Theoretical Tools for Gravitational Wave Physics

Nov 3, 2025, 8:40 AM → Nov 5, 2025, 5:00 PM Europe/Zurich

E51, HIT building (ETH, Zurich)

 

As we progress into the era of gravitational wave (GW) astronomy, robust theoretical frameworks are essential for interpreting data and uncovering new physics. This workshop is dedicated to the deployment of tools from theoretical physics for GW phenomenology.

Key topics include:

1. Modern theoretical approaches for modeling GWs from compact binaries, such as effective field theories and techniques inspired by quantum field theory.

2. Tidal response and quasi-normal modes of compact objects, which provide insights into their internal structure and offer potential signatures of new physics.

3. Gravitational wave memory effects, linked to asymptotic symmetries and soft graviton theorems, provide further tests of GR and insights into cosmology.

Bringing together experts from across gravitational theory and related fields, the workshop aims to deepen our theoretical understanding required to fully exploit the potential of incoming GW observations.

List of invited speakers: Paolo Creminelli Stefano Foffa Carlo Heissenberg Lam Hui Alex Kehagias Justin Khoury Alessandro Podo Radu Roiban Luca Santoni Laura Sberna Chia-Hsien Shen Enrico Trincherini Ingrid Vazquez-Holm Flippo Vernizzi Jann Zosso

Organisers: Davide Racco Borna Salehian Giovanni Tambalo Leonardo Senatore

 

Recording of the talks are also available on Youtube.

Security Warning: Please be cautious of unofficial emails offering accommodation or other services. All official workshop communication will only come from the organizers. If in doubt, please contact us directly.

 

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Mon, November 3

1 Effective field theory post-Newtonian approach to the two-body inspiral dynamics

Over the past two decades, ideas and techniques originating from particle physics—and more broadly from quantum and classical field theory—have been increasingly and successfully applied to the study of compact binaries in general relativity.
Non-Relativistic General Relativity is one of the precursors of this trend; in this talk I will describe its contributions to the field from the simplest cases to the latest developments.

Speaker: Stefano Foffa (U. Geneva)

FoffaETH25.pdf

Recording

Break: Coffee Break

2 Overview of QFT methods in GR

Abstract TBA

Speaker: Radu Roiban (Pennsylvania State University)

Recording

Lunch

3 Black hole modes

I will discuss black hole modes (quasinormal and quasibound/superradiant), their properties (orthogonality and (in)completeness) and their application in modelling the black hole ringdown and scalar clouds.

Speaker: Laura Sberna (Nottingham U.)

BHmodes_lsberna.pdf

Recording

Lightning talks

Conveners: Anna Chrysostomou, E. Şeyma Kutluk, Mateja Boskovic, Sk Jahanur Hoque

4 Worldline effective field theory for gravitational atoms

Superradiant instabilities of rotating black holes can give rise to extended bosonic clouds surrounding them - gravitational atoms. These long-lived structures serve as natural laboratories for probing a wide range of parameter space for putative ultralight bosons in nature. The presence of a companion can significantly modify both the cloud's evolution and the orbital dynamics, leaving a trail of feedback effects that require detailed modeling. Building on the worldline effective field theory approach, we develop a systematic framework for binaries on generic (eccentric and inclined) orbits, capturing both resonant and non-resonant interactions—and thus correcting, even qualitatively, previous conclusions based on balance laws. Furthermore, we can systematically include other effects such as the self-gravity of the cloud and the tidal response of the cloud to the companion, relevant in parts of the parameter space. This approach allows us to provide a more precise characterization of the imprints of ultralight particles in gravitational-wave signals from binary BHs - signatures that are within reach of upcoming observatories such as LISA, Cosmic Explorer and the Einstein Telescope.

Speaker: Mateja Boskovic (DESY, Hamburg)

Boskovic.pdf

5 A semianalytic treatment of quasinormal excitation factors in the eikonal regime

Semianalytical quasinormal mode (QNM) computations have largely focused on quasinormal frequencies (QNFs), since constructing the eigenfunctions of the underlying QNM boundary-value problem is technically challenging. Yet in black hole merger events, identifying individual QNMs within the observed post-merger superposition of modes is essential for waveform modeling, tests of general relativity, and the broader black hole spectroscopy programme. Because mode identification depends on relative excitation, a precise and perturbation-independent quantification of QNM excitations is crucial. In this talk, we present a higher-order extension of the Dolan–Ottewill inverse multipolar expansion method, enhanced by corrections from from the WKB technique of Iyer and Will, which enables the systematic and efficient computation of QNMs and their quasinormal excitation factors (QNEFs). This framework substantially reduces errors - particularly at low multipolar numbers - while maintaining excellent agreement with computationally intensive methods, thereby offering a practical tool for future gravitational-wave modeling and tests of the no-hair conjecture.

Speaker: Anna Chrysostomou (LPTHE, Sorbonne Université)

Chrysostomou.pdf

6 Gravitational memory effect and asymptotic symmetries in de Sitter space-times

Gravitational memory effect is the permanent displacement in the relative separation between freely falling particles resulting from the passage of gravitational wave train. We obtain a closed form expression for the linearized perturbation upto quadrupolar order around de Sitter space-times generated by spatially compact sources. We demonstrate that such a source causes a displacement memory effect close to future infinity. We also discuss a correspondence between memory effect and asymptotic symmetries of de Sitter.

Speaker: Sk Jahanur Hoque (Université Libre de Bruxelles, Belgium)

Jahanur.pdf

7 Gravitational Radiation and Charges on de Sitter

We write a closed form expression for the metric perturbation around de Sitter that describes gravitational radiation from a compact and slowly varying source, in terms of a consistent multipolar expansion at quadrupolar order. We show that the corresponding displacement memory effect with both the even and odd parities is at a higher order in the radial expansion compared to their flat counter-parts. Using the form of the metric perturbation we obtained, we write expressions for SO(1,4) charges at future infinity that reduce to the correct expressions in the flat limit, which also includes a definition of mass that strictly decreases in the presence of gravitational radiation.

Speaker: E. Şeyma Kutluk (Scuola Normale Superiore)

Seyma-Kutluk.pdf

Break

Lightning talks

Conveners: Davide Perrone, Francesco Serra

8 Love Numbers from Effective Field Theory

I will discuss a framework which allows to study Dynamical Love Numbers through an Effective Field Theory, using exact solutions for a scalar field in a Schwarzschild background. I will explain how to obtain results up to O(G9) for the first Love Numbers and how to extend the work to include general compact objects.

Speaker: Davide Perrone (University of Geneva)

Perrone.pdf

9 Testing Lorentz symmetry through black hole physics

Testing Lorentz symmetry through black hole physics

Speaker: Dr Francesco Serra (Johns Hopkins University)

Serra.pdf

10 Tidal deformations of Kerr black holes and their impact on EMRIs

Tidal interactions play a fundamental role in shaping binary systems and affect their gravitational-wave (GW) signals, which is crucial for future detectors such as LISA and ET. While tidal effects on binaries are often studied in a weak-field approximation, a fully relativistic description is needed to capture the role of spin and curvature in shaping orbital dynamics and GW emission.

In this work, we present the first explicit metric for a tidally deformed Kerr black hole, valid up to generic quadrupolar tidal deformations. Expressed in terms of electric and magnetic tidal moments, our construction applies to any external vacuum perturbation in the small-tide approximation. The metric is obtained via reconstruction techniques based on the Teukolsky Master Equation and incorporates spin–tidal couplings, providing a relativistic framework to quantify environmental effects around black-hole spacetimes.

Using this solution, we analyze the secular dynamics of a test particle orbiting the tidally deformed Kerr black hole, focusing on the innermost stable circular orbit (ISCO) and the light ring (LR). We compute tidal-induced shifts in their location and frequencies, and show how these are affected by the black hole’s spin.

These effects accumulate over the long inspiral of EMRIs, leading to measurable phase shifts in the GW signal, while also impacting black-hole spin inference and the quasinormal-mode spectrum. Our findings open new observational pathways for detecting environmental effects in strong gravity, offering potential smoking-gun signatures for LISA and other future detectors.

Speaker: Marta Cocco (Center of Gravity, Niels Bohr Institute and University of Perugia)

Cocco.pdf

11 Dynamical tidal response of non-rotating black holes: connecting the MST formalism and worldline EFT

The tidal response of black holes (BHs) encodes key information about gravity in the strong-field regime and directly affects gravitational waveforms from binary inspirals. We study the dynamical tidal response of static, spherically symmetric BHs in the low-frequency regime, where analytic techniques provide valuable insights. Using the Regge–Wheeler equation and the Mano–Suzuki–Takasugi (MST) method, we derive the small-frequency expansion of BH perturbations and match the results to the worldline effective field theory (EFT) framework.
　This matching enables us to extract the conservative part of the dynamical tidal response, known as dynamical tidal Love numbers, and to clarify their renormalization properties within general relativity (GR). We confirm the universal flow under renormalization but also identify ambiguities in the finite part, which depend on the renormalization scheme or initial conditions.
　Our framework naturally extends to compare BHs in GR with other compact objects or predictions from alternative gravitational theories. The notion of “bare” tidal Love numbers provides a means to distinguish such scenarios and could leave detectable imprints on inspiral waveforms. Incorporating these effects into post-Newtonian waveform modeling offers a new avenue to probe compact object structure and to test GR in the strong-field regime.

Speaker: Hajime Kobayashi (Yukawa Institute for Theoretical Physics, Kyoto University)

Hajime.pdf

12 Black hole in four dimensional general relativity with non-zero Love numbers.

Love numbers of compact objects quantify their tidal deformability against external perturbations. It is known that Love numbers of asymptotically flat black holes (BHs) in General Relativity are identically zero. We show that, quite contrary to common expectations, the tidal Love numbers of asymptotically de Sitter black holes in four dimensions are non-zero.

Speaker: Sreejith Nair (Indian Institute of Technology Gandhinagar)

Nair.pdf

13 Observable signature of magnetic tidal coupling in hierarchical triple systems

Compact objects can form binary systems in the vicinity of a supermassive black hole (SMBH), resulting in a hierarchical triple system. The presence of the SMBH can leave a detectable imprint on the gravitational waves emitted by the binary, which may be observed by next-generation gravitational wave detectors. One such imprint arises from resonances induced in the binary’s evolution by the presence of the SMBH. Among these, we focus on precession resonances, which occur when the periastron precession frequency of the binary becomes commensurate with the orbital frequencies governing the binary's motion around the SMBH.
We model the binary system up to 0.5 post-Newtonian (PN) order, while the SMBH is treated using the fully relativistic Schwarzschild metric. This framework allows us to include magnetic tidal moments in the coupling between the binary and the SMBH, which are purely relativistic and have no Newtonian analogue. These magnetic moments introduce a distinct relativistic signature of the SMBH in the binary's dynamics.
We demonstrate that relativistic effects and magnetic tidal moments give rise to a new resonance condition for precession resonances, specifically in the case of a quasi-circular orbit around the SMBH. This leads to multiple eccentricity kicks in the orbit of the compact binary over time, features that can be detectable by LISA, and results in an accelerated merger of the two compact objects. To the best of our knowledge, this represents the first observable effect of magnetic tidal moments in resonances within triple systems.

Speaker: Davide Panella (Niels Bohr Institute & University of Perugia)

Panella.pdf

14 Ladder Symmetry as a Sufficient Condition for vanishing static Tidal Love numbers

The vanishing of static Tidal Love numbers for asymptotically flat black holes in four dimensions has been explained using the "Ladder Symmetry", which is a symmetry for the static perturbation equations around such backgrounds. The existence of a Ladder structure among the solutions is known to be a necessary condition for the Love numbers to vanish. Still, it remains an open question whether it is a sufficient condition for vanishing Love numbers. In this talk, employing the "parametrized formalism for computing tidal Love numbers (as in Phys. Rev. D 109 (2024) 4, 044067)," we will establish that for four-dimensional static, spherically symmetric black-hole spacetimes, and a particular class of stationary rotating four-dimensional black-hole spacetimes (a restricted class of the Konoplya-Rezzolla-Zhidenko spacetimes), any deviation from a Ladder symmetric solution would necessarily result in a non-zero static tidal Love number. In essence, this proves that the existence of Ladder symmetry in the solutions of perturbation equations is a necessary and sufficient condition for the vanishing of static tidal Love numbers in four-dimensional stationary asymptotically-flat black hole solutions belonging to the classes mentioned above.

Speaker: Shuvayu Roy (Indian Institute of Technology, Gandhinagar)

Roy.pdf

Lightning talks: Poster gong show

Conveners: Davide Panella, Hajime Kobayashi, Marta Cocco, Shuvayu Roy, Sreejith Nair

Break

15 Black hole perturbation theory: from Love numbers to ringdown

A time honored way to study black holes is to perturb them. We will give an overview of several recent developments, including the tidal deformability of black holes (characterized by their Love numbers), and their ringdown after a binary merger. We will go over the surprising symmetries governing black hole perturbatioins, and discuss the latest progress into the nonlinear regime.

Speaker: Lam Hui (Columbia U.)

Recording

Tue, November 4

16 Static Quadratic Love Numbers

Tidal Love numbers quantify the deformability of compact objects under external tidal fields. They are key quantities in gravitational‑wave astronomy for accurately modeling waveforms during the final orbits of an inspiral and are tightly connected to the microphysics of the compact object. I will present a framework for computing tidal Love numbers beyond linear order by matching relativistic perturbation theory of compact objects (Schwarzschild black holes and neutron stars) with the worldline effective field theory approach used to define their tidal deformability.

Speaker: Filippo Vernizzi (U. Paris-Saclay)

ETH_NSLN.pdf

Break: Coffee Break

17 Nonlinear Tails in Black Hole Ringdown

Black holes relax toward stationarity through gravitational‐wave ringdown, followed at fixed radius by late–time power-law tails. Recent work suggests that nonlinear tails sourced by quadratic self-couplings can outshine the familiar linear tails. In this talk I present a compact derivation of these nonlinear tails using the near-horizon AdS2×S2 perspective. The method treats the late-time waveform as a response of the AdS2 scalar sector to an effective quadratic source and maps the tail amplitude to horizon data. In particular, I show that the overall normalization of the dominant nonlinear tail is controlled by Aretakis constants, providing a direct link between conserved horizon charges and observable late-time decay.

Speaker: Alex Kehagias (Natl. Tech. U. Athens)

Kehagias_slides.pdf

Recording

Lunch

18 Dynamical Tidal Response of Schwarzschild Black Holes

The tidal deformability of a gravitating object is characterized by a set of coefficients that quantify its response to an external field perturbation. It is well known that the zero-frequency response coefficients—also known as the static tidal Love numbers—of Schwarzschild black holes vanish identically in four-dimensional general relativity. At subleading order in the adiabatic expansion, the dissipative and conservative response coefficients become nonzero, capturing, respectively, absorption across the horizon and frequency-dependent corrections to the tidal Love numbers. Using the framework of the point-particle effective field theory, I will present the calculation of the dynamical Love numbers of Schwarzschild black holes up to second order in frequency. In addition to the previously known logarithmic renormalization-group running, I will derive the scheme-dependent finite terms.

Speaker: Luca Santoni (APC, U. Paris-Cité)

Recording

Santoni_slides.pdf

19 Naturalness of vanishing black-hole tides

I will present a symmetry argument for the vanishing and non-renormalization of static Love numbers for spherically symmetric black holes at nonlinear order, in D=4 classical General Relativity. The symmetry is realized both in full GR and in the worldline EFT, allowing for a unified treatment. This clarifies the naturalness of vanishing static Love numbers in the worldline EFT when including non-linearities, and extends previous vanishing results to all nonlinear static tides. When extended to higher dimensional gravity, this also explains the pattern of vanishing and running static Love numbers of electric and tensor type, and predicts new results at the nonlinear order. Based on joint work with Julio Parra-Martinez.

Speaker: Alessandro Podo (IHES Paris)

Podo_slides.pdf

Recording

Break: Coffee Break

20 Nonlinear gravitational memory from scattering amplitudes

Scattering amplitudes offer a convenient tool to recast gravitational-wave observables in terms of gauge-invariant building blocks. Amplitude methods are particularly relevant in the the post-Minkowskian (PM) regime, in which sources are far apart and interact weakly. A complementary approach is provided by soft theorems, which apply for small frequencies while retaining exact information on the hard dynamics.
In this talk, I will discuss recent progress achieved by combining these two methods to calculate the waveform sourced by the scattering of two compact objects. In particular, I will show how the nonlinear memory effect emerges from amplitudes in the NNLO PM waveform, to leading order in the soft limit, and illustrate how its explicit calculation can be reduced to cut two-loop integrals.

Speaker: Carlo Heissenberg (IPhT Saclay)

Recording

Slides_Heissenberg.pdf

21 Positivity in Renormalization: from BSM to Love Numbers

Renormalization group (RG) evolution is a fundamental feature of quantum field theory, playing a pivotal role in both its formal structures and phenomenological applications in particle physics and beyond. While the framework for calculating RG flows is well established, unexpected patterns continue to emerge. In this talk, I will leverage fundamental principles—unitarity and analyticity—to constrain a large sector of RG evolution in effective field theory. This result can be understood as an infrared avatar of positivity bounds. For phenomenological applications, I will discuss examples in the context of the SMEFT. For the second half of the talk, I will discuss the analogous cases of Love numbers renormalization for classical black holes and neutron stars. In both quantum and classical EFTs, the RG evolution exhibits an interesting positive direction.

Speaker: Chia-Hsien Shen (NTU Taiwan)

ETH_seminar.pdf

Wed, November 5

22 Gravitational memory and soft theorems for equal-time correlators

Gravitational memory describes the lasting change in the separation and relative velocity of freely-falling detectors after the passage of gravitational waves. The phenomenon is intimately related to Weinberg's soft graviton theorems and BMS symmetries at future null infinity. In this talk, I will elucidate the relation between BMS transformations and the description of gravitational memory in synchronous coordinates, commonly used in gravitational wave detectors like LISA. I will show that gravitational memory corresponds to large residual diffeomorphisms in this gauge, such as volume-preserving spatial rescalings. I will then derive the associated soft theorems for equal-time correlation functions. These turn out to be the flat space analogues of the well-known inflationary consistency relations.

Speaker: Justin Khoury (U. Pennsylvania)

Khoury.pdf

Break: Coffee Break

23 Is Love positive?

I will show that in the regime of weak gravity, all electric Love numbers are strictly positive. The statement is based on a Kramers-Kronig dispersion relation and it requires some assumptions that I will discuss with several examples.

Speaker: Paolo Creminelli (ICTP Trieste)

Creminelli.pdf

Lunch

24 Asymptotic Causality

Speaker: Enrico Trincherini (SNS Pisa)

Break: Coffee Break

25 Towards claiming a detection of gravitational memory

After offering an introduction into the topic of gravitational memory, I will present the ongoing research of the LISA fundamental physics working group towards a first single event detection of the phenomenon: This includes an assessment of the proper theoretical modeling of the gravitational memory signal, a study of the LISA response to memory, as well as continuing efforts towards a realistic estimate of the detectability from supermassive black hole mergers.

Speaker: Jann Zosso (NBI Copenhagen)

JZ - Towards claiming a detection of gravitational memory.pptx

Zosso.pdf