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1. Sudden breakdown of effective field theory near cool Kerr-Newman black holes

   https://doi.org/10.1007/JHEP05(2024)122  

   Horowitz, Gary T; Kolanowski, Maciej; Remmen, Grant N; Santos, Jorge E (May 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> It was recently shown that (near-)extremal Kerr black holes are sensitive probes of small higher-derivative corrections to general relativity. In particular, these corrections produce diverging tidal forces on the horizon in the extremal limit. We show that adding a black hole charge makes this effect qualitatively stronger. Higher-derivative corrections to the Kerr-Newman solution produce tidal forces that scale inversely in the black hole temperature. We find that, unlike the Kerr case, for realistic values of the black hole charge large tidal forces can arise before quantum corrections due to the Schwarzian mode become important, so that the near-horizon behavior of the black hole is dictated by higher-derivative terms in the effective theory. 

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2. Asymptotic Analysis of Harmonic Maps with Prescribed Singularities

   https://doi.org/10.1007/s40818-026-00247-1  

   Han, Qing; Khuri, Marcus; Weinstein, Gilbert; Xiong, Jingang (December 2026, Annals of PDE)

   Abstract This is the first in a series of two papers to establish the mass-angular momentum inequality for multiple black holes. We study singular harmonic maps from domains of 3-dimensional Euclidean space to the hyperbolic plane having bounded hyperbolic distance to extreme Kerr harmonic maps. We prove that every such harmonic map admits a unique tangent harmonic map at the extreme black hole horizon. The possible tangent maps are classified and shown to be shifted ‘extreme Kerr’ geodesics in the hyperbolic plane that depend on two parameters, one determined by angular momentum and another by conical singularities. In addition, rates of convergence to the tangent map are established. Similarly, expansions in the asymptotically flat end are presented. These results, together with those of Li-Tian [24, 25] and Weinstein [35, 36], provide a complete regularity theory for harmonic maps from$$\mathbb {R}^3\setminus z\text {-axis}$$ $R^3\backslash z\text{-axis}$ to$$\mathbb {H}^2$$ $H^2$ with these prescribed singularities. The analysis is additionally utilized to prove existence of the so called near horizon limit, and to compute the associated near horizon geometries of extreme black holes. 

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3. Electromagnetic–gravitational perturbations of Kerr–Newman black hole

   https://doi.org/10.1142/S0218271825400036  

   Giorgi, Elena (January 2025, International Journal of Modern Physics D)

   Black holes are important objects in our understanding of the universe, as they represent the extreme nature of General Relativity. The Kerr–Newman black hole is the most general asymptotically flat black hole solution and its stability properties have long been elusive due to the interaction between gravitational and electromagnetic radiations. We illustrate the main conjectures regarding the stability problem of known black hole solutions and present some recent theorems regarding the evolution of the Kerr–Newman black holes to coupled perturbations. 

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4. Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry

   Younsi, Z.; Psaltis, D.; Ozel, F. (October 2021, ArXivorg)

   The image of a supermassive black hole surrounded by an optically-thin, radiatively-inefficient accretion flow, like that observed with the Event Horizon Telescope, is characterized by a bright ring of emission surrounding the black-hole shadow. In the Kerr spacetime this bright ring, when narrow, closely traces the boundary of the shadow and can, with appropriate calibration, serve as its proxy. The present paper expands the validity of this statement by considering two particular spacetime geometries: a solution to the field equations of a modified gravity theory and another that parametrically deviates from Kerr but recovers the Kerr spacetime when its deviation parameters vanish. A covariant, axisymmetric analytic model of the accretion flow based on conservation laws and spanning a broad range of plasma conditions is utilized to calculate synthetic non-Kerr black-hole images, which are then analysed and characterized. We find that in all spacetimes: (i) it is the gravitationally-lensed unstable photon orbit that plays the critical role in establishing the diameter of the rings observed in black-hole images, not the event horizon or the innermost stable circular orbit, (ii) bright rings in these images scale in size with, and encompass, the boundaries of the black-hole shadows, even when deviating significantly from Kerr, and (iii) uncertainties in the physical properties of the accreting plasma introduce subdominant corrections to the relation between the diameter of the image and the diameter of the black-hole shadow. These results provide theoretical justification for using black-hole images to probe and test the spacetimes of supermassive black holes. 

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5. Electro-magnetic energy extraction from rotating black holes in AdS

   https://doi.org/10.1007/JHEP12(2020)018  

   Callebaut, Nele; Rodriguez, Maria J.; Verlinde, Herman (December 2020, Journal of High Energy Physics)

   null (Ed.)

   A bstract Force-Free Electrodynamics for black holes in Anti de Sitter is considered. We present new, energy extracting solutions of Force-Free Electrodynamics in Anti de Sitter-Near Horizon Extremal Kerr and Super-Entropic Near Horizon Extremal Kerr geometries. The relevant equations of motion are derived from an action for force-free plasma surrounding spinning black holes with generic asymptotics. We consider the energy flux of electrodynamic fields in rotating frames to argue that the correct measure for energy extraction is the energy flux measured by a rotating observer in the near horizon region. We illustrate this procedure by application to near horizon solutions in Kerr, AdS-Kerr and BTZ. 

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