Invited talk (30 + 5 min) Jean-Luc Lehners

• Jean-Luc Lehners (MPI Gravitational Physics)

Room: Hall 206 Title: Complex metrics and the no-boundary wave function Abstract: I will provide an introduction to the no-boundary wave function, and review its implementation in simple inflationary models. Earlier treatments indicated that the no-boundary wave function would favour very short inflationary phases, in strong tension with observations. Recent investigations have shown the need for a more careful definition, especially with regards to the question of which metrics and field configurations should be summed over in gravitational path integrals. This leads both to a deeper understanding of gravitational path integrals, and to the consequence that, under certain circumstances, long inflationary phases are required. I will discuss the implications for observational predictions, in particular for the size of the universe, and I will comment on the relation to string theory.

Invited talk (30 + 5 min) Bianca Dittrich

• Bianca Dittrich (Perimeter I)

Room: Hall 206 Title: From spacetime quanta to area metric dynamics Abstract: Spin foams constitute a path integral approach to quantum gravity, based on a rigorous notion of quantum geometry, resulting in the notion of space time quanta. I will present recent progress on understanding the continuum limit of spin foams. This includes the proposal of gravity described by an area metric action, arising as an effective continuum description for spin foam dynamics. This opens the door to the exploration of phenomenological effects of spin foams.

Invited talk (30 + 5 min) Benjamin Knorr

• Benjamin Knorr (Heidelberg U)

Room: Hall 206 Title: Tackling quantum gravity non-perturbatively Abstract: In this talk I will describe asymptotically safe quantum gravity: an approach to quantum gravity that is based solely on non-perturbative quantum field theory. After a brief introduction, I will discuss achievements and challenges of the programme.

Invited talk (30 + 5 min) Jakub Gizbert-Studnicki

• Jakub Gizbert-Studnicki (Jagiellonian U)

Room: Hall 206 TItle: Causal Dynamical Triangulations. From the infrared to the ultraviolet Abstract: One of key questions of quantum gravity is whether it can be formulated as a non-perturbatively renormalizable quantum field theory valid up to arbitrarily large energy scale as predicted by the, so-called, asymptotic safety conjecture. Causal Dynamical Triangulations (CDT) is a lattice quantum field theory of gravity based on the formalism of Regge Calculus and Feynman path integrals. It is studied by numerical Monte Carlo simulations. In my talk I will briefly introduce the four-dimensional CDT and present its most important results, including the non-trivial phase structure and the dynamically emerging semiclassical universe. I will discuss how one can define the infrared and ultraviolet limits of CDT and thus provide evidence in favour of asymptotic safety.

Invited talk (30 + 5 min) Tanja Hinderer

• Tanja Hinderer (Utrecht U)

Room: Hall 206 Title: Tidal and higher curvature effects in gravitational waves from black hole binary binary inspirals in quadratic gravity Abstract: A major science goal with gravitational waves (GWs) from merging black holes is to probe strong-field gravity in nonlinear, dynamical regimes. This requires accurate predictions of gravitational waveforms in theories beyond General Relativity to understand potential signatures of new gravitational physics, degeneracies with other effects, and gain deeper insights into the information on fundamental gravitational physics contained in various GW parameters. This talk will focus on an example of quadratic gravity theories where black holes develop scalar hair. I will discuss the effects of higher-curvature nonlinearities and tidal interactions on the GW signals from binary inspirals and consequences of including self-interactions of the scalar field in the theory. I will conclude with an outlook to future prospects and remaining challenges, as larger and more precise gravitational-wave dataset will become available and their science exploitation will start to be limited by the theoretical modeling.

Invited talk (30 + 5 min) Jan Plefka

• Jan Plefka (Humboldt U, Berlin)

Room: Hall 206 TItle: High-precision black hole scattering from worldline quantum field theory Abstract: Predicting the outcome of scattering processes of elementary particles in colliders is the central achievement of relativistic quantum field theory applied to the fundamental (non-gravitational) interactions of nature. While the gravitational interactions are too minuscule to be observed in the microcosm, they dominate the interactions at large scales. As such the inspiral and merger of black holes and neutron stars in our universe are now routinely observed by gravitational wave detectors. The need for high precision theory predictions of the emitted gravitational waveforms has opened a new window for the application of perturbative quantum field theory techniques to the domain of gravity. Here we report on a new highest-precision analytical result for the scattering angle, radiated energy, and recoil of such a black hole or neutron star scattering encounter at the fifth order in Newton’s gravitational coupling G, assuming a hierarchy in the two masses. This is achieved by synergistically porting state-of-the-art techniques for the scattering of elementary particles in colliders to this classical physics problem in our universe. Our results show that in the radiated energy a new class of mathematical functions related to Calabi-Yau manifolds appear for the first time in nature, which so far have been studied in mathematics and compactifications within string theory.

Invited talk (30 + 5 min) Agata Trovato

• Agata Trovato (U Trieste, INFN Trieste)

Room: Hall 206 Title: Machine-learning in gravitational wave analyses Abstract: Machine-learning (ML) techniques are now widely used in many fields of research, including Gravitational Wave (GW) analyses of many different types. In same cases, ML algorithms are used to produce the published LIGO-Virgo-KAGRA results. Besides that, a long list of proposed applications can be found in the literature. I will try to summarise the main use cases and focus one some interesting examples.

Invited talk (30 + 5 min) Bernd Brügmann

• Bernd Brügmann (U Jena)

Room: Hall 206 Title: Gravity, computation, and machine learning - Numerical methods for relativity Abstract: Numerical relativity started with the goal to solve Einstein's equations on the computer, which translates into mathematical and numerical methods for partial differential equations, and simulations on supercomputers. Current efforts have expanded into multi-physics simulations that are by now directly connected to observations, in particular gravitational waves and multimessenger events like kilonovae. Alternative models beyond astrophysics are being explored as well, including e.g. boson stars. We will give a compact review of the numerical issues involved, and we will also mention in passsing recent applications of machine learning to gravitational wave data analysis.

Invited talk (30 + 5 min) Roland Steinbauer

• Ronald Steinbauer (U Vienna)

Room: Hall 206 Title: Synthetic curvature for GR and beyond Abstract: Synthetic methods have profoundly transformed Riemannian geometry in recent decades, extending core concepts and results beyond smooth manifolds. More precisely, bounds on sectional and Ricci curvature, using triangle comparison and optimal transport respectively, have proven to be so robust as to be independent of any differentiable structure. Recently, the foundations for an analogous synthetic Lorentzian geometry have been laid based on the core notion of Lorentzian length spaces. This talk explains the basics of this new geometry, outlines initial results, and explores potential applications in general relativity and discrete approaches to quantum gravity.

Invited talk (30 + 5 min) Igor Khavkine

• Igor Khavkine (Czech Academy of Sciences)

Room: Hall 206 Title: Invariants: a perspective for mathematical and numerical relativity Abstract: Curvature invariants have long been a focus of the mathematical classification of Riemannian and Lorentzian geometries, but have been outside of the primary interest in numerical and mathematical relativity. I will discuss how invariants could help with some interesting questions in those areas that would be difficult to answer in other ways.

Invited talk (30 + 5 min) Bičák Memorial Talk (Oldrich Semerák, Pavel Krtouš and David Kubiznak) Room: Hall 206 Abstract: Oldrich Semerák, Pavel Krtouš and David Kubiznak will recall the life and work of Jiří Bičák, an internationally well-known and influential scholar of general relativity, relativistic astrophysics and cosmology and former professor at Charles University, who died in January 2024.

Invited talk (30 + 5 min) Toby Wiseman

• Toby Wiseman (Imperial College, London)

Room: Hall 206 Title: Dynamical evolution of massive gravity: analytic and numerical progress Abstract: Ghost free massive gravity is a very interesting and well motivated modification of Einstein's gravity that simply adds a mass to the graviton. While much studied, until recently there was little progress on understanding whether the theory behaves as GR does on scales well below the graviton mass, as must be the case for it to pass observational tests such as those at solar system scales. One of the main stumbling blocks was that gravitational collapse could not be studied as there was no practical dynamical formulation of the theory (ie. an analog of ADM for GR). I will review recent progress on a new dynamical formulation, its well posedness, and show results from computations of spherically symmetric gravitational collapse. The results are interesting and reveal a subtle theory that will require 3+1 numerical studies to properly test its viability.

Invited talk (30 + 5 min) Sascha Husa

• Sascha Husa (U Balearic Islands)

Room: Hall 206 Title: Modelling the gravitational waves from compact binary coalescence Abstract: In this talk I will review the challenges and state of the art of modelling the gravitational wave signals from the coalescence of compact binaries - the only type of gravitational wave signal that has been identified to date. The talk will in particular focus on the challenges to solve the Einstein equations numerically, and to provide computationally efficient waveform models across the parameter space of astrophysically plausible sources.

Invited talk (30 + 5 min) Agnieszka Janiuk

• Agnieszka Janiuk (Polish Academy of Sciences)

Room: Hall 206 Title: Relativistic MHD simulations of merging and collapsing stars Abstract: Compact binary mergers and the collapse of massive stars can produce intense transients observable across high-energy wavelengths. Events such as gamma-ray bursts and kilonova emissions are often accompanied by gravitational wave detections, making them crucial sources for multi-messenger astrophysics. To explore these phenomena theoretically, state-of-the-art approaches like numerical relativity and GR magnetohydrodynamic simulations are used. In this talk, I will review the current progress in simulations of mergers and collapsars, and present recent findings from my team, achieved using European High-Performance Computing facilities.

Invited talk (30 + 5 min) Maria J. Rodriguez

• Maria J. Rodriguez (Utah State U & UA Madrid)

Room: Hall 206 Title: Revisiting tidal deformations in black holes Abstract: Recent advancements suggest that tidal forces, traditionally expected to vanish in extremal black holes, may play a more fundamental role than previously thought. In this talk, we review the tidal deformations of black holes, focusing on the role of Love numbers in characterizing these deformations, and explore the unique properties of tidal interactions in extremal black holes.

Invited talk (30 + 5 min) Niels Warburton

• Niels Warburton (University College Dublin)

Room: Hall 206 TItle: Gravitational waveform models for extreme mass ratio binaries via the self-force approach Abstract: Extreme mass ratio binaries, where a stellar-mass compact object orbits a supermassive black hole, are key sources of gravitational waves for the future LISA mission. Detecting and characterising these sources requires template models that are rapid to generate and accurately track the phase evolution of the binary. In this talk I will overview how the perturbative "self-force" approach can be used to model these binaries. I will also highlight how recent results pushing the expansion to second-order in the mass ratio allows modelling of intermediate mass-ratio binaries with mass ratios as small as ~10:1.

Invited talk (30 + 5 min) Eva Hackmann

• Eva Hackmann (ZARM, U Bremen)

Room: Hall 206 Title: General relativistic geodesy Abstract: The intersection of General Relativity and geodesy represents a new frontier in Earth sciences. A major task of geodesy is to determine the gravity field of the Earth, e.g. to monitor mass variations. Due to recent advancements in high precision clock comparison, General Relativity introduced an entirely new measurement concept to geodesy based on the gravitational redshift. We present the basics of a genuinely general relativistic framework for geodesy, generalising the traditional (post-)Newtonian geodetic concepts. Moreover, we outline the analytical and computational tools used to explore these new exciting applications of clocks on ground and in space.

Invited talk (30 + 5 min) Tomáš Ledvinka

• Tomáš Ledvinka (Charles U)

Room: Hall 206 Title: Near-critical spacetimes of collapsing axisymmetric gravitational waves Abstract: The process of creating a black hole by collapsing gravitational waves is a long-studied problem in numerical relativity, particularly as a pure-gravity model of critical collapse. One of the reasons for the rather slow progress in understanding critical collapse of gravitational waves is that usual hyperbolic slicing conditions, such as the 1+log slicing, break down in this context. We show that once the breakdown of coordinate conditions is overcome, we can study several families of axisymmetric asymptotically flat initial data families, for which a strength parameter can be fine-tuned between dispersal into empty space and collapse into a black hole, similarly to the well-known discovery by Choptuik. We find that such near-critical spacetimes exhibit behavior similar to scalar-field collapse: for different families of initial data, we observe universal “echoes” represented by approximate scaled copies of the same spacetime region. In contrast to the very regular behavior observed in the collapse of spherically symmetric massless