From Black Holes to the Cosmos: Matt Visser’s journey through space and time

Aug 24, 2026, 9:00 AM → Aug 28, 2026, 6:00 PM Europe/Rome

SISSA (Miramare Campus)

This conference honors the scientific legacy of Matt Visser, whose pioneering work has profoundly influenced our understanding of general relativity, quantum field theory in curved spacetime, wormholes, energy conditions, analogue gravity, and beyond. Gathering colleagues, collaborators, and students from around the world, the meeting will highlight recent developments at the interface of geometry, gravitation, and quantum theory, while celebrating Visser's distinctive contributions to these fields. The conference will be hosted in the Aula Magna of SISSA Miramare Campus.

Invited speakers:
Matt Visser, Carlos Barceló, Ivan Booth, Fay Dowker, Ted Jacobson, Eleni-Alexandra Kontou, Stefano Liberati, Francisco Lobo, Robert Mann, Prado Martín Moruno, Eric Poisson, Ralf Schützhold, Thomas Sotiriou, Silke Weinfurtner, David Wiltshire
 

 

Sponsors:

          

 

poster1.pdf

poster2.pdf

Mon, August 24

9:00 AM → 9:30 AM Welcome Reception

9:30 AM → 10:30 AM Stefano Liberati --- A Visser's guide to the edge of spacetime

Liberati.pdf

10:30 AM → 11:00 AM Coffee Break

11:00 AM → 12:00 PM Thomas Sotiriou --- Black hole hair and gravitational waves as probes of new physics

The last 10 years gravitational wave observations have allowed us to finally probe the strong field, nonlinear dynamics of gravity and have produced interesting bounds on deviations from General Relativity and the existence of new light fields. In the same period our theoretical understanding of how such deviation can arise and get imprinted on waveform have also progressed significantly. I will draw some lesson from this theoretical progress and discuss how it can inform the way we search of new physics with gravitational waves.

12:00 PM → 1:00 PM Ivan Booth --- Black hole evolutions: lessons from bifurcation theory

Apparent horizons are the best-known examples of marginally outer trapped surfaces (MOTS). However, it is now clear that most MOTS are not apparent horizons. Large, likely infinite, families of MOTS are found in the interior of black hole mergers and these engage in a complex set of interactions and evolutions, including the ultimate dissolution of the original apparent horizons inside the final black hole. The key theoretical tool that brings order to these evolutions is the MOTS stability operator, the modern version of which was introduced by Andersson, Mars and Simon in 2005. It is best understood as the linearization of the outward null expansion for surfaces “near” an existing MOTS. It both identifies whether or not a MOTS can be understood as a (local) apparent horizon (forming a boundary between trapped and untrapped regions) and also determines how a MOTS will evolve in a changing spacetime: if the stability operator is invertible then that evolution is unique.

In this talk, I will review that background and then focus on MOTS with non-invertible stability operators and so non-unique evolutions. The MOTS pair-creations and annihilations observed during black hole mergers are the best-known examples of non-unique evolutions but these are not the only possibilities. Understanding the MOTS as fixed points of the outward null expansion equations, a generalization of standard, dynamical system, bifurcation theory can be used to classify all possible non-unique evolutions. MOTS pair-creation/annihilations are then understood as examples of saddle-node bifurcations. There are other possibilities, including pitchfork and transcritical bifurcations. I apply analytical and numerical tools to identify examples of the various bifurcations in a variety of spacetimes. This theory depends only on the geometry of a MOTS and its surrounding spacetime. Hence the classification results apply not only to possible bifurcations observed in numerical time evolutions but also those that occur for any other deformation of the spacetime. In particular, the constraints on possible bifurcations are not restricted to general relativity but apply to any geometric theory of gravity.

Booth.pdf

1:00 PM → 2:30 PM Lunch- held at the ICTP Canteen

Lunch is held at the ICTP Canteen (https://www.ictp.it/building/leonardo-building/room/leonardo-building-cafeteria). The tables outside are reserved for our conference.

2:30 PM → 2:50 PM Uwe R. Fischer - (Volume-law scaling for entanglement negativity from the Hawking radiation of analogue black holes)

The quantum information content of Hawking radiation holds the key to understanding black-hole evaporation and the fate of unitarity. Motivated by recent advances in cold-atom experiments, we develop a lattice-regularization approach aimed at simulating the coarse-grained entanglement scaling of a quantum field in a 1+1D analogue black-hole background. We provide the first concrete demonstration that logarithmic negativity --- an entanglement monotone that typically exhibits a UV-divergent log-scaling for the conformal vacuum --- acquires a UV-finite volume term from the nonlocal correlations seeded by Hawking radiation. We show that this volume term encodes both the number density and spatial distribution of entangled Hawking pairs along the black-hole interior and exterior. We highlight its prospective detection in currently realizable experiments as well as its implications beyond the analogue paradigm, in particular for black-hole thermodynamics. [Preprint https://arxiv.org/abs/2604.02075][1] [1]: https://arxiv.org/abs/2604.02075

Fischer.pdf

2:50 PM → 3:10 PM Goffredo Chirco - (Metric-Deformed Poisson Sigma Models: Dissipative Extensions of JT Gravity and Boundary Dynamics)

I will discuss a metric deformation of the Poisson Sigma Model as a controlled departure from its purely topological and conservative dynamics. Focusing on the SL(2,R) formulation of Jackiw–Teitelboim gravity, I will argue that such a deformation may provide an effective description of non-conservative, coarse-grained, or quantum-gravitational corrections to near-AdS2 dynamics. In particular, the model suggests a possible way to interpolate between ideal near-extremal horizon boundary dynamics and more dissipative, Schwarzschild-like regimes.

3:10 PM → 3:30 PM Valentin Pomakov - (Analog regular black holes and black hole mimickers in a Bose-Einstein condensate)

With the goal of investigating inner horizon and inner light ring instabilities of static, spherically symmetric regular black holes and black hole mimickers, respectively, we show how to emulate their corresponding spacetime possessing a radius-dependent mass function $m(r)$ in an analog-gravity platform based on a Bose-Einstein condensate (BEC). We compare the sonic and the gravitational line elements and show a correspondence between $m(r)$ and the ratio of radial flow velocity and sonic speed. This allows, in theory, to emulate the entire spacetime in the analog platform, and shows which are the condensate analog quantities for gravitational parameters like the regularization parameter $\ell$ and the ADM mass $M$ in different areas of the condensate. Resolving this correspondence relation along with the Bernoulli and continuity equation, we show how to experimentally realize the central and the asymptotic region of the gravitational spacetime in the BEC by means of a given profile $U(r)$ of the external potential and interaction strength $g(r)$, tuned by means of the Feshbach resonance with a radially varying magnetic field $B(r)$. We propose a concrete experimental setup and, using experimental values of the relevant quantities from existing BEC experiments, provide a reality-check which demonstrates the viability and realizability of our analog-gravity program.

Pomakov.pdf

3:30 PM → 3:50 PM Adolfo Cisterna - (New Dynamical Black Hole Solutions: Extending the Spectrum of Exact Primordial Objects)

Constructing exact solutions describing compact objects embedded in dynamical cosmological environments remains a major challenge in General Relativity, with most known results restricted to highly symmetric settings. In this talk, we present new exact solutions and solution-generating techniques that extend the analytical description of dynamical black holes in cosmological backgrounds. First, we introduce a novel method for the self-interacting Einstein–scalar system that generates non-stationary, axisymmetric geometries. Using this approach, we obtain the first exact solution describing a dynamical axisymmetric black (or white) hole embedded in an expanding or contracting cosmology. Its dynamical trapping horizons are characterized using the mean curvature vector, which generalizes the Kodama vector beyond spherical symmetry and allows a foliation-independent identification of trapped and untrapped regions. We then construct a new exact solution of the Einstein–scalar–Maxwell system describing a dynamical black hole interacting with a time-dependent external electromagnetic field. The configuration is obtained by dressing a Schwarzschild black hole with a radially and temporally varying scalar field within the Fonarev framework, producing a time-dependent generalization of the Fisher–Janis–Newman–Winicour solution. An external electromagnetic field is subsequently generated through a Lie point symmetry extending the Harrison transformation to dynamical settings. The resulting spacetime features a dynamical horizon, an axisymmetric electromagnetic field, and asymptotics combining Friedmann–Lemaître–Robertson–Walker and Levi–Civita geometries. We analyze the geometric and physical properties of these solutions, including their horizon structure and asymptotic behavior. Notably, time dependence can cloak curvature singularities that would otherwise be naked in stationary limits. These results provide new analytical tools to explore dynamical compact objects in cosmology, with potential applications to primordial black holes.

3:50 PM → 4:20 PM Coffee Break

4:20 PM → 4:40 PM Massimiliano Rinaldi - (Aspects of cosmological coupling)

The idea that compact objects are coupled to the expansion of the universe was put forward almost a century ago by McVittie, who introduced a specific metric that, however, exhibits naked singularities. Since then, many authors have attempted to improve the McVittie solution to eliminate the singularity, with limited success. In recent years, the cosmological coupling of compact objects has resurfaced prominently, driven by new theoretical arguments and observations. In this talk, I will offer a brief survey of this topic and present some recent insights.

Rinaldi.pdf

4:40 PM → 5:00 PM Johanna Borissova - (Regular black holes in quasi-topological gravity)

Integrable $2D$ dilaton theories play a central role in discussions of black holes in a variety of approaches to classical and quantum gravity. They can be obtained from the spherical reduction of $d\geq 4$ quasi-topological gravities, and in particular onshell configurations for the $2D$ metric and scalar field represent genuine $d$-dimensional vacuum solutions. This talk will discuss the reconstruction of generic $d$-dimensional static spherically symmetric black holes satisfying $g_{tt}g_{rr}=-1$ in Schwarzschild gauge as quasi-topological vacuum solutions, and use this reconstruction to establish a unified framework for their black hole thermodynamics. I will illustrate that the generating function determining $f(r) = -g_{tt}$ in the integrated equation of motion provides the thermodynamic mass in a first law which can be derived for any such black hole by an application of Wald's Noether charge formalism.

Borissova.pdf

5:00 PM → 5:20 PM Antonio Panassiti - (Null convergence condition in regular black-hole spacetimes with Minkowski core)

Recent works in the literature have addressed how (i) black holes with de Sitter core circumvent the Hawking–Penrose (1970) singularity theorem, and (ii) geometries with either a de Sitter or Minkowski core can be obtained from effective models of asymptotically safe gravitational collapse, based on the high-energy behavior of the vanishing Newton coupling. Here, we investigate how generic black holes with Minkowski core specifically bypass the hypothesis of the singularity theorem on the convergence condition for geodesics. We prove that, independently of the generating dynamics, they always violate already the null convergence condition (NCC), differently from geometries with de Sitter core which always violate only the timelike convergence condition (TCC). Upon giving concrete examples of such non-singular black holes--typically characterized by exponential mass functions which can be either analytically expandable or not, we discuss the deeper physical feature encoded in the violation of the NCC. In particular, ingoing trajectories of the congruence of radial timelike geodesics approaching the Minkowski core of a black-hole spacetime undergo an additional phase where their focusing slows down to the point of reaching a state of fully parallel flow.

5:20 PM → 5:40 PM Justin Feng - (Event horizon termination and the emergence of Lorentz signature)

In this talk, I describe in detail how one might understand the termination of the event horizon of a black hole in terms of a quasiregular singularity characterized by points possessing two future-directed light cones and two past-directed light cones (in fact this spacetime is conformal to a region of the 1+1 trousers spacetime). I then discuss Euclidean signature shift-symmetric scalar-tensor theories from which one can extract a Lorentzian structure, and show how this theory can provide a microscopic description for the aforementioned singularities. I discuss some recent works on the emergence of Lorentzian dispersion relations and a model for the big bang, as well as some preliminary and ongoing work on understanding compact objects in this class of theories.

Tue, August 25

9:30 AM → 10:30 AM Eleni-alexandra Kontou --- Wormholes as time machines

Wormhole solutions—bridges connecting different regions of spacetime—were proposed early in the history of General Relativity, and it was soon realized that they could potentially act as time machines, violating causality. It was soon shown that traversable wormholes necessarily violate the classical energy conditions, which impose non-negativity constraints on contractions of the stress-energy tensor. Since quantum fields can violate these conditions, it was initially hoped that wormholes might be realizable within semiclassical gravity. However, negative energy densities in quantum field theory are themselves subject to constraints. Quantum energy inequalities and averaged energy conditions place bounds on renormalized negative energies when averaged along geodesics, thereby imposing restrictions on the construction and maintenance of traversable wormholes. Recently, long wormholes—wormholes for which traversal takes longer than traveling between the same endpoints through the exterior spacetime—have been proposed as an alternative. Do quantum energy inequalities constrain long wormholes in the same way? Can long wormholes ever become time machines, or does their extended traversal time prevent causality violation? In this talk, I will review the construction of long wormholes, investigate whether they can be converted into time machines, and discuss the constraints imposed by quantum energy inequalities.

10:30 AM → 11:00 AM Coffee Break

11:00 AM → 12:00 PM Prado Martín Moruno --- Breaking symmetries ma non troppo: transverse diffeomorphisms, interacting dark fluids, and unified dark sector.

In this talk the implications of a restricted symmetry breaking in the gravitational sector, where standard diffeomorphism invariance (Diff) is reduced to transverse diffeomorphisms (TDiff) through the matter action, will be analysed. The idea of restoring the full diffeomorphism invariance by the introduction of additional fields will be also explored and reformulations with the full symmetry will be presented. Furthermore, interesting applications of this framework to the dark sector of the Universe will be discussed. On the one hand, multi-field dynamics naturally induces an effective interaction and energy exchange between fields. Thus, multi-field scenarios provide a robust, symmetry-principled foundation for interacting dark fluids. On the other hand, the silent unified dark fluid will be presented. This is a novel class of single scalar field-theory models that provide a unified description of the dark sector as a single perfect fluid with a vanishing speed of sound.

MartinMoruno.pdf

12:00 PM → 1:00 PM Eric Poisson --- Stability of thin shells: From Matt to now

Matt loves thin shells, and who doesn't? But thin shells tend to be dynamically unstable to nonradial perturbations, and this limits their use in terms of modeling exotic astrophysical bodies (such as black-hole mimickers and even wormholes). After telling some stories about Matt and how we came to work together, I will describe some recent work (with Tristan Pitre and Berend Schneider) that establishes that self-gravitating thin shells are dynamically unstable in both Newtonian gravity and general relativity.

Poisson.pdf

1:00 PM → 2:00 PM Lunch- held at the ICTP Canteen

Lunch is held at the ICTP Canteen (https://www.ictp.it/building/leonardo-building/room/leonardo-building-cafeteria). The tables outside are reserved for our conference.

2:00 PM → 2:30 PM Poster Session

2:30 PM → 2:50 PM Raúl Carballo-Rubio - (Black holes in regular gravitational theories)

The study of gravitational theories in which the singular character of general relativity is alleviated is an active area of research. In particular, a great deal of effort has been devoted to finding theories in which black hole interiors are regular. Regularizing vacuum black hole solutions can provide guidance in the exploration of the landscape of theories beyond general relativity. In this talk, I will discuss explorations of the behavior of these theories in non-vacuum and dynamical situations.

Carballo-Rubio.pdf

2:50 PM → 3:10 PM Jacopo Mazza - (Beyond circles: stationary axisymmetric black holes and the breaking of circularity)

Circularity is an accidental symmetry of the Kerr metric, one that is widely assumed when searching for rotating black hole solutions in modified gravity as well as when constructing models of Kerr mimickers. Though extremely enticing, circularity is often an excessively restrictive assumption, and understanding the consequences of its loss is thus crucially relevant. In this seminar, I wish to present some recent results on the subject: after describing in detail what this symmetry entails, I will show how to construct stationary and axisymmetric spacetimes exhibiting a controlled breaking of circularity; then, I will describe the impact of circularity breaking on the hole’s horizon, focusing in particular on the laws of black hole mechanics. This discussion is thus going to be pertinent for anyone with an interest in compact astrophysical objects and their phenomenology, in general relativity and beyond.

Mazza.pdf

3:10 PM → 3:30 PM Finnian Gray - (The hidden symmetries of slowly rotating black holes)

I will discuss a recently proposed class of Generalized Lense–Thirring spacetimes in all dimensions. The class represent an ansatz for slowly rotating black holes which can be applied to solve the equations of motion, to linear order in the rotation parameters, for a wide variety of gravitational theories including Einstein--Gravity. In this case it is equivalent to the original Lense--Thiring solution to linear order. However, the class has the following remarkable properties which go beyond ordinary Lense--Thrring spacetimes: 1) It is regular on the horizon, 2) it can be put into Painlevé--Gullstrand (in-falling) coordinates, and most remarkably 3), it has a growing tower of hidden symmetries which in higher dimensions is greater than the number of explicit isometries. In this construction, the key ingredient is the non-commutativity of the underlying Killing vectors of the base space. The growing rank irreducible Killing tensors are determined by the structure constants of the corresponding Lie algebra. These hidden symmetries and the resultant constants of motion underpin the integrability and separability of test particles and fields in these spacetimes, thus allowing physical observables to be easily characterized.

3:30 PM → 3:50 PM Hamed Barzegar - (Warp Drive: a Gedankenexperiment taken too far)

In this talk, I provide a systematic classification and critical evaluation of warp-drive spacetimes, to which Prof. Visser has made significant contributions. Moving beyond the popular "creative" approach, I adopt a rigorous "agonistic" methodology to address the recent renaissance of claims regarding physically feasible, positive-energy warp drives. By applying a basic principle, which mandates mathematical consistency within General Relativity (GR) prior to physical speculation, we identified fundamental errors and misconceptions in existing literature. I will present a model hierarchy distinguishing between Restricted Warp (R-Warp)--encompassing most proposals--and more generalized configurations. The presentation focuses on demystifying these models through several no-go theorems, e.g.: 1) It is impossible to construct a superluminal R-Warp model that is globally hyperbolic. 2) The ADM energy for R-Warp models is generically zero, exposing the mathematical ambiguity of "total mass" calculations used in many studies. 3) I will contrast the "creative" approach (Synge G-method) with the "realistic" one, highlighting how the former often leads to "fantastic" but unphysical results. Finally, by recalling a proof that R-Warp models fundamentally violate the null energy condition (due to Santiago, Schuster, and Visser 2022), we conclude that warp drives are best viewed as Gedankenexperiments for probing the boundaries of GR rather than viable technological blueprints. Time permitting, I will link these spacetimes to cosmological models.

3:50 PM → 4:20 PM Coffee Break

4:20 PM → 4:40 PM Sebastian Schuster - (Science-Fiction Double Feature: Of Time Travel and Warp Drives)

General relativity and cosmology have had a generous influx of ideas from science-fiction, with Matt contributing and influencing these developments significantly. The metric reverse-engineering that is at the core of this, is frequently called into question on diverse grounds—some better, some less so. Warp drives in particular have seen a recent resurgence with substantial controversy. In this talk, I will explain why reverse-engineering is a fine, time-honoured tradition beyond general relativity; why point-wise energy conditions are middling, at best; and why the the fate of these speculative metrics needs to be decided by the (so-far) equally speculative field of quantum gravity. After a summary of recent work on warp drives and their extensions, I will give an example of what sort of problem quantum gravity would have to solve in such contexts: Time travel through the lens of relational quantum dynamics. I will wrap up with some ideas on how to extent these first toy models to unwieldy physics besides time travel.

4:40 PM → 5:00 PM Jonathan Barenboim - (Evaporating regular black holes in 2D gravity)

Quantum field theory in curved spacetime predicts that black holes evaporate through Hawking radiation, but without a full quantum treatment of gravity the endpoint of the process is not yet understood. This talk will present a general model of evaporating black holes in 2D dilaton gravity, with a focus on a Bardeen-like regular black hole model. The formation and evaporation of a black hole, including backreaction, is simulated numerically. We find that the apparent horizons evaporate smoothly in finite time and that the final spacetime is free of pathologies such as singularities, event horizons, or Cauchy horizons. These results suggest that resolving the singularity could be a viable solution to the black hole information loss problem

5:00 PM → 5:20 PM Massimiliano Spadafora - (Semiclassical Black Hole - White Hole transition: an analytical treatment)

Recent progress in semiclassical gravity is suggesting that, in spherical black holes with outer and inner horizons, the Hawking process drives the complete evaporation of the trapped region in timescales shorter than the Hawking time. However, this does not mark the end of the story, as this process is accompanied by the formation of an anti-trapped region, or white hole. In this work, we present a novel analytic treatment of black hole evaporation. Within simplified two-dimensional models describing the formation of charged and regular black holes, we show that the emergence of the anti-trapped region is unavoidable and is caused by the amplification of negative energy fluxes created along the \textit{outgoing} direction inside the black hole. This picture suggests that the energy fluxes originated by the white hole along the \textit{ingoing} direction can trigger the subsequent formation of a black hole, producing a cascade of black-to-white hole transitions which might terminate in a spacetime free of horizons. The analytic techniques developed in this work set the grounds to investigate this scenario in an exhaustive way.

Spadafora.pdf

5:20 PM → 5:40 PM Noa Zilberman - (The semiclassical energy outflux emerging from a collapsing shell)

When a compact object collapses to form a black hole, quantum field theory predicts the emission of an energy outflux to future null infinity, which later relaxes to Hawking radiation. Within the semiclassical framework, we derive a simple, closed form, analytical expression for the energy outflux emitted from a spherical thin null shell collapsing to form a black hole (in 4D). In particular, this energy outflux vanishes (quadratically in r-2M) as the shell approaches the horizon. This result refutes claims that the Hawking energy outflux originates from the collapsing body, showing instead that it develops in a broad strong-field region. Additionally, this vanishing implies that semiclassical backreaction cannot prevent or significantly affect the classical process of gravitational collapse and horizon formation (as sometimes claimed). This talk is based on the joint work arXiv:2503.00622 with Amos Ori.

Zilberman.pdf

Wed, August 26

9:30 AM → 10:30 AM David Wiltshire --- Parallel journeys in space, time and the timescape

Matt and I have been asking similar questions for 4 decades, for a time as collaborators and for longer as colleagues in Aotearoa / New Zealand. I will tell the story of our superposition of states, with a focus on the cosmos. In the last 2 decades, I have developed the timescape cosmology to explain apparent cosmic acceleration in the statistical regime of general relativity. As inhomogeneities in density and expansion grow, they back-react on average cosmic expansion, which differs from conventional FLRW models. Average homogeneity and isotropy is not assumed but is explained as a consequence of extending Einstein's strong equivalence principle from local frames to quasilocal regions. Observationally, dynamical spatial curvature arises as time-varying gradients of the kinetic energy of expansion, depending directly on the volume fraction of cosmic voids. The timescape expansion history is close to ΛCDM, but with differences at a precision which we are now finally probing. Whereas ΛCDM is increasingly challenged, independent observational tests now favour timescape. I will survey the current status relative to foundational questions I believe we still have to answer before arriving at a full theory of quantum gravity: What is the geometrical description of the origin of scale? How is infinity physically characterized when fitting one geometry in another? How is quasilocal energy to be accounted for in energy conditions?

Wiltshire.pdf

10:30 AM → 11:00 AM Coffee Break

11:00 AM → 12:00 PM Fay Dowker --- Meeting Matt in the Middle

Good books on GR agree on certain aspects of the theory that we can consider to be its core features. Beyond the core, however, we do not know exactly what GR — and semiclassical gravity more generally — should allow or not allow physically. For example, should we consider spacetimes with closed causal loops as part of physical GR or not? Are spacetimes with certain types of boundaries or singularities part of GR?  In this fertile arena Matt has played a leading rôle in pushing back the Veil of Unknowing, elucidating the physics of spacetime beyond mere "axioms". Such questions can also be addressed from the direction of quantum gravity in which Lorentzian spacetime is not assumed but must be derived. Within the framework of the gravitational path integral and a Feynmanian heuristic for the recovery of the classical approximation I will use the example of causal set quantum gravity to sketch out how predictions about the properties of physically allowed spacetimes might arise.

Dowker.pdf

12:00 PM → 1:00 PM Carlos Barceló --- From black holes to fundamental gravity and back: analogue gravity and the harmonic background paradigm

In this talk, I will take a journey through the main themes that have guided my research over the years. My work has been shaped by the interplay between understanding the black hole singularity problem and some of the essential aspects of general relativity, leading me to explore three main avenues: analogue and emergent gravity, semiclassical gravity, and a particular version of bimetric gravity, more appropriately described as a new paradigm —the so-called Harmonic Background Paradigm. All of these explorations have been greatly influenced by Matt’s work.

Barcelo.pdf

1:00 PM → 2:30 PM Lunch- held at the ICTP Canteen

Lunch is held at the ICTP Canteen (https://www.ictp.it/building/leonardo-building/room/leonardo-building-cafeteria). The tables outside are reserved for our conference.

2:30 PM → 2:50 PM Ana Alonso Serrano - (Insights into the flow of information and the black hole information problem)

In this talk, I present our work on the information flow encoded in evaporation according to standard thermodynamics and on methods for performing a continuous entropy grouping (classical and quantum). I then present a similar analysis applied to the black hole information problem and explain how the study of a multipartite system can offer a new approach that does not introduce any new exotic physics on the path to solving the problem.

Alonso-Serrano.pdf

2:50 PM → 3:10 PM Alexander Kamenshchik - (Cosmological Singularities and Quantum Particles)

We study if there is an opportunity to describe quantum particles in the vicinity of three types of cosmological singularities, big bang-big crunch, big rip and big brake. Writing down the Dirac equation for spinors, and choosing a convenient parametrization for basis functions of the spinor field, we show that the corresponding second-order differential equation has two independent solutions which are non-singular in the case of all three types of singularities. That permits us to construct the Fock space for the spinor particles and to interprete this fact as their opportunity to cross these cosmological singularities. We compare the results of this study with previous results obtained for scalar particles.

Kamenshchik.pdf

3:10 PM → 3:30 PM Roberto Casadio - (Gravitational collapse and integrable singularities)

The collapse of a regular compact object could undergo a stage containing integrable curvature singularities that can be properly described only using quantum physics. I will present a model for this stage in which the inner horizon shrinks and disappears, possibly leaving behind a matter core of macroscopic size. A model for the latter based on the quantisation of dust particle trajectories will also be briefly reviewed.

Casadio.pdf

4:20 PM → 6:50 PM Social Activity

Thu, August 27

9:30 AM → 10:30 AM Francisco Lobo --- Sculpting Spacetimes: Matt Visser’s Journey from Thin-Shell Wormholes to Black-Bounces

Traversable wormholes provide a remarkable laboratory for probing the interplay between geometry, matter, stability, and causality in general relativity. Matt Visser has played a central role in transforming these objects from speculative constructions into precisely calculable geometries. After a brief historical introduction, we will focus on Matt’s thin-shell programme: cut-and-paste wormholes, Israel-Lanczos junction conditions, the localization and quantification of exotic matter, and the Poisson-Visser stability analysis. We will then discuss the geometric characterization of static and dynamical throats, chronology issues, generic thin shells and gravastars, and the more recent black-bounce programme, where regular black holes and traversable wormholes emerge within a common geometrical framework. The unifying theme is characteristic of Matt’s approach to physics: turning bold questions about spacetime into explicit calculations of geometry, exoticity, stability, and causal structure -- in short, “sculpting spacetimes” while keeping the physical interpretation of each geometrical construction fully explicit.

Lobo.pdf

10:30 AM → 11:00 AM Coffee Break

11:00 AM → 12:00 PM Ted Jacobson --- Guiding center quantization of a quantum Hall analog of Hawking radiation

The talk will revisit Michael Stone's quantum Hall analog of Hawking radiation, in which a Fermi sea of electrons occupying half of a plane, subjected to a quadrupolar electric potential, gives rise to analog Hawking radiation of chiral edge modes. I'll explain the classical guiding center theory, quantize it, and apply it in this setting. The quantum dynamics takes place on a non-commutative plane, where localization of the Fermi sea to the half-plane leads directly to the Hawking radiation on the edge. The radiation is thermal with respect to laboratory time, which is equal (up to a constant factor) to the boost angle in the analog Minkowski spacetime in which the chiral edge modes propagate, so in fact it corresponds to analog Unruh radiation.

Jacobson.pdf

12:00 PM → 1:00 PM Robert Mann --- Probing the Analog Vacuum

Amongst Matt Visser’s many and varied contributions to physics, a clear stand-out is his pioneering efforts in analog gravity. This once esoteric subject has now grown into a full-fledged sub discipline that involves a healthy and exciting interplay between theory and experiment. In recent years it has become appreciated that the vacuum state — the analog vacuum — can be used to test ideas in Relativistic Quantum Information, or RQI. The quantum vacuum is simulated by phononic surface fluctuations of a pancake-shaped Bose-Einstein condensate (the analog vacuum), coupled to a laser that can serve as a 2-level Unruh-DeWitt detector. After outlining the basics of RQI, I will describe RQI phenomena that could be tested using the analog gravity approach. These include the harvesting of entanglement from the analog vacuum and detector superposition. In this latter case, the response function corresponding of a detector in a superposition of locations and states of motion appears in the difference-photocurrent power spectrum. Operating beyond the standard quantum limit using squeezed light, the signal-to-noise ratio is estimated as larger than 10 for extracting the response function over a broad set of baseband frequencies.

1:00 PM → 2:00 PM Lunch- held at the ICTP Canteen

Lunch is held at the ICTP Canteen (https://www.ictp.it/building/leonardo-building/room/leonardo-building-cafeteria). The tables outside are reserved for our conference.

2:00 PM → 2:30 PM Poster Session

2:30 PM → 2:50 PM Antonia Micol Frassino - (Semi-classical black holes: recent advances and future prospects)

Semi-classical gravity provides a controlled framework for studying quantum effects in black hole physics in the absence of a complete theory of quantum gravity. In this talk, I will focus on recent developments in braneworld models of black holes, in which holography provides a way to account for the backreaction of quantum fields on the geometry. I will describe the resulting semiclassical black hole solutions and discuss how they can be used to investigate thermodynamic properties and geometric constraints beyond the classical regime. Particular attention will be given to the role of extended thermodynamics and to the constraints imposed by geometric inequalities. Finally, I will comment on some open directions and future developments motivated by these results.

Frassino.pdf

2:50 PM → 3:10 PM Lorenzo Pisani - (Semiclassical effects near the Cauchy horizon of a Reissner-Nordström black hole)

While a formal solution to the problem of renormalization in semiclassical gravity has been understood for several decades, efficient computational prescriptions for implementing the renormalization in black hole spacetimes have only been developed in recent years. Since computing the renormalized expectation value of the stress-energy tensor is an essential step in solving the semiclassical Einstein equations, it is important that we have efficient schemes for its numerical computation. Working within the framework of the Taylor-Breen-Ottewill extended coordinate method, we present an extension of that method to the interior of spherically symmetric black holes that possess a Cauchy horizon. We obtain the renormalized vacuum polarization and stress-energy tensor in the interior of a Reissner-Nordström black hole and derive their behavior on the Cauchy horizon, highlighting how the mass of the field affects the regularity of the renormalized vacuum polarization on this horizon.

Pisani.pdf

3:10 PM → 3:30 PM Marco de Cesare - (Gravity from a modified Bekenstein entropy law: cosmological consequences)

Different approaches to quantum gravity predict logarithmic corrections to the Bekenstein entropy. Generalizing Jacobson’s derivation of the gravitational field equations from spacetime thermodynamics, Alonso-Serrano and Liška have derived effective dynamics incorporating quantum gravity corrections. The resulting field equations are traceless, as in unimodular gravity, and feature a single free parameter which controls deviations from general relativity. We analyze the evolution of the cosmological background and primordial perturbations in this model, and discuss theoretical constraints.

deCesare.pdf

3:30 PM → 3:50 PM Marion Cromb - (Curved spacetimes and accelerated observers in experimental analogues)

In the Gravity Laboratory we use fluid systems to investigate curved spacetime and fundamental physics effects that are otherwise difficult to probe directly. Previously the group has used macroscopic water and superfluid vortices to measure analogues of black hole rotational superradiance. Now we aim to test the Unruh effect: the idea that an accelerating observer sees a quantum field - even the quantum vacuum - differently to an inertial one. We'll use the surface waves on a thin film of superfluid helium as our observed 'quantum field'. In particular I will discuss the different optical detection methods planned for such an experiment.

Cromb.pdf

3:50 PM → 4:20 PM Coffee Break

4:20 PM → 4:40 PM Jessica Santiago - (Primordial Black Holes, Charge, and Dark Matter: Rethinking Evaporation Limits)

Earlier studies investigating the allowed fraction of dark matter as primordial black holes (PBHs) tend to completely rule out PBHs with masses smaller than ~10^{-15} solar masses. This is due to the lack of evidence for Hawking radiation coming from the final evaporation stages of such small PBHs. These limits, however, make the key assumption that these PBHs can be modelled as uncharged, non-rotating Schwarzschild black holes. This talk concerns changes to these lower mass bounds when charge is included i.e., by going to Reissner–Nordström black holes as models for PBHs. In particular, the analysis presented here assumes a "dark" U(1) charge (assumed to be present in the early universe), and employs the Hiscock and Weems model for charged black hole evaporation to properly account for the Schwinger effect. By assigning a heavier dark electron mass and/or increasing its charge, this approach not only suppresses Hawking radiation but also mitigates Schwinger pair production, effectively extending the lifespan of PBHs beyond the age of the universe, bypassing the conventional wisdom that such small-mass black holes would have evaporated entirely by now.

Santiago.pdf

4:40 PM → 5:00 PM Ilia Musco - (New perspectives on primordial black hole formation)

Primordial black holes (PBHs) can form from the collapse of large cosmological perturbations in the early Universe. In this talk, I will discuss recent advances in our understanding of PBH formation in different cosmological scenarios, including the radiation-dominated era, the electroweak and QCD phase transitions, and phases dominated by a massless scalar field. The resulting PBH population can span a broad range of masses, making PBHs compelling candidates for dark matter, potential sources of gravitational waves from binary mergers in the lower mass gap, and possible progenitors of the supermassive black holes observed at high redshift.

Musco.pdf

5:00 PM → 5:20 PM Marco Bruni - (Supermassive BHs seeds from direct collapse of CDM-curvature peaks in ΛCDM)

In the first part of this talk I will present results from numerical relativity simulations of a toy-model cosmic web  of over-densities,   voids and filaments  with the Einstein Toolkit,  evolving from standard growing-mode perturbative CDM  initial conditions  in the matter era, at z ~ 300. I will show  how  the first collapse  of peaks of over-densities is very well predicted by the simple "top hat” model (AKA Oppenheimer-Sneyer collapse). Although this simple cosmic web structure doesn’t assume any symmetries, the collapse of peaks is quasi-spherical, which naturally  leads to the conjecture that a horizon can form very early. In the second part I will briefly present current numerical relativity efforts to identify a horizon enclosing these CDM peaks, as well as analytic results  from Galoppo, Bruni and Harada 2605.30145 (based on exact solutions of Einstein equations) showing  that supermassive black hole seeds can form  from the direct collapse of the peaks of overdensities in the CDM distribution, with masses M ~ 10^3-10^6 at z ~ 10, or even earlier.

Bruni.pdf

5:20 PM → 5:40 PM Diego Sáez-Chillón Gómez - (Compact objects physics in the realm of multi-messenger astronomy)

The reconstruction of the images of the supermassive objects at the centres of the M87 and Milky Way galaxies show two objects characterized by a central depression in luminosity (the so-called shadow) and a ring-shaped light structure around them. Both properties are closely linked to the emission intensity profile of the accretion disk and to the structure of spacetime. In addition, the detection of gravitational waves has made possible to study compact objects during the merger of binary systems of -mainly- black holes in order to test the Kerr family of solutions. In this talk, I will explain, on the one hand, the spacetime effects that influence the image obtained and how it may be used in the future to discern the existence of objects beyond the Kerr paradigm. Finally, I will also present the correspondence that exists between the shadow images and the spectrum of quasinormal modes of gravitational waves when such objects are perturbed. This is expected to play a fundamental role for the future multi-messenger astronomy of ultracompact objects.

Saez-Chillon.pdf

8:00 PM → 10:00 PM Social Dinner

Fri, August 28

9:30 AM → 10:30 AM Ralf Schützhold --- Stimulated Emission or Absorption of Gravitons by Light

We study the exchange of energy between gravitational and electromagnetic
waves in an extended Mach-Zehnder or Sagnac type geometry that is analogous
to an “optical Weber bar.”
In the presence of a gravitational wave (such as the ones measured by the
Laser Interferometer Gravitational Wave Observatory), we find that it should
be possible to observe (via interference or beating effects after a delay line)
signatures of stimulated emission or absorption of gravitons with present-day
technology.
Apart from marking the transition from passively observing to actively
manipulating such a natural phenomenon, this could also be used as a
complementary detection scheme.
Nonclassical photon states may improve the sensitivity and might even allow
us to test certain quantum aspects of the gravitational field.
[Phys. Rev. Lett. 135, 171501 (2025)]

schuetzhold.pdf

10:30 AM → 11:00 AM Coffee Break

11:00 AM → 12:00 PM Silke Weinfurtner --- From Analogue Spacetimes to Quantum Fields in the Laboratory: Twenty Years After a PhD with Matt

Nearly twenty years ago, during my PhD with Matt Visser in Wellington, New Zealand, I was introduced to the idea that fundamental questions in gravitation, cosmology and quantum field theory could be explored in laboratory systems. In this talk, I trace how three research directions that originated in those years, analogue models for massive scalar fields, rotating black-hole analogues, and analogue cosmological horizons, evolved into major strands of my later research. Looking back, these projects reveal the remarkable reach of ideas first explored during my PhD and the profound influence Matt has had on my scientific outlook and enduring fascination with quantum field theory in the laboratory.

12:00 PM → 1:00 PM Matt Visser --- Reminiscences and Retrodictions: Physical horizons versus mathematical horizons.

I shall first present some historical remarks, and then in the technical part focus on the physics of horizons.
The event horizons, and Cauchy horizons, of mathematical General Relativity are mathematical idealisations
that are not particularly well suited to questions that physicists and astronomers like to ask.
For very many purposes dynamical trapping horizons are much more suitable.
I shall give a brief overview of the current situation; with some (possibly controversial) hints for the future...

1:00 PM → 2:30 PM Lunch- held at the ICTP Canteen

Lunch is held at the ICTP Canteen (https://www.ictp.it/building/leonardo-building/room/leonardo-building-cafeteria). The tables outside are reserved for our conference.