We investigate the properties of charged black hole geometries in nonlinear electrodynamics. We focus on the recently reported analytic charged black hole solutions to illustrate the consequences of a non-monotonic lapse function that exists for a wide range of black hole solutions. The spacetime admits stable light-rings, static near-horizon observers, and trapped near horizon photon orbits. We also show that although these modifications near the horizon are screened from afar, they nonetheless lead to additional branches of quasinormal modes for the black hole that are longer lived than the canonical Einstein branches.

We take a fresh look at the viability of physically realistic extremal black holes within our (non-supersymmetric) low energy physics. By incorporating prefactors and volume effects, we show that Schwinger discharge in charge neutral environments is far more efficient than commonly assumed. Using ionisation estimates for neutral hydrogen, we obtain a new and robust lower bound on the mass of an extremal electrically charged black hole, exceeding 10^14 M_⊙. For magnetic black holes, we compute the Lee-Nair-Weinberg instability and revisit early universe pair creation rates, including singular instantons that substantially enhance production, to demonstrate that the extreme charges required for stability are cosmologically implausible. Finally, we speculate that an extremal Kerr black hole could shed angular momentum via superradiant scattering from the stochastic gravitational wave background. Taken together, our results provide a unified picture that extremal black holes are unlikely to persist in our universe.

We investigate the properties of charged black hole geometries in nonlinear electrodynamics. We focus on the recently reported analytic charged black hole solutions to illustrate the consequences of a nonmonotonic lapse function that exists for a wide range of black hole solutions. The spacetime admits stable light-rings, static near-horizon observers, and trapped near horizon photon orbits. We also show that although these modifications near the horizon are screened from afar, they nonetheless lead to additional branches of quasinormal modes for the black hole that are longer lived than the canonical Einstein branches.

Rotating black holes can amplify incident waves through superradiant scattering. When these waves are confined, repeated amplification gives rise to the black-hole bomb instability, whose nonlinear evolution remains poorly understood despite its central role in models of bosonic clouds around astrophysical black holes. Here, we reproduce the black-hole bomb mechanism in a laboratory setting using a gravity simulator based on a draining vortex in superfluid helium. Surface waves propagating on the superfluid interface experience an effective rotating spacetime and undergo repeated superradiant amplification within a cylindrical cavity. By tuning the temperature and flow parameters, we achieve exponential growth of a low-frequency resonant mode, followed by the arrest of the instability and the formation of a long-lived non-equilibrium steady state. Using spatially and temporally resolved measurements, we identify nonlinear frequency shifts, harmonic generation, and coherent three- and four-wave mixing that redistribute energy among interacting modes. This novel end state of the experimental black-hole bomb highlights the role of nonlinear wave interactions in quenching the runaway growth expected from linear theory and governing the system's late-time dynamics. Our results establish a laboratory framework for investigating the nonlinear evolution of black-hole bombs, with implications for analogous phenomena involving ultralight bosonic fields around rotating black holes.

Draining vortices provide a powerful platform for simulating black hole phenomena in tabletop experiments. In realistic fluid systems confined within a finite container, low-frequency waves amplified by the vortex are reflected at the walls, rendering the system unstable. This process, known in the gravitational context as the black hole bomb, manifests as a sloshing motion of the free surface. The analogy, however, becomes more nuanced when a realistic vortex core with a non-singular vorticity distribution is considered. We investigate this by analysing a non-draining Rankine vortex in the shallow-water and inviscid limits. At low circulation, the sloshing corresponds to an instability of the vorticity field, whereas at high circulation where fluid is expelled from the vortex core, the destabilising mechanism coincides with that of the black hole bomb. Our variational framework distinguishes the energetic contributions of vorticity and irrotational perturbations, offering new insight into the rotating-polygons instability reported by, e.g. Jansson et al. (2006). From the analogue-gravity perspective, we identify hollow core vortices as an optimal regime for exploring black-hole-like instabilities in fluids.

Black-hole spectroscopy aims to infer physical properties of black holes by detecting the spectrum of quasinormal modes (QNMs) they emit while settling towards equilibrium. Unlike normal modes, which are resonances of energy-conserving systems, QNMs are damped oscillations arising when a system loses energy due to open boundaries or via dissipation. The detection of the full QNM spectrum of black holes is challenging due to rapidly decaying amplitudes of these resonances, limiting observations only to the longest-lived mode. Theoretical and numerical studies suggest that environmental confinement due to surrounding plasma or dark matter modify the QNM spectrum. Here, we employ black-hole spectroscopy to show how spatial confinement similarly affects the spectrum of nanometre-scale interface waves surrounding a giant quantum vortex in superfluid helium-4, an experimentally accessible quantum system that emulates dynamics in rotating curved spacetime. In the available parameter space, we observe regimes in which multiple QNMs emerge from the interface noise spectrum. In agreement with theoretical predictions, their real and imaginary frequencies are shifted with respect to those expected in the unbounded system. Our results demonstrate the critical role of spatial confinement in shaping the QNM spectrum, highlighting the importance of environmental effects on spectral stability of astrophysical compact objects.

We investigate dyonic black holes in a weak field expansion of non-linear electrodynamics. The breadth of parameter space permits a rich thermodynamic structure, additional turning points and intricate phase phenomena. Energy conditions are employed to ensure the physical viability of solutions. Analytic special cases illustrate novel properties of black holes in non-linear electrodynamics, including modified extremal limit behaviour. Numerical solutions offer the most elaborate thermodynamic landscape, culminating in up to five turning points, and multiple reentrant phase transitions.

We investigate ultra slow-roll inflation with a seed black hole in a de Sitter background. By numerically tracking transitions from slow-roll to ultra slow-roll inflation, we find that quasi-normal mode solutions of the scalar field are excited following the decay of the slow-roll attractor, depending on the mass of the black hole. For small black holes, the picture is similar to standard inflation with the usual damping of the scalar field; with a large black hole, we find that the ringing modes dominate. It is believed that the transition to ultra slow-roll in the pure inflationary case enhances the peak of the primordial power spectrum, thereby increasing the likelihood of primordial black hole formation. We comment on how the novel ringing behaviour due to the seed black hole might impact on cosmological perturbations.

We briefly overview the case for using black holes as a discriminator for theories of gravity. The opportunities and challenges for the various observational experiments are outlined, and key questions for the community identified. This note summarises the discussion from the roundtable on the third day of Black Holes Inside and Out.

Astrophysical black holes are open systems which, when perturbed, radiate quasinormal modes (QNMs) to infinity. By contrast, laboratory analogs are necessarily finite sized, presenting a potential obstacle to exciting QNMs in experiments. We explore how the QNM spectrum of a toy-model black hole changes when enclosed by a partially reflecting wall with adjustable reflectivity. Our results reveal a continuous connection between the QNM spectra of open and finite-sized systems. Additionally, we demonstrate that QNMs in this setup are easily excited by incoherent background noise. This Letter opens new avenues for studying QNMs of black holes and compact objects in laboratory settings, where finite-size effects and noise are unavoidable.

Higher derivative terms in the gravitational action are natural from the perspective of quantum gravity, but are perceived as leading to a lack of well-posedness. The Gauss-Bonnet term has second-order equations of motion, but does not impact gravitational dynamics in 4D, so one might expect that it is not physically relevant. We discuss how signatures can show up in tunnelling processes and whether these will likely be physically accessible in Higgs vacuum decay.

We investigate ultra slow-roll inflation in a black hole background finding a correspondence between scalar solutions of ultra slow-roll inflation and quasi-normal modes of the cosmological black hole spacetime. Transitions from slow-roll to ultra slow-roll can enhance the peak of the primordial power spectrum increasing the likelihood of primordial black hole formation. By following such a transition in a black hole background, we observe a decay of the slow-roll attractor solution into the quasi-normal modes of the system. With a large black hole, the ringing modes dominate, which could have implications for the background of cosmological scalar perturbations and peak enhancement.

The gravitational physics landscape is evolving rapidly, driven by our ability to study strong-field regions, in particular black holes. Black Holes Inside and Out gathered world experts to discuss the status of the field and prospects ahead. We hope that the ideas and perspectives are a source of inspiration. Structure: Black Hole Evaporation - 50 Years by William Unruh The Stability Problem for Extremal Black Holes by Mihalis Dafermos The Entropy of Black Holes by Robert M. Wald The Non-linear Regime of Gravity by Luis Lehner Black Holes Galore in D > 4 by Roberto Emparan Same as Ever: Looking for (In)variants in the Black Holes Landscape by Carlos A. R. Herdeiro Black Holes, Cauchy Horizons, and Mass Inflation by Matt Visser The Backreaction Problem for Black Holes in Semiclassical Gravity by Adrian del Rio Black Holes Beyond General Relativity by Enrico Barausse and Jutta Kunz Black Holes as Laboratories: Searching for Ultralight Fields by Richard Brito Primordial Black Holes from Inflation by Misao Sasaki Tests of General Relativity with Future Detectors by Emanuele Berti Black Holes as Laboratories: Tests of General Relativity by Ruth Gregory and Samaya Nissanke Simulating Black Hole Imposters by Frans Pretorius Black Hole Spectroscopy: Status Report by Gregorio Carullo VLBI as a Precision Strong Gravity Instrument by Paul Tiede Testing the nature of compact objects and the black hole paradigm by Mariafelicia De Laurentis and Paolo Pani Some Thoughts about Black Holes in Asymptotic Safety by Alessia Platania Black Hole Evaporation in Loop Quantum Gravity by Abhay Ashtekar How the Black Hole Puzzles are Resolved in String Theory by Samir D. Mathur Quantum Black Holes: From Regularization to Information Paradoxes by Niayesh Afshordi and Stefano Liberati

Gravity simulators 1 are laboratory systems in which small excitations such as sound 2 or surface waves 3 , 4 behave as fields propagating on a curved spacetime geometry. The analogy between gravity and fluids requires vanishing viscosity 2 – 4 , a feature naturally realized in superfluids such as liquid helium or cold atomic clouds 5 – 8 . Such systems have been successful in verifying key predictions of quantum field theory in curved spacetime 7 – 11 . In particular, quantum simulations of rotating curved spacetimes indicative of astrophysical black holes require the realization of an extensive vortex flow 12 in superfluid systems. Here we demonstrate that, despite the inherent instability of multiply quantized vortices 13 , 14 , a stationary giant quantum vortex can be stabilized in superfluid 4 He. Its compact core carries thousands of circulation quanta, prevailing over current limitations in other physical systems such as magnons 5 , atomic clouds 6 , 7 and polaritons 15 , 16 . We introduce a minimally invasive way to characterize the vortex flow 17 , 18 by exploiting the interaction of micrometre-scale waves on the superfluid interface with the background velocity field. Intricate wave–vortex interactions, including the detection of bound states and distinctive analogue black hole ringdown signatures, have been observed. These results open new avenues to explore quantum-to-classical vortex transitions and use superfluid helium as a finite-temperature quantum field theory simulator for rotating curved spacetimes 19 .

Astrophysical black holes are open systems which, when perturbed, radiate quasi-normal modes (QNMs) to infinity. By contrast, laboratory analogues are necessarily finite-sized, presenting a potential obstacle to exciting QNMs in the lab. In this study, we investigate how the QNM spectrum of a toy-model black hole is modified when the system is enclosed by a partially reflecting wall. Counter to expectation, we demonstrate that QNMs not only persist in finite-sized systems, but the number of accessible modes increases. Furthermore, we show that QNMs in this set-up can be easily excited by incoherent background noise. Our findings align with studies exploring the spectral stability of black holes, such as those examining small modifications of the surrounding gravitational field. Importantly, our work paves the way for exploring spectral stability in laboratory systems, enabling experimental investigation of black hole features in closed systems.

Quasinormal modes (QNMs) are essential for understanding the stability and resonances of open systems, with increasing prominence in black hole physics. We present here the first study of QNMs of optical potentials. We show that solitons can support QNMs, deriving a soliton perturbation equation and giving exact analytical expressions for the QNMs of fiber solitons. We discuss the boundary conditions in this intrinsically dispersive system and identify novel signatures of dispersion. From here, we discover a new analogy with black holes and describe a regime in which the soliton is a robust black hole simulator for light-ring phenomena. Our results invite a range of applications, from the description of optical pulse propagation with QNMs to the use of state-of-the-art technology from fiber optics to address questions in black hole physics, such as QNM spectral instabilities and the role of nonlinearities in ringdown.

Multiply quantised vortices (MQVs) within single component Bose-Einstein condensates are unstable and decay rapidly. We show that MQVs can be stabilised by adding a small number of atoms of a second species to the vortex cores, and that these atoms remain in the vortex core as the system evolves. A consequence of the stabilisation is that nearby co-rotating vortices can orbit in the opposite sense to their individual rotations when enough of the second species is present. This has implications concerning the imaging of vortices, as well as quantum turbulence and vortex nucleation in two-component condensates, such as those involving mixtures of $^{87}$Rb and $^{133}$Cs.

This paper studies the holographic description of 2 + 1-dimensional accelerating black holes. We start by using an ADM decomposition of the coordinates suitable to identify boundary data. As a consequence, the holographic CFT lies in a fixed curved background which is described by the holographic stress tensor of a perfect fluid. We compute the Euclidean action ensuring that the variational principle is satisfied in the presence of the domain wall. This requires including the Gibbons-Hawking-York term associated with internal boundaries on top of the standard renormalised AdS3 action. Finally, we compute the entanglement entropy by firstly mapping the solution to the Rindler-AdS spacetime in which the Ryu-Takayanagi surface is easily identifiable. We found that as the acceleration increases the accessible region of the conformal boundary decreases and also the entanglement entropy, indicating a loss of information in the dual theory due to acceleration.

Abstract Gravity simulators [1] are laboratory systems where small excitations like sound [2] or surface waves [3] behave as fields propagating on a curved spacetime geometry. The analogy between gravity and fluids requires vanishing viscosity [2, 3], a feature naturally realised in superfluids like liquid helium or cold atomic clouds [4-6]. Such systems have been successful in verifying key predictions of quantum field theory in curved spacetime [6-9]. In particular, quantum simulations of rotating curved spacetimes indicative of astrophysical black holes require the realisation of an extensive vortex flow [10] in superfluid systems. Despite the inherent instability of multiply quantised vortices [11, 12], here we demonstrate that a stationary giant quantum vortex can be stabilised in superfluid 4He. Its compact core carries thousands of circulation quanta, prevailing over current limitations in other physical systems such as magnons [4], cold gases [5, 6] and polaritons [13, 14]. We introduce a minimally invasive way to characterise the vortex flow [15, 16] by exploiting the interaction of micrometre-scale waves on the superfluid interface with the background velocity field. Intricate wave- vortex interactions, including the detection of bound states and distinctive analogue black hole ringdown signatures, have been observed. These results open new avenues to explore quantum-to-classical vortex transitions and utilise superfluid helium as a finite temperature quantum field theory simulator for rotating curved spacetimes [17].

In U(1) U(1) Abelian gauge theory coupled to fermions, the non-conservation of the axial current due to the chiral anomaly is given by a dynamical operator F_{\mu\nu} \tilde{F}^{\mu\nu} FμνF̃μν constructed from the field-strength tensor. We attempt to describe this physics in a universal manner by casting this operator in terms of the 2-form current for the 1-form symmetry associated with magnetic flux conservation. We construct a holographic dual with this symmetry breaking pattern and study some aspects of finite temperature anomalous magnetohydrodynamics. We explicitly calculate the charge susceptibility and the axial charge relaxation rate as a function of temperature and magnetic field and compare to recent lattice results. At small magnetic fields we find agreement with elementary hydrodynamics weakly coupled to an electrodynamic sector, but we find deviations at larger fields.