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The authors show that in the f(Q)$f(Q)$ gravity with a non‐metricity scalar Q$Q$, the curvatures in Einstein's gravity, that is, the Riemann curvature constructed from the standard Levi‐Civita connection, could not be excluded or naturally appear. The first observation is that even in f(Q)$f(Q)$ gravity, the conservation of the matter energy‐momentum tensor is not described by the covariant derivatives in the non‐metricity gravity but that is given by the Levi‐Civita connection. The commutator of the covariant derivatives in Einstein's gravity inevitably induces the Riemann curvature. There is no symmetry nor principle which prohibits the Riemann curvature in non‐metricity gravity. Based on this observation, the authors propose and investigate fQ,G$f\left(Q, \mathcal {G} \right)$ gravity with the Gauss–Bonnet invariant G$\mathcal {G}$ and its generalizations. The authors also show how fQ,G$f\left(Q, \mathcal {G} \right)$ models realizing any given the Friedmann–Lemaître–Robertson– Walker (FLRW) spacetime can be reconstructed. The reconstruction formalism to cosmology is applied. Explicitly, the gravity models which realize slow roll or constant roll inflation, dark energy epoch as well as the unification of the inflation and dark energy are found. The dynamical autonomous system and the gravitational wave in the theory under investigation are discussed. It is found the condition that the de Sitter spacetime becomes the (stable) fixed point of the system. [ABSTRACT FROM AUTHOR]

A spherically symmetric black hole solution in the conformal Weyl gravity was found by Mannheim and Kazanas in 1988. While the quasinormal modes (QNMs) of these black holes have been considered in a few works, here we complement these studies by considering in detail the regime of vanishing cosmological constant and negative values of the Weyl parameter for which we find quasinormal frequencies for scalar, Dirac and electromagnetic perturbations with the time-domain integration and WKB methods. In particular, we derive the compact and remarkably accurate analytic formula for the frequencies in the form of expansion in terms of the inverse multipole number. Comparison with the time-domain integration shows that the 6th order WKB method with the Padé approximants is quite accurate, unless the black hole is in the near extreme state. [ABSTRACT FROM AUTHOR]

While the quantum realm seems hidden, it can also reach examples of infinite energy, especially when a part of space is roughly removed until it disappears, possibly forever. Since it follows that Nothing can enter a region where the space is missing, the quantum realm, as seen now in affine quantization, will automatically come to help everything else by creating colossal “quantum walls” that will ensure that everything stays out of all black holes. In this paper, we show that the expanded quantum realm allows Nothing to ever fall into a black hole. [ABSTRACT FROM AUTHOR]

The butterfly velocity of four-dimensional rotating charged asymptotically AdS black hole is calculated to probe chaos using localized rotating shock waves. In this work, we obtain the angular momentum dependence of the butterfly velocity due to rotation in the shock wave probes. In general, the angular momentum L of the shock waves increases the butterfly velocity. The localized shocks also generate butterfly velocities which vanish when we approach extremality, indicating no entanglement spread near extremality. One of the butterfly velocity modes is well bounded by both the speed of light and the Schwarzschild-AdS result, while the other may become superluminal. Aside from the logarithmic behavior of the scrambling time which indicates chaos, the Lyapunov exponent is also positive and bounded by κ = 2 π T H / (1 - μ L) . The Kerr–NUT–AdS and Kerr–Sen–AdS solutions and their ultraspinning versions are used as examples to attain a better understanding of the chaotic phenomena in rotating black holes, especially those with extra conserved charges. [ABSTRACT FROM AUTHOR]

We investigate solutions to the Tolman-Oppenheimer-Volkoff equations and their implications, deriving an equation of state for the star’s surface as a function of two geometric parameters, λ (r) and ν (r) . We find that the parameter λ (r) is essential for explaining fluctuations in the baryon number density and baryonic chemical potential within and outside a neutron star, enabling simulations of regions with varying neutron matter densities. We also highlight the significant role of the ν (r) parameter for describing the explosive transition of a neutron star to a black hole, establishing a direct connection between the neutron star’s radius and the event horizon’s radius. The parameter λ (r) is crucial for understanding how the baryon number density and baryonic chemical potential fluctuate inside and outside a neutron star. The core is characterized by λ > 0 , and the outer crust is defined by λ < 0 . Finally, by analyzing the polytropic equation of state, we find that the neutron stars are composed of two distinct regions: a low-density outer crust and a very high-density core, separated by a sharp boundary. The mixing region between the two layers is known as the inner crust and outer core. [ABSTRACT FROM AUTHOR]

We show that the first several overtones could be calculated by the time-domain integration method for asymptotically de Sitter black holes already at the lowest multipole numbers of gravitational and electromagnetic perturbations. This is not possible for asymptotically flat black holes, for which extraction of frequencies with the Prony method is usually possible with reasonable accuracy only for the fundamental mode. The reason for much better efficiency in the de Sitter case is the absence of power-law tails: the quasinormal modes dominate the signal not only at the intermediate stage, but also at asymptotically late times. [ABSTRACT FROM AUTHOR]

In this work, we investigate the quasinormal modes (QNMs), shadow radius, gray-body factor, Hawking radiation and absorption cross-section of the Reissner–Nordström black hole surrounded by a cloud of strings in Rastall gravity for a massless scalar field. We use the sixth-order Wenzel, Kramers and Brillouin (WKB) and unstable circular null geodesic methods to calculate QNMs. Moreover, the values obtained by the two methods are in very good agreement. The real part and absolute value of imaginary part of quasinormal frequencies decrease with the increase of a. As q increases, the real part of quasinormal frequencies increases while the absolute value of imaginary part first increases and then decreases. For β > 0, they decrease with the increase of β. While they first increase and then decrease with β increasing at small value of a when β < 0. Then using the unstable circular null geodesic method, we obtain the shadow radius of black hole. We find that shadow radius decreases first and then increases as the negative parameter β increases. We also calculate the gray-body factor. According to it, Hawking radiation and absorption cross-section are calculated. We explore the influence of a, β and q on the energy emission rate. When other parameters are fixed, the black hole lives longer as a, positive parameter β and q increase. To obtain absorption cross-section, we adopt the partial wave method and sinc approximation method, and the results obtained by two methods have excellent consistency at mid-high frequency region. Further, we assess the effect of a, β and q on the absorption cross-section. [ABSTRACT FROM AUTHOR]

A view of the global structure of active galactic nuclei (AGN) is presented herein following the detection of blue-shifted warm absorber (WA) and ultrafast outflow (UFO) absorption features in their X-ray spectra. A straightforward interpretation of these features suggests the presence of magnetohydrodynamic winds off the underlying accretion disks spanning a wide range of a few to ~ 106 Schwarzschild radii. UFOs are associated with wind segments closest to a black hole, with decreasing ionization absorber species associated with these wind segments at increasingly larger distances; eventually, the wind segments at the largest distances are sufficiently cool and dusty to be associated with the AGN tori, as suggested in the past. Furthermore, spectroscopic X-ray observations at a sufficient resolution allow estimates of the mass fluxes of these winds, showing that they increase with radius. As a consequence, the mass flux of the underlying accretion disk must decrease toward the accreting black hole, eventually reaching a value smaller than that needed to convert the flow into an X-ray hot advection-dominated accretion flow; it is suggested that this hot segment of the accretion flow is responsible for the observed AGN X-rays (and galactic X-ray binaries) in place of the ad hoc corona assumed thus far. This work indicates that the properties of this component that are reflected in its relative luminosity to the viscous disk O-UV component depend on the source luminosity in broad agreement with the observations. [ABSTRACT FROM AUTHOR]

Discovered as an apparent pattern, a universal relation between geometry and information called the holographic principle has yet to be explained. This relation is unfolded in the present paper. As it is demonstrated there, the origin of the holographic principle lies in the fact that a geometry of physical space has only a finite number of points. Furthermore, it is shown that the puzzlement of the holographic principle can be explained by a magnification of grid cells used to discretize geometrical magnitudes such as areas and volumes into sets of points. To wit, when grid cells of the Planck scale are projected from the surface of the observable universe into its interior, they become enlarged. For that reason, the space inside the observable universe is described by the set of points whose cardinality is equal to the number of points that constitute the universe's surface. [ABSTRACT FROM AUTHOR]

The Event Horizon Telescope (EHT) is an array of radio telescopes around the world that uses the technique of Very Long Baseline Interferometry (VLBI) to create a virtual Earth‐sized telescope. Using this technique, the EHT released the first image of a black hole on April 10, 2019, accelerating black hole science and creating enormous public impact. Now, a team of scientists and engineers is looking ahead to the next horizon: movies of black holes. In this paper, we give an overview of how the next‐generation EHT (ngEHT) Project has adopted a systems engineering approach to the design of future improvements to the EHT array, developing clear traceability from community‐defined key science goals. Scheduling, operations, site selection, observing bandwidth, data transport, and data management are some of the many factors that require careful consideration and balance and therefore benefit from intentional systems engineering methods underpinned by a "single source of truth" system model. [ABSTRACT FROM AUTHOR]

Since the launch in June 2017, Insight-HXMT has been in service smoothly in orbit and particularly productive in 2023 and 2024. Among a total number of 238 papers published so far based on Insight-HXMT data, 63 were published in 2023, and 32 in the early 2024 (till 2024 April), accounting for 40% of the total. These studies cover a variety of scientific subjects including the basic properties of black holes and neutron stars, the outburst of accreting black hole and neutron star X-ray binaries, thermal nuclear burst probes, isolated pulsars, quasi periodical oscillations, cyclotron resonant scattering features, fast radio bursts and gamma-ray bursts, etc. This paper introduces the overall progress with focus on some potential breakthroughs.

In this work, we mainly focus on testing the spacetime properties around black holes surrounded by a dark fluid, which are potential candidates for dark energy described by the Chaplygin-like equation of state through its shadow. To do this, we first study the black hole's horizon structure and shadow for the non-rotating case. Then, we obtain a rotating black hole solution in the presence of a dark fluid using the generalized Newman–Janis algorithm and study the effects of the black hole spin and the fluid parameters on the black hole horizons. Also, we obtained the shadow cast of the rotating black hole using celestial coordinates and showed that the presence of the dark fluid causes an increase in shadow size. Moreover, we use the shadow size of supermassive black holes Sagittarius A* and M87* from Event Horizon Telescope observations to obtain constraints on the spin, black hole charge, and dark fluid parameters. Lastly, we investigate the energy emission rate of a charged black hole surrounded by a Chaplygin-like dark fluid, comparing it to both rotating and non-rotating cases. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the properties of a five-dimensional Ricci-flat black hole within the framework of the Space–Time–Matter theory. By employing the Klein–Gordon equation, we derive the wave function equations for both the mass dimensional coordinate and the radial dimensional coordinate. We present the analytical solutions for the wave function on the mass dimensional coordinate under two distinct scenarios. Remarkably, we observe the absence of quantum effects on the mass dimensional coordinate, which aligns with the absence of a barrier in the wave function equation. For the radial component, we employ the B-spline method to numerically solve its wave function. Our findings reveal the existence of Hawking radiation between the event horizon and the cosmological horizon, with the transmission and reflection probabilities being determined by the presence of a barrier. Furthermore, we discover a significantly higher probability of particle detection near the cosmological horizon, while the probability near the event horizon approaches zero. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the influence of perfect fluid dark matter and quantum corrections on the thermodynamics of nonlinear magnetic-charged black hole. We consider the metric of the static nonlinear magnetic-charged black hole in the background of perfect fluid dark matter. Starting with the black hole temperature and the corrected entropy, we use the event horizon propriety in order to find the temperature, and based on the surface gravity definition, we find the uncorrected entropy. However, using the definition of the corrected entropy due to thermal fluctuation, we find and plot the entropy of the black hole. We find that the entropy is affected for smaller nonlinear magnetic-charged black holes. Afterwards, we study the thermodynamic stability of the black hole by computing and plotting the evolution of heat capacity. The results show that second-order phase transition occurs, which appears more later as the dark matter parameter decreases, and leads the black hole to move from the stable phase to the unstable phase. Furthermore, we show that the heat capacity for smaller black holes are also affected, since it appears not being only an increasing function. We also find that the behavior of Gibbs energy is modified when taking into account quantum corrections. [ABSTRACT FROM AUTHOR]

In this paper, null geodesics for a symmergent black hole are investigated. The geodesic equation of this space time is solved analytically according to the Weier-strass elliptic function. Also, the effective potential is obtained and plotted. Finally, using the form of effective potential and obtained analytical solution of geodesic equations, some possible types of orbits related to null geodesics are demonstrated. [ABSTRACT FROM AUTHOR]

In this paper, we consider an α' corrected Reissner-Nordström AdS black hole to study thermodynamics. We study the P -- V criticality and thermodynamical stability of the black hole. We obtained a first-order phase transition, which may be interpreted as the large/small black hole phase transition. Therefore, we obtained a van der Waals behaviour and obtained critical points. Finally, we calculate quantum work used to resolve the information loss paradox. [ABSTRACT FROM AUTHOR]

We investigate a spherically symmetric exact solution of Einstein's gravity with cosmological constant in (2 + 1) dimensions, non-minimally coupled to a scalar field. The solution describes the gravitational field of a black hole, which is free of curvature singularities in the entire spacetime. We use the formalism of geometrothermodynamics to investigate the geometric properties of the corresponding space of equilibrium states and find their interpretation from the point of view of thermodynamics. It turns out that, as a result of the presence of thermodynamic interaction, the space of equilibrium states is curved with two possible configurations, which depend on the value of a coupling constant. In the first case, the equilibrium space is completely regular, corresponding to a stable thermodynamic system. The second case is characterized by the presence of two curvature singularities, which are shown to correspond to locations where the system undergoes two different phase transitions, one due to the breakdown of the thermodynamic stability condition and the second one due to the presence of a divergence at the level of the response functions. [ABSTRACT FROM AUTHOR]

We consider the hypothesis that the sources of dark energy (DE) could be black holes (BHs) or more exotic objects, such as naked singularities or gravastars. We propose a definition of the presence of DE in the Universe and a criterion for what can be considered the source of this dark energy. It is based on the idea of the accelerated expansion of the Universe, which requires antigravity caused by large negative pressure. A recently proposed hypothesis, that the mass of BHs increases with time according to the same law as the volume of the part of the Universe containing it and the population of BHs can mimic DE, is examined. We demonstrate the reasons why it cannot be accepted, even if all the assumptions on which this hypothesis is based are considered true. [ABSTRACT FROM AUTHOR]

The process of the gravitational collapse might lead not only to a black hole but also to naked singularity formation. In this paper, we consider the generalized Vaidya spacetime with polytropic and generalized polytropic equations of state. We solve the Einstein and Einstein–Maxwell equations to obtain the explicit form of a mass function. We consider the limiting cases of solutions and find out, that generalized Vaidya spacetime might behave like Vaidya–de Sitter and Bonnor–Vaidya–de sitter solutions. Moreover, we explicitly show, that the part of solution, which depends on the polytropic index, is similar to cosmological fields surrounding both Vaidya and Bonnor–Vaidya black holes. The process of the gravitational collapse has been then considered. We have found out that the conditions of the naked singularity formation don't depend on the polytropic index. [ABSTRACT FROM AUTHOR]

This study focuses on the newly suggested Bardeen BH concept with a cloud of strings around it. Isenthalpic curves in the T i − P i plane are explored concurrently with the investigation of the Joule–Thomson coefficient and inversion temperature. A comparison between the behavior of the BH and the Van der Waals fluid is made in order to shed light on the system and clarify its similarities and differences. A crucial component of our research entails determining corrected entropy in order to comprehend thermal fluctuations. Based on corrected entropy, the thermal stability debate shows that there is no fluctuation when it comes to small BHs. Additionally, we investigate the energy emission process from the Bardeen BH solution with a cloud of strings in order to clarify this important thermodynamic feature. [ABSTRACT FROM AUTHOR]

We study the weak cosmic censorship conjecture by overspinning the Kerr–Newman–Kasuya black hole with test particle. We consider the geodesic equation of motion and find the certain conditions on the angular momentum of the black hole. We find the expression of δ J min and δ J max and then check whether the inequality satisfies or not. The results suggest that the Kerr–Newman–Kasuya black hole can be overspun showing the violation of cosmic censorship conjecture. [ABSTRACT FROM AUTHOR]

The dark energy star is a hypothetical model proposed as an alternative to address problems related to the structure of black holes (BHs), such as the presence of singularities. While dark energy star models have been previously explored, their application to BHs remains unexplored. This paper aims to investigate the concept of dark energy stars and compare their properties to BHs. The primary objective is to explore the physical profiles of dark energy stars and evaluate their similarity to the physical properties of BHs described by the Schwarzschild solution. To achieve this, specific properties of the dark energy star models need to be satisfied in the context of BHs, and their physical profiles are studied. The metric function g r r proposed by M. R. Finch and J. E. F. Skea [Class. Quantum Grav. 6, 467 (1989)], as adopted by A. Banerjee, M. K. Jasim and A. Pradhan [Mod. Phys. Lett. A 35, 2050071 (2020), arXiv:1911.09546 [gr-qc]], is used and parametrized, making it close to BH spacetimes. The findings show that the model exhibits properties similar to BHs in terms of the stellar radius, compactness, surface redshift, and nature of gravity. Specifically, the dark energy star model behaves like BHs with a dark energy parameter ω = − 1 ∕ 3 , satisfying all energy conditions. However, it should be noted that the investigation is limited to static spherically symmetric cases and further studies are required to explore the model in rotating cases. Overall, this study sheds light on the potential of dark energy star models in explaining BH properties and presents promising avenues for further research in understanding the nature of BHs and dark energy. [ABSTRACT FROM AUTHOR]

The recent expansion of the Internet of Things (IoT) owes a lot to the significant contribution of the 6LoWPAN protocol, which has been extensively employed in low-power and lossy networks. To facilitate communication in 6LoWPAN networks, the Internet Engineering Task Force (IETF) has suggested the usage of the Routing Protocol for Low-Power and Lossy Networks (RPL). Despite its usefulness, the open and restricted nature of the RPL protocol renders it susceptible to both internal and external attacks. Since IoT devices connected through the RPL protocol have limited resources like processing power, battery life, memory, and bandwidth, ensuring their security is of the utmost importance. One of the primary obstacles to IoT networks is RPL routing attacks, which disrupt the network's normal routing activities and structure. This study investigates the impact of five RPL routing attacks, namely Blackhole, Sybil, Selective Forwarding (SF), Sinkhole, DIO suppression, and DIS flooding, on the IoT networks' performance. The study evaluated the network's performance for normal and five routing attack scenarios using numerous performance metrics including Link throughput, No. of packets generated (control and data), Sensor data throughput, Packet Delivery Ratio (PDR), and Delay in packet delivery. This work conducted simulations using the Tetcos NetSim v12.1 IoT network simulator tool and is the first to analyze IoT network performance under multiple routing assault scenarios with various performance measures. The analysis showed that the performance metrics of PDR, Sensor data throughput, and No. of data packets transmitted decreased significantly in attack scenarios compared to the normal scenario, with an average decreased percentage of 70%, 70%, and 39.4%, respectively. In contrast, the metrics Link throughput, Delay, and No. of control packets transmitted increased in attack scenarios compared to the normal scenario, with average values supplemented by a factor of 35, 255, and 36, respectively. Additionally, the Destination-Oriented Directed Acyclic Graph (DODAG) real-time formation under different scenarios was provided. [ABSTRACT FROM AUTHOR]

We have explored the exponential surface brightness profile (SBP) of stellar disks, a topic extensively discussed by many authors yet seldom integrated with the study of correlations between black holes, bulges, and entire disks. Building upon our prior work in the statistical mechanics of disk-shaped systems and aligning with methodologies from other research, we analyze the influence of the central body. This analysis reveals analytical relationships among black holes, bulges, and the entire stellar disk. Additionally, we incorporate a specific angular momentum distribution (SAMD) that aligns more closely with observational data, showing that for the self-gravitating disk, with the same surface density, a reduction in its spin results in only a slight decrease in its radius, whereas with the same SAMD, an increment in its spin significantly limits its extent. A key feature of our model is its prediction that the surface density profile of an isolated disk will invariably exhibit downbending at a sufficient distance, a hypothesis that future observations can test. Our refined equations provide a notably improved fit for SBPs, particularly in the central regions of stellar disks. While our findings underscore the significance of statistical mechanics in comprehending spiral galaxy structures, they also highlight areas in our approach that warrant further discussion and exploration. [ABSTRACT FROM AUTHOR]

In this work, we aim at the question of holographic phase transitions in two-dimensional systems with Lifshitz scaling. We consider the gravity side candidate for a dual description as the black hole solution of new massive gravity (NMG) with Lifshitz scaling. We discuss the effects due to the Lifshitz scaling in the AGGH (Ayon-Beato-Garbarz-Giribet-Hassaïne) solution in comparison with the BTZ (Bañados-Teitelboim-Zanelli) black hole. Likewise, we compute the order parameter and it indicates a second-order phase transition in a (1 + 1) dimension Lifshitz boundary. [ABSTRACT FROM AUTHOR]

In this work, we will study the boundary behaviors of a spacelike positive constant mean curvature surface Σ in the Schwarzschild spacetime exterior to the black hole. We consider two boundaries: the future null infinity I + and the horizon. Suppose near I + , Σ is the graph of a function - P (y , s) in the form v ¯ = - P , where v ¯ is the retarded null coordinate with s = r - 1 and y ∈ S 2 . Suppose the boundary value of P (y , s) at s = 0 is a smooth function f on the unit sphere S 2 . If P is C 4 at I + , then f must satisfy a fourth order PDE on S 2 . If P is C 3 , then all the derivatives of P up to order three can be expressed in terms of f and its derivatives on S 2 . For the extrinsic geometry of Σ , under certain conditions we obtain decay rate of the trace-free part of the second fundamental forms A ˚ . In case A ˚ decays fast enough, some further restrictions on f are given. For the intrinsic geometry, we show that under certain conditions, Σ is asymptotically hyperbolic in the sense of Chruściel–Herzlich (Pac J Math 212(2):231–264, 2003). Near the horizon, we prove that under certain conditions, Σ can be expressed as the graph of a function u which is smooth in η = 1 - 2 m r 1 2 and y ∈ S 2 , and all its derivatives are determined by the boundary value u at η = 0 . In particular, a Neumann-type condition is obtained. This may be related to a remark of Bartnik (in: Proc Centre Math Anal Austral Nat Univ, 1987). As for intrinsic geometry, we show that under certain conditions the inner boundary of Σ given by η = 0 is totally geodesic. [ABSTRACT FROM AUTHOR]

In this paper, we explore gravitational lensing effects associated with rotating black holes within the framework of loop quantum gravity. Utilizing the Gauss-Bonnet theorem as extended by Ono et al., we compute the light deflection angle in the weak field limit for a lens that is finitely distanced from both the source and the observer. Our findings indicate that the weak deflection angle for rotating black holes in LQG is smaller than that observed for the classical Kerr black holes, albeit with minimal deviations. In the strong field limit, we determine the photon sphere radius, the light deflection angle, and lensing observables, including the image position θ∞, angular separation s, magnification rmag, and temporal delays among various relativistic images. By considering supermassive black holes, such as Sgr A* and M87*, within the LQG framework, we calculate these observables and investigate the influence of the quantum parameter Aλ on them, compared with the Kerr black hole outcomes. Our comparative analysis reveals that the image position θ∞ and separation s for Sgr A* consistently exceed those for M87*, whereas M87* exhibits considerably greater time delays than Sgr A*. These distinctions could be important in differentiating between rotating black holes in LQG and classical Kerr black holes in future astronomical observations.

This research investigates the properties of charged T-duality black holes within the context of string theory. The investigation focuses on the implications of T-duality, electric charge, and thermodynamic parameters on the curvature and behavior of a black hole. The findings demonstrate that physical variables like μ and H have significant impacts on the position of the event horizon, photon radius as well as shadow radius. The study calculates the values of quasinormal modes and observes through the graphical behavior. Additionally, the paper investigates the entropy and free energies of the system by considering simple logarithmic correction and also demonstrates how the T-duality parameter enhances the thermodynamic stability of the Bardeen black hole geometry. We explore the emission energy of the considered black hole. Overall, this research contributes to our understanding of black holes and their role in the universe within the context of string theory.

Recent observations of black hole shadows have revolutionized our ability to probe gravity in extreme environments. This manuscript presents a novel analytic method to calculate, in leading-order terms, the key parameters of photon sphere and shadow radius. This method offers advantages for complex metrics where traditional approaches are cumbersome. We further explore the impact of black hole mass on the photon sphere radius, providing insights into black hole interactions with their surroundings. Our findings contribute significantly to black hole physics and gravity under extreme conditions. By leveraging future advancements in observations, such as the next-generation Event Horizon Telescope (ngEHT), this work paves the way for even more precise tests of gravity near black holes.

We consider the hypothesis that the sources of dark energy (DE) could be black holes (BHs) or more exotic objects, such as naked singularities or gravastars. We propose a definition of the presence of DE in the Universe and a criterion for what can be considered the source of this dark energy. It is based on the idea of the accelerated expansion of the Universe, which requires antigravity caused by large negative pressure. A recently proposed hypothesis, that the mass of BHs increases with time according to the same law as the volume of the part of the Universe containing it and the population of BHs can mimic DE, is examined. We demonstrate the reasons why it cannot be accepted, even if all the assumptions on which this hypothesis is based are considered true.

In this paper we continue our study of the non-linear response of a Schwarzschild black hole to an ingoing gravitational wave by computing the Newman–Penrose (NP) constants. The NP constants are five complex, supertranslation-invariant quantities defined on null infinity I + and although put forward in the 60's, they have never been computed in a non-stationary setting. We accomplish this through a numerical implementation of Friedrich's generalized conformal field equations whose semi-global evolution yields direct access to I + . The integral expressions for the NP constants as given originally are expressed in a general gauge to enable their computation from numerically generated data. Canonical methods of fixing inherent degrees of freedom in their definitions are discussed. The NP expressions are then computed for a variety of different ingoing wave profiles in axisymmetry, and then with no symmetry assumptions in a general 3 + 1 setting for which all five complex quantities are non-zero. We discuss the consequence of the (numerical) constancy of the NP expressions for the (non)-smoothness of null-infinity. [ABSTRACT FROM AUTHOR]

In this work, the quasinormal mode, greybody factors, and absorption cross section of de Sitter Reissner‐Nordström black hole surrounded by quintessence field in Rastall gravity are studied. The violation of energy‐momentum conservation has a non‐linear effect on the quasinormal modes. With an increase in the black hole charge, both real parts of quasinormal modes i.e. oscillation frequency of ring‐down Gravitational Waves (GWs) and damping or decay rate of GWs increase non‐linearly. A similar observation is made for the black hole structural parameter also, however in this case the variation is almost linear. The remnant mass of a black hole depends on different physical parameters of a black hole. In the case of greybody factors also, both parameters have similar impacts are observed. With an increase in these parameters, greybody factors decrease. Moreover, the null geodesics and the impact of Rastall gravity on the light trajectory are also investigated. Our study suggests that the presence of a surrounding quintessence field may shadow the existence of black hole charges in such black hole configurations. [ABSTRACT FROM AUTHOR]

Searches for gravitational waves from compact binary mergers, which to date have reported ∼100 observations, have previously ignored binaries whose components are consistent with the mass of neutron stars (1–2 M⊙) and have high dimensionless spin >0.05. While previous searches targeted sources that are representative of observed neutron star binaries in the Galaxy, it is already known that neutron stars can regularly be spun up to a dimensionless spin of ∼0.4, and in principle reach up to ∼0.7 before breakup would occur. Furthermore, there may be primordial black hole binaries or exotic formation mechanisms to produce light black holes. In these cases, it is possible for the binary constituent to be spun up beyond that achievable by a neutron star. A single detection of this type of source would reveal a novel formation channel for compact binaries. To determine whether there is evidence for any such sources, we use pycbc to conduct a targeted search of LIGO and Virgo data for light compact objects with high spin. Our analysis detects previously known observations GW170817 and GW200115; however, we report no additional mergers. The most significant candidate, not previously known, is consistent with the noise distribution, and so we constrain the merger rate of spinning light binaries. [ABSTRACT FROM AUTHOR]

During particle collisions in the vicinity of the horizon of black holes, it is possible to achieve energies and temperatures corresponding to phase transitions in particle physics. It is shown that the sizes of the regions of the new phase are of the order of the Compton length for the corresponding mass scale. The lifetime is also on the order of the Compton time. It is shown that the inverse influence of the energy density in the electro-weak phase transition in collisions on the space–time metric can be neglected. [ABSTRACT FROM AUTHOR]

Karl Schwarzschild’s landmark paper revealed the first ever exact solution to Einstein’s gravitational field equation. This was a major scientific achievement. However, his solution for the metric of a spherically symmetric spacetime is different than the one found in textbooks today— because Schwarzschild assumed, like in Newtonian gravity, that only one singularity could exist. Given the newness of Einstein’s general theory of relativity and the existing paradigm of Newtonian gravity, his assumption was natural. While other authors have previously pointed out this shortcoming and corrected it, the contribution of this paper is to show how Schwarzschild might have discovered it himself. A simple geometric proof indicates how Schwarzschild, with a different assignment of a single constant of integration, would have arrived at the metric found in textbooks today. [ABSTRACT FROM AUTHOR]

Theories on the bosonic nature of dark matter are a promising alternative to the cold dark matter model. Here we consider a dark matter halo in the state of a Bose–Einstein condensate (BEC), subject to the gravitation of a black hole. In the low energy limit, we bring together the general relativity in the Schwarzschild metric and the quantum description of the BEC. The model is solvable in the Fermi normal coordinates with the so called highly nonlocal approximation and describes tidal deformations in the condensate wave function. The black hole deforms the localized condensate until the attraction of the compact object overcomes the self-gravitation and destabilizes the solitonic dark matter. Moreover, the model can be implemented as a gravitational analog in the laboratory; the time-dependent potential generated by the galactic black hole can be mimicked by an optical trap acting on a conventional condensate. The results open the way to new laboratory simulators for quantum gravitational effects. [ABSTRACT FROM AUTHOR]

The Scalar Field Dark Matter model has been known in various ways throughout its history; Fuzzy, BEC, Wave, Ultralight, Axion-like Dark Matter, etc. All of them consist in proposing that dark matter of the universe is a spinless field Φthat follows the Klein-Gordon (KG) equation of motion □Φ-dV/dΦ=0, for a given scalar field potential V. The difference between different models is sometimes the choice of the scalar field potential V. In the literature we find that people usually work in the non-relativistic, weak-field limit of the Klein- Gordon equation, where it transforms into the Schrödinger equation and the Einstein equations into the Poisson equation, reducing the KG-Einstein system, to the Schrödinger-Poisson system. In this paper, we review some of the most interesting achievements of this model from the historical point of view and its comparison with observations, showing that this model could be the last answer to the question about the nature of dark matter in the universe. [ABSTRACT FROM AUTHOR]

We discuss the problem of canonical quantization of a free massive spinor field in the Schwarzschild spacetime. It is shown that a consistent procedure of canonical quantization of the field can be carried out without taking into account the internal region of the black hole, the canonical commutation relations in the resulting theory hold exactly and the Hamiltonian has the standard form. Spin sums are obtained for solutions of the Dirac equation in the Schwarzschild spacetime. [ABSTRACT FROM AUTHOR]

This paper studies the Rindler trajectories in the Cloud of strings in 3rd-order Lovelock gravity. According to the generalization of the Letaw–Frenet equations for curved spacetime (ST), the trajectory will continue to accelerate linearly and uniformly throughout its motion. The ST of the Cloud of strings in 3rd-order Lovelock gravity, a boundary is established on the bound of the accelerated magnitude |a| for radially inward traveling trajectories in the expression of the BH mass m which is represented by | a | ≤ b + 1 3 / 2 3 3 m . For a certain selection of asymptotic initial data h, the linearly uniformly accelerated trajectory always enters the BH for acceleration |a| greater than the bound value. To study the bound value by |a|, the radial linearly uniformly accelerated trajectory can only travel to infinity within a small radius or the distance of the closest approach. However, it is observed that when the bound | a | = b + 1 3 / 2 3 3 m is saturated, and this distance approaches its lowest value of r b = 3 m b + 1 . We also demonstrate that the value of the acceleration has a limited constraint, there is always an extension of the closest approach r b > 2 m b + 1 for | a | ≤ B (m , h) , for each set of finite asymptotic initial data h. [ABSTRACT FROM AUTHOR]

The firewall transformation put forward by 't Hooft in recent years has made ambitious claims of solving the firewall problem and the black hole information paradox while maintaining unitary evolution. However, the theory has received limited attention from the community, especially in regards to its foundations in purely classical gravitational physics. This paper investigates the underlying assumptions of 't Hooft's firewall transformation before quantization. We find that the limiting procedure used by 't Hooft in order to obtain an identification of the quantum operators for ingoing and outgoing particles near a black hole is not consistent. We propose a correction, which involves a more relaxed approximation regime. In the new approximation regime, we find a new classical analog for the firewall transformation for spherical shells, which allows evolving the spherical shells' dynamics past their point of collision. In the classical theory, no firewall is removed, as both ingoing and outgoing matter is present on every spacelike hypersurface, and it does not appear that any firewalls will be removed after a canonical quantization. [ABSTRACT FROM AUTHOR]

The mini‐outburst of black hole X‐ray binaries (BHXBs) is a re‐bright event of light curve, which is often detected in BHXBs with the low orbital period, such as XTE J1650‐500 (Porb≈7.69±0.02hr$$ {P}_{\mathrm{orb}}\approx 7.69\pm 0.02\kern0.22em \mathrm{hr} $$), GRO J0422+32 (Porb≈5.091850±0.000005hr$$ {P}_{\mathrm{orb}}\approx 5.091850\pm 0.000005\kern0.22em \mathrm{hr} $$) and Swift J1753.5‐0127 (Porb≈3.244±0.001hr$$ {P}_{\mathrm{orb}}\approx 3.244\pm 0.001\kern0.22em \mathrm{hr} $$). Because of its low X‐ray luminosity, the physics properties of mini‐outburst is still unclear. GX 339‐4 is a famous BHXBs with the orbital period Porb=16.9hr$$ {P}_{\mathrm{orb}}=16.9\kern0.22em \mathrm{hr} $$ and present many normal outburst and failed outburst over the past decades. However, the mini‐outburst is not still detected in GX 339‐4. In this work, using the Rossi X‐ray Timing Explorer (RXTE) observations, we investigated the energy spectrum properties of the GX 339‐4 outburst in 2004 and confirmed that the outburst in 2004 is a mini‐outburst. It is very interest that the mini‐outburst has been through a whole state transition from Low/Hard state to High/Soft state. We also give a comparison between the normal outburst in 2002 and the mini‐outburst in 2004. And we find that the two outburst present a similar Hardness‐Intensity diagram and spectral properties, which suggest that the normal outburst and mini‐outburst have a similar physics. We think that the mini‐outburst of GX 339‐4 in 2004 can be well explained by the disk instability model. [ABSTRACT FROM AUTHOR]

In this work, we try to find the volume inside the power-Maxwell charged black hole in Rastall gravity based on the definition and method proposed by Christodoulou and Rovelli. We find the interior volume as well as CR volume for this black hole. We also analyze our work numerically and show that our result is compatible with previous results obtained by various authors. [ABSTRACT FROM AUTHOR]

This work focuses on the examination of a regular black hole within Verlinde's emergent gravity, specifically investigating the Hayward-like (modified) solution. The study reveals the existence of three horizons under certain conditions, i.e. an event horizon and two Cauchy horizons. Our results indicate regions which phase transitions occur based on the analysis of heat capacity and Hawking temperature. To compute the latter quantity, we utilize three distinct methods: the surface gravity approach, Hawking radiation, and the application of the first law of thermodynamics. In the case of the latter approach, it is imperative to introduce a correction to ensure the preservation of the Bekenstein–Hawking area law. Geodesic trajectories and critical orbits (photon spheres) are calculated, highlighting the presence of three light rings. Additionally, we investigate the black hole shadows. Furthermore, the quasinormal modes are explored using third- and sixth-order Wentzel–Kramers–Brillouin approximations. In particular, we observe stable and unstable oscillations for certain frequencies. Finally, in order to comprehend the phenomena of time-dependent scattering in this scenario, we provide an investigation of the time-domain solution. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the influence of the angular momentum of the charged particle around non-extremal and extremal charged Taub–NUT black holes on Lyapunov exponent, and find spatial regions where chaos bound is violated. The exponent is determined by the effective potential. When the particle's charge is fixed, circular orbits gradually move away from the event horizons with the angular momentum's increases. We find that the bound is always violated at a certain distance from the horizons when the angular momentum is greater than a certain value. Especially when the black holes' charges are small enough, the Lyapunov exponent's violation always exists for any value of the particle's angular momentum. [ABSTRACT FROM AUTHOR]

With his seminal and pioneering work on the stability of the Schwarzschild black hole and its interaction with gravitational radiation, Vishu had opened a new window on black hole astrophysics. One of the interesting conjectures that soon followed in John Wheeler pronouncing "a black hole has no hair"; it is entirely specified by three parameters—mass, spin and charge—and nothing more. The discovery of gravitational waves in 2016 produced by the merger of two black holes and observed by the LIGO–VIRGO collaboration, carried the definitive signature of quasinormal modes, the phenomenon of black hole ringdown, exactly as predicted by Vishu in his 1970 Nature paper (see the introduction to Classics by R. Isaacson in this issue) 46 years ago. This was the crowning glory. [ABSTRACT FROM AUTHOR]