arXiv subjects
Publications and source records attributed to Rong-Jia Yang.
Using gravitational decoupling under the requirements of a well-defined event horizon and the source matter satisfying the weak energy condition, we construct nonsingular hairy black holes with spherical or axial symmetry. These solutions emerge from a deformation of the Minkowski vacuum, bridging the novel hairy geometries and the classical Schwarzschild and Kerr solutions at the maximum deformation in their respective sectors.
We investigate black hole solutions in quadratic $f(R,T)=R+kT^2$ gravity coupled with nonlinear electrodynamics $\mathcal{L}(F)=α- F/(4π) + γF^2$. Solving the field equations yields an exact magnetically charged (anti-)de Sitter black hole metric featuring novel $r^{-6}$ and $r^{-14}$ correction terms. The horizon structure can exhibit up to four horizons depending on the parameters. Thermodynamic analysis reveals that the heat capacity exhibits critical phase transitions, with the critical point significantly shifted by the coupled corrections. Using the effective metric formalism, we study the photon sphere and black hole shadow, finding that the magnetic charge $Q$ shrinks the shadow while the nonlinear parameter $γ$ enlarges it. Constraints from Event Horizon Telescope observations of M87* and Sgr A* place the parameter $γ$ at the order of $10^3$ at $1σ$ and $2σ$ confidence levels. Our results demonstrate that quadratic $f(R,T)$ gravity combined with nonlinear electrodynamics provides a consistent framework for strong-field gravity phenomenology.
We propose a parameterized equation of state for dark energy and perform observational tests with the Hubble parameter measurements, the Pantheon supernova sample, and DESI DR2 data. We obtain the best-fit values for the parameters as: $H_0 = 73.28\pm 0.15~\mathrm{km\,s^{-1}\, Mpc^{-1}}$, $Ω_{\rm m} = 0.316\pm 0.011$, and $α= 0.00032\pm 0.00046$, demonstrating that the model exhibits a high degree of consistency with astronomical observations and provides a promising parameterized method for addressing the Hubble tension.
We investigate the shadows, accretion disks, and observational images of rotating Ayón-Beato--Garc\'ıa (ABG) black holes with mass $M$, spin $a$, and nonlinear-electrodynamics (NLED) charge parameter $ζ$, treating photons as neutral test particles on null geodesics of the background metric so as to obtain the purely geometric shadow. The shadow shrinks with increasing $ζ$ and develops a ``D''-shaped morphology for near-extremal spin. The thermal disk properties follow from the Novikov--Thorne model with inner edge at the innermost stable circular orbit (ISCO), whereas the images use a separate, phenomenological optically thin emission model extending to the horizon. The image asymmetry and redshift maps depend strongly on $(a, ζ)$ and the inclination. Comparing the geometric shadow diameters with Event Horizon Telescope observations of M87$^{*}$ and Sgr A$^{*}$ gives the indicative estimates $ζ\lesssim 0.21\,M$ and $ζ\lesssim 0.43\,M$, respectively; since $ζ$ characterizes each black hole individually, these are quoted separately rather than combined, and the Kerr limit $ζ= 0$ remains fully consistent with both.
We investigate the shadow and optical appearance of Bardeen black hole (BH) immersed in perfect fluid dark matter (PFDM). Using EHT observations of M87* and Sgr A*, we constrain the DM parameter to a narrow range $b/M \sim \mathcal{O}(10^{-1}-10^{-2})$ for M87* and to $\mathcal{O}(10^{-2}-10^{-3})$ for Sgr A*. From these constraints we derive a rough prediction for the PFDM density near the shadow scale ($R_{\mathrm{sh}}\sim5M$): $ρ_{\mathrm{PFDM}} \sim 0.27$-$2.67\,\mathrm{g/cm^3}$ for Sgr A*, dropping to $\sim10^{-24}$-$10^{-25}\,\mathrm{g/cm^3}$ at 100 pc. Moreover, increasing $b$ substantially enlarges the photon sphere, impact parameter, shadow radius, and suppresses the observed disk brightness, while the magnetic charge $g$ produces only negligible corrections completely masked by PFDM on macroscopic scales. Subsequently, we investigate the primary/secondary images, flux, and redshift profiles for the PFDM-Bardeen BH using the Novikov-Thorne disk model, and compare these quantities with those of NFW, Dehnen-type and Moore DM BHs. The four BH types exhibit distinct densities at the shadow radius and at 100 pc, offering a potential distinguishing signature. Furthermore, for all DM BH models, blueshift appears in the primary image as inclination increases, while the secondary image remains redshift dominated even at high inclinations. Hence, if significant blueshifted emission were detected at low inclination, the predictions of these four DM models would be seriously challenged.
Dark matter (DM), a fundamental cosmic component, motivates the study of its influence on black hole (BH) shadows, especially for spinning BHs confirmed by EHT observations. This work generalizes the Schwarzschild BH surrounded by DM to an axisymmetric Kerr BH using the Newman-Janis Algorithm (NJA), investigating the resulting event horizon and ergosphere structures. Employing null geodesics, we examine the effects of DM mass ($Δ$M) on BH shadow, including its radius, distortion, and the associated energy emission rate. Our analysis reveals that DM has a negligible effect below a critical mass, once this threshold is surpassed, all BH structures expand significantly. Furthermore, DM robustly contributes to the shadow maintaining a near circular shape, even for highly spinning BHs. This pronounced structural expansion under high DM mass may potentially exceed current observational constraints, suggesting that DM must either be absent in the immediate vicinity of the BH or its localized mass must remain below this critical value to be consistent with astrophysical observations.
We investigate the shadow, timelike geodesic structure, radiation properties of thin accretion disks, and optical appearance of a static spherically symmetric regular black hole, constructed based on the Dehnen-type density profile. Using observational data from M87* and Sgr A*, we constrain the model parameter $a$ at both $1σ$ and $2σ$ confidence levels. Based on the Page--Thorne model, we calculate the local radiative flux, redshift factor distribution, and the radiation flux received by a distant observer, systematically examining the effects of the parameter $a$ and the viewing angle on the black hole image. The results show that larger $a$ will enlarge the effective radiation area of the accretion disk and significantly enhance the asymmetry and Doppler boosting effects of the direct and secondary images at large viewing angles.
We obtain a new general solution for the gravitational field equations in quantum fluctuation modified gravity, which reduces to different classes of black holes surrounded by fluids, by taking some specific values of the parameter of the equation of state. We discuss the strong energy condition in a general way and also for some special cases of different fluids. In addition, the Hawking temperature associated to the horizons of solutions and constraints on the parameter characterizing the fluctuation of metric are taken into account in our analysis.
We analyze DESI DR2 data with a model-independent method and find that: (a) the expansion of the universe may speed up with a confidence level more than 2.3 $σ$ at redshift $z_{51}\in (0.51, 0.955)$; (b) the expansion of the universe may speed down with a confidence level greater than 1.7 $σ$ at redshift $z_{75}\in (0.955, 1.484)$; (c) $w_{\rm{x}}\leq w_{\rm{t}}<-1$ with confidence level exceeding 1.6 $σ$ at redshift $z_{53}\in (0.922, 0.955)$.
We investigate the steady, axisymmetric, force-free magnetosphere of Kerr-Sen black hole (BH) within the framework of the Einstein-Maxwell-dilaton-axion (EMDA) theory. By perturbatively solving the nonlinear Grad-Shafranov (GS) equation, we determine the magnetic field configuration and quantify the influence of the dilaton parameter $r_2$ on the energy extraction rate and radiative efficiency. Our results show that both the energy extraction power and the radiative efficiency increase with $r_2$, exceeding those of the standard Kerr BH, whereas the extraction efficiency remain consistent with the Kerr case. In addition, we perform $χ^2$ statistical analysis using observational data from six binary BH systems, which indicates that the Kerr BH currently provides a better fit for bulk Lorentz factors $Γ= 2$ and $5$.
Based on the classical limit of relativistic scattering amplitudes, we compute the coupling between a general super-renormalizable gravity and massive scalar particles. This allows us to derive the $D$-dimensional metric corrections at both tree-level and one-loop level-the latter containing the first calculation by using newly derived three-graviton Feynman rules. By introducing Newtonian potential function $Φ\equiv\frac{h_{00}}{2}$, we reproduce at tree level, $\mathcal{O}(G)$, the well-known result proportional to the error function in four-dimensional spacetime. Furthermore, we obtain the loop-level contributions to the potential function at order $\mathcal{O}(G^2)$ and perform a numerical analysis of its behavior at large distances. Our results indicate that, at the one-loop level, the magnitude of the potential increases gradually with distance in the asymptotic regime.
We constructed the ghost-free condition for nonlocal gravity using de-Sitter background field expansion and identified the structure of the nontrivial form factors. Our analysis shows that the particle spectrum of this model is nearly equivalent to general relativity(GR), with the potential addition of a scalar particle with positive mass m. Additionally, by employing recursion relations, we established the equivalence between nonlocal gravity and higher-derivative gravity. Moreover, we provided a comprehensive proof of the stability of de-Sitter solution within the nonlocal framework.
Using a model-independent analysis method which bases on the Lagrange mean value theorem for obtaining the derivative of the Hubble function, we analyze $H(z)$ parameter data with some restrictive conditions. We find that: (a) the Universe may experience an accelerated expansion with a confidence level greater than 5 $σ$ at redshift $z_{101}\in (0, 0.36)$ and greater than 1.9 $σ$ at redshifts $z_{3835}\in (1.3, 1.53)$ and $z_{3836}\in (1.43, 1.53)$, where $z_j<z_{ij}<z_i$ and $i$ marks the $i$-th Hubble parameter data we consider; (b) the Universe may experience a decelerated expansion with a confidence level greater than 1.5 $σ$ at redshift $z_{2012}\in (0.40, 0.52)$; (c) $w_{\rm{x}}\leq w_{\rm{t}}<-1$ with confidence level great than 1.6 $σ$ at redshift $z_{3836}\in (1.43, 1.53)$. These results indicate that the evolution of dark energy may be oscillatory.
Within a framework requiring a well-defined event horizon and matter obeying the weak energy condition, we employ gravitational decoupling method to construct non-singular hairy black holes: spherically or axially symmetric. These solutions arise from a deformation of the Minkowski vacuum, where the maximum deformation can yield the Schwarzschild metric for the static case, and the Kerr geometry for the stationary case, respectively.
This study investigates the properties of the thin accretion disk around a black hole in quantum fluctuation modified gravity (QFMGBH) using the Novikov-Thorne model. We restrict the parameter \( α\) characterizing the quantum fluctuation of metric and compute the ISCO radius, examining its effect on the energy flux, the radiation temperature, the luminosity spectrum, the energy efficiency, and the shadow size for various values of the parameter \( ω\) characterizing the matter around the black hole. Our findings show that the ISCO radius increases with \( α\) for \( ω= 1/3, 0, -2/3 \), but decreases for \( ω= -4/3 \). As \( α\) increases, the energy flux and the temperature show distinct trends, with changes in the luminosity spectrum and the efficiency. The shadow size increases for \( ω= 1/3, 0, -2/3 \), and decreases for \( ω= -4/3 \). The accretion disk around the QFMGBH is smaller, hotter, and brighter than that surrounded the Schwarzschild BH in GR, suggesting that the observable differences can potentially distinguish quantum fluctuation modified gravity from standard GR.
We establish a framework to construct spherically symmetric and static solutions in $f(R)$ gravity coupled with nonlinear electromagnetic fields. We present two new specific solutions and discuss the energy conditions. We calculate some thermodynamic quantities of the obtained black holes like entropy and energy and investigate their thermodynamic topology.
We explore a square-law k-inflation using the Hamilton-Jacobi approach. Focusing on scenarios where the Hubble parameter exhibits a power-law dependence on the k-field, our analysis encompasses the computations of crucial observables, such as the scalar power spectrum, the tensor-to-scalar ratio, and the scalar spectral index. We further constrain the model's parameters using Planck data and present a specific form of the potential. Our results demonstrate that the model aligns well with observational data.
Extreme-mass-ratio inspirals (EMRIs), consisting of a massive black hole and a stellar compact object, are one of the most important sources for space-borne gravitational wave detectors like TianQin. Their population study can be used to constrain astrophysical models that interpret the EMRI formation mechanisms. In this paper, as an initial attempt, we employ a parametrization method to describe the EMRI population model in the loss cone formation channel. This approach, however, can be extended to other models such as the accretion disc driven formation channel. We present the phenomenological characteristic of the MBH mass, spin, and redshift distributions. Then, we investigate the posterior distribution of the hyper-parameters that describe this population model. The optimistic results show that TianQin could recover almost all the posterior of the hyper-parameters within $1σ$ confidence interval. The hyper-parameters $α_1, α_2, b$, which describe the MBH mass distribution, could be measured with an accuracy of $46.4\%$, $12.6\%$, and $3\%$, respectively. The hyper-parameters $μ_z$, and $σ_z$, which describe the redshift distribution, could be measured with an accuracy of $15.4\%$ and $21.1\%$. With this estimation accuracy, the EMRI population characteristics can be effectively demonstrated, potentially serving as evidence for EMRI formation in the future studies. Furthermore, with an increasing number of detectable events, the parameter estimation for the hyper-parameters will improve and the confidence intervals will be narrowed.