This study examines how the concepts of "farthest" and "nearest" points change depending on the geometric structure of the universe. The research comparatively analyzes three fundamental cosmological models: (1) the static closed universe model (Ω > 1), (2) the flat or semi-open universe model (Ω ≤ 1), and (3) the dynamic expanding universe model (ΛCDM). The philosophical proposition expressed as "The point infinitely approached is the farthest point, and at the same time the nearest point" has been evaluated within each model framework. Analysis results reveal that this paradoxical statement is mathematically valid only in the closed universe model, remains conceptually meaningless in the flat universe model, and becomes physically impossible in the dynamic expanding universe due to the cosmological event horizon. The study utilizes Planck Collaboration (2020) observational data, Einstein (1915, 1917) field equations, and Friedmann (1922) cosmology equations as the theoretical framework.

In this work, we investigate the shadow properties of the Kerr–Newman–Anti-de Sitter black hole coupled to nonlinear electrodynamics. The shadow is constructed by employing the celestial coordinate approach for an observer located at a finite distance, which is required due to the non-asymptotically flat structure of the spacetime. The size, distortion, area, and oblateness of the shadow are analyzed in terms of the black hole parameters, namely, the spin, the effective charge, and the nonlinearity parameter. We show that the nonlinear electrodynamics significantly modifies the photon region and therefore changes the shadow observables, while the rotation mainly controls the deformation of the silhouette. We further confront the theoretical results with the Event Horizon Telescope observations of M87* and Sgr A* in order to constrain the parameter space of the model. The allowed ranges of the effective charge depend sensitively on the nonlinearity parameter, and the combination of both sources leads to tighter and physically more consistent bounds. In addition, we study the energy emission rate derived from the shadow radius and the Hawking temperature and discuss how it is affected by the rotation and the nonlinear electromagnetic field. Our analysis shows that the considered black hole solution provides a consistent extension of the Kerr geometry in a non-asymptotically flat background and that the shadow observables can be used as an efficient tool to test the effects of nonlinear electrodynamics in strong gravity.

Abstract In this paper, we study the polymer black hole solution surrounded by a quintessence field. The influence of quintessence on the polymer black hole is investigated through its thermodynamic properties, such as the Hawking temperature, entropy, and specific heat, which allow us to address the question of thermodynamic stability. We then calculate bounds on the electromagnetic greybody factors and photon emission rates of the black hole, highlighting the interplay between quintessence and quantum gravity effects in determining these phenomena. We also examine the effects of quintessence and quantum gravity on the geodesics and shadows of massless particles around the black hole. Our results are further compared with observational data of both the Sagittarius A* and M87* black holes from the Event Horizon Telescope (EHT) collaboration.

We prove that, in the framework of the Oppenheimer-Snyder collapse, the Schwarzschild exterior maximizes the event horizon formation time Δ T eh = 19 6 m among all asymptotically flat, static, spherically-symmetric black holes with the same ADM mass m that satisfy the weak energy condition. This bound extends the typical black hole inequalities–such as the Penrose inequality, which constrains spatial geometry–to temporal setting.

We investigate the dynamical stability of circular orbits around a Kerr black hole embedded in a Dehnen-type dark matter halo. The effective spacetime metric of the combined system is constructed using the Newman–Janis algorithm, and the effective potential for test-particle motion in the equatorial plane is derived. The stability of circular orbits is analyzed through the Hessian matrix of the effective potential, while the stability strength and restoring-force distribution are employed to quantify the orbital response to small perturbations. Our results show that the presence of the dark matter halo significantly alters the spatial structure of stable circular orbits, leading to non-continuous stable regions whose location and extent depend sensitively on the halo’s characteristic density, scale radius, and the black hole spin. The innermost stable circular orbit (ISCO) is shifted relative to the vacuum Kerr case, with its position determined by the combined effects of the spin and halo parameters. Two-dimensional heatmaps, parameter scans, and three-dimensional visualizations systematically illustrate how the black hole spin and dark matter halo properties influence the ISCO and the distribution of stable orbits. Finally, we analyze the influence of the dark matter halo on the structure of the black hole event horizon. These results provide a detailed theoretical investigation of orbital dynamics around rotating black holes in dark-matter-rich environments.

We present an analytic study of charged particle dynamics and charge assisted energy extraction around nonrotating black holes surrounded by perfect fluid dark matter in Kalb-Ramond gravity. The dark matter contribution and the Kalb-Ramond sector deform the near horizon geometry in a controlled way and generate a generalized electromagnetic ergoregion outside the event horizon, within which charged particles can access negative energy states under a monotone electrostatic potential. We characterize how the size of this region depends on the perfect fluid dark matter parameter and the Kalb-Ramond coupling, and we show that negative energy trajectories exhibit a universal behavior: they admit at most one radial turning point and inevitably terminate at the black hole. Building on this structure, we formulate a charged Penrose type mechanism and derive compact analytic bounds on the escaping energy and on the maximum local efficiency, together with feasibility regions in the parameter space of the splitting radius and the particle charge. We also provide an illustrative time dependent extension with charge decay and possible mass accretion, which tracks the evolution of the horizon and of the negative energy domain. These results offer a tractable framework for assessing how dark matter can reshape strong field charged energetics in modified gravity scenarios and for identifying potential imprints on high energy processes near black holes.

Abstract In a recent analysis (Misra et al. in npj Quantum Inf 10:34, 2024. 10.1038/s41534-024-00817-w), it has been shown that Hawking radiation is the main source of energy to empower a coherent signal pulse. In this work, we have explored the same effect for a case where the time derivative of the scalar field mode of the redirected Hawking radiation appears explicitly in the interaction Hamiltonian. We have considered a stream of two-level atoms falling freely towards the event horizon of a black hole. The Hawking radiation redirected from an orbiting mirror interacts with the atoms which make a transition between the ground state and the excited state through the emission of a signal photon. The signal pulse is amplified by the mechanical work done by the redirected Hawking mode. The whole set up works as a black hole powered quantum heat engine. We have shown that this amplification depends on the frequency of both the signal mode and the Hawking mode, the flux of the redirected Hawking mode and the lapse function of the black hole. In contrast to the result obtained in Misra et al. (npj Quantum Inf 10:34, 2024. 10.1038/s41534-024-00817-w), we observe in our analysis, that due to the coupling of the momentum degrees of freedom of the Hawking radiation modes with the freely falling detector, the power output depends inversely with the lapse function of the black hole and is proportional to the frequency of the emitted Hawking radiation. As a result the maximum output power enhances significantly if the atom is very close to the event horizon of the black hole. We have analyzed this effect for two types of detectors attached to the cavity. At first we considered a point-like detector and then we have done the analysis from the perspective of a detector with smearing.

Abstract It is commonly expected that maximally multipartite entangled states provide the richest quantum correlations and thus represent optimal quantum resources in the Schwarzschild black hole background. Here, by analyzing the genuine N-partite entanglement of fermionic modes near the event horizon, quantified via concurrence, we identify a counterintuitive phenomenon: in this curved spacetime, the genuine multipartite entanglement of a maximally entangled state can be lower than that of a suitably chosen non-maximally entangled state. This observation implies that, from the perspective of quantum resource efficiency, non-maximally entangled multipartite states may outperform their maximal counterparts for quantum information tasks under gravitational effects. Therefore, preparing appropriately engineered non-maximally entangled multipartite states as initial resources provides a more practical and effective approach to quantum information processing in Schwarzschild spacetime.

This paper presents a study on gravitational vacuum stars (gravastars) with an isotropic matter distribution in de Rham–Gabadadze–Tolley (dRGT) massive gravity incorporating Buchdahl metric function. Here we have conducted an analysis on thin-shell singularity-free gravastar configuration. The study demonstrates the viability of gravastars as alternatives to black holes (BHs) in this massive gravity. Our research yields singularity-free analytical solutions for gravastar interior and without event horizon. Our discussion focuses on the properties of the thin shell of ultra-relativistic stiff fluid viz., length, energy, entropy and the massive gravity’s impact on these physical properties. The junction conditions have been carefully examined and the study of surface redshift analysis implies regularity of the model. Our paper also includes energy condition analysis supporting the thin shell formation. Thus our solutions eliminate singularities and information paradoxes and the implications of these solutions are seemingly noteworthy and exhibit physical desirable properties.

Abstract We present independent imaging analyses of Event Horizon Telescope (EHT) observations of the active galactic nuclei in radio galaxy Centaurus A and quasar 3C 279 using Generative Deep learning Image Reconstruction with Closure Terms (G en DIR e CT), a recently developed machine-learning framework built on conditional diffusion models that uses interferometric closure invariants as primary observables. For Centaurus A, our reconstruction reveals two prominent emission ridges (≃ 80 μas each) along the jet sheath with a brightness ratio of 1.4 ± 0.1 and an opening angle of 12.3 ± 0.3 deg. For 3C 279, we identify three distinct components in the image, with the southern jet ejecta on sub-parsec scale exhibiting a proper motion of 4.6 ± 1.0 μas over ≈ 5.39 days away from the northern components, corresponding to an apparent superluminal velocity of ≃ 10 ± 2 times light speed. These measurements are consistent with those reported by the EHT Collaboration. The results are significant because we demonstrate that: (1) imaging from interferometric aperture synthesis data, especially in VLBI and most acutely in extremely sparse arrays like the EHT, remains a severely ill-posed and challenging inverse problem, yet closure invariants preserve robust morphological information that can strongly constrain structural features, and (2) more importantly, closure-invariant imaging largely avoids calibration systematics, thus providing a fundamentally independent view of spatial structure with very high angular resolution. The generative nature of G en DIR e CT further allows us to sample and characterise clusters of plausible image solutions for each dataset. As a calibration-independent, generative imaging approach, G en DIR e CT offers a robust and truly independent blind-imaging tool for current and future VLBI experiments.

Abstract This study investigates the polarization characteristics of solitonic boson stars surrounded by a thin accretion disk. By comparing their polarization images with corresponding optical images, we find a positive correlation between the polarization intensity distribution in the polarization images and the brightness in the optical images. Consequently, the strongest polarization occurs at the location corresponding to the direct image. The influence of the coupling strength of the sixtic potential on the polarization intensity distribution is not monotonic, under strong coupling, the polarization will concentrated on the left side of the image as the coupling strength increases, whereas under weak coupling, it is more evenly distributed across the entire direct image as the coupling strength increases. Moreover, we find that as the initial scalar field increases, both the lensing image and photon ring become more prominent. However, the polarization intensity at these regions remains weak. Due to the absence of the event horizon in solitonic boson stars, the polarization vector can penetrate the stellar interior, unlike in black holes, where no polarization signals exist within the event horizon. Our numerical simulations clearly reveal this phenomenon, suggesting that polarization features may serve as an effective tool for distinguishing solitonic boson stars from black holes.

Abstract We investigate energy extraction from a rotating black hole spacetime modified by a quantum correction parameter $$ \alpha .$$ α . Focusing on the Penrose process, we analyze particle splitting within the ergoregion and evaluate the extraction efficiency $$ \eta $$ η as a function of the spin parameter a and the quantum correction $$ \alpha .$$ α . Our analysis shows that quantum effects effectively enlarge the ergoregion by shifting both the static limit and the event horizon toward smaller radii. Numerical results indicate that, within the range of spin values listed in Table 1, an event horizon exists only for the critical value $$ \alpha = 1.1663 ,$$ α = 1.1663 , beyond which the horizon disappears. A direct comparison with the classical Kerr case reveals that these geometric modifications significantly enhance the efficiency of energy extraction. In particular, the maximum efficiency of the Penrose process reaches approximately $$ 2.85\% $$ 2.85 % for $$ \alpha = 1.1663,$$ α = 1.1663 , compared to $$ 1.72\% $$ 1.72 % for the Kerr black hole. In addition, we derive an expression for the irreducible mass, emphasizing its role in constraining the extractable rotational energy. Overall, our results demonstrate that quantum corrections have a pronounced impact on black hole energetics, leading to measurable deviations from standard Kerr predictions.

There has been considerable effort to mimic analogue black holes and wormholes in solid state systems. Lattice realisations in particular present specific challenges. One of those is that event horizons in general have both white and black hole (grey hole) character, a feature guaranteed by the Nielsen-Ninomiya theorem. We here explore and extend the capability of superconducting circuit hardware to implement on-demand spacetime geometries on lattices, combining the nonreciprocity of gyrators with the non-linearity of Josephson junctions. We demonstrate the possibility of the metric sharply changing within a single lattice point, thus entering a regime where the modulation of system parameters is “trans-Planckian”, and the Hawking temperature ill-defined. Instead of regular Hawking radiation, we find an instability in the form of an exponential burst of charge and phase quantum fluctuations over short time scales – a robust signature even in the presence of an environment. Moreover, we present a loop-hole for the typical black/white hole ambiguity in lattice simulations: exceptional points in the dispersion relation allow for the creation of pure black (or white) hole horizons, at the expense of a radical change in the dynamics of the wormhole interior.

A bstract We define a notion of ‘quantum entanglement index’ with the aim to compute it for black hole horizons in string theory at one-loop order using the stringy replica method. We consider the horizon of BTZ black holes to construct the relevant conical orbifolds, labeled by an odd integer N , and compute the partition function as a function of N , corresponding to the fractional indexed Rényi entropy. We show that it is free of tachyons and naturally finite both in the ultraviolet and the infrared, even though it is generically ultraviolet divergent in the field theory limit. Thus, the index provides a useful diagnostic of the entanglement structure of string theory without the need for analytic continuation in N .

Abstract We investigate a rotating black hole embedded in a five-dimensional flat bulk by extending the Kerr metric through the Gürses-Gürsey line element. Employing Boyer-Lindquist coordinates, we reinterpret the black hole as a charged-like object in five dimensions and analyze its horizon structure and shadow morphology. Our results reveal that the shadow is shaped by an axially symmetric gravitational field modulated by an extrinsic curvature term arising from the higher-dimensional embedding. Simulations demonstrate that the visibility amplitude and shadow profile of the Gürses-Gürsey black hole align with Event Horizon Telescope observations of $$\hbox {M87}^{*}$$ M87 ∗ in the Kerr limit, while also allowing measurable deviations that could be probed by future high-resolution experiments.

Abstract We investigate how deviations from the Bekenstein–Hawking entropy modify black-hole spacetimes through the recently proposed entropy-geometry correspondence. For four representative modified entropies, namely Barrow, Rényi, Kaniadakis, and logarithmic, we derive the corresponding effective metrics and analyze their thermodynamic and topological classification using the off-shell free energy and winding numbers. We show that Barrow and Rényi entropies yield a single unstable sector with global charge $$W=-1$$ W = - 1 , while logarithmic and Kaniadakis corrections produce canceling defects with $$W=0$$ W = 0 , revealing topological structures absent in the Schwarzschild case. Using the modified metrics, we further calculate the photon-sphere radius and shadow size, showing that each modified entropy relation induces characteristic optical shifts. Thus, by comparing with Event Horizon Telescope observations of Sgr A $$^*$$ ∗ , we extract new bounds on all entropy-deformation parameters. Our results demonstrate that thermodynamic topology, together with photon-sphere phenomenology, offers a viable way to test generalized entropy frameworks and probe departures from the Bekenstein-Hawking area law.

In this paper, we study thermodynamics and its applications of a family of static charged dilaton black holes in 2+1 dimensions found by Chan and Mann [1] and Xu [2]. There is a dimensionless parameter [Formula: see text] in the black hole solutions presented: it is related to the coupling constant for the dilaton with the electromagnetic field and the gravitational field. Black hole horizons exist only for [Formula: see text]. [Formula: see text] black hole is a solution to low energy string theory. Thermodynamics is studied in the canonical ensemble where charge is constant as well as in grand canonical ensemble where the potential is constant. The cosmological constant is considered as a thermodynamical variable where the pressure [Formula: see text]. We computed the first law and the Smarr relations for the black hole and introduced two new thermodynamical parameter in order to satisfy the first law. We computed temperature, thermodynamic volume, specific heat capacities, Gibbs free energy and studied local and global stability of the black hole. Thermodynamic volume differs from the geometric volume. In the canonical ensemble, we noticed that thermodynamic behavior falls into two broad categories: For [Formula: see text], small black holes are locally stable and large black holes are not. For [Formula: see text] the black hole is locally and globally stable for all values of the horizon radius. In order to demonstrate the two broad categories, we have presented [Formula: see text] and [Formula: see text] black holes in detail. There were no phase transitions for the above values of [Formula: see text]. In the grand canonical ensemble, we noticed that there is a Hawking-Page phase transition for the black hole with [Formula: see text]. We have also studied the Joule-Thomson expansion and the Reverse Isoperimetric Inequality of these black holes. We made the observation that the charged dilaton black hole does not violate the Reverse Isoperimetric Inequality for certain values of the parameters of the theory. Finally, we have suggested future work.

In this work, we explored the dynamics of black holes within the framework of Kalb-Ramond gravity, emphasizing the effects of spontaneous Lorentz symmetry breaking on their perturbative and thermodynamic properties. Starting with a modified spherically symmetric black hole solution, we derived and analyzed the spin-dependent Regge-Wheeler potentials for scalar, vector, and tensor perturbations, uncovering significant deviations from the Schwarzschild black hole due to the Lorentz symmetry-breaking parameter. Our findings revealed that the parameter strongly influences the stability of the black hole and the propagation of perturbative modes. Additionally, we calculated GFs to investigate the transmission coefficients of black hole radiation and demonstrated their dependence on both the spin of the fields and the symmetry-breaking effects. Thermodynamic analyses showed modifications to the temperature, entropy, and specific heat, which highlighted the critical role of Lorentz symmetry-breaking effects on black hole stability. Furthermore, we discussed the area quantization of the black hole horizon, revealing its direct dependence on the symmetry-breaking parameter and providing insights into the quantum nature of black holes. These results collectively enrich our understanding of black hole physics under modified gravity theories and offer potential observational implications.

Abstract Restricted by event horizon suppression, tidal disruption events (TDEs) provide a unique window into otherwise hidden supermassive black holes (SMBHs) at the lower end of the mass spectrum, allowing the connection between star formation and SMBH mass to be explored across a broad stellar mass range. We derive stellar masses and specific star formation rates using Prospector fits to UV–mid-infrared broadband spectral energy distributions (SEDs) for 42 TDE hosts, together with a high-mass comparison sample, and combine these with SMBH mass estimates from the literature. We first verify our approach by reproducing the established result that quenched galaxies host more massive SMBHs than star-forming systems at fixed stellar mass, a result widely interpreted as evidence for SMBH growth driving the blue-to-red sequence transition. However, examining the TDE sample in isolation reveals a trend reversal at lower masses, uncovering a surprising population of low-mass (10 9.6 ≲ M gal ≲ 10 10.5 M ⊙ ), quenched galaxies hosting SMBHs systematically less massive ( M BH ≲ 10 6.5 M ⊙ ) than those in star-forming galaxies of comparable stellar mass. After ruling out degeneracies in our SED fits, we conclude that this reflects a physical difference in the quenching mechanism between these TDE hosts and the more massive galaxies. This is unlikely to be driven by active galactic nucleus feedback, and could instead result from environmental processes, which can end star formation and hinder SMBH growth. We also show that the quenched and poststarburst population within the TDE sample is likely underrepresented due to selection biases, suggesting the true fraction could be even higher than observed.

Abstract The coevolution of supermassive black holes (SMBHs) and their host galaxies remains one of the central open questions in cosmology, rooted in the coupling between accretion, feedback, and the multiscale physics that links the event horizon to the circumgalactic medium. Here we bridge these scales by embedding a first-principles, GRMHD-informed prescription for black hole accretion and feedback—derived from multizone simulations that self-consistently connect inflows and outflows from the horizon to the Bondi radius—within cosmological magnetohydrodynamic zoom-in simulations of ∼10 14 M ⊙ halos. These GRMHD results predict a “suppressed Bondi” regime in which magnetic stresses and relativistic winds strongly reduce effective accretion rates in a spin-dependent manner. We find that black holes cannot grow efficiently by accretion until they exceed ∼10 7 M ⊙ , regardless of the feedback strength. Beyond this threshold, systems bifurcate: low-spin ( η ∼ 0.02) black holes continue to accrete without quenching star formation, while high-spin ( η ≳ 0.3) black holes quench effectively but become starved of further growth. Early, massive seeding partially alleviates this tension through merger-driven assembly, yet an additional cold or super-Eddington accretion mode appears essential to reproduce the observed SMBH population and the empirical black hole–galaxy scaling relations. Our results demonstrate that GRMHD-informed feedback models can account for the maintenance-mode behavior of low-luminosity active galactic nuclei like M87*, but cannot by themselves explain the full buildup of SMBH mass across cosmic time. A unified, multiregime framework is required to capture the evolving interplay between spin-dependent feedback, cold inflows, and mergers in driving coevolution.