TBA

TBA

This talk will review results on light rings, focusing in particular on topological approaches developed under various assumptions. I will discuss the main conclusions of these results and their implications for the existence and structure of light rings in spacetime geometries.

Binary mergers of stellar-mass black holes are among the best probes of the strong-field regime of gravity, and in particular of the high-curvature corrections typically introduced within the effective field theory framework. In this talk, I will discuss possible deviations in the inspiral–merger–ringdown waveform, with a focus on the detectability of beyond-GR effects in each of these phases. I will also discuss the accumulated gravitational wave memory and its role in future tests of strong gravity.

At the threshold of black hole formation, gravitational collapse exhibits universal critical behavior, most famously in the form of Choptuik’s discretely self-similar solution. In this talk, I will present recent progress on the analytic, numerical, and geometric structure of these critical spacetimes. Using the large-D expansion, one can construct an infinite family of analytic discretely self-similar solutions of the Einstein–massless-Klein–Gordon system, providing closed-form insight into a phenomenon previously known only numerically. I will then discuss how these solutions extend to arbitrary continuous spacetime dimension D>3 by numerically constructing the critical spacetimes directly, yielding a family of “critical spacetime crystals” whose echoing period and Choptuik exponent vary nontrivially with D. Finally, I will describe a newly identified geometric observable of the critical solution: the angle at which null energy condition saturation lines intersect at the center, separating alternating regions of positive and negative curvature, and which can be computed in closed form for any D>3. Taken together, these results provide a broader analytic and numerical framework for understanding critical collapse across spacetime dimensions.

Kerr Black Holes (BHs) with Scalar Hair, introduced by Herdeiro and Radu in 2014 [1], are stationary and axisymmetric spacetimes where a rotating horizon is surrounded by a synchronized distribution of a massive minimally coupled scalar field. Across their parameter space, they bridge the gap between scalar clouds around Kerr Black Holes (vanishing scalar field) and rotating Boson Stars (vanishing horizon). The latter were previously shown to be dynamically unstable against non-axisymmetric modes (Sanchis-Gual et al. 2019 [2]).
In a recently published work (Nicoules et al. 2026 [3]), we showed with numerical evolutions that very hairy BHs also suffer from similar instabilities. Meanwhile, BHs with lower amount of hair in the coexistence region do not suffer from such instabilities, within the time scales of the simulations. We continue here this line of work by further probing the parameter space of the Kerr Black Holes with Scalar Hair. This also includes considering excited modes or self-interaction terms.

[1] DOI:10.1103/PhysRevLett.112.221101
[2] DOI:10.1103/PhysRevLett.123.221101
[3] DOI:10.1103/1btp-58hy

Black holes endowed with synchronised bosonic hair, in both their scalar and Proca realisations, are stationary axisymmetric solutions of general relativity coupled to a massive bosonic field. We demonstrate that magnetised equilibrium tori can be consistently constructed in such spacetimes, following the Komissarov prescription with a purely toroidal magnetic field. The construction is presented for representative configurations of both families, spanning regimes from mild to extreme hairiness. Their stability is then assessed through GRMHD evolutions, providing the first such study of magnetised tori in synchronised bosonic-hair backgrounds.

We investigate the absorption of massless scalar waves by three distinct hairy black hole solutions obtained through the gravitational decoupling method, considering the weak, the strong or the dominant energy conditions. Remarkably, in certain configurations of hairy black holes associated with the fulfillment of the weak energy condition, trapped modes may appear, resulting in Breit-Wigner-like resonances in their absorption profile. These long-lived modes (and consequently the spectral lines in the absorption spectrum) are commonly related to stable light rings in the spacetime, a structure often associated with horizonless exotic compact objects, such as wormholes . We investigate how the gravitational decoupling method introduces novel light ring structures in hairy black holes and influences the absorption spectra through its deformation parameters. Our numerical results show excellent agreement with well-known approximations.

Axially symmetric static double-black-hole (BH) configurations can be constructed, in vacuum General Relativity, as Bach-Weyl solutions. However, these vacuum solutions are not regular, since conical singularities along the symmetry axis are required to balance the mutual gravitational attraction between the two BHs. On the other hand, asymptotically flat balanced configurations of two spinning BHs with synchronised scalar hair (2sBHs) are possible. These are constructed within a generalized Bach-Weyl framework and arise from two spinning boson stars (2sBSs) by placing a horizon at the center of each component. Here, we investigate the effects of quartic scalar self-interactions on this family of solutions, comprising the 2sBSs, the 2sBHs, and an intermediate configuration—single spinning BHs with quadrupolar scalar hair (1sBHs). For 2sBSs, the additional repulsive force introduced by the self-interactions drives a topological transition of the ergoregion, from a single torus to a double torus, in the strong-gravity regime. For 1sBHs, as the self-interaction coupling strength increases, the solutions become “hairier” but their horizons cannot become heavier; moreover, the self-interactions broaden the regime in which an analytical effective model accurately describes these solutions. For 2sBHs, increasing the coupling reshapes the bifurcation structure of the solution sequences and, as in the 1sBH case, repulsive self-interactions cannot make the horizons heavier; horizons carrying a larger mass fraction are obtained only when attractive self-interactions are considered.

Regular black holes and horizonless black-hole mimickers offer
mathematically consistent alternatives to address the challenges posed by
standard black holes. However, the formation mechanism of these
alternative objects is still largely unclear and constitutes a
significant open problem since understanding their dynamical formation
represents a first step to assess their existence. We here investigate,
for the first time and without invoking higher-curvature corrections,
the dynamical formation of a well-known horizonless black-hole mimicker,
namely, a gravastar. More specifically, starting from the collapse of a
uniform dust sphere as in the case of the Oppenheimer-Snyder collapse, we
demonstrate that, under fine-tuned conditions, a
gravastar can form from the nucleation and expansion of a de-Sitter
region with initial zero size at the center of the collapsing
sphere. Furthermore, the de-Sitter expansion naturally slows down near
the Schwarzschild radius, where it meets the collapsing dust surface and
gives rise to a static equilibrium. Interestingly, we also find a maximum
initial compactness of the collapsing star of C= 3/8, above
which the collapse to a black hole is inevitable.

TBA

TBA

Astrophysical turbulence characterizes a variety of systems, from the heliosphere to the interstellar medium and compact object environments, spanning a wide range of length- and timescales, from large-scale eddies to sub-electron structures. Although often associated with randomness and unpredictability, turbulence can generate persistent coherent structures that emerge from chaotic backgrounds and travel undisturbed over long timescales. While persistent structures have been extensively studied in classical viscous fluids, much less is known about their collisionless, magnetized counterparts, where such “magnetic vortices” may play a crucial role in particle energization and dissipation. Their internal structure remains poorly understood, primarily due to the complex coupling between macroscopic dynamics and characteristic plasma scales. The study of turbulent relativistic plasmas around compact objects, such as black holes and neutron stars, is key to integrating high-energy astrophysics, particle kinetics, and MHD. These plasmas may consist of multiple particle species, including electrons, protons, and positrons, with relative abundances that depend sensitively on the astrophysical context. While relativistic MHD captures large-scale dynamics, it fails to account for essential kinetic effects such as nonthermal particle distributions and fast magnetic reconnection. For this purpose, PIC simulations provide the most accurate framework for studying the dynamics of charged particles in electromagnetic fields. However, most PIC studies are limited to simplified two-species plasma models (electron-proton or electron-positron), neglecting the role of additional charge carriers. Including a third species can significantly influence local charge neutrality, current structures, and reconnection efficiency, yet its effects remain largely unexplored.

Turbulence in curved spacetimes in general, and in the vicinity of black holes in particular, remains a poorly understood phenomenon that is often analyzed using techniques developed for flat spacetimes. We propose a novel approach to study turbulence in strong gravitational fields, based on the computation of structure functions on generic manifolds and therefore applicable to arbitrary curved spacetimes. In particular, we introduce a formalism to compute characteristic properties of turbulence, such as the second-order structure function and the power spectral density, in terms of proper lengths and volumes rather than coordinate lengths and volumes, as customarily done. By applying this approach to the turbulent rest-mass density field from simulations of magnetized disk accretion onto a Kerr black hole, we rigorously examine turbulence close to the event horizon, as well as in the disk, wind, and jet. We demonstrate that the new approach can capture the typical behavior of an inertial-range cascade and that differences up to 40–80% emerge in the vicinity of the event horizon with respect to the standard flat spacetime approach. While these differences become smaller at larger distances, our study highlights that special care needs to be paid when analyzing turbulence in strongly curved spacetimes. Building on this framework, we also investigate the dynamical role of multiple density inhomogeneities by comparing simulations initialized with and without localized overdense structures. This controlled setup allows us to quantify how small-scale perturbations seed and modify turbulent cascades and to test whether they leave detectable imprints on global observables. In this way, our work connects small-scale variability, generated by localized structures and instabilities, to macroscopic signatures relevant for interpreting observations of accreting black holes.

We present high-resolution General Relativistic Magnetohydrodynamic (GRMHD) simulations investigating the transition between Magnetically Arrested Disks (MAD) and high-efficiency transient accretion events (Super MAD). We distinguish these phenomena from traditional flux eruption events, focusing instead on regimes of extreme radiative and kinetic efficiency. Drawing parallels between stellar-mass and supermassive black hole dynamics, we analyze long-duration radio transients as signatures of intermediate-mass black hole activity. Furthermore, we propose a novel interpretation of the high-redshift “Little Red Dot” population as hyper-accreting sources where jets are suppressed or “choked” by dense environments.

The image of a black hole (BH) in an optically thin environment is expected to be characterized by a dark central region surrounded by a bright ring of a characteristic size. The BH shadow and the photon ring are believed to play an important role in producing these features, both of which are observed in horizon-scale images of supermassive BH candidates. In this talk, we show that a dynamically stable object with no event horizon or capture cross-section (i.e., no shadow), no light rings (i.e., not ultracompact), and only moderate gravitational redshift is able to reproduce a similar appearance. As an example, we perform general relativistic magnetohydrodynamic simulations of accretion onto a boson star with a sextic potential. We find that a ring of stalled matter forms at a radius which, after gravitational lensing, matches the size of the bright ring expected from a black hole of the same mass. We discuss the robustness of this mechanism and the lifetime of such a structure, showing that it is possible for non-ultracompact exotic compact objects to mimic the expected appearance of black hole shadows in horizon-scale images.

The electromagnetic duality is a symmetry of Maxwell’s equations in vacuum that can be used as a method to generate dyonic or magnetic solutions from the purely charged case. Since this symmetry does not make any assumptions regarding the underlying geometry, it also applies to dynamical spacetimes. Here, we show that the emitted electromagnetic radiation differs between solutions in the same dual family and explore this duality in dynamical spacetimes.

The pseudo-complex version of the Friedmann–
Lemaître-Robertson-Walker model (pcFLRW) is presented
within the framework of pseudo-complex General
Relativity (pcGR). In this approach, dark energy emerges
as a geometric consequence of the pseudo-complex structure,
leading to a specific functional form for the Hubble
parameter H(z) characterized by a single geometric parameter
β. This parameter governs the effective dark-energy
to the present-day time derivative of the Hubble parameter.
Using recent DESI BAO data, we constrain β = 1.0426 ± 0.0144,
which yields a positive value for teh time.change of teh Hubble parameter,
namely (0.94 ± 0.32) × 10−17 (km/s^2)/Mpc.
The best-fit value also implies a deceleration
parameter q = −0.9361 ± 0.0216. Using the exact
Sandage–Loeb relation, the predicted redshift drift over 20
years for a source at z = 4 is _v _ −11.1 cms−1, in close
agreement with the_CDM prediction but arising from a distinct
geometric origin. 99