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1. 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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2. Gravitational footprints of black holes and their microstate geometries

   https://doi.org/10.1007/JHEP10(2021)138  

   Bah, Ibrahima; Bena, Iosif; Heidmann, Pierre; Li, Yixuan; Mayerson, Daniel R. (October 2021, Journal of High Energy Physics)

   A bstract We construct a family of non-supersymmetric extremal black holes and their horizonless microstate geometries in four dimensions. The black holes can have finite angular momentum and an arbitrary charge-to-mass ratio, unlike their supersymmetric cousins. These features make them and their microstate geometries astrophysically relevant. Thus, they provide interesting prototypes to study deviations from Kerr solutions caused by new horizon-scale physics. In this paper, we compute the gravitational multipole structure of these solutions and compare them to Kerr black holes. The multipoles of the black hole differ significantly from Kerr as they depend non-trivially on the charge-to-mass ratio. The horizonless microstate geometries (that are comparable in size to a black hole) have a similar multipole structure as their corresponding black hole, with deviations to the black hole multipole values set by the scale of their microstructure. 

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3. An infinity of black holes

   https://doi.org/10.1088/1361-6382/ac994b  

   Horowitz, Gary T; Wang, Diandian; Ye, Xiaohua (October 2022, Classical and Quantum Gravity)

   Abstract In general relativity (without matter), there is typically a one parameter family of static, maximally symmetric black hole solutions labeled by their mass. We show that there are situations with many more black holes. We study asymptotically anti-de Sitter solutions in six and seven dimensions having a conformal boundary which is a product of spheres cross time. We show that the number of families of static, maximally symmetric black holes depends on the ratio, λ , of the radii of the boundary spheres. As λ approaches a critical value, λ c , the number of such families becomes infinite. In each family, we can take the size of the black hole to zero, obtaining an infinite number of static, maximally symmetric non-black hole solutions. We discuss several applications of these results, including Hawking–Page phase transitions and the phase diagram of dual field theories on a product of spheres, new positive energy conjectures, and more. 

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4. Ladder symmetries of black holes. Implications for love numbers and no-hair theorems

   https://doi.org/10.1088/1475-7516/2022/01/032  

   Hui, Lam; Joyce, Austin; Penco, Riccardo; Santoni, Luca; Solomon, Adam R. (January 2022, Journal of Cosmology and Astroparticle Physics)

   Abstract It is well known that asymptotically flat black holes in generalrelativity have a vanishing static, conservative tidal response. We show that this is a result of linearly realized symmetries governingstatic (spin 0,1,2)perturbations around black holes. The symmetries have a geometric origin: in the scalar case, they arise from the (E)AdS isometries of a dimensionally reduced black hole spacetime. Underlying the symmetries is a ladder structure which can be used to construct the full tower of solutions,and derive their general properties: (1) solutions that decay withradius spontaneously break the symmetries, and mustdiverge at the horizon;(2) solutions regular at the horizon respect the symmetries, andtake the form of a finite polynomial that grows with radius.Taken together, these two properties imply that static response coefficients — and in particular Love numbers — vanish. Moreover, property (1) is consistent with the absence of black holes with linear (perturbative) hair. We also discuss the manifestation of these symmetries in the effective point particle description of a black hole, showing explicitly that for scalar probesthe worldline couplings associated with a non-trivial tidal response and scalar hair must vanish in order for the symmetries to be preserved. 

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5. Non-BPS floating branes and bubbling geometries

   https://doi.org/10.1007/JHEP02(2022)162  

   Heidmann, Pierre (February 2022, Journal of High Energy Physics)

   A bstract We derive a non-BPS linear ansatz using the charged Weyl formalism in string and M-theory backgrounds. Generic solutions are static and axially-symmetric with an arbitrary number of non-BPS sources corresponding to various brane, momentum and KKm charges. Regular sources are either four-charge non-extremal black holes or smooth non-BPS bubbles. We construct several families such as chains of non-extremal black holes or smooth non-BPS bubbling geometries and study their physics. The smooth horizonless geometries can have the same mass and charges as non-extremal black holes. Furthermore, we find examples that scale towards the four-charge BPS black hole when the non-BPS parameters are taken to be small, but the horizon is smoothly resolved by adding a small amount of non-extremality. 

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