arXiv subjects

Chao-Jun Feng

Publications and source records attributed to Chao-Jun Feng.

At least 19 recordsLinked to original sources

arXiv2026

Analytic motions of spinning particles in Schwarzschild-(anti-)de Sitter spacetime

The motion of a spinning test particle in a static, spherically symmetric spacetime with a cosmological constant is studied within the Mathisson-Papapetrou-Dixon formalism, working to linear order in the particle's spin. By exploiting the symmetries of the background, the radial and latitudinal dynamics are reduced to first-order equations governed by a spin-deformed quintic polynomial. A complete classification of the real root structure of this quintic is carried out, from which the allowed orbital types for spinning particles in the Schwarzschild-anti-de Sitter and Schwarzschild-de Sitter geometries follow directly. The parameter dependence of the root structure and of the stable circular orbit interval is analyzed. In particular, for values of the cosmological constant beyond the Stuchl\'ik limit, where spinless particles admit no bound orbits, spinning particles can still remain bound. The radial motion is expressed piecewise in terms of Lauricella hypergeometric functions, with the simple positive real roots of the quintic acting as branch cuts. The latitudinal motion consists of small oscillations about an equatorial plane, and its phase, governed by the same quintic, admits an analogous analytic representation.

gr-qc↗

arXiv2026

Exact dynamics and the spin wall for large-spin particles in Schwarzschild spacetime

The motion of a spinning test particle in a curved spacetime is governed by the Mathisson--Papapetrou--Dixon (MPD) equations and deviates from geodesic motion already at first order in the spin. While essentially all existing studies truncate the dynamics at linear order in the spin, we present an exact, nonperturbative treatment of planar motion in Schwarzschild spacetime under the Tulczyjew--Dixon spin supplementary condition: eliminating the four-velocity recasts the MPD system into a closed algebraic form and reduces the radial motion to an effective-potential problem, with no expansion in the spin at any stage. This exact framework uncovers qualitative features that are absent from---and in fact unattainable within---the linearized description. Most notably, for sufficiently large spin the effective potential develops a double root at a characteristic radius determined solely by the particle mass and spin, marking an impenetrable \emph{spin wall} of purely spin origin; beyond a critical spin the wall lies outside the event horizon and shields it from generic infalling particles. Moreover, the wall is a filter for particle: only orbits with a specific combinations of spin, angular momentum and energy can penetrate it, all others being reflected before reaching the horizon. In addition, the innermost stable circular orbit, which in linear treatments merely shifts continuously with spin, is obtained in closed form in the weak-field limit and is shown to cease to exist at sufficiently large spin. We further compute the spin correction to the perihelion precession in the weak-field limit and verify that all results reduce to the standard ones at vanishing spin. The spin wall and its filtering rule are genuine nonperturbative phenomena, invisible to any finite-order expansion in the spin, with potential observational signatures in accretion flows around compact objects.

gr-qc↗

arXiv2025

Dynamical analysis of the H^2 + H^-2 Dark Energy model considering viscosity and interaction

In this study, we further developed and investigated the dual parameter phenomenological dark energy model (H^2 + H^-2 dark energy model) derived from Kaniadakis holographic dark energy. On the theoretical basis of the original H^2 + H^-2 dark energy model (HHDE), four types of viscosities and seven types of interactions were introduced. These were combined pairwise, and a dynamical analysis was conducted on a total of 35 Modified H^2 + H^-2 Viscous Interacting Dark Energy (MHH-VIDE) models. The advantage of the HHDE model and MHH-VIDE models is that these models can greatly relieve the Hubble tension and cicumventing the potential issue of a 'big rip', and the dark energy is Quintom-like. In this article, we performed a three-dimensional dynamical analysis of the aforementioned models with interactions and viscosity, testing their viability. The results suggest that the nature of this dark energy is closer to a property of spacetime than a cosmological component. The phase diagram analysis reveals a modified radiation-dominated epoch, a transitional matter-dominated phase, and a late-time attractor corresponding to the dark-energy-driven acceleration phase.

astro-ph.CO↗

arXiv2025

Images of nonsingular nonrotating black holes in conformal gravity

The accretion disk around a black hole and its emissions play an essential role in theoretical analysis of the black hole image. In the literature, two analytical toy models of accretions are widely adopted: the spherical model and the thin disk model. They are different geometrically but both thin optically. We polish them for free-falling accretions around static spherical black holes. As an application, we investigate the images of a class of nonsingular black holes conformally related to the Schwarzschild black hole. These black holes are vacuum solutions of a family of conformal gravity theories. Results are compared with the Schwarzschild black hole of the same mass. Our results indicate that the conformal factor does not affect the shadow radius seen by distant observers, but it leaves an imprint on the intensity image of black hole.

gr-qc↗

arXiv2025

Higher order analysis of the gravitational wave velocity memory effect between two free-falling gyroscopes in the plane wave spacetime

In the plane wave spacetime, when gravitational waves pass by, an angular deviation exists between two free-falling gyroscopes, which naturally corresponds to the velocity memory effect. In the shackwave spacetime background, the angular deviation between two free-falling gyroscopes is calculated, which is also found to correspond to the velocity memory effect. In the plane wave spacetime, with linear polarization taken into account, no contribution is made by the first-order terms of the initial separation distance $L$ (or the initial separation velocity $v_0$), $\bm{P}$ (or $\bm{M}$) of two free-falling gyroscopes to the velocity memory effect, while contributions are initiated from the second-order terms. Under certain circumstances, the second-order contribution of the initial separation distance $L$ is of the same order of magnitude as the first-order contribution of the initial separation velocity $v_0$. When both + polarization and $\times$ polarization are taken into account, in the context of the merger of supermassive black holes and with the initial separation velocity approaching the speed of light, the order of magnitude of the angle is $10^{-16}$ rads.

gr-qc↗

arXiv2024

Thermodynamics of the $H^2 + H^{-2}$ Dark Energy model

In this study, we introduced a phenomenological dark energy model $H^{2}+H^{-2}$ model) that incorporates the first-order approximation of Kaniadakis holographic entropy dark energy and utilizes the Hubble horizon, $1/H$ as the infrared cutoff. The advantage of this model is that it can relieve the Hubble tension issue and cicumventing the potential issue of a "big rip". In this article, we will study the thermodynamics of the model and obtain the corrected temperature. In addition, we found that the model modified the entropy area relationship by adding an area cubic term in addition to the area term.

gr-qc↗

arXiv2024

Acceleration of the Universe without the Hubble tension with Kaniadakis holographic dark energy using the Hubble horizon as the IR cut-off

We introduce a holographic dark energy model that incorporates the first-order approximate Kaniadaski entropy, utilizing the Hubble horizon, $1/H$, as the infrared cutoff. We investigate the cosmological evolution within this framework. The model introduces an extra parameter relative to the $Λ$CDM model. It posits a Universe that is initially dominated by dark matter, which then evolves to a phase where dark energy becomes the predominant component, with this transition occurring at a redshift of approximately $z \sim 0.419$. The energy density of dark energy is ultimately expected to become constant, thereby circumventing the potential issue of a "big rip". Employing the most recent Type Ia supernova and Hubble parameter data, we constrain the model's parameters and find a Hubble constant of $H_0=72.8$ km/s/Mpc, thereby resolving the Hubble tension issue. The estimated age of the Universe, based on the best-fit parameter values, is $14.2$ Gyr. Furthermore, we predict the number of strong gravitational lenses and conduct statefinder and $Om$ diagnostic analyses to validate and characterize the model.

astro-ph.CO↗

arXiv2024

Image of Kerr-de Sitter black holes illuminated by equatorial thin accretion disks

To explore the influence of the cosmological constant on black hole images, we have developed a comprehensive analytical method for simulating images of Kerr-de Sitter black holes illuminated by equatorial thin accretion disks. Through the application of explicit equations, we simulate images of Kerr-de Sitter black holes illuminated by both prograde and retrograde accretion disks, examining the impact of the cosmological constant on their characteristic curves, relative sizes, and observed intensities. Our findings reveal that, in comparison to Kerr black holes, the cosmological constant not only diminishes the relative size of a black hole but also amplifies its luminosity. Moreover, an observer's relative position in the universe ($r_0/r_C$) can influence both the relative size and luminosity of a black hole, where $r_0$ is the distance from the observer to the black hole, $r_C$ is the cosmological horizon determined by the value of the cosmological constant $Λ$.

gr-qc↗

arXiv2024

Geometric deformation and redshift structure caused by plane gravitational waves

The curved spacetime induced by gravitational waves can give rise to visual effects such as geometric distortions and redshift structures in the observed image. By establishing a mapping from the object's surface coordinates to the observer's screen coordinates, we study these effects in the context of plane gravitational waves. The simulation reveals that the image of an object doesn't merely seem compressed or stretched, but rather appears twisted and wobbled. Furthermore, the redshift structure on the object's surface appears to rotate as a whole. This outcome offers an intuitive depiction of the lensing effect in plane gravitational wave spacetimes.

gr-qc↗

arXiv2023

Reduced Kiselev black hole

The Kiselev model describes a black hole surrounded by a fluid with equations of state $p_r/ρ=-1$ and $p_t/ρ=(3w+1)/2$ respectively in radial and tangential directions. It has been extensively studied in the parameter region $-1 0$, then a new horizon of black hole type will emerge. This case has been mentioned in Kiselev's pioneer work but seldom investigated in the literature. Referring to it as reduced Kiselev black hole, we revisit this case with attention to its causal structure, thermodynamics, shadow cast and weak-field limit. An alternative interpretation and extensions of the black hole are also discussed.

gr-qc↗

arXiv2023

Viscous effect in the late time evolution of phantom universe

We investigate the cosmological implications of a phantom dark energy model with bulk viscosity. We explore this model as a possible way to resolve the big rip singularity problem that plagues the phantom models. We use the latest type Ia supernova and Hubble parameter data to constrain the model parameters and find that the data favor a significant bulk viscosity over a non-constant potential term for the phantom field. We perform a dynamical analysis of the model and show that the only stable and physical attractor corresponds to a phantom-dominated era with a total equation of state that can be greater than $-1$ due to the viscosity. We also study the general effect of viscosity on the phantom field and the late time evolution of the universe. We apply the statefinder diagnostic to the model and find that it approaches a nearby fixed point asymptotically, indicating that the universe can escape the big rip singularity with the presence of bulk viscosity. We conclude that bulk viscosity can play an important role in affecting the late-time behavior as well as alleviating the singularity problem of the phantom universe.

gr-qc↗

arXiv2023

Spin vector deviation and the gravitational wave memory effect between two free-falling gyroscopes in the plane wave spacetime

In the plane wave spacetime, we find that there will be a precession angle deviation between two free-falling gyroscopes when gravitational waves passed through. This kind of angle deviation is closely related to the well-known standard velocity memory effect. Initial conditions such as the separation velocity or displacement between the two gyroscopes will affect this angle deviation. The evolutions of the angle deviation are calculated for different cases. We find that in some extreme circumstance, the angle deviation's order of magnitude produced by a rotating compact binary source could be $10^{-14}$ rads. Therefore, this memory effect caused by the gravitational wave is likely to be detected in the future.

gr-qc↗

arXiv2022

Latest data constraint of some parameterized dark energy models

Using various latest cosmological datasets including Type-Ia supernovae, cosmic microwave background radiation, baryon acoustic oscillations, and estimations of the Hubble parameter, we test some dark energy models with parameterized equations of state and try to distinguish or select observation-preferred models. We obtain the best fitting results of the six models and calculate their values of the Akaike Information Criteria and Bayes Information Criterion. And we can distinguish these dark energy models from each other by using these two information criterions. However, the $Λ$CDM model remains the best fit model. Furthermore, we perform geometric diagnostics including statefinder and Om diagnostics to understand the geometric behaviour of the dark energy models. We find that the six DE models can be distinguished from each other and from $Λ$CDM, Chaplygin gas, quintessence models after the statefinder and Om diagnostics were performed. Finally, we consider the growth factor of the dark energy models with comparison to $Λ$CDM model. Still, we find the models can be distinguished from each other and from $Λ$CDM model through the growth factor approximation.

astro-ph.CO↗

arXiv2022

Photons generated by gravitional waves in the near-zone of a neutron star

When a gravitational wave or a graviton travels through an electric or magnetic background, it could convert into a photon with some probability. In this paper, a dipole magnetic field is considered as this kind of background in both the Minkowski spacetime and the curved spacetime in the near-zone of a neutron star. In the former case, we find that the graviton traveling vertically rather than parallel to the background magnetic field could be more effectively converted into an electromagnetic radiation field. In the latter case, we focus on the situation, in which the graviton travels along the radial direction near a neutron star. The radius of a neutron star is about ten kilometers, so the gravitational wave with long wavelength or low frequency may bypass neutron stars by diffraction. For high frequency gravitational wave, the conversion probability is proportional to the distance square as that in the static electric or magnetic background case. The smaller the inclination angle between the dipole field and the neutron star north pole is, the larger magnetic amplitude will be. The term that described curved spacetime will slightly enhance this kind of probability. We estimate that this value is about the order of $\sim 10^{-14}- 10^{-10}$. Therefore, it is expectable that this kind of conversion process may have a potential to open a window for observing high frequency gravitational waves.

gr-qc↗

arXiv2020

Dynamics of Viscous Phantom Universe

The phantom dark energy remarkably boosts our prehension of the accelerating Universe. Various models are widely discussed in the phantom Universe without bulk viscosity. From the hydrodynamics' point of view, it is natural to introduce the nonperfect fluid in the study of the Universe, as an ideal fluid just an approximation to the real world after all and using the generalized equation of state (EoS) with bulk viscosity, the early inflationary universe and the accelerated expansion of the late-time universe are described by many authors. In this paper, in order to investigate how the viscosity will influence the evolution of the Universe, we study a class of phantom dark energy models with bulk viscosity by the method of dynamical analysis technique. We show that there are different cosmic late-time behaviors and the stability also brings some constraints on the models. We also plot the evolutionary trajectories of this model in the statefinder parameter-planes to see the different behaviors of the models from the statefinder viewpoint.

gr-qc↗

arXiv2020

Solar System Tests of a New Class of $f(z)$ Theory

Recently, a new kind of $f(z)$ theory is proposed to provide a different perspective for the development of reliable alternative models of gravity in which the $f(R)$ Lagrangian terms are reformulated as polynomial parameterizations $f(z)$. In the previous study, the parameters in the $f(z)$ models have been constrained by using cosmological data. In this paper, these models will be tested by the observations in the solar system. After solving the Ricci scalar as a function of the redshift, one could obtain $f(R)$ that could be used to calculate the standard Parameterized-Post-Newtonian (PPN) parameters. First, we fit the parametric models with the latest cosmological observational data. Then the tests are performed by solar system observations. And last we combine the constraints of solar system and cosmology together and reconstruct the $f(R)$ actions of the $f(z)$ parametric models.

astro-ph.CO↗

arXiv2020

Artificial Neural Network Spectral Light Curve Template for Type Ia Supernovae and its Cosmological Constraints

The spectral energy distribution (SED) sequence for type Ia supernovae (SN Ia) is modeled by an artificial neural network. The SN Ia luminosity is characterized as a function of phase, wavelength, a color parameter and a decline rate parameter. After training and testing the neural network, the SED sequence could give both the spectrum with wavelength range from 3000Å~to 8000Å~ and the light curve with phase from 20 days before to 50 days after the maximum luminosity for the supernovae with different colors and decline rates. Therefore, we call this the Artificial Neural Network Spectral Light Curve Template (ANNSLCT) model. We retrain the Joint Light-curve Analysis (JLA) supernova sample by using the ANNSLCT model and obtain the parameters for each supernova to make a constraint on the cosmological $Λ$CDM model. We find that the best fitting values of these parameters are almost the same as those from the JLA sample trained with the Spectral Adaptive Lightcurve Template 2 (SALT2) model. So we believe that the ANNSLCT model could be used to analyze a large number of SN Ia multi-color light curves measured in the current and future observational projects.

astro-ph.CO↗

arXiv2020

Generalized Quantum Spring

Recently, it was found that after imposing a helix boundary condition on a scalar field, the Casimir force coming from the quantum effect is linearly proportional to $r$, which is the ratio of the pitch to the circumference of the helix. This linear behavior of the Casimir force is just like that of the force obeying the Hooke's law on a spring. In this paper, inspiring by some complex structures that lives in the cells of human body like DNA, protein, collagen etc., we generalize the helix boundary condition to a more general one, in which the helix consists of a tiny helix structure, and makes up a hierarchy of helix. After imposing this kind of boundary condition on a massless and a massive scalar, we calculate the Casimir energy and force by using the so-called zeta function regularization method. We find that the Hooke's law with the generalized helix boundary condition is not exactly the same as usual one. In this case, the force is proportional to the cube of $r$ instead. So we regard it as a generalized Hooke's law, which is complied by a \emph{generalized quantum spring}.

hep-th↗