the photon ring's contribution is negligible due to extreme demagnification. The influence of the cloud of strings and MCDF parameters on all results is analyzed throughout the study.

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arXiv2024

Geodesic structure, shadow and optical appearance of black hole immersed in Chaplygin-like dark fluid

In this study, we focus on a black hole immersed in a cosmological Chaplygin-like dark fluid (CDF), characterized by the equation of state $p=-B/ρ$ and an additional parameter $q$ influencing the energy density of the fluid. We investigate the geodesic structure, shadow, and optical appearance of such a black hole. Through analysis on the effective potential and the epicyclic frequencies, it is found that the existence of innermost/outermost stable circular orbits for a timelike particle is governed by the CDF parameters. The behaviors of the orbital conserved quantities and Keplerian frequency are also examined. Due to the existence of pseudo-cosmological horizon, the determination of the shadow radius depends significantly on the position of the observer. By placing the static observer at an approximately flat position between the event and pseudo-cosmological horizons, we constrain the CDF parameters using EHT observations. We investigate the effect of CDF on the shadows and optical images of the black hole, surrounded by various profiles of accretions. For the thin disk accretion, the light trajectories are categorized into direct emission, lensing ring, and photon ring based on impact parameters. Due to the existence of outermost stable circular orbits, outer edges could exist in the direct and lensing ring images. The observed brightness is mainly due to direct emission, with a minor contribution from the lensing ring, while the contribution from the photon ring is negligible due to extreme demagnetization. In the case of spherical accretion, we consider both static and infalling accretion models. The images obtained under infalling accretion are slightly darker than those under static accretion, attributed to the Doppler effect. Throughout the study, we analyze the influence of the parameters $B$ and $q$ on the results.

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arXiv2019

Dark matter minispike: a significant enhancement of eccentricity for intermediate-mass-ratio-inspirals

When a stellar massive compact object, such as a black hole (BH), inspirals into an intermediate massive black hole (IMBH), an intermediate-mass-ratio-inspiral (IMRI) system forms. Such kind of systems are important sources for space-based gravitational wave detectors including LISA, Taiji and Tianqin. Dark matter (DM) minispikes may form around IMBHs. We study the effect of dynamical friction against DM minispike on the evolution of eccentric IMRI. For such investigation we construct the dynamical equations which describes the evolution of eccentric IMRI under the effect of dynamical friction. As dynamical friction is large for small velocity, the dissipation of energy near apastron is more than that near periastron. This will greatly enhance the eccentricity. For instance, with an initial semi-latus rectum of $1\rm AU$, even a moderate DM minispike can make the eccentricity grow from $0.3$ to $0.85$. In the extremal case the eccentricity could be enhanced to near $1$. We also study a specific case which corresponds to an IMRI in the center of a globular cluster (GC) and find the eccentricity can keep its value above $0.95$ until the IMRI enters LISA band. These gravitational wave with enhanced eccentricity by DM minispikes can be easly distinguished from that without DM at $10^{-3}\rm Hz$ due to the eccentricity difference. These anticipations can be tested by future space-based GW detectors such as LISA.

astro-ph.HE↗

arXiv2019

Dark matter: an efficient catalyst for intermediate-mass-ratio-inspiral events

Gravitational waves (GWs) can be produced if a stellar compact object, such as a black hole (BH) or neutron star, inspirals into an intermediate-massive black hole (IMBH) of $(10^3 \sim 10^5)\,M_\odot$. Such a system may be produced in the center of a globular cluster (GC) or a nuclear star cluster (NSC), and is known as an intermediate- or extreme-mass-ratio inspiral (IMRI or EMRI). Motivated by the recent suggestions that dark matter minispikes could form around IMBHs, we study the effect of dynamical friction against DM on the merger rate of IMRIs/EMRIs. We find that the merger timescale of IMBHs with BHs and NSs would be shortened by two to three orders of magnitude. As a result, the event rate of IMRIs/EMRIs are enhanced by orders of magnitude relative to that in the case of no DM minispikes. In the most extreme case where IMBHs are small and the DM minispikes have a steep density profile, all the BH in GCs and NSCs might be exhausted so that the mergers with NSs would dominate the current IMRIs/EMRIs. Our results suggest that the mass function of the IMBHs below $10^4 \,M_\odot$ would bear imprints of the distribution of DM minispikes because these low-mass IMBHs can grow efficiently in the presence of DM minispikes by merging with BHs and NSs. Future space-based GW detectors, like LISA, Taiji, and Tianqin, can measure the IMRI/EMRI rate and hence constrain the distribution of DM around IMBHs.

gr-qc↗

arXiv2018

Gravitational waves with dark matter minispikes: the combined effect

It was shown that the dark matter(DM) minihalo around an intermediate mass black hole(IMBH) can be redistributed into a cusp, called the DM minispike. We consider an intermediate-mass-ratio inspiral consisting of an IMBH harbored in a DM minispike with nonannihilating DM particles and a small black hole(BH) orbiting around it. We investigate gravitational waves(GWs) produced by this system and analyze the waveforms with the comprehensive consideration of gravitational pull, dynamical friction and accretion of the minispike and calculate the time difference and phase difference caused by it. We find that for a certain range of frequency, the inspiralling time of the system is dramatically reduced for smaller central IMBH and large density of DM. For the central IMBH with $10^5M_\odot$, the time of merger is ahead, which can be distinguished by LISA, Taiji and Tianqin. We focus on the effect of accretion and compare it with that of gravitational pull and friction. We find that the accretion mass is a small quantity compared to the initial mass of the small BH and the accretion effect is inconspicuous compared with friction. However, the accumulated phase shift caused by accretion is large enough to be detected by LISA, Taiji and Tianqin, which indicate that the accretion effect can not be ignored in the detection of GWs.

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arXiv2014

Loop Quantum Cosmology of Bianchi I Model in $\barμ$ and $\barμ'$ Schemes with Higher Order Holonomy Corrections

The detailed formulation of loop quantum cosmology with higher order holonomy corrections has been constructed recently in the homogeneous and isotropic spacetime, yet it is important to extend the higher order holonomy corrections to include the effects of anisotropy which typically grow during the collapsing phase. In this paper we investigate the Bianchi I model in $\barμ'$ scheme which truly captures the regularization of the Hamiltonian constraint. To compare with the earlier works and provide a comparison with the $\barμ'$ scheme, we also investigate the $\barμ$ scheme although it has many disadvantages. First we construct the effective dynamics with higher order holonomy corrections in a massless scalar field, then we extend it to the inclusion of arbitrary matter. Besides that, we also analyze the behavior of the anisotropy during the evolution of the universe. We find that in the $\barμ'$ scheme, the singularity is never approached and the quantum bounce is generic as in the isotropic case, regardless of the order of the holonomy corrections. Some differences in the bouncing phase of the two schemes are also found out. It is also shown that in the two schemes the behavior of the anisotropy is not the same before and after the bounce.

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arXiv2013

Power spectrum and anisotropy of super inflation in loop quantum cosmology

We investigate the scalar mode of perturbation of super inflation in the version of loop quantum cosmology in which the gauge invariant holonomy corrections are considered. Given a background solution, we calculate the power spectrum of the perturbation in the classical and LQC conditions. Then we compute the anisotropy originated from the perturbation. It is found that in the presence of the gauge invariant holonomy corrections the power spectrum is exponentially blue and the anisotropy also grows exponentially in the epoch of super inflation.

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