5th EPS Conference on Gravitation

9 Dec 2024, 08:00 → 11 Dec 2024, 20:00 Europe/Amsterdam

Hall 206 (Czech Academy of Sciences, Prague)

R. Loll (Department of High-Energy Physics, Radboud University), A. Pravdová (Institute of Mathematics, Czech Academy of Sciences)

Aim and Motivation

General relativity is a beautiful, but highly complex theory in terms of its field content, invariant structure and the nature of its dynamical equations. Describing gravitational phenomena beyond a weak-field regime and in the absence of global symmetries requires refined computational methods. Progress in understanding black hole properties, binary systems, the early universe, and astrophysical environments depends critically on further developing our computational tool box. 

Unlocking Gravity Through Computation will bring together gravity researchers from different backgrounds. We are delighted to be hosting a broad range of keynote speakers working at the forefront of developing and applying numerical approximation and simulation techniques, as well as new analytical methods. We aim for a constructive and interactive meeting, including discussions on the future challenges and opportunities for computation in classical and quantum gravity. We invite the submission of abstracts for contributed talks and posters and encourage especially junior researchers to participate. 

Confirmed Invited Speakers

• Bernd Brügmann, Jena University
• Bianca Dittrich, Perimeter Institute
• Jakub Gizbert-Studnicki, Jagiellonian University
• Eva Hackmann, Bremen University
• Tanja Hinderer, Utrecht University
• Sascha Husa, University of the Balearic Islands
• Agnieszka Janiuk, Polish Academy of Sciences
• Igor Khavkine, Czech Academy of Sciences
• Benjamin Knorr, Nordita, Stockholm
• Tomáš Ledvinka, Charles University Prague
• Jean-Luc Lehners, Max-Planck-Institute for Gravitational Physics
• Jan Plefka, Humboldt University Berlin
• Maria J. Rodriguez, Utah State University & Autonomous University of Madrid
• Roland Steinbauer, Vienna University
• Agata Trovato, University of Trieste & INFN Trieste
• Niels Warburton, University College Dublin
• Toby Wiseman, Imperial College, London

Location and Venue

Prague is the capital and largest city of the Czech Republic, the historical capital of the Kingdom of Bohemia. The historic centre of Prague has been included in the UNESCO list of World Heritage Sites. Various notable physicists spent part of their career in Prague, including Johannes Kepler, Marcus Marci, Christian Doppler, Ernst Mach and Albert Einstein.The conference will take place close to Prague's city centre in one of the conference halls of the Academy of Sciences of the Czech Republic, built in the mid 19th century.

5thEPS_poster.pdf

Monday 9 December

08:00 → 08:45 Registration

08:45 → 09:00 Introduction & Welcome: Renate Loll, Mairi Sakellariadou, Alena Pradová

09:00 → 09:35 Invited talk (30 + 5 min) Jean-Luc Lehners

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.

Speaker: Jean-Luc Lehners (MPI Gravitational Physics)

PragueEPS_Lehners_241209.pdf

09:35 → 10:10 Invited talk (30 + 5 min) Bianca Dittrich

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.

Speaker: Bianca Dittrich (Perimeter I)

2024 Prague-BDitrich.pdf

10:10 → 10:40 Coffee break

10:40 → 11:15 Invited talk (30 + 5 min) Benjamin Knorr

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.

Speaker: Benjamin Knorr (Heidelberg U)

Knorr_AS-overview_and_comp_challenges.pdf

11:15 → 11:50 Invited talk (30 + 5 min) Jakub Gizbert-Studnicki

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.

Speaker: Jakub Gizbert-Studnicki (Jagiellonian U)

JGizbert Studnicki_Prague_2024.pdf

11:50 → 13:20 Lunch

13:20 → 13:55 Invited talk (30 + 5 min) Tanja Hinderer

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.

Speaker: Tanja Hinderer (Utrecht U)

Hinderer.pdf

13:55 → 14:55 Contributed talks (15+5 min each)

13:55 Unlocking the spin foam path integral

Spin foam models are a non-perturbative, background independent approach to quantum gravity derived from general relativity formulated as a constrained topological field theory. The sum over histories is codified in group theoretic data associated to a triangulation, which can be understood as area and angle variables. Extracting physics from these models requires efficient numerical techniques. In this talk I will give a brief overview of the models and outline the numerical challenges. I will present the progress in recent years in computing its fundamental amplitudes and discuss symmetry-reduced cosmological models as first tests to make contact with (semi-)classical physics.

Speaker: Sebastian Steinhaus (FSU Jena)

Steinhaus_unlocking_spin_foams.pdf

14:15 Open issues in the construction of non-singular black holes

Within the context of general relativity, black holes must contain a singular core. It is widely believed that singularities are not truly a physical feature, but rather a sign indicating the breakdown of the general relativistic description. Among the possible non-singular scenarios, regular black holes and or horizonless ultracompact stars have taken a particular preeminence. In this talk, I am going to discuss generic instability mechanisms that must be addressed to make these objects viable models of non-singular spacetimes.

Speaker: Francesco Di Filippo (Charles U)

Di Filippo.pdf

14:35 Do we live inside a Hayward black hole?

I will discuss a (quantum mechanically) modified model for the Oppenheimer-Snyder collapse scenario where the exterior of the collapsing dust ball is a Hayward black hole spacetime and the interior is a dust Friedmann-Robertson-Walker cosmology. This interior cosmology is entirely determined by the junction conditions with the exterior black hole. It turns out to be non-singular, displaying a power-law contraction which precedes a de Sitter phase or, reversely, a power-law expansion followed by a de Sitter era. We will learn that (cosmic) inflation in the time-reversed setting is a (quantum) mechanism that decelerates collapsing matter and prevents it from singularity formation. I will also discuss the global causal structure, in particular, the numerically computed conformal diagram and the viability of the model.

Speaker: Michal Bobula (U Wroclaw)

prague_talk_bobula.pdf

14:55 → 15:25 Coffee break

15:25 → 16:00 Invited talk (30 + 5 min) Jan Plefka

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.

Speaker: Jan Plefka (Humboldt U, Berlin)

plefka WQFT_Prague.pdf

16:00 → 17:00 Contributed talks (15+5 min each)

16:00 Linear perturbations of the Kerr spacetime in quadratic gravity

The extension of General Relativity by including quadratic curvature invariants in the Einstein-Hilbert action is one of the most extensively studied alternative gravity theories today. In this contribution, we employ the quadratic gravity field equations, formulated using the Newman-Penrose formalism, to analyse extensions of the Kerr black hole spacetime going beyond GR. We derive constraints on the linear gravitational perturbations of this background. Here, we summarise the key concepts leading to the general form of these constraints, while detailed explicit examples will be presented in a separate contribution.

Speaker: Robert Svarc (Charles U)

svarcQG_Kerr_perturbation.pdf

16:20 Hamiltonian charges and boundary effects in gravitational systems

In this work, we compute the Hamiltonian surface charges of gravitational systems under a range of conservative boundary conditions, including Dirichlet, Neumann, and York’s mixed boundary conditions, where the conformal induced metric and the trace of the extrinsic curvature are fixed. We demonstrate that for all these boundary conditions, the canonical approach produces results consistent with those obtained from covariant phase space methods enhanced by an added boundary Lagrangian—a recently proposed technique in the literature, which our findings support. This study further explores the impact of boundary conditions on energy, showing that both quasi-local and asymptotic energy expressions are sensitive to the choice of boundary condition. Additionally, our approach suggests a novel integrable charge for the Einstein-Hilbert Lagrangian that deviates from the Komar charge when considering non-Killing and non-tangential diffeomorphisms, offering a potential alternative in such contexts.

Speaker: Gloria Odak (Charles U)

Odak PragueDec24.pdf

16:40 Covariant phase spaces beyond diffeomorphism invariance

The covariant phase space construction first introduced by J. Lee and R. Wald represents a versatile computational tool. It allows us to compute Noetheresque conserved quantitites in a number of different settings, and also to straightforwardly compare various theories of gravity. In my talk, I show how to extend this formalism beyond the class of diffeomorphism invariant theories, in particular to theories invariant only under volume-preserving (transverse) diffeomorphisms. Such theories require the introduction of some background, non-dynamical structure, which has unexpected effects on the covariant phase space formalism. Nevertheless, all the results available for the diffeomorphism invariant theories, e.g. the expressions for mass and for black hole entropy, have a direct analogy in this setting.

Speaker: Marek Liška (Dublin IAS)

Liska 5th EPS.pdf

17:00 → 18:00 Round table: Day 1 (Convener: Dionysios Anninos)

18:00 → 19:00 Poster session & Reception

Tuesday 10 December

09:00 → 09:35 Invited talk (30 + 5 min) Agata Trovato

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.

Speaker: Agata Trovato (U Trieste, INFN Trieste)

Trovato_EPSGravity.pdf

09:35 → 10:10 Invited talk (30 + 5 min) Bernd Brügmann

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.

Speaker: Bernd Brügmann (U Jena)

10:10 → 10:40 Coffee break

10:40 → 11:15 Invited talk (30 + 5 min) Roland Steinbauer

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.

Speaker: Ronald Steinbauer (U Vienna)

eps5-steinbauer.pdf

11:15 → 11:50 Invited talk (30 + 5 min) Igor Khavkine

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.

Speaker: Igor Khavkine (Czech Academy of Sciences)

khavkine-eps5.pdf

11:50 → 13:20 Lunch

13:20 → 13:55 Invited talk (30 + 5 min) Bičák Memorial Talk (Oldrich Semerák, Pavel Krtouš and David Kubiznak)

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.

13:55 → 14:55 Contributed talks (15+5 min each)

13:55 Homogenous symmetry operators in Kerr-NUT-AdS spacetimes

Kerr-NUT-AdS spacetimes possess hidden symmetries encoded in the so-called principal Killing-Yano tensor. Focusing on 4 dimensions, we present a number of symmetry operators for scalar, vector, and tensor perturbations, that are of degree 2 (to be defined) and homogeneous in the principal tensor.

Speaker: David Kubiznak (Charles U)

UnlockingGravity_Kubiznak_Final.pptx

14:15 Post-merger gravitational wave signals from binary neutron stars: effect of the magnetic field

The oscillation modes of neutron star (NS) merger remnants, as encoded by the kHz post-merger gravitational wave (GW) signal, hold great potential for constraining the as-yet undetermined equation of state (EOS) of dense nuclear matter. Previous works have used numerical relativity simulations to derive quasi-universal relations for the key oscillation frequencies, but most of them omit the effects of a magnetic field. We conduct fully general-relativistic, magnetohydrodynamics simulations of NSNS mergers with two different masses, two different EOSs (SLy and ALF2), three different magnetic field topologies just prior to merger (pure poloidal and pure toroidal, both confined to the interior, and "pulsar-like" poloidal extending from the interior to the exterior), and with four different initial magnetic field strengths. We find that magnetic braking and magnetic turbulent viscosity drive the merger remnants towards uniform rotation and increase the overall angular momentum loss. This loss causes the remnant to contract, resulting in a time-dependent increase in the fundamental quadrupole f2 GW frequency. The overall shift is up to ~ 200 Hz for the strongest magnetic field considered and ≲ 50 Hz for the median case. This shift will be degenerate with competing effects coming from the equation of state, mass ratios, and prior spins. Isolating an f2 ≲ 100 Hz shift from any individual effect will be challenging for current or future GW observations, which should include the magnetic field in their analyses.

Speaker: Milton Ruiz (U Valencia)

ruiz EPS_conference.pdf

14:35 Compact binary formation history from cross-correlation studies

Binary compact object mergers are the result of typically tens to hundreds of million years of evolution. Such evolution follows specific pathways, the details of which at current state-of-art are not yet completely understood. These gaps in our understanding are expected to leave residual ambiguities in the prediction of the astrophysical merger rate which can potentially turns out to bias any population study which uses the merger rate as input. These astrophysical biases can be picked up in cross-correlation (XC) studies. On the astrophysical side, I will investigate two aspects of the compact binary formation channel which are not yet unambiguously understood. I will begin with a study of the effect of mass transfer function. This function relates progenitor and remnant mass across the endpoint of stellar lifespan, and I will use two different transfer functions to quantify the modelling bias. Moving on, I will also investigate another aspect of the formation channel, the so called 'delay function'. Here again, I will consider and contrast four different delay functions. In each case I will discuss implications w.r.t current and upcoming network of GW detectors. XCs between species can also be boosted by the use of suitable techniques. I will also discuss how different beaming functions can boost the XCs between detected GW events and large scale structure. Once again, the implications with different detector networks will be discussed.

Speaker: Kabir Chakravarti (Czech Academy of Sciences)

talk_EPS-Prague_Kabir chakravarti.pdf

14:55 → 15:25 Coffee break

15:25 → 16:00 Invited talk (30 + 5 min) Toby Wiseman

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.

Speaker: Toby Wiseman (Imperial College, London)

TalkWiseman.pdf

16:00 → 17:00 Contributed talks (15+5 min each)

16:00 Repeating nuclear transients as Extreme-Mass Ratio Inspirals counterparts

In recent years, a mysterious new class of astrophysical objects has been uncovered. These objects are spatially coincident with the nuclei of external galaxies and exhibit X-ray variations that repeat over timescales ranging from minutes to months. They manifest in three distinct ways in the data: stable quasi-periodic oscillations (QPOs), quasi-periodic eruptions (QPEs), and quasi-periodic outflows (QPOuts). QPOs are systems that display smooth, recurrent variations in X-ray brightness, while QPEs are characterized by sudden changes resembling eruptions. QPOuts represent systems that show repeating outflows moving at mildly relativistic speeds of approximately $0.1-0.3c$, where c is the speed of light. The underlying physical mechanisms driving these phenomena, collectively called as repeating nuclear transients (RNTs), are a topic of intense debate, with most models suggesting that they originate from either instabilities within the inner accretion flow or from orbiting objects. There is significant excitement surrounding the latter class of models, as it has been proposed that some repeating systems could host extreme mass-ratio inspirals, potentially detectable by upcoming space-based gravitational wave interferometers. This could pave the way for a new era of “persistent” multi-messenger astronomy. I will briefly summarize the most important observational facts about RNTs and discuss the most promising EMRI scenarios, also presenting supporting GRMHD simulations.

Speaker: Petra Suková (Czech Academy of Sciences)

EPS_Praha_December_2024_Sukova.pdf

16:20 Spin signatures on the solution and emission spectra of accretion flows around black holes

We investigate accretion flows around rotating black holes (BHs) and obtain self-consistent transonic solutions in full general relativistic prescription. The flow is assumed to be viscous and radiative. Viscosity helps in the removal of angular momentum outwards, allowing matter to get accreted inwards. In addition, viscous heat dissipated makes the matter hotter. On the other hand, radiation mechanisms like bremsstrahlung, synchrotron, and their inverse-Comptonisations cools down the matter. Thus, the solution depends highly on the interplay between heating and cooling processes. In our work we investigate the entire energy–angular momentum parameter space and obtain both shocked and shock-free accretion solutions. Because of the spin in Kerr black holes, the event horizon is dragged to a region <2GM/c^2, increasing the efficiency of accretion process. Ample of works showed a rotating BH to yield high temperature solutions compared to a Schwarzschild BH. This suggests higher emission. Interestingly we have found a distinct annihilation line present only in extremely rotating BHs arising from regions very close to the central object. We have investigated further the other effects of spin on the spectrum obtained from accretion flows around BHs. We find efficiencies reaching >30% for maximally rotating BHs.

Speaker: Shilpa Sarkar (Harish Chandra Research I)

ShilpaSarkar_EPS-compressed.pdf

16:40 Measuring the spacetime of SMBH with pulsar timing

Future observations with next-generation radio telescopes are expected to discover radio pulsars closely orbiting around Sagittarius A (Sgr A), the supermassive black hole (SMBH) at our Galactic Center (GC). Such a system can provide a unique laboratory for measuring the spacetime of SMBH and testing gravity theories. We provide a numerical timing model for the pulsar-SMBH systems based on the post-Newtonian (PN) equation of motion and use it to explore the prospects of measuring the BH properties with pulsar timing. We forecast the measurement precision of BH spin and quadrupole moment and thus the test of the No-hair theorem in GR. We further investigate the possibility of probing the vector charge of Sgr A* in the bumblebee gravity model. Related paper: https://doi.org/10.1103/PhysRevD.108.123034; https://doi.org/10.1088/1475-7516/2024/04/087.

Speaker: Zexin Hu (Tübingen U)

Zexin Hu-Measuring the spacetime of SMBH with pulsar timing.pdf

17:00 → 18:00 Round table: Day 2 (Convener: Toni Font)

19:15 → 20:15 Organ Concert for Conference Participants

20:15 → 22:30 Conference dinner

Wednesday 11 December

09:00 → 09:35 Invited talk (30 + 5 min) Sascha Husa

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.

Speaker: Sascha Husa (U Balearic Islands)

Prague2024_Husa.pdf

09:35 → 10:10 Invited talk (30 + 5 min) Agnieszka Janiuk

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.

Speaker: Agnieszka Janiuk (Polish Academy of Sciences)

EPS_AJaniuk.pdf

10:10 → 10:40 Coffee break

10:40 → 11:15 Invited talk (30 + 5 min) Maria J. Rodriguez

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.

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

rodriguez Prague Talk 2024.pdf

11:15 → 11:50 Invited talk (30 + 5 min) Niels Warburton

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.

Speaker: Niels Warburton (University College Dublin)

Warburton-EPS-Prague.pdf

11:50 → 13:20 Lunch

13:20 → 13:55 Invited talk (30 + 5 min) Eva Hackmann

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.

Speaker: Eva Hackmann (ZARM, U Bremen)

hackman 2024_12_EPS_Prague.pdf

13:55 → 14:55 Contributed talks (15+5 min each)

13:55 Solving Teukolsky equation to compute extreme mass ratio inspirals

Teukolsky equation allows the study of perturbations of the Kerr metric in linear approximation. We shall discuss how the solution of this equation can be used to compute the evolution of an extreme mass ratio inspiral, where a stellar compact object inspirals towards a supermassive black hole due to gravitational radiation reaction. We will address both time domain and frequency domain approaches solving Teukolsky equation.

Speaker: Georgios Loukes-Gerakopoulos (Czech Academy of Sciences)

5EPS_Grav_ggloukes.pdf

14:15 An effective-one-body model for large-mass-ratio black hole binaries

The effective-one-body (EOB) approach is a powerful formalism that maps the two-body problem in general relativity into the motion of a single body in an effective metric. EOB-based waveform models are nowadays providing fast and accurate templates for comparable-mass coalescing compact binaries, where the latter are the events observed by the currently operating detectors. Third-generation detectors will instead allow us to detect different signals, like the ones coming from black hole binaries with a larger mass ratio. I will present an EOB waveform model that has been adapted to better suit the large-mass-ratio regime, that also includes eccentricity and precessing spins.

Speaker: Angelica Albertini (Charles U)

Albertini_EPS_Dec24.pdf

14:35 Post-Minkowskian self-force in the low-velocity limit: scalar field scattering

Self Force Analytical computations has been used in the past years as an useful tool to calibrate the numerical simulations of waveforms for EMRIs. At the same time, in the field of Scattering Amplitudes, a lot of parallel work has been done in trying to compute observables related to the unbound two-bodies scattering. It has also emerged a theoretical argument that shows how it is possible to map information from scattering orbits results to the bounded case, through the so-called unbound-to-bound mapping. Because of this, also in the Self Force community, it is increased the interest in the description of the unbound motion. I will present the approach to compute analytic correction to the unbound motion in the Self-Force formalism, relying on a double Post-Minkowskian and Post-Newtonian (low velocity) expansion. We have developed a general strategy to study both gravitational and non-gravitational Self Force for any type of orbit that lies on the equatorial plane. We demonstrate our approach by computing the Self Force for a scalar charge that scatters off a Schwarzschild black hole. These results have been obtained up to 5 Post-Minkowskian and 4.5 Post-Newtonian level, showing how it is possible, with this methodology to get to the state of the art of Post-Minkowskian results. For what concerns the Post-Newtonian expansion, I will show what are the main difficulties to get to higher orders, which are fundamental for the comparison with numerical results. In conclusion, I will present some recent development in the gravitational case.

Speaker: Davide Usseglio (SSM Napoli)

EPS_Prague_Usseglio.pdf

14:55 → 15:25 Coffee break

15:25 → 16:00 Invited talk (30 + 5 min) Tomáš Ledvinka

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

Speaker: Tomáš Ledvinka (Charles U)

EPS_Ledvinka.pdf

16:00 → 17:00 Contributed talks (15+5 min each)

16:00 Searching for gravitational waves in real LIGO noise using neural networks

Conventional searches for gravitational wave signals in detector data are computationally demanding and struggle when certain transient noise sources are present. Recently, machine-learning algorithms have been proposed to address current and future challenges. We present a neural-network based algorithm to search for binary black hole waveforms. We also apply our algorithm to real O3b data and recover the relevant events of the GWTC-3 catalog.

Speaker: Ondřej Zelenka (Czech Academy of Sciences)

zelenka_slides.pdf

16:20 Reconstruction of the WLP metric for stationary and axially symmetric gravitation perturbation

The study of black hole perturbations typically follows two main approaches: the direct metric perturbation and perturbation via the Newman-Penrose (NP) or Geroch-Held-Penrose (GHP) formalism. In the latter case, a reconstruction method is required, such as the Debye (or Hertz) potential method, to obtain the corresponding metric perturbation. However, the reconstructed metric is expressed in the radiation gauge, which is not always optimal. In this paper, we analyze stationary and axially symmetric perturbations of the Kerr black hole within both frameworks and derive an explicit gauge transformation between the radiation gauges and the standard Weyl-Lewis-Papapetrou (WLP) form of the metric. Additionally, we express the linearised metric coefficients of the WLP metric directly in terms of the Debye potential.

Speaker: David Kofron (Charles U)

2024_5thEPS, Kofron.pdf

16:40 Quasinormal modes of black holes encircled by a disk

When a black hole rings due to some external perturbation it emits gravitational waves described by quasi normal modes (QNMs) – a series of exponentially damped harmonic oscillations. If the final state is fully described by the Kerr(-Newman) metric, this ring-down carries a unique signature of its parameters encoded in the modes' complex frequencies. However, astrophysical black holes are rarely isolated. Due to attraction forces, black holes bound matter from their environment, often resulting in the formation of disc-like structures. I present our work, where we studied the quasinormal modes of a black hole surrounded by gravitating thin disc. We found that within the physical parameter space, the presence of the disc universally reduces oscillating frequencies while prolonging dumping time. If such a universal relation holds generically, it may help distinguish shifts in quasinormal modes caused by environmental effects (such as the presence of an accretion disc) from those predicted by other putative effects from theories beyond GR.

Speaker: Petr Kotlařík (Czech Academy of Sciences)

kotlarik eps-prague-2024.pdf

17:00 → 18:00 Round table: Day 3 (Convener: Philippe Jetzer)

18:20 → 19:20 Guided City Walking Tour for Conference Participants