arXiv subjects

Benrong Mu

Publications and source records attributed to Benrong Mu.

At least 19 recordsLinked to original sources

arXiv2025

Gravitational Lensing by Black Holes in Einstein-nonlinear Electrodynamic Theories with Multiple Photon Spheres

In this paper, we study the gravitational lensing effects of non-linear electrodynamic black holes. Non-linear electrodynamic black holes serve as typical models for multi-event horizon black holes. Depending on the choice of metric parameters, these black holes can possess more than five event horizons. Consequently, within certain parameter ranges, black holes can have more than three photon spheres of varying sizes outside the event horizon. Specifically, we focus on the strong gravitational lensing effects near the triple photon spheres, particularly the formation of higher-order images of point sources and celestial spheres. The presence of one, two, or three or more photon spheres significantly increases the number of higher-order images of a point source. When a black hole is illuminated by a celestial sphere, the three photon spheres generate three critical curves in the black hole image, with the smallest critical curve coinciding with the shadow's edge. Additionally, since non-linear electrodynamic black holes are models of multi-event horizon black holes, we can infer the gravitational lensing effects and the changes in celestial images for black holes with more than three photon spheres by analyzing the distinctions and patterns between the gravitational lensing effects of one, two, and three photon spheres.

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arXiv2025

Schwarzschild Black Holes Immersed in Born-Infeld Magnetic Fields and Their Observational Signatures

We investigate the influence of Born-Infeld (BI) nonlinear electrodynamics on magnetic field configurations, photon orbits, and black hole shadows for Schwarzschild black holes immersed in magnetic fields. Assuming that the BI magnetic fields are asymptotically uniform and aligned with the polar axis, we solve the nonlinear magnetic field equations numerically using pseudospectral methods. Our analysis shows that nonlinear electromagnetic effects become prominent near the event horizon, particularly in the polar regions, where the magnetic field strength is significantly enhanced. This enhancement leads to closed photon orbits on the meridional plane becoming prolate, with noticeable stretching along the polar axis. Simulations of black hole images reveal that, at high observer inclinations, the shadow, which is circular in the Maxwell limit, becomes increasingly elongated along the polar direction as the nonlinear effects increase.

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arXiv2024

Quasinormal modes of Schwarzschild-like black hole surrounded by the pseudo-isothermal dark matter halo

The merger of binary black holes produces a series of decaying oscillations, during which energy is radiated in gravitational waves. The characteristic signal in the ringdown phase can be described by complex oscillation frequencies called quasinormal modes (QNMs). In this paper, we investigate the ringdown spectrum resulting from scalar field perturbations of black holes surrounded by pseudo-isothermal dark matter halos. The complex frequencies of these quasinormal modes are numerically computed using the sixth-order WKB approximation. Additionally, the time evolution of the scalar perturbations is examined using the finite difference method, considering various multipole numbers and dark matter halo parameters. For a static, spherically symmetric black hole, the photon sphere--composed of circular null geodesics--plays a crucial role in analyzing the black hole shadow. Furthermore, the connection between the black hole shadow and QNMs is explored in the eikonal limit.

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arXiv2023

Thermodynamic instability and the violation of reverse isoperimetric inequality in Torus-like black hole

Recently, conjectures have been made that there is a correlation between thermodynamic instability and super-entropy~\cite{{Johnson:2019mdp},{Cong:2019bud}}. In this paper, we started with torus-like black holes and verified the correlation mentioned above. After solving the specific analytical solutions of the specific heat capacity at constant volume $C_{V}$, the specific heat capacity at constant pressure $C_{P}$ and plotting the figures, we obtained the results which are consistent with the hypothesis from Ref. \cite{Cong:2019bud}. To further verify the generality of conjectures, we explored the BTZ black hole in nonlinear electrodynamics and phantom AdS black hole. And then the results are summarized via a table. Ultimately, the comparison among these three black holes leads us to the conclusion that there is no essential connection between thermodynamic instability and super-entropy.

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arXiv2023

Echoes from charged black holes influenced by quintessence

In this paper, we investigate the effective potential and echoes from the dyonic black hole with quintessence. For a dyonic black hole, the quasi-topological electromagnetism provides proper matter energy-momentum tensor to curve the spacetime, and quintessence strengthens this force. We find that when the effect of quintessence becomes stronger, the black hole potential transforms between single-peak and double-peak, which will influence the existence of black hole echoes. In particular, we find that observer will receive a sudden vanishment of high-frequency echoes when quintessence remains a relatively strong effect.

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arXiv2023

Thermodynamic instabilities of Kerr-Newman Ads black holes

In~\cite{Johnson:2019mdp}, Clifford put forward that the specific heat at constant volume $C_{V}$ is always negetive for the super-entropy black hole. Later in~\cite{Cong:2019bud}, Robert et al. found $C_{V}>0$ in certain region for the generalized exotic BTZ black holes. Futhermore, they proposed a new conjecture that as the $C_{V}>0$, the specific heat at constant pressure $C_{P}$ always satisfies $C_{P}<0$ in super-entropy black hole. In this paper, we examine the both conjectures with regard to the thermodynamic instability of super-entropy black hole. By means of a super-entropy black hole, the Kerr-Newman-AdS black hole in a coordinate system that rotates at infinity, we detailedly analyze its character and notice there exists the region where $C_{V}>0$ and meanwhile $C_{P}>0$. Hence, we find a counterexample to the two conjectures.

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arXiv2023

Thermodynamic Instabilities of Conformal Gravity Holography in Four Dimensions

Recently, a conjecture has been proposed, which indicates a correlation between super-entropy black holes and the thermodynamic instability \cite{Cong:2019bud}. W.Cong et al. suggested that the $C_{V}$ (specific heat capacity at constant volume) and $C_{P}$ (specific heat capacity at constant pressure) of a super-entropy black hole could not be greater than zero simultaneously as set in the extended phase space, which implies that the black hole is unstable in extended thermodynamics. This conjecture is intriguing and meaningful. Therefore, we did a study on that. After deriving the equations of specific heat capacities as well as plotting the relevant curves of the four-dimensional conformal black holes (a kind of super-entropy black hole), we obtained regions on the graphs where $C_{V}$ and $C_{P}$ are simultaneously greater than zero, which contradicts the conjecture that super-entropy black hole corresponds to its thermodynamic instability. Thus far, we have provided a counterexample to the hypothesis in \cite{Cong:2019bud}.

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arXiv2023

Lyapunov Exponents and Phase Transitions of Born-Infeld AdS Black Holes

In this paper, we characterize the phase transitons of Born-Infeld AdS black holes in terms of Lyapunov exponents. We calculate the Lyapunov exponents for both null and timelike geodesics. It is found that black hole phase transitions can be described by multiple-valued Lyapunov exponents. And its phase diagram can be characterized by Lyapunov exponents and Hawking temperature. Besides, the change of Lyapunov exponents can be considered as order parameter, and exists a critical exponent $1/2$ near critical point.

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arXiv2022

Chaos bound of charged particles around phantom AdS black hole

In this paper, we investigated the chaos of the phantom AdS black hole in near-horizon regions and at a certain distance from the black hole, where the Lyapunov exponent was calculated by the Jacobian determinant. We found that the angular momenta of charged particles nearby the black hole affect the position of the equilibrium orbit as well as the Lyapunov exponent. When the charge is large enough and the angular momenta take particular values, the bound is violated at a certain distance from the event horizon. Concurrently, we compared the differences by analyzing tables and figures after taking different values of $\eta$, where $\eta$ is a constant indicating the nature of the electromagnetic field. When $\eta=1$, it corresponds to the Reissner-Nordstr$\ddot{o}$m-AdS black hole (RN-AdS black hole) and it represents the phantom AdS black hole when $\eta=-1$. In this way, we can draw the following conclusions. Under different values of $\eta$, the variation trends of curves related to $\lambda^{2}-\kappa^{2}$ could be different, where it can be judged whether the constraint is violated by determining if $\lambda^{2}-\kappa^{2}$ is greater than zero. In addition, the corresponding numerical values of angular momenta violating the bound in the phantom AdS black hole are much smaller than the case of RN-AdS black hole. But cases in these different black holes have the same rules to violate the bound. The large absolute values of charge and $\Lambda$ are more likely to violate the bound.

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arXiv2022

Chaos bound and its violation in the torus-like black hole

In this paper, we have studied the variation of the chaos bound in two regions of the torus-like black hole, i.e., the region close to the black hole horizon and the region at a certain distance from the black hole horizon. The angular momentum of the particle affects the effective potential and influences the magnitude of the chaotic behavior of the particle. Therefore, the angular momentum of particle is important in the study. The angular momentum of a particle not only affects the particle equilibrium orbital position, but also affects the Lyapunov exponent. As the angular momentum of the particle increases, the particle equilibrium position gradually moves away from the black hole horizon. In the near black hole horizon region, the chaos bound is not violated, however, at the far black hole horizon region, the chaos bound is violated. In addition unlike the charged AdS black hole which has a spherical topology of the horizon, the torus-like black hole has a toroidal topology of the horizon.

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arXiv2022

Temporal and Spatial Chaos of RN-AdS Black Holes Immersed in Perfect Fluid Dark Matter

We investigate the thermodynamic chaos of RN-AdS black holes immersed in Perfect Fluid Dark Matter by considering the dynamical equations of the fluid system evolved in the spinodal region. Based on the Melnikov method, it is shown that there exists a critical amplitude that affects the temporal chaos. And the influence of black holes charge and state parameter on the critical amplitude is investigated with specific initial temperature. Then, for inevitable spatial chaos, three different types of portraits are discssued according to the difference between the phase transition pressure and the ambient pressure. Additionally, we check the local equilibrium near saddle points which shows that spatial chaos always exists regardless of the perturbation intensity.

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arXiv2022

Joule-Thomson expansion of the lower-dimensional black hole in rainbow gravity

In this paper, we extend Joule-Thomson expansion to the low-dimensional regime in rainbow gravity by considering the rainbow rotating BTZ metric in the (2+1)-dimensional spacetime. After the metric of the black hole is obtained, we get the Joule-Thomson expansion of the black hole, including the Joule-Thomson coefficient, inversion curves, and isenthalpic curves. We find that a rainbow rotating BTZ black hole does not have $P-V$ critical behavior. The effects of rainbow gravity are to slow down the trend of the increase of the Joule-Thomson coefficient and make its zero point larger. Moreover, the rainbow gravity slows down the inverse temperature of the black hole, meaning that a rainbow rotating BTZ black hole tends to change its heating or cooling action at a lower temperature, which can be attributed to the topology of the black hole.

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arXiv2022

Stationary and Free-fall frame Kerr black hole in gravity's rainbow

Doubly special relativity (DSR) is an effective model for encoding quantum gravity in flat spacetime. To incorporate DSR into general relativity, one could use gravity's rainbow, where the spacetime background felt by a test particle would depend on its energy. In this paper, we investigate the thermodynamics of rainbow Kerr black hole in the scenario with the stationary(ST) orthonormal frame and free-fall(FF) orthonormal frame. After the rainbow metric in ST frame and FF frame is deduced, the Hamilton-Jacobi method is used to acquire the modified Hawking temperature, specific heat and corresponding the modified entropy to each scenario, then the thermodynamic properties are discussed. We find that the effects of rainbow gravity on Kerr black holes are quite model-dependent. In other words, the value of parameter $\eta$ and $n$ with Amelino-Camelia's proposal are crucially important and worth discussing. Specificly, with most widly accepted choice ($n=2,\eta >0$), the effects of rainbow gravity tend to decrease the Hawking temperature but increase the black hole entropy in ST frame, and increase the Hawking temperature but decrease the black hole entropy in FF frame conversely.

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arXiv2022

Thermodynamics of the RN-AdS black hole with cloud of strings and quintessence in stationary and free-fall frame in rainbow gravity

In this paper, we investigate the thermodynamic properties of the RN-AdS black hole with cloud of strings and quintessence in rainbow gravity with the stationary (ST) orthonormal frame and the free-fall (FF) orthonormal frame. After the SF and the FF rainbow metric is obtained, we get the Hawking temperature and the entropy, and their physical meanings are discussed. We find that, for the ST rainbow RN-AdS black hole with cloud of strings and quintessence, the effect of rainbow gravity is to increase the Hawking temperature but decrease the entropy of the black hole. However, for the FF rainbow case, rainbow gravity turns out to decrease the Hawking temperature but increase the entropy of the black hole, which seems that the effects rainbow gravity has are quite model-dependent.

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arXiv2022

Probing Phase Structure of Black Holes with Lyapunov Exponents

We conjecture that there exists a relationship between Lyapunov exponents and black hole phase transitions. To support our conjecture, Lyapunov exponents of the motion of particles and ring strings are calculated for Reissner-Nordstr\"{o}m-AdS black holes. When a phase transition occurs, the Lyapunov exponents become multivalued, and branches of the Lyapunov exponents coincide with black hole phases. Moreover, the discontinuous change in the Lyapunov exponents can be treated as an order parameter, and has a critical exponent of $1/2$ near the critical point. Our findings reveal that Lyapunov exponents can be an efficient tool to study phase structure of black holes.

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arXiv2022

Joule-Thomson expansion of d-dimensional charged AdS black holes with cloud of strings and quintessence

Herein, we focus on the study of Joule-Thomson expansion corresponding to a d-dimensional charged AdS black hole with cloud of strings and quintessence. Then its relevant solution and some thermodynamic properties are investigated. Specifically, we evaluate its Joule-Thomson expansion from four important aspects, including the Joule-Thomson coefficient, inversion curve, isenthalpic curve, and ratio $\frac{T_{i}^{min}}{T_{c}}$. After analysis, different dimensions with strings of cloud and quintessence parameters have different effects on the Joule-Thomson coefficient (the same situation are found for the inversion curve, isenthalpic curve, and ratio $\frac{T_{i}^{min}}{T_{c}}$).

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arXiv2021

Joule-Thomson expansion of Reissner-Nordström-Anti-de Sitter black holes with cloud of strings and quintessence

The Joule-Thomson expansion is studied for Reissner-Nordström-Anti-de Sitter black holes with cloud of strings and quintessence, as well as its thermodynamics. The cosmological constant is treated as thermodynamic pressure, whose conjugate variable is considered as the volume. The characteristics of the Joule-Thomson expansion are studied in four main aspects with the case of $ω=-1$ and $ω=-\frac{2}{3}$, including the Joule-Thomson coefficient, the inversion curves, the isenthalpic curves and the ratio between $T_{i}^{min}$ and $T_{c}$. The sign of the Joule-Thomson coefficient is possible for determining the occurrence of heating or cooling. The scattering point of the Joule-Thomson coefficient corresponds to the zero point of the Hawking temperature. Unlike the van der Waals fluids, the inversion curve is the dividing line between heating and cooling regions, above which the slope of the isenthalpic curve is positive and cooling occurs, and the cooling-heating critical point is more sensitive to $Q$. Concerning the ratio $\frac{T_{i}^{min}}{T_{c}}$, we calculate it separately in the cases where only the cloud of strings, only quintessence and both are present.

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arXiv2021

Thermodynamics with pressure and volume of black holes based on two assumptions under scalar field scattering

Recently, a new assumption was proposed in [Phys. Rev. D 100, no.10, 104022 (2019)]. This assumption considers that the energy of the particle changes the enthalpy of the black hole after throwing the particle into the black hole. Using the energy-momentum relation, the results show that the second law of thermodynamics of the black hole is valid in extended phase space. In this paper, we discuss the validity of the laws of thermodynamics and the stability of the horizon of the charged AdS black hole by scalar field scattering under two assumptions, i.e., the energy flux of the scalar field $dE$ changes the internal energy of the black hole $dU$ and the energy flux of the scalar field $dE$ changes the enthalpy of the black hole $dM$.

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