arXiv subjects

Ruth Gregory

Publications and source records attributed to Ruth Gregory.

At least 19 recordsLinked to original sources

arXiv2026

End state of the experimental black-hole bomb

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.

gr-qc↗

arXiv2026

Acceleration and Rotation in Three Dimensions

We present a new family of exact solutions representing accelerating, rotating, point particle and black hole solutions in three-dimensional Einstein gravity. We investigate their geometric properties and the global embedding in three dimensional anti-de Sitter space. Our results extend the catalogue of exact solutions in three-dimensional Einstein gravity and provide a useful setting for the study of black hole thermodynamics, asymptotic symmetries, and holography.

gr-qc↗

arXiv2026

Properties of black holes in non-linear electrodynamics

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.

hep-th↗

arXiv2025

Astrophysical viability of extremal black holes

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 $10^{14} M_\odot$. 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.

gr-qc↗

arXiv2025

When is a sloshing vortex an analogue black hole bomb?

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.

physics.flu-dyn↗

arXiv2025

Thermodynamics of dyonic black holes in non-linear electrodynamics

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.

hep-th↗

arXiv2025

Black-hole spectroscopy from a giant quantum vortex

Black-hole spectroscopy aims to infer the fundamental properties of black holes by analysing the spectrum of gravitational waves emitted as they settle into equilibrium. These resonances, known as quasinormal modes (QNMs), decay rapidly, which limits the time-domain analysis of gravitational-wave data or numerical simulations to the longest-lived mode, except for a particularly loud event. Owing to the analogy between fields in curved spacetime and waves propagating in a flowing medium, QNMs can be equally excited in a laboratory. In these finite-sized systems, the QNM spectrum is expected to alter: compared to their counterparts in unbounded settings, the real frequencies of QNMs shift while their damping rates (imaginary frequencies) reduce, thereby enhancing their detectability. Here we show that multiple QNMs can be extracted from noise-driven interface waves surrounding a giant quantum vortex in superfluid helium-4, which emulates a spacetime geometry indicative of a rotating black hole. By resolving waves with different azimuthal periodicity, we find that both fundamental modes and their higher-frequency overtones are excited, and oscillate at frequencies given by the size of our system. Since similar effects may arise in astrophysical scenarios due to the interstellar medium or dark matter, gravity simulators now complement numerical and observational approaches to black-hole spectroscopy.

gr-qc↗

arXiv2024

Ultra slow-roll with a black hole

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.

hep-th↗

arXiv2024

Black Holes Inside and Out 2024: visions for the future of black hole physics

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

gr-qc↗

arXiv2024

On the origin of quasinormal modes in semi-open systems

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 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 work opens new avenues for studying QNMs of black holes and compact objects in laboratory settings, where finite-size effects and noise are unavoidable.

gr-qc↗

arXiv2024

Testing Higher Derivative Gravity Through Tunnelling

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.

hep-th↗

arXiv2023

Primordial Black Holes and Higgs Vacuum Decay

Phase transitions are part of everyday life, yet are also believed to be part of the history of our universe, where the nature of particle interactions change as the universe settles into its vacuum state. The discovery of the Higgs, and measurement of its mass suggests that our vacuum may not be entirely stable, and that a further phase transition could take place. This article is based on a talk in the Oldenberg Series, and reviews how we find the probability of these phase transitions, discussing past work on how black holes can dramatically change the result! Apart from a brief update at the end, this article mostly follows the content of the talk.

hep-th↗

arXiv2023

Quasinormal Modes of Optical Solitons

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.

physics.optics↗

arXiv2023

Rotating Curved Spacetime Signatures from a Giant Quantum Vortex

Gravity simulators are laboratory systems where small excitations like sound or surface waves behave as fields propagating on a curved spacetime geometry. The analogy between gravity and fluids requires vanishing viscosity, a feature naturally realised in superfluids like liquid helium or cold atomic clouds. Such systems have been successful in verifying key predictions of quantum field theory in curved spacetime. In particular, quantum simulations of rotating curved spacetimes indicative of astrophysical black holes require the realisation of an extensive vortex flow in superfluid systems. Here we demonstrate that despite the inherent instability of multiply quantised vortices, a stationary giant quantum vortex can be stabilised in superfluid $^4$He. Its compact core carries thousands of circulation quanta, prevailing over current limitations in other physical systems such as magnons, atomic clouds and polaritons. We introduce a minimally invasive way to characterise the vortex flow 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.

gr-qc↗

arXiv2023

Accelerating Black Holes in $2+1$ dimensions: Holography revisited

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 AdS$_{3}$ 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.

hep-th↗

arXiv2023

Seeded vacuum decay with Gauss-Bonnet

We investigate false vacuum decay catalysed by black holes under the influence of the higher order Gauss-Bonnet term. We study both bubble nucleation and Hawking-Moss types of phase transition in arbitrary dimension. The equations of motion of ''bounce'' solutions in which bubbles nucleate around arbitrary dimensional black holes are found in the thin wall approximation, and the instanton action is computed. The headline result that the tunnelling action for static instantons is the difference in entropy of the seed and remnant black holes is shown to hold for arbitrary dimension. We also study the Hawking-Moss transition and find a picture similar to the Einstein case, with one curious five-dimensional exception (due to a mass gap). In four dimensions, we find as expected that the Gauss-Bonnet term only impacts topology changing transitions, i.e. when vacuum decay removes the seed black hole altogether, or in a (Hawking-Moss) transition where a black hole is created. In the former case, topology changing transitions are suppressed (for positive GB coupling $\alpha$), whereas the latter case results in an enhanced transition.

hep-th↗

arXiv2023

Stability of quantised vortices in two-component condensates

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.

cond-mat.quant-gas↗

arXiv2022

Regge pole description of scattering by dirty black holes

We study the problem of plane monochromatic scalar waves impinging upon a Schwarzschild dirty black hole -- a Schwarzschild black hole surrounded by a thin spherical shell of matter -- using the complex angular momentum approach. We first recall general results concerning the differential scattering cross section in the classical limit through a null geodesic analysis by exploring different configurations of the shell. In particular, we show that dirty black hole spacetimes may exhibit various critical effects for geometrical optics. We compute the Regge pole spectrum for various shell configurations and show that it exhibits two or three distinct branches of poles, labelled inner surface waves, broad resonances and outer surface waves. In the latter, two sub-families have been identified, the surface waves associated with the outer light-ring and the creeping modes associated with the surface of the shell. We show, using WKB analysis, that the position of the shell sets the real part of the broad resonances while its energy-momentum and the discontinuity of the potential at the shell's surface set their imaginary part. Next, we provide the complex angular momentum representation of the differential scattering cross section and examine the role of the different Regge pole branches. We compute the differential scattering cross section for various configurations at several frequencies and show a very good agreement with the partial-wave calculations. Finally, we highlight the role of the critical effects, i.e., orbiting, glory, grazing and rainbow scattering, and their impact on the differential scattering cross section.

gr-qc↗