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Zeldovich amplification of classic waves carrying orbital angular momentum (OAM) from a rotating absorber is an extension of Penrose superradiance from a rotating black hole. The demonstration of Zeldovich amplification in recently published experiments showed the possibility of extracting energy from a spinning black hole or a rotating absorber. However, it remains unclear whether extracting energy from non-absorbent bodies is possible. Here, we experimentally demonstrate the amplification of acoustic OAM from rotating impellers made of non-absorbent materials. We develop a multichannel least-mean-square algorithm to emit high-charge acoustic OAM beams into three types of impellers. The acoustic gains (more than 20 dB) have been measured by both a static microphone and a microphone array working as a virtual rotating receiver. The results indicate that the acoustic gain from the impeller with a large windward area is much higher than the ones with a small area. Our work is worthwhile in proposing the experimental method to study the phenomenon of acoustic OAM amplification and showing prospects in industrial applications such as amplifying acoustic signals by commonly used impellers. Our work also discusses a possible way of extracting energy from non-absorbent celestial systems, such as the orbiting planets of the Solar system, which are much less absorbent to light but much closer to the Earth than a black hole. [ABSTRACT FROM AUTHOR]

A new class of acoustic black hole (ABH) waveguides is presented, which relies in its operation on an array of optimally designed functionally graded perforated rings (FGPRs). In this manner, the developed ABH is provided with built-in energy dissipation characteristics generated by virtue of the flow through perforations, which enhances its acoustic absorption behavior and makes the speed of the propagating waves vanish faster when reaching the end of the waveguide. Furthermore, the particular design of the rings enables sandwiching of additional porous absorbing layers between the rings to further boost the absorption characteristics of the proposed ABH. Accordingly, the operating principle of the new class of ABH is radically different from that of the conventional ABH that employs sequential solid-flat rings of decreasing inner radii to create a virtual power law taper necessary for generating the black hole effect, but through reactive means rather than the effective dissipative means of the proposed ABH. Therefore, this paper develops a transfer matrix modeling (TMM) approach to model the absorption and reflection characteristics of the new class of ABH, in an attempt to predict its behavior, optimize the selection of its design parameters, and more importantly, demonstrate its merits as effective means for controlling sound propagation. Numerical examples are presented to highlight the merits and behavior of the proposed ABH. Predictions of the TMM are validated against experimental results that are available in the literature for one and two micro-perforated plates. Comparisons are also established between the ABH with FGPR and the conventional ABH in order to distinguish the behavior and underlying principles of their operations. [ABSTRACT FROM AUTHOR]

In this paper, we applied AdS/CFT correspondence to carefully study images of quantum-corrected AdS black hole from the viewpoint of the wave optics. The result shows that the absolute amplitude of response function always displays an interference pattern that comes from the diffraction of the scalar wave. The strength of interference increases with quantum parameter a, while decreases with the event horizon rh and source parameters (ω,σ). For the images of dual black hole, one can see at position θobs=0 that it is a perfect Einstein ring with a series of concentric striped patterns around it, while it will be broken at other positions. This result well coincides with the geometrical optics, i.e. the radius of ring. The feature of images of dual black hole is closely related to the parameters of black hole, source, and optics system. Especially, the effect of quantum parameter a on images of AdS black hole is very important. For instance, it gives rise to the fact that the image would evolve into one light spot at θobs=π/2, rather than two which is usually found in Schwarzschild AdS black hole. Finally, since the holographic images constructed from observables of the quantum system focus on testing the existence of dual black hole, we conclude that the holographic images of quantum corrected AdS black hole presented in this paper would be more helpful for this target. [ABSTRACT FROM AUTHOR]

The Bogoliubov transformation connecting the standard inertial frame mode functions to the standard mode functions defined in the Rindler frame R 0, leads to the result that the inertial vacuum appears as a thermal state with temperature T 0 = a 0 / 2 π where a 0 is the acceleration parameter of R 0. We construct an infinite family of nested Rindler-like coordinate systems R 1 , R 2 , ... within the right Rindler wedge, with time coordinates τ 1 , τ 2 , ... , and acceleration parameters a 1 , a 2 , ... by shifting the origin along the inertial x -axis by amounts ℓ 1 , ℓ 2 , ⋯ . We show that, apart from the inertial vacuum, the Rindler vacuum of the frame Rn also appears to be a thermal state in the frame R n + 1 with the temperature a n + 1 / 2 π . In fact, the Rindler frame R n + 1 attributes to all the Rindler vacuum states of R 1 , R 2 , ... R n , as well as to the inertial vacuum state, the same temperature a n + 1 / 2 π . We further show that our result is discontinuous in an essential way in the coordinate shift parameters. For a Rindler frame Ri, this thermality turns on with smallest non-zero ℓ i allowed in the semiclassical framework and remains insensitive to (ℓ i , a i − 1) thereafter, indicating its universal Planckian origin. Similar structures can be introduced in the right wedge of any spacetime with bifurcate Killing horizon, like, for e.g. Schwarzschild spacetime. Apart from providing unsuppressed observables capturing Planck scale effects, these results have important implications for quantum gravity (QG) when flat spacetime is treated as the ground state of QG. Furthermore, a frame with the shift ℓ and the corresponding acceleration parameter a (ℓ) can be thought of as a Rindler frame which is instantaneously comoving with the Einstein's elevator moving with a variable acceleration. Our result suggests that the quantum temperature associated with such an Einstein's elevator is the same as that defined in the comoving Rindler frame. The implications of these results are wide ranging, from having a definitive signature of Planck shifts in the horizon to the existence of a new set of observers in black hole exterior having thermodynamic description of the horizon they perceive. [ABSTRACT FROM AUTHOR]

The Milky Way galaxy is estimated to host up to a billion stellar-mass solitary black holes (BHs). The number and distribution of BH masses can provide crucial information about the processes involved in BH formation, the existence of primordial BHs, and the interpretation of gravitational wave (GW) signals detected in LIGO–VIRGO–KAGRA. Sahu et al. recently confirmed one solitary stellar-mass BH in our galaxy using astrometric microlensing. This work proposes a novel mechanism to identify such BH by analysing the frequency and damping of the quasi-normal modes of GW generated from the interaction of the BH and EM wave originating from a transient electromagnetic (TEM) source. The incoming EM waves distort the curvature of a BH, releasing GWs as it returns to a steady state. Using the covariant semitetrad formalism, we quantify the generated GWs via the Regge – Wheeler tensor and relate the GW amplitude to the energy of the TEM. We demonstrate that isolated BHs at a distance of 50 pc from Earth can be detected by LIGO A+ and 100 pc by Cosmic Explorer/Einstein Telescope. Additionally, we discuss the observational implications for orphan afterglows associated with GRBs, highlighting the potential for further discoveries. [ABSTRACT FROM AUTHOR]

This paper focuses on the examination of stable and unstable geometrical structures of thin-shell wormholes constructed from Henneaux–Martinez–Troncoso–Zanelli black holes through the Visser cut-and-paste approach. We use the Israel thin-shell formalism for evaluating the stress–energy tensor components of the matter distribution near the wormhole throat. Equations of state, specifically the phantom-like and generalized Chaplygin gas model for exotic matter, are used to conduct an extensive investigation into the stability of the thin-shell. The radial perturbation around the equilibrium throat radius is also considered to explore the stable configuration for specific values of physical parameters. Our results show that the stable zones of these thin-shell wormholes increase as the scalar field parameter ℰ approaches −1. This study gives light on the behavior and dynamics of these wormholes throw light on their potential stability and lead the way for additional theoretical physics research. [ABSTRACT FROM AUTHOR]

We study the thermodynamic geometry of two types of asymptotically anti-de Sitter black holes in four-dimensional Einsteinian cubic gravity, including the uncharged rotating and charged non-rotating black holes, applying the Ruppeiner thermodynamic geometry method. The divergence of the scalar curvature of the Ruppeiner metric ( R R ) is found to be related to the vanishing of the heat capacity. The stability of the solutions is shown to be affected by the Einsteinian cubic gravity coupling constant λ. In the unstable phase of the rotating black hole, the R R appeared to possess additional diverging points, where the number of these points and the behavior of R R are controlled by the angular momentum parameter. Also, for the charged non-rotating black hole case, we show that depending on the values of λ , the solution may enjoy two types of phase transition and R R behaviors. The characteristic behavior of R R of these two types of black holes enabled us to recognize the types of interactions between microscopic degrees of freedom. [ABSTRACT FROM AUTHOR]

Using the AdS/CFT correspondence, this paper investigates the holographic images of a charged black hole within the context of Lorentz symmetry breaking massive gravity. The photon rings, luminosity-deformed rings, or light points from various observational perspectives are obtained. We also study the influences of both the chemical potential and temperature on the Einstein ring. Unlike the previous work, which primarily examines the effect of chemical potential on ring radius at high temperatures and find no change in the radius with varying chemical potential, we also investigate the effect of chemical potential on the ring radius at low temperature besides at high temperature. Our findings indicate that at low temperatures, the photon ring radius decreases with increasing of chemical potential, while at high temperatures, the results are consistent with previous studies. Additionally, we explore the impact of the model parameter λ on the Einstein ring radius and find the the ring radius increases as the model parameter λ increases. More interestingly, for the large chemical potential, u = 1, the temperature dependence of the photon ring radius is reversed for λ = 2 and λ = 4. Conversely, for a small chemical potential u = 0.1, the temperature dependence of the Einstein ring stays the same as λ = 2 and λ = 4. [ABSTRACT FROM AUTHOR]

Oscillations of black hole spacetimes exhibit divergent behavior near the bifurcation sphere and spatial infinity. In contrast, these oscillations remain regular when evaluated near the event horizon and null infinity. The hyperboloidal approach provides a natural framework to bridge these regions smoothly, resulting in a geometric regularization of time-harmonic oscillations, known as quasinormal modes (QNMs). This review traces the development of the hyperboloidal approach to QNMs in asymptotically flat spacetimes, emphasizing both the physical motivation and recent advancements in the field. By providing a geometric perspective, the hyperboloidal approach offers an elegant framework for understanding black hole oscillations, with implications for improving numerical simulations, stability analysis, and the interpretation of gravitational wave signals. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the circular motion and chaotic behavior of charged particles in the dyonic global monopole spacetime surrounded by a perfect fluid. We classified the black hole into three special regimes: dark matter, dust, and radiation. We precisely calculated the Lyapunov exponent for each regime as an eigenvalue of the Jacobian matrix. We examine, through numerical and graphical analysis, the circular motion and chaos bound violation across all the regimes. In the dark matter regime, stable orbits conform to the chaos bound. Even though the bound brings orbits with small charges and those far from the event horizon closer, they never violate it. In the dust regime, there can be more than one orbit for a fixed mass, charge, topological defect, and fluid parameter, especially when the angular momentum is small. At this point, the orbits are unstable, and those that are closer to the event horizon violate the bound. Similarly, in the radiation regime, orbits that are closer to the event horizon are unstable and chaotic, especially with greater angular momentum. In fact, regardless of the charge, topological defect, and fluid parameter, all orbits, whether they are far from or close to the horizon, become unstable and violate the bound when the angular momentum is significantly large. [ABSTRACT FROM AUTHOR]

Recent research has shown that generalized entropies are suitable for describing the thermodynamic properties of gravitational systems. In 2016, Giddings proposed that the spectrum of Hawking radiation, which characterizes evaporating semiclassical black holes, originates from a quantum “atmosphere” that lies beyond a black hole’s horizon. Subsequent studies confirmed that this proposal is possible if the Bekenstein–Hawking entropy of the Schwarzschild black hole is taken to be the Tsallis entropy, and that this result is also compatible with the generalization of the Tsallis entropy to the Rényi entropy. In our paper, we show that the Sharma–Mittal entropy, which is another generalization of the Tsallis entropy, rejects the possibility of Hawking’s proposal but can be compatible with Giddings’ suggestion. So, in this paper, we consider the Sharma–Mittal entropy and study its behavior related to Giddings’ suggestion. So we specify a range for the upper limit of the (δ−R) parameter, which enables Giddings’ suggestion in the presence of the Sharma–Mittal entropy. We also understood that the Sharma–Mittal entropy, similar to the Tsallis and Rényi entropies, exhibits the same behavior under some approximation of its parameters, in accordance with Giddings’ suggestion. [ABSTRACT FROM AUTHOR]

Detecting dual active galactic nuclei (DAGNs) in observations and understanding theoretically which massive black holes (MBHs) compose them and in which galactic and large-scale environment they reside are becoming increasingly important questions as we enter the multimessenger era of MBH astronomy. This paper presents the abundance and properties of DAGN produced in nine large-scale cosmological hydrodynamical simulations. We focus on DAGN powered by AGN with |$L_{\rm bol}\geqslant 10^{43}\, \rm erg\, s^{-1}$| and belonging to distinct galaxies, i.e. pairs that can be characterized with current and near-future electromagnetic observations. We find that the number density of DAGN separated by a few to 30 proper kpc varies from |$10^{-8}$| (or none) to |$10^{-3} \, \rm comoving\, Mpc^{3}$| in the redshift range |$z=0\!-\!7$|⁠. At a given redshift, the densities of the DAGN numbers vary by up to two orders of magnitude from one simulation to another. However, for all simulations, the DAGN peak is in the range |$z=1\!-\!3$|⁠ , right before the peak of cosmic star formation or cosmic AGN activity. The corresponding fractions of DAGN (with respect to the total number of AGN) range from 0 per cent to 6 per cent. We find that simulations could produce too few DAGN at |$z=0$| (or merge pairs too quickly) compared to current observational constraints while being consistent with preliminary constraints at high redshift (⁠|$z\sim 3$|⁠). Next-generation observatories (e.g. Advanced X-Ray Imaging Satellite [AXIS]) will be of paramount importance to detect DAGN across cosmic times. We predict the detectability of DAGN with future X-ray telescopes and discuss DAGN as progenitors for future Laser Interferometer Space Antenna (LISA) gravitational wave detections. [ABSTRACT FROM AUTHOR]

A major open question in astrophysics is the mechanisms by which massive black holes (BHs) form in the early Universe, which pose constraints on seeding models. We study BH formation and evolution in a flexible model combining the cosmological IllustrisTNG (TNG) simulations with semi-analytic modelling in post-processing. We identify our TNG model hosts based on various criteria including a minimum gas mass of |$10^7$| – |$10^9$| |${\rm M}_{\odot }$|⁠ , total host mass of |$10^{8.5}$| – |$10^{10.5}$| |${\rm M}_{\odot }$|⁠ , and a maximum gas metallicity of 0.01–0.1 |$\mathrm{Z}_{\odot }$|⁠. Each potential host is assigned a BH seed with a probability of 0.01–1. The populations follow the TNG galaxy merger tree. This approach improves upon the predictive power of the simple TNG BH seeding prescription, narrowing down plausible seeding parameter spaces, and it is readily adaptable to other cosmological simulations. Several model realizations predict |$z\lesssim 4$| BH mass densities that are consistent with empirical data as well as the TNG BHs. However, high-redshift BH number densities can differ by factors of |$\sim$| 10 to |$\gtrsim$| 100 between seeding parameters. In most model realizations, |$\lesssim 10^5$| |${\rm M}_{\odot }$| BHs substantially outnumber heavier BHs at high redshifts. Mergers between such BHs are prime targets for gravitational-wave detection with Laser Interferometer Space Antenna. The |$z=0$| BH mass densities in most realizations of the model agree well with observations, but our strictest seeding criteria fail at high redshift. Our findings strongly motivate the need for better empirical constraints on high- z BHs, and they underscore the significance of recent active galactic nucleus discoveries with JWST. [ABSTRACT FROM AUTHOR]

X-ray reflection spectroscopy is a powerful tool to study the strong gravity region of black holes. The next generation of astrophysical X-ray missions promises to provide unprecedented high-quality data, which could permit us to get very precise measurements of the properties of the accretion flow and of the space–time geometry in the strong gravity region around these objects. In this work, we test the accuracy of the relativistic calculations of the reflection model relxill and of its extension to non-Kerr space–times relxill_nk in view of the next generation of X-ray missions. We simulate simultaneous observations with Athena /X-IFU and LAD of bright Galactic black holes with a precise and accurate ray-tracing code and we fit the simulated data with the latest versions of relline and relline_nk. While we always recover the correct input parameters, we find residuals in the fits when the emission from the inner part of the accretion disc is higher. Such residuals disappear if we increase the number of interpolation points on the disc in the integral of the transfer function. We also simulate full reflection spectra and find that the emission angle from the accretion disc should be treated properly in this case. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the star formation histories of massive-quenched galaxies at |$z=3$| in three semi-analytical models (Shark, gaea, Galform) and three cosmological hydrodynamical simulations (Eagle, IllustrisTNG, Simba). We study the predicted number density and stellar mass function of massive-quenched galaxies, their formation and quenching time-scales and star formation properties of their progenitors. Predictions are disparate in all these diagnostics, for instance: (i) some simulations reproduce the observed number density of very massive-quenched galaxies (⁠|$\gt 10^{11}\, \rm {\rm M}_{\odot }$|⁠) but underpredict the high density of intermediate-mass ones, while others fit well the lower masses but underpredict the higher ones; (ii) in most simulations, except for gaea and Eagle , most massive-quenched galaxies had starburst periods, with the most intense ones happening at |$4\lt z\lt 5$|⁠ ; however, only in Shark and IllustrisTNG we do find a large number of progenitors with star formation rates |$\gt 300\rm \, {\rm M}_{\odot }\, yr^{-1}$|⁠ ; (iii) quenching time-scales are in the range |$\approx 20\!-\!150$| Myr depending on the simulation; among other differences. These disparate predictions can be tied to the adopted active galactic nucleus (AGN) feedback model. For instance, the explicit black hole (BH) mass dependence to trigger the 'radio mode' in IllustrisTNG and Simba makes it difficult to produce quenched galaxies with intermediate stellar masses, also leading to higher baryon collapse efficiencies (⁠|$\approx 15\!-\!30$|  per cent); while the strong bolometric luminosity dependence of the AGN outflow rate in gaea leads to BHs of modest mass quenching galaxies. Current observations are unable to distinguish between these different predictions due to the small sample sizes. However, these predictions are testable with current facilities and upcoming observations, allowing a 'true physics experiment' to be carried out. [ABSTRACT FROM AUTHOR]

We investigate high-frequency traders' behavior in the context of the fastest and most extreme price movements (EPMs) that can be observed in the market, specifically ultra-fast flash events, challenging the methodologies employed in the academic and practitioner literature for identifying sudden liquidity black holes. To refine the price-shock identification methodology, we introduce a new approach called sequence-based flash events (SFEs), which relies on tick sequences instead of predetermined fixed-time intervals within which all flash events in the sample are assumed to occur. This alternative methodology offers the advantage of pinpointing the exact time and duration of a crash, which, in turn, provides a way to more accurately define the observation windows around it. We compare our sample of SFEs with both the so-called "mini flash crashes", as identified by the Nanex detection algorithm, and the so-called EPMs, as identified by Brogaard et al. (2018). We use close and open prices, as well as high and low prices. Based on our sample of SFEs, we find no evidence that HFTs trigger extreme price shocks. However, we find that HFTs exacerbate SFEs by increasing the net imbalance in the direction of these shocks as they occur. Finally, we show that the choice of the price-shock identification methodology is critical. Thus, we urge regulators to exercise caution and avoid hasty conclusions regarding HFTs' contribution to price stability in stressful market conditions. [ABSTRACT FROM AUTHOR]

In the recent years, primordial black holes (PBHs) have emerged as one of the most interesting and hotly debated topics in cosmology. Among other possibilities, PBHs could explain both some of the signals from binary black hole mergers observed in gravitational-wave detectors and an important component of the dark matter in the Universe. Significant progress has been achieved both on the theory side and from the point of view of observations, including new models and more accurate calculations of PBH formation, evolution, clustering, merger rates, as well as new astrophysical and cosmological probes. In this work, we review, analyze and combine the latest developments in order to perform end-to-end calculations of the various gravitational-wave signatures of PBHs. Different ways to distinguish PBHs from stellar black holes are emphasized. Finally, we discuss their detectability with LISA, the first planned gravitational-wave observatory in space. [ABSTRACT FROM AUTHOR]

In our quest toward a theory of gravity beyond General Relativity, black holes with their strong gravitational fields represent an important testing ground. Among the numerous alternative theories of gravity, much work in recent years has focused on a set of scalar–tensor theories, where the scalar field couples to higher curvature terms. The properties of the resulting black holes in such theories may differ distinctly from those of the Schwarzschild and Kerr black holes as, for instance, revealed by their shadows or their gravitational wave spectra. [ABSTRACT FROM AUTHOR]

The Page curve exhibits a sharp peak at the Page time corresponding to a phase transition from an empty quantum extremal surface to a nonempty one. This study delves into the impact of this phase transition on the informational content of black hole radiation through constructing a smooth Page curve which represents a gradual transition instead. We utilize replica trick, apply holography in a specific form, and introduce mathematical identities inherent to the curve’s smoothness. Our results suggest that a gradual transition could lead to a significant increase in the informational content of the radiation. [ABSTRACT FROM AUTHOR]

By expanding in powers of the inverse multipole number, we derive compact analytic expressions for the quasinormal modes of scalar and electromagnetic perturbations around regular Simpson–Visser black holes. These analytic formulas offer sufficient accuracy for the first and higher multipoles, except when the black hole is in a near-extremal state. The correspondence between null geodesics and high-frequency quasinormal modes is confirmed for the case under consideration. As an application, we show that Simpson–Visser black holes respect the Hod's bound on the damping rate of the fundamental quasinormal mode. [ABSTRACT FROM AUTHOR]

In this work we explore some aspects of the spectral instability of back hole quasi-normal modes, using a specific model as an example. The model is that of a small bump perturbation to the effective potential of linear axial gravitational waves on a Schwarzschild background, and our focus is on three different aspects of the instability: identifying and distinguishing between the two different types of instabilities studied previously in the literature, quantifying the size of the perturbations applied to the system and testing the validity of the pseudospectral numerical method in providing a convergent result for this measure, and finally, relating the size and other features of the perturbation to the degree of destabilisation of the spectrum. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the thermodynamic properties of the Schwarzschild–Tangherlini black hole in D-dimension in a generic noncommutativity setting in which geometry and matter are affected. A mass density and radial coordinate (r) are used to carry out this assignment. At this point, the transition on the radial coordinate r is really performed in the same space contrary to the Bopp shift, which requires us to operate in phase space. By contrasting our model with that of earlier research on noncommutative inspired geometry, we are able to see how two-fold deformation affects our black hole’s thermodynamic characteristics. [ABSTRACT FROM AUTHOR]

We study the geometrothermodynamics of black holes within the framework of the five-dimensional Einstein–Gauss–Bonnet (EGB) theory and its various extensions, including Einstein–Maxwell–Gauss–Bonnet (EMGB), the inclusion of a cosmological constant in EMGB and Einstein–Yang–Mills–Gauss–Bonnet (EYMGB). To expand the scope of our research, we utilize Tsallis entropy, an extended form of entropy that surpasses the traditional Boltzmann–Gibbs entropy. We first obtain the relations for the Hawking temperature, specific heat and scalar curvature in terms of Tsallis entropy and then show that incorporating Tsallis entropy into the description of black holes allows us to explore their thermodynamic characteristics beyond conventional limits, providing valuable insight into the stability of the black holes. We analyze that in the scenario of Tsallis entropy, the stability regions and phase transition points of specific heat coincide with the divergence points of scalar curvatures which confirm the consistency of the results. We also provide a comparative examination of our findings for the considered models of black holes. This comparative analysis deepened our understanding of the thermodynamic behavior exhibited by these diverse black holes within the extended EGB framework. [ABSTRACT FROM AUTHOR]

In this research paper, we discuss the energy–momentum-squared gravity model ℱ(ℛ,풯2) coupled with perfect fluid. We obtain the equation of state for the perfect fluid in ℱ(ℛ,풯2)-gravity model. Furthermore, we deal with the energy–momentum-squared gravity model ℱ(ℛ,풯2) coupled with perfect fluid, which admits the Ricci solitons with a conformal vector field. We provide a clue in this series to determine the density and pressure in the radiation and phantom barrier periods, respectively. In addition, we investigate the different energy conditions, black holes and singularity conditions for perfect fluid attached to ℱ(ℛ,풯2)-gravity in terms of Ricci soliton. Finally, we derive the Schrödinger equation for energy–momentum-squared gravity model ℱ(ℛ,풯2) coupled with perfect fluid. [ABSTRACT FROM AUTHOR]

In this paper, we extend the vacuum Schwarzschild solution to obtain new black hole solutions within the framework of the Rastall theory of gravity using minimal gravitational decoupling technique. Through the addition of an extra source which induces anisotropy in static spherically symmetric configuration, the field equations are obtained which are then decomposed into two simpler arrays by virtue of a deformation strictly on the radial metric component. The first of these sets corresponds to the seed source (which we take as the vacuum) and is specified by the metric components of the Schwarzschild spacetime. For the second set, a solution is determined by imposing a constraint on the anisotropic extra source via a linear equation of state. For specified values of the Rastall and decoupling parameters, two extended solutions are obtained. The nature of the matter that constitutes the extra source is investigated through the energy conditions. We also study the asymptotic behavior of both solutions by analyzing the deformed radial metric component. It turns out that the additional source in both solutions is exotic due to the violation of the dominant energy conditions. The asymptotic flatness is preserved only for the solution corresponding to the conformally symmetric extra source. [ABSTRACT FROM AUTHOR]

In an attempt to simulate black hole echoes (generated by potential quantum-gravitational structure) in numerical relativity, we recently described how to implement a reflecting boundary outside of the horizon of a black hole in spherical symmetry. Here, we generalize this approach to spacetimes with no symmetries and implement it numerically using the generalized harmonic formulation. We cast the evolution equations and the numerical implementation into a Summation By Parts scheme, which seats our method closer to a class of provably numerically stable systems. We implement an embedded boundary numerical framework that allows for arbitrarily shaped domains on a rectangular grid and even boundaries that evolve and move across the grid. As a demonstration of this framework, we study the evolution of gravitational wave scattering off a boundary either inside, or just outside, the horizon of a black hole. This marks a big leap toward the goal of a generic framework to obtain gravitational waveforms for behaviors motivated by quantum gravity near the horizons of merging black holes. [ABSTRACT FROM AUTHOR]

Supermagnified stars are gravitationally lensed individual stars that are located close to a caustic of a lensing galaxy cluster, and have their flux magnified by a large enough factor (typically |${\sim} 1000$|⁠) to make them detectable with present telescopes. The maximum magnification is limited by microlensing caused by intracluster stars or other compact objects, which create a network of corrugated critical lines with an angular width proportional to the surface density of microlenses. We consider a set of nine cases of supermagnified stars reported in the literature, and derive an upper limit on the surface density of compact objects, such as primordial black holes, that might be present as a fraction of the dark matter in addition to known intracluster stars. Any such additional compact objects would widen the corrugated critical line network and therefore the width of the distribution of supermagnified stars around the modelled critical lines of the lens. We find that any compact objects, including primordial black holes, with masses above |${\sim} 10^{-6} \, {\rm M}_\odot$| (below which the microcaustics are closer together than the typical angular size of supermagnified stars) cannot account for more than |${\sim} 2$|  per cent of the dark matter. [ABSTRACT FROM AUTHOR]

Comparing the dynamical and stellar masses of Milky Way (MW) globular clusters (GCs) reveals a discrepancy exceeding a factor of 2. Since this substantial invisible mass is concentrated in the cluster centre, it is attributed to stellar remnants. The majority of mass in remnants consists of white dwarfs (WDs). Allocating over half of a GC's current mass to WDs could significantly restrict the dynamical evolution scenarios governing stellar clusters. As the most massive stars in GCs, black holes (BHs) exert a substantial effect on the escape rate of lower mass stars, such as WDs. This paper aims to identify which scenarios of BH natal kicks can accurately reproduce the notable dark remnant fraction observed in MW GCs. We compare the observed remnant fraction of MW GCs with a comprehensive grid of direct N -body simulations while adjusting the natal kick received by BHs. Our results reveal that simulations employing low natal kicks to BHs are the only ones capable of mirroring the remnant fraction of MW GCs. According to the Spitzer instability, the presence of a BH population prompts the formation of a BH sub-system (BHSub) at the centre of a star cluster. The BHSub serves as an energetic power plant, continually releasing kinetic energy through few-body encounters between single and binary BHs, and transferring the generated energy to the entire stellar population. This energy induces a significant difference in the ejection rate of stellar remnants and luminous stars, ultimately increasing the fraction of dark remnants within the star cluster. [ABSTRACT FROM AUTHOR]

Astrophysical black holes (BHs) have two fundamental properties: mass and spin. While the mass-evolution of BHs has been extensively studied, much less work has been done on predicting the distribution of BH spins. In this paper, we present the spin evolution for a sample of intermediate-mass and massive BHs from the NewHorizon simulation, which evolved BH spin across cosmic time in a full cosmological context through gas accretion, BH–BH mergers and BH feedback including jet spindown. As BHs grow, their spin evolution alternates between being dominated by gas accretion and BH mergers. Massive BHs are generally highly spinning. Accounting for the spin energy extracted through the Blandford–Znajek mechanism increases the scatter in BH spins, especially in the mass range |$10^{5}{-}10^{7}\,\rm M_\odot$|⁠ , where BHs had previously been predicted to be almost universally maximally spinning. We find no evidence for spin-down through efficient chaotic accretion. As a result of their high spin values, massive BHs have an average radiative efficiency of |$\lt \varepsilon _{\rm r}^{\rm thin}\gt \approx 0.19$|⁠. As BHs spend much of their time at low redshift with a radiatively inefficient thick disc, BHs in our sample remain hard to observe. Different observational methods probe different sub-populations of BHs, significantly influencing the observed distribution of spins. Generally, X-ray-based methods and higher luminosity cuts increase the average observed BH spin. When taking BH spin evolution into account, BHs inject, on average, between three times (in quasar mode) and eight times (in radio mode) as much feedback energy into their host galaxy as previously assumed. [ABSTRACT FROM AUTHOR]

In an accompanying publication, the MeerKAT Pulsar Timing Array (MPTA) Collaboration reports tentative evidence for the presence of a stochastic gravitational wave background, following observations of similar signals from the European and Indian Pulsar Timing Arrays, the North American Nanohertz Observatory for Gravitational Waves, the Parkes Pulsar Timing Array, and the Chinese Pulsar Timing Array. If such a gravitational wave background signal originates from a population of inspiraling supermassive black hole binaries, the signal may be anisotropically distributed in the sky. In this paper, we evaluate the anisotropy of the MPTA signal using a spherical harmonic decomposition. We discuss complications arising from the covariance between pulsar pairs and the regularization of the Fisher matrix. Applying our method to the |$4.5 \hbox{-}\text{yr}$| data set, we obtain two forms of sky maps for the three most sensitive MPTA frequency bins between |$7 \ {\rm and} \ 21 \, {\rm nHz}$|⁠. Our 'clean maps' estimate the distribution of gravitational wave strain power with minimal assumptions. Our radiometer maps answer the question: Is there a statistically significant point source? We find a noteworthy hotspot in the |$7 \, \mathrm{nHz}$| clean map with a p -factor of |$p=0.015$| (not including trial factors). Future observations are required to determine if this hotspot is of astrophysical origin. [ABSTRACT FROM AUTHOR]

Pulsar timing arrays search for nanohertz-frequency gravitational waves by regularly observing ensembles of millisecond pulsars over many years to look for correlated timing residuals. Recently the first evidence for a stochastic gravitational wave background has been presented by the major arrays, with varying levels of significance (⁠|${\sim} 2\sigma \!-\! 4\sigma$|⁠). In this paper, we present the results of background searches with the MeerKAT Pulsar Timing Array. Although of limited duration (4.5 yr), the |${\sim} 250\,000$| arrival times with a median error of just |$3 \, \mu {\rm s}$| on 83 pulsars make it very sensitive to spatial correlations. Detection of a gravitational wave background requires careful modelling of noise processes to ensure that any correlations represent a fit to the underlying background and not other misspecified processes. Under different assumptions about noise processes, we can produce either what appear to be compelling Hellings–Downs correlations of high significance (⁠|$3\sigma \!-\! 3.4\sigma$|⁠) with a spectrum close to that which is predicted, or surprisingly, under slightly different assumptions, ones that are insignificant. This appears to be related to the fact that many of the highest precision MeerKAT Pulsar Timing Array pulsars are in close proximity and dominate the detection statistics. The sky-averaged characteristic strain amplitude of the correlated signal in our most significant model is |$h_{{\rm c}, {\rm yr}} = 7.5^{+0.8}_{-0.9} \times 10^{-15}$| measured at a spectral index of |$\alpha =-0.26$|⁠ , decreasing to |$h_{{\rm c}, {\rm yr}} = 4.8^{+0.8}_{-0.9} \times 10^{-15}$| when assessed at the predicted |$\alpha =-2/3$|⁠. These data will be valuable as the International Pulsar Timing Array project explores the significance of gravitational wave detections and their dependence on the assumed noise models. [ABSTRACT FROM AUTHOR]