Black Hole Spacetimes Research Articles

Probing hidden topology with quantum detectors

We consider the transition rate of a static Unruh-DeWitt detector in two (2+1)-dimensional black hole spacetimes that are isometric to the static Bañados-Teitelboim-Zanelli black hole outside the horizon but have no asymptotically locally anti–de Sitter exterior behind the horizon. The spacetimes are the RP2 geon, with spatial topology RP2\{point at infinity}, and the Swedish geon of Åminneborg ., with spatial topology T2\{point at infinity}. For a conformal scalar field, prepared in the Hartle-Hawking-type state that is induced from the global vacuum on the anti–de Sitter covering space, we show numerically that the detector’s transition rate distinguishes the two spacetimes, particularly at late exterior times, and we trace this phenomenon to the differences in the isometries that are broken by the quotient construction from the universal covering space. Our results provide an example in which information about the interior topology of a black hole is accessible to a quantum observer outside the black hole. Published by the American Physical Society 2025

Radiative properties and QPOs around charged black hole in Kalb–Ramond gravity

Studies of accretion disc luminosities and quasiperiodic oscillations around black holes may help us understand the gravitational properties of black hole spacetime. This work is devoted to studying the radiation properties of the accretion disk around the black holes in Kalb–Ramond gravity. We investigate the event horizon of the black hole spacetime and calculate the effective gravitational mass of the spacetime. Also, we analyze the circular motion of test particles in the black hole spacetime. The effects of the black hole charge and KR parameters on the particles’ effective mass, energy, and angular momentum at circular orbits and innermost stable circular orbits are studied. The frequency of Keplerian orbits and the radial and vertical oscillations of the particles along stable orbits are calculated and applied to analyze the existence of QPO in relativistic precession, warped disc, and epicyclic resonance models. QPO orbits’ locations with ratios of upper and lower frequencies of twin-peaked QPOs 3:2, 4:3, and 5:4 are analyzed compared to ISCO. We also obtain constrain values for the black hole mass, charge, KR field parameter, and QPO orbits found using Markovian chain Monte Carlo (MCMC) simulations for stellar mass (XTE J1550, GRS 1915+105), intermediate mass (M82-X1), and supermassive black holes (Sgr A*). Finally, we explore the radiative properties of the accretion disk around the charged black hole in KR gravity, such as the total radiation flux, accretion disc temperature, and differential luminosity.

The confluent Heun functions in black hole perturbation theory: a spacetime interpretation

Abstract This work provides a spacetime interpretation of the confluent Heun functions within black hole perturbation theory (BHPT) and explores their relationship to the hyperboloidal framework. In BHPT, the confluent Heun functions are solutions to the radial Teukolsky equation, but they are traditionally studied without an explicit reference to the underlying spacetime geometry. Here, we show that the distinct behaviour of these functions near their singular points reflects the structure of key geometrical surfaces in black hole spacetimes. By interpreting homotopic transformations of the confluent Heun functions as changes in the spacetime foliation, we connect these solutions to different regions of the black hole’s global structure, such as the past and future event horizons, past and future null infinity, spatial infinity, and even past and future timelike infinity. We also discuss the relationship between the confluent Heun functions and the hyperboloidal formulation of the Teukolsky equation. Although neither confluent Heun form of the radial Teukolsky equation can be interpreted as hyperboloidal slices, this approach offers new insights into wave propagation and scattering from a global black hole spacetime perspective.

Particle dynamics and QPOs around a black hole coupled with nonlinear electrodynamics and cloud of strings

Particle dynamics and QPOs around a black hole coupled with nonlinear electrodynamics and cloud of strings

Lorentz violation alleviates gravitationally induced entanglement degradation

Lorentz violation is a significant phenomenon in the framework of quantum physics, with implications for fundamental symmetries. In this paper, we explore the effects of Lorentz violation on quantum entanglement through a black hole spacetime that is coupled with a Lorentz-violating field. We establish the relationship between the Hartle-Hawking vacuum state and the Boulware number states for this case, and employ the near horizon approximation in an appropriate form to rewrite the black hole metric into a Rindler-like form. Subsequently, using this revised metric, the analytical forms of logarithmic negativity and mutual information are derived and plotted as functions of Rob’s distance from the r = 0 point. Based on the results, we find that the coupling between spacetime and the Lorentz-violating vector field alleviates gravity-induced entanglement degradation. At high mode frequencies, the effects of Lorentz violation are negligible, but they become significant at low frequencies. This suggests that investigating Lorentz violation at astrophysical scales requires low-frequency detectors, as the low energy of these fields enhances the significance of the Lorentz-violating field’s non-zero vacuum expectation value.

Periodic orbits and gravitational wave radiation in short hair black hole spacetimes for an extreme mass ratio system

For a short hair black hole(BH) that bypasses the “no-short-hair” theorem, its hair parameters have a significant impact near the event horizon and may thus carry the internal information of the BH. This paper studies the influence of the hair parameter Qm and the structural parameter k of the short hair BH on the periodic orbits of particles and gravitational wave radiation in the extreme mass ratio(EMR) system. The research results show that Qm has a significant impact on the radius of the orbit, angular momentum, and the E-L space. A larger value of k weakens this effect and makes it degenerate with the Schwarzschild BH. In addition, an increase in Qm or a decrease in k will lead to a reduction in the period of gravitational waves and a significant enhancement of the signal amplitude. These results provide new observational means for distinguishing short hair BH from classical BH and also offer new insights for verifying the no-hair theorem and understanding the physical behaviors near the event horizon.

Magnetic reconnection and energy extraction from a Konoplya–Zhidenko rotating non-Kerr black hole

Recently, magnetic reconnection has attracted considerable attention as a novel energy extraction mechanism, relying on the rapid reconnection of magnetic field lines within the ergosphere. We have investigated the properties of the energy extraction via magnetic reconnection in a Konoplya–Zhidenko rotating non-Kerr black hole spacetime with an extra deformation parameter. Our results show that the positive deformation parameter expands the possible region of energy extraction and improves the maximum power, maximum efficiency, and the maximum ratio of energy extraction between magnetic reconnection and the Blandford–Znajek process. This means that in the Konoplya–Zhidenko rotating non-Kerr black hole spacetime one can extract more energy via magnetic reconnection than in the Kerr black hole case. These effects of the deformation parameter may provide valuable clues for future astronomical observations of black holes and verification of gravity theories.

Penrose and super-Penrose energy extraction from a Reissner-Nordström black hole spacetime with a cosmological constant through the Bañados-Silk-West mechanism

Penrose and super-Penrose energy extraction from a Reissner-Nordström black hole spacetime with a cosmological constant through the Bañados-Silk-West mechanism

Entanglement entropy in quantum black holes

Abstract We discuss the entanglement entropy for a massive scalar field in two Schwarzschild-like quantum black hole spacetimes, also including a nonminimal coupling term with the background scalar curvature. To compute the entanglement entropy, we start from the standard spherical shell discretization procedure, tracing over the degrees of freedom residing inside an imaginary surface. We estimate the free parameters for such quantum metrics through a simple physical argument based on Heisenberg uncertainty principle, along with alternative proposals as asymptotic safety, trace anomaly, and graviton corpuscular scaling. Our findings reveal a significant decrease in entropy compared to the area law near the origin for the quantum metrics. In both scenarios, the entanglement entropy converges to the expected area law sufficiently far from the origin. We then compare these results to the entropy scaling in regular Hayward and corrected-Hayward spacetimes to highlight the main differences with such regular approaches.

Holographic reconstruction of black hole spacetime: machine learning and entanglement entropy

We investigate the bulk reconstruction of AdS black hole spacetime emergent from quantum entanglement within a machine learning framework. Utilizing neural ordinary differential equations alongside Monte-Carlo integration, we develop a method tailored for continuous training functions to extract the general isotropic bulk metric from entanglement entropy data. To validate our approach, we first apply our machine learning algorithm to holographic entanglement entropy data derived from the Gubser-Rocha and superconductor models, which serve as representative models of strongly coupled matters in holography. Our algorithm successfully extracts the corresponding bulk metrics from these data. Additionally, we extend our methodology to many-body systems by employing entanglement entropy data from a fermionic tight-binding chain at half filling, exemplifying critical one-dimensional systems, and derive the associated bulk metric. We find that the metrics for a tight-binding chain and the Gubser-Rocha model are similar. We speculate this similarity is due to the metallic property of these models.

Novel topological phenomena of timelike circular orbits for charged test particles

Abstract The topological approach has recently been successfully employed to investigate timelike circular orbits (TCOs) for massive neutral test particles. The observed vanishing topological number implies that these TCOs occur in pairs. However, the behavior of charged test particles in this regard remains unexplored. To address this issue, our study focuses on examining the influence of particle charge on the topology of TCOs within a spherically symmetrical black hole spacetime holding a nonvanishing radial electric field. We consider four distinct cases based on the charges of the particle and the black hole: unlike strong charge, unlike weak charge, like weak charge, and like strong charge. For each case, we calculate the corresponding topological number. Our results reveal that when the charge is large enough, the topological number takes a value of -1 instead of 0, which differs from the neutral particle scenario. Consequently, in cases of small charges, the TCOs appear in pairs, whereas in cases of larger charges, an additional unstable TCO emerges. These findings shed light on the influence of the particle charge on the topological properties and number of TCOs.

Off-equatorial deflections and gravitational lensing. II. In general stationary and axisymmetric spacetimes

Abstract In this work, we develop a general perturbative procedure to find the off-equatorial plane deflections in the weak deflection limit in general stationary and axisymmetric spacetimes, allowing the existence of the generalized Carter constant. Deflections of both null and timelike rays, with the finite distance effect of the source and detector taken into account, are obtained as dual series of $M/r_0$ and $r_0/r_{s,d}$. These deflections allow a set of exact gravitational lensing equations from which the images' apparent angular positions are solved. The method and general results are then applied to the Kerr-Newmann, Kerr-Sen, and rotating Simpson-Visser spacetimes to study the effect of the spin and characteristic (effective) charge of the spacetimes and the source altitude on the deflection angles and image apparent angles. It is found that, in general, both the spacetime spin and charge only affect the deflections from the second non-trivial order, while the source altitude influences the deflection from the leading order. Because of this, it is found that, in gravitational lensing in realistic situations, it is hard to measure the effects of the spacetime spin and charge from the images' apparent locations. We also presented the off-equatorial deflections in the rotating Bardeen, Hayward, Ghosh, and Tinchev black hole spacetimes.

Astrophysical insights into magnetic Penrose process around parameterized Konoplya–Rezzolla–Zhidenko black hole

In this study, we investigate the parameterized Konoplya–Rezzolla–Zhidenko (KRZ) black hole (BH) spacetime in the presence of an external asymptotically uniform magnetic field. We first examine the innermost stable circular orbit (ISCO) radii for both neutral and charged test particles, demonstrating that the deformation parameters, δ1 and δ2, reduce the ISCO values. Subsequently, we assess the energy efficiency of the magnetic Penrose process (MPP) for an axially symmetric parameterized BH, analyzing the effects of the deformation parameters and the magnetic field on the energy extraction process. Our findings indicate that the rotational deformation parameter δ2 is crucial for the efficiency of energy extraction from the BH. The synergy between the rotational deformation parameter and the magnetic field significantly boosts the energy extraction efficiency, with values exceeding 100%. Interestingly, for extremal BHs with negative δ2 values, the energy efficiency increases, in contrast to Kerr BHs where the MPP effect diminishes. Additionally, we explore the astrophysical implications of the MPP by deriving the maximum energy of a proton escaping from the KRZ parameterized BH due to the beta decay of a free neutron near the horizon. Our results show that negative δ2 values require stronger magnetic fields to achieve equivalent energy levels for high-energy protons, providing deeper insights into high-energy astrophysical phenomena around the parameterized BH.

Hawking Radiation of Renormalization Group Improved Regular Black Holes

AbstractA renormalization group approach based on the idea that the primary contribution to the Schwarzschild‐like black hole spacetime arises from the value of the gravitational coupling is considered. The latter depends on the distance from the origin and approaches its classical value in the far zone. However, at some stage, this approach introduces an arbitrariness in choosing an identification parameter. There are three approaches to the identification: the modified proper length (the Bonanno–Reuter metric), the Kretschmann scalar (the Hayward metric), and an iterative, and, in a sense, coordinate‐independent procedure (Dymnikova solution). Using the Wentzel–Kramers–Brillouin method, gray‐body factors are calculated for the Standard Model massless test fields and their corresponding energy emission rates. For all of these solutions, it is found that the intensity of Hawking radiation of massless fields is significantly suppressed by several or more orders once the quantum correction is taken into consideration. This indicates that the effect of suppression of the Hawking radiation may be appropriate to the quantum corrected black holes in asymptotically safe gravity in general and is independent on the particular choice of the identification parameter.

α -surfaces and global coordinates in black hole spacetimes

We present a discussion of the periodicity in imaginary time of maximally extended black hole spacetimes without reference to Euclidean manifolds. As motivation, we first demonstrate our approach for the Rindler geometry in flat space before then applying it for the case of Schwarzschild time in the maximally extended Kruskal geometry. One notable advantage of our approach is that it can be utilized in the Kerr case, again without reference to a Euclidean manifold, and without complexifying the angular momentum parameter, a. One unusual feature of our application in the Kerr case is that, even for the purely Lorentzian geometry, it is developed in terms of explicitly complex coordinates which are associated with distinct families of α-surfaces. Moreover, coordinatization of these gives a single set of coordinates which cover the entire geometry outside the inner horizon. Published by the American Physical Society 2024

Geodesics in Carrollian Reissner-Nordström black holes

In this work, we study the geodesics in different Carrollian limits of RN (Reissner-Nordström) black holes, considering the motions of both neutral and charged particles. We use the geodesic equations in the weak Carrollian structure and analyze the corresponding trajectories projected onto the absolute space, and find that the geodesics are well defined. In particular, we examine the electric-electric and magnetic-electric limit of the RN black hole, focusing on their geodesic structures. We find that the global structures of the usual RN black holes get squeezed under the ultrarelativistic limit. More precisely, the nonextreme magnetic-electric RN spacetime has two different asymptotic flat patches while the extreme black hole spacetime consists of only one patch. For the magnetic-electric RN spacetime, the Carrollian extremal surfaces (CESs) divide the spacetime into several geodesically complete regions, and the geodesics can only travel in one of these regions. For the charged particles, we extend the analysis by considering their interactions with the electromagnetic field in the Carrollian RN spacetimes and find that their trajectories are significantly different from the neutral geodesics. Published by the American Physical Society 2024

Towards a Unitary Formulation of Quantum Field Theory in Curved Space-Time: The Case of the Schwarzschild Black Hole

Abstract We argue that the origin of unitarity violation and the information loss paradox in our understanding of black holes (BHs) lies in the standard way of doing quantum field theory in curved space-time (QFTCS), which is heavily biased on intuition borrowed from classical general relativity. In this paper, with the quantum-first approach, we formulate a so-called direct-sum QFT (DQFT) in BH space-time based on a novel formulation of discrete space-time transformations in gravity that potentially restores unitarity. By invoking the quantum effects associated with the gravitational backreaction, we show that the Hawking quanta emerging outside of the Schwarzschild radius ($r_S=2GM$) cannot be independent of the quanta that continue to be inside $r_S$. This enables information to be carried by Hawking quanta, but in the BH DQFT formalism, we do not get any firewalls. Furthermore, DQFT leads to the BH evaporation involving only pure states. This means the quantum mechanical effects at the BH horizon produce two components of a maximally entangled pure state in geometric superselection sector Hilbert spaces. This construction enables pure states to evolve into pure states, restoring unitarity and observer complementarity. Finally, we discuss how our framework leaves important clues for formulating a scattering matrix and probing the nature of quantum gravity.

Null geodesics around a black hole with weakly coupled global monopole charge

Null geodesics around a black hole with weakly coupled global monopole charge

Shadows and quasinormal modes of rotating black holes in Horndeski theory: Parameter constraints using EHT observations of M87* and Sgr A*

Shadows and quasinormal modes of rotating black holes in Horndeski theory: Parameter constraints using EHT observations of M87* and Sgr A*

Signatures from the observed jet power and the radiative efficiency for rotating black holes in loop quantum gravity

We investigate the radiative efficiency and jet power in the spacetime of a rotating black hole within the framework of loop quantum gravity (LQG), which includes an additional LQG parameter. The results show that as the LQG parameter increases, the radiative efficiency decreases for slowly rotating black holes while it increases for rapidly rotating black holes. Furthermore, the jet power is found to increase for different black hole spins. With the observed data from the well-known sources A0620-00, H1743-322, XTE J1550-564, GRS1124-683, GRO J1655-40, and GRS1915+105, we make some constraints on the black hole spin parameter and the LQG parameter. The presence of the LQG parameter broadens the allowed range of the black hole spin parameter for sources A0620-00, H1743-322, XTE J1550-564 and GRO J1655-40. However, for the source GRS 1915+105, there is no overlap between the allowed parameter regions, which implies that the rotating LQG black hole cannot simultaneously account for the observed jet power and the radiative efficiency as in other black hole spacetimes