IBS CTPU-CGA 2025 Workshop on Quasi Normal Mode and Black Hole Perturbation

26 May 2025, 09:00 → 30 May 2025, 18:00 Asia/Seoul

B438 (IBS headquater)

Black hole physics is a vibrant field of research that has seen numerous significant advancements in recent years, playing an increasingly crucial role in fundamental physics. The study of black hole perturbation is crucial in various aspects of black hole phenomena, including gravitational wave theory, binary black hole inspiral, ring-down processes, black hole superradiance, etc.

The purpose of this workshop is to exchange ideas, to promote collaboration, and to discuss recent developments in black hole physics.

 

The list of invited speakers: 

Yeong-Bok Bae (Chung-Ang University)

Emanuele Berti (Online) (Johns Hopkins University)

Swetha Bhagwat (University of Birmingham)

Pablo Cano (University of Barcelona)

Vitor Cardoso (Online) (Niels Bohr Institute)

Francisco Duque (Albert Einstein Institute)

Yasuyuki Hatsuda (Rikkyo University) 

Young-Hawn Hyun (Chung-Ang University)

Masashi Kimura (Daiichi Institute of Technology)

Caio Macedo (Federal University of Pará)

Keefe Mitman (Cornell University)

Naritaka Oshita (YITP)

Jaime Redondo-Yuste (Niels Bohr Institute)

Giada Santoro (Institut de Fisica d'Altes Energies)

Thomas Spieksma (Niels Bohr Institute) 

Takahiro Tanaka (Kyoto University)

Gimmy Tomaselli (Princeton IAS)

Sebastian Völkel (Albert Einstein Institute)

Helvi Witek (University of Illinois Urbana-Champaign)

Huan Yang (Tsinghua University)

Zihan Zhou (Online) (Princeton IAS)

Huiyu Zhu (IBS)

 

 

If you wish to attend this workshop, please register by April 20th,23:59 KST. We will also accept a small number of contributed talks, which will be assigned to May 29th. If you wish to give a contributed talk, please register it with the application of a contributed talk (with its talk title and abstract) by April 20th, 23:59 in KST. The selection will be made by the organizing committee.  Please note that due to staff shortages, we will not be able to support VISA applications from non-invited speakers except in special cases. 

 

The list of organizers: 
Emanuele Berti (Johns Hopkins)
Vitor Cardoso (Niels Bohr Institute) 
Masashi Kimura (Daiichi Inst. Tech.)
Helvi Witek (UIUC)
Masahide Yamaguchi (IBS) 
Huiyu Zhu (IBS) 

Monday, 26 May

09:50 Opening

1 Invited Talk: Takahiro Tanaka (Title: Evolution of super-radiant axion clouds)

Title: Evolution of super-radiant axion clouds

May25_Takahiro Tanaka.pdf

Session: Superradiance: (Chair: Masahide Yamaguchi)

2 Invited Talk: Gimmy Tomaselli (Title: Gravitational atoms and black hole binaries)

Title: Gravitational atoms and black hole binaries

Abstract: Various models of physics beyond the Standard Model predict the existence of ultralight bosons. These particles can be produced through superradiant instabilities, which create boson clouds around rotating black holes, forming so-called "gravitational atoms". In this talk, I review a series of papers that study the interaction between a gravitational atom and a binary companion. The companion can induce transitions between bound states of the cloud (resonances), as well as transitions from bound to unbound states (ionization). These processes back-react on the binary’s dynamics and leave characteristic imprints on the emitted gravitational waves (GWs), providing direct information about the mass of the boson and the state of the cloud.

May26_Giovanni Thomaselli.pdf

3 Invited Talk: Thomas Spieksma (Title: Black hole superradiance: a gateway to new physics)

Title: Black hole superradiance: a gateway to new physics

Abstract: Black holes in our Universe are not expected to reside in vacuum. Their environments—such as accretion disks or dark matter distributions—can significantly impact black hole physics, whether in isolation or in binary systems. A particularly intriguing environment arises from black hole superradiance: a process through which a spinning black hole transfers energy and angular momentum to a surrounding bosonic field. This interaction gives rise to a dense, atom-like condensate of ultralight particles. Such systems differ markedly from other astrophysical environments and may produce unique observational signatures. In this talk, I will explore two scenarios where these bosonic “clouds” could have observational consequences. First, I examine whether couplings to the electromagnetic sector could lead to detectable light emission from an isolated black hole-cloud system. Second, I investigate how these clouds influence binary evolution during an inspiral, with a focus on future gravitational wave detectors like LISA.

May26_Thomas Spieksma.pdf

Session: Superradiance: (Chair: Takahiro Tanaka)

4 Invited Talk: Huiyu ZHU (Title: Survival of the Fittest: Testing Superradiance Termination with Simulated Binary Black Hole Statistics )

Title: Survival of the Fittest: Testing Superradiance Termination with Simulated Binary Black Hole Statistics

Abstract: The superradiance instability of rotating black holes leads to the formation of an ultralight boson cloud with distinctive observational signatures, making black holes an effective probe of ultralight bosons. However, around black holes in a binary system, the superradiance effect of such clouds can be terminated by tidal perturbations from the companion, leading to cloud depletion. In this study, we focus on the superradiance of a scalar boson and perform the first analysis of the impact of this termination effect on superradiant black hole binaries which are realistically modeled after their statistics in our Galaxy. Working with a dataset of approximately 10^7 black hole binaries simulated using the Stellar EVolution for N-body (SEVN) population synthesis code, we identify the superradiant candidates and those that manage to survive the termination effect. We then calculate the cloud survival rate for various boson masses and black hole spin models. Our findings reveal that the l=m=1 cloud modes are generally stable against termination, whereas the l=m=2 modes can be significantly affected, with survival rates dropping below 10% for boson masses below approximately 0.5\times 10^{-12} eV. In addition, our analysis indicates that clouds that overcome termination typically exhibit a higher superradiant growth rate and therefore a higher detectability.

May26_Huiyu ZHU.pdf

5 Invited Talk: Swetha Bhagwat (Title: Mode Amplitudes: Linear and Nonlinear Dynamics In Ringdowns)

Title: Mode Amplitudes: Linear and Nonlinear Dynamics In Ringdowns

May26_Swetha Bhagwat.pdf

Session: QNM: (Chair: Keefe Mitman)

6 Invited Talk: Yeong-Bok Bae (Title: Ringdown Gravitational-Waves from the Close Scattering of Two Black Holes )

Title: Ringdown Gravitational-Waves from the Close Scattering of Two Black Holes

Abstract: In this talk, we present fully general-relativistic simulations showing that two equal-mass, nonspinning black holes on close-scattering hyperbolic trajectories can emit a clear ringdown gravitational-wave signal without merging. This signal arises purely from the dynamical tidal deformations during the close encounter. By analyzing the gravitational radiation after the flyby, we identify oscillatory features whose frequencies match the quasinormal modes of a Schwarzschild black hole to within a few percent. Our results demonstrate that such nonmerging encounters can excite genuine ringdown modes, offering new insights into the dynamics of strong-field gravity beyond the context of black hole mergers.

18:00 Welcome Dinner

Tuesday, 27 May

7 Invited Talk: Emanuele Berti (online)

May27_Emanuele Berti.pdf

Session: QNM: (Chair: Swetha Bhagwat)

8 Invited Talk: Huan Yang (Title: Nonlinear black hole ringdowns)

Title: Nonlinear black hole ringdowns

Abstract: In this talk, I will discuss recent progress in characterizing second order quainormal modes of rotating black holes. This includes two independent ways of computing the mode-mode coupling coefficients, which lead to consistent results. A parameter survey shows that multiple 2nd-order order modes may be detectable for 3G ground-based detectors and space-borne gravitational wave detectors. If time permits, I will also mention some ongoing work on quasinormal mode tomography.

May27_Huan Yang.pdf

9 Invited Talk: Keefe Mitman (Title: Leveraging binary black hole ringdown excitations as probes of astrophysics and tests of GR )

Title: Leveraging binary black hole ringdown excitations as probes of astrophysics and tests of GR

Abstract: With the commencement of the LIGO-Virgo-KAGRA Collaboration's fourth observing run, the field of gravitational wave (GW) physics is uniquely poised to collect even more precise data from compact binary coalescences. Consequently, we will soon be able to perform even more stringent tests of general relativity. One such test that is particularly promising is trying to verify black hole perturbation theory's predictions using the ringdown excitations imprinted on the merger's remnant black hole. Doing so, however, requires that we have a robust understanding of these excitations through numerical relativity simulations as well as a means to model them across binary black hole parameter space. In this talk, I will highlight recent advancements regarding our understanding of quasi-normal modes (QNMs) that have made performing this test even more alluring. In particular, I will show how QNMs probe the intrinsic geometry of black holes and even reveal information about the nonlinear nature of Einstein's equations. Following this, I will then explain how we can model QNMs using Gaussian process regression and will provide updates on the current status of these models and how they can be used in gravitational wave data analysis to probe both astrophysics as well as the nature of gravity.

Session: QNM: (Chair: Huan Yang)

10 Invited Talk: Jaime Redondo-Yuste (Title: Nonlinear effects in perturbation theory)

Title: Nonlinear effects in perturbation theory

Abstract:
General relativity is a nonlinear theory, and understanding its nonlinear features is crucial for many physical phenomena. In this talk I will motivate the study of subleading orders in perturbation theory. I will first review some generalities of perturbation theory in symmetric spacetimes, and then focus on black hole backgrounds. Nonlinear effects become relevant during the ringdown phase, where they may be observable with upcoming gravitational wave detectors. I will outline different theoretical approaches to this problem, including a recent formulation using symmetric spinors. Finally I will present new results on the excitation of quadratic quasinormal modes in the high frequency limit, and discuss its implications for the stability of black hole spacetimes.

May27_Jaime Redondo Yuste.pdf

11 Invited Talk: Vitor Cardoso (online) (Title: The quest for horizons)

Title: The quest for horizons

Abstract: One of the most remarkable possibilities of General Relativity concerns gravitational collapse to black holes. Is the strong field dynamical regime of gravity well described by General Relativity? I will summarize the status of black hole spectroscopy and attempts at probing near horizon physics.

May27_Vitor Cardoso.pdf

Session: ECO & QNM: (Chair: Huiyu ZHU)

12 Invited Talk: Naritaka Oshita (Title: Excitability, convergence and (in)completeness of quasi-normal modes)

Title: Excitability, convergence and (in)completeness of quasi-normal modes

Abstract: After the merger of a binary black hole system, the remnant black hole undergoes free oscillations, emitting gravitational waves known as the ringdown signal.
These ringdown waveforms contain multiple quasi-normal modes (QNMs), and it is expected that black hole spectroscopy—resolving these modes individually—will enable tests of gravity in the strong-field regime.
In this talk, I will explain how the multiple QNMs exhibit a universal and destructive interference pattern characterized by the black hole greybody factor or its S-matrix.
I will also discuss the convergence and (in)completeness of the QNM expansion of ringdown and the potential implications of these findings for testing strong gravity.

May27_Naritaka Oshita.pdf

Wednesday, 28 May

13 Invited Talk: Zihan Zhou (online) (Title: When black hole perturbation theory meets scattering amplitudes and conformal field theory )

Title: When black hole perturbation theory meets scattering amplitudes and conformal field theory

Abstract:
The study of black hole perturbation theory has seen a resurgence in recent years after the observation of the gravitational waves generated by the coalescence of binary black holes. In this talk, I will summarize some of the recent progress in understanding black hole perturbation theory from the aspect of scattering amplitude and conformal field theory (CFT). First of all, I will review the three analytic methods, i.e. Mano-Suzuki-Takasugi (MST), CFT and monodromy methods of solving Teukolsky equations. Then, I will discuss the physics behind the solutions via the gravitational Raman scattering amplitudes and apply them in the waveform resummation.

May28_Zihan Zhou.pdf

Session: Mathematical Relativity: (Chair: Huiyu ZHU)

14 Invited Talk: Yasuyuki Hatsuda (Title: Heun connection formulae and quasinormal modes)

Title: Heun connection formulae and quasinormal modes

Abstract:
Recently, analytic formulae of connection coefficients in Heun's differential equation were conjectured. In this talk, I will explain how to use them to analyze quasinormal mode problems for four-dimensional asymptotically de Sitter black holes.

May28_Yasuyuki Hatsuda.pdf

13:30 Free Discussion

18:00 Banquet

Thursday, 29 May

15 Invited Talk: Masashi Kimura

Session: Scalarization, modified theories of gravity, ringdown detection: (Chair: Young-Hwan Hyun)

16 Invited Talk: Pablo Cano (Title: Eikonal quasinormal modes of rotating black holes beyond GR: a window into extremality )

Title: Eikonal quasinormal modes of rotating black holes beyond GR: a window into extremality

Abstract:
The computation of quasinormal modes of rotating black holes in modified theories of gravity has been recently made possible thanks to the development of new techniques, like a modified Teukolsky equation and spectral methods. However, no method so far has been able to peek into the highly rotating regime — close to extremality. In this talk, I will consider a newly identified higher-curvature modification of GR that preserves the isospectrality of quasinormal modes in the eikonal limit. In this theory, eikonal perturbations can be described in terms of an effective scalar equation, and solving it we will obtain the corrections to the eikonal Kerr quasinormal modes for arbitrary rotation. For moderate rotation, we check that the eikonal computation gives a good approximation to the exact QNMs obtained from the modified Teukolsky equation, even for low harmonics. For high rotation, we discover that the corrections to GR become much larger and can lead to dramatic effects. Our results suggest that the observation of the ringdown of a highly rotating black hole would be a “golden event” to search for new physics.

May29_Pablo Cano.pdf

17 Invited Talk: Sebastian Völkel (Title: Advances in the Parametrized QNM Framework and New Perspectives from Bound States)

Title: Advances in the Parametrized QNM Framework and New Perspectives from Bound States

Abstract:
Black hole spectroscopy, applied to the ringdown phase of compact binary mergers, is one of the most promising tools to test the nature of black holes. It allows us to quantify whether black holes and their perturbative dynamics are well described by general relativity and, thus, serve as a magnifier to explore fundamental physics. The parametrized quasi-normal mode framework has been developed as a powerful tool to capture small changes in black hole dynamics caused by small deviations from general relativity. It provides a unique role to mediate between theory-specific approaches and fully agnostic data-driven ones. In this talk, I will first review some of the seminal works and contributions of the framework. Then, I will focus on recent progress, including the framework's first systematic study in the time domain and its extension to rotating black holes. Finally, I will use the intriguing idea to relate quasi-normal modes with the bound states of an inverted potential to address ongoing discussions of spectral stability and the excitation of quasi-normal modes.

May29_Sebastian Vokel.pdf

Session: Scalarization, modified theories of gravity, ringdown detection: (Chair: Masashi Kimura)

18 Invited Talk: Young-Hawn Hyun

19 Contributed Talk: Miok Park (Title: Hairy Black Holes by Spontaneous Symmetry Breaking in ESGB theory )

Title: Hairy Black Holes by Spontaneous Symmetry Breaking in ESGB theory

Abstract: We study hairy black hole solutions in Einstein-(Maxwell)-scalar-Gauss-Bonnet theory. The scalar coupling function includes quadratic and quartic terms, so the gravitational action has a U(1) symmetry. We argued that when the effective mass of the scalar field is at the critical value, hairy black holes are generated in a symmetry-broken vacuum via spontaneous symmetry breaking, where the Goldstone bosons are trivial. To examine such process, we introduce a scalar field perturbation on the newly formed background spacetime. We solve the linearized perturbation equation using Green’s function method. We begin by solving the Green’s function analytically, incorporating the branch cut contribution and then proceed to calculate the quasinormal modes (QNMs) numerically, which arise from the presence of poles in the Green’s function. These numerical results provide strong evidence for a dynamical process that unstable black holes without hairs transition into stable hairy black holes in the symmetry-broken phase through the spontaneous symmetry breaking. Moreover, we found that the spontaneous symmetry breaking associated with local U(1) is unlikely to occur in this theory.

Session: QNM: (Chair: Yeong-Bok Bae)

20 Contributed Talk: Theodoros Nakas (Title: Black holes in string-inspired Euler-Heisenberg theory: Doppelgänger black holes)

Title: Black holes in string-inspired Euler-Heisenberg theory: Doppelgänger black holes

Friday, 30 May

21 Invited Talk: Helvi Witek (online)

Session: Environmental effect & Superradiance: (Chair: Giada Santoro)

22 Invited Talk: Francisco Duque

May30_Francisco Duque.pdf

23 Invited Talk: Caio Macedo (Title: Black holes and bosonic fields: modes and binary systems)

Title: Black holes and bosonic fields: modes and binary systems

Abstract
Bosonic fields around rotating black holes can grow exponentially due to superradiant instabilities, producing detectable dynamical signatures in gravitational waves. We explore relativistic configurations of bosonic fields and black holes, analyzing their quasinormal modes and inspiral dynamics. Our findings provide key insights into potential observational signatures for next-generation gravitational wave detectors.

May30_Caio Macedo.pdf

Session: Environmental effect & Superradiance: (Chair: Francisco Duque)

24 Contributed Talk: Ao Guo (Title: Common Envelope Evolution of Ultralight Boson Clouds )

Title: Common Envelope Evolution of Ultralight Boson Clouds

Abstract: Ultralight bosons, as potential dark matter candidates, can form dense clouds around black holes via superradiant instabilities. In binary black hole systems, these clouds evolve under mutual tidal interactions, and form a shared structure that envelopes the entire binary—termed a gravitational molecule —as orbital separation approaches the size of the cloud. We investigate the dynamics of ultralight boson cloud common envelopes, with non-adiabatic evolution driven by orbital eccentricity and energy-level crossings of gravitational molecules in comparable-mass black hole binaries. And this interaction induces significant backreaction on the binary's orbital parameters, accelerating/decelerating the inspiral and imprinting distinct phase shifts in gravitational wave signals.

May30_Ao Guo.pptx

25 Contributed Talk: Ziyu Tang (Title: A photon cloud induced from an axion cloud )

Title: A photon cloud induced from an axion cloud

Abstract: It is known that the coupling between axions (m a = µ ℏ) and photons can lead to a superradiant growth and cause electromagnetic (EM) bursts with frequency µ /2 when the coupling constant ka multiplied with the amplitude of the axion clould ϕ 0 larger than a critical value. In the first-order of the coupling constant, we studied how the existence of an axion cloud influences the distribution of EMs around a Kerr black hole, and found that even for very small couplings, as long as there is a background EM field (satisfying the field equation in Kerr background like Wald vector potential valid in realistic astrophysical environment), there would exist an oscillating photon (EM) cloud with frequency µ same as the axion cloud. The evolution of the photon (EM) cloud with time t and azimuthal angle φ is obtained analytically while the distribution on the cross section is solved numerically. Intriguingly, the induced EMs have very different symmetries in contrast with the background EMs, which may provides as a feature for the existence of both an axion cloud and the axion-photon coupling. Also, for near horizon region, we compare the induced EMs (photons as fields) with the photon region (photons as particles) and find some coincides. Essentially, the induced photon cloud is sourced by the axion cloud via the coupling, not related to superradiance effects.

May30_Ziyu Tang.pdf

Session: Environmental effect & Superradiance: (Chair: Caio Macedo)

26 Invited Talk: Giada Santoro

May30_Giada Caneva Santoro.pdf