2nd New Horizons for Psi Workshop

23 Aug 2026, 14:00 → 28 Aug 2026, 18:00 Asia/Shanghai

The 2nd New Horizons for Psi Workshop will be held in Beijing, China, from 24–28 August 2026, hosted by the International Centre for Theoretical Physics Asia-Pacific (ICTP-AP).

The New Horizons for Psi Workshop is an international conference series devoted to exploring fundamental fields in the vicinity of compact objects, and to understanding how these extreme gravitational environments can serve as powerful laboratories for fundamental physics. The growing interplay between black hole astrophysics, gravitational-wave observations, and particle physics has opened new opportunities to probe phenomena at the intersection of these fields. The workshop brings together researchers working on black holes, gravitational waves, and particle physics, fostering cross-disciplinary dialogue.

In addition to plenary talks, the program will include contributed talks and focused mini-workshop sessions. Abstract submissions for contributed talks are due by 24 July 2026.

 

Invited Speakers

Haipeng An (Tsinghua U)
Enrico Barausse (SISSA)
Xiaojun Bi (IHEP, CAS)
Diego Blas (ICREA/IFAE)
Kfir Blum (Weizmann Institute of Science)
Richard Brito (IST)
Arianna Foschi (LESIA)
Hyungjin Kim (IPhT)
Eugene Lim (KCL) 
Andrea Maselli (GSSI)
Hidetoshi Omiya (Kyoto U) 
Paolo Pani*(Sapienza U of Rome)
Gilad Perez (Weizmann Institute of Science)
Jing Ren (IHEP, CAS)
Nils Siemonsen (Princeton U)
Volodymyr Takhistov (KEK & IPMU)
Giovanni Tomaselli (IAS)
Rodrigo Vicente (U of Amsterdam)
Shao-Jiang Wang (ITP, CAS)
Hai-Bo Yu (UC Riverside)
Hong Zhang (Shandong U)

*=TBD

 

International Advisory Board

Gianfranco Bertone, Diego Blas, Richard Brito, Vitor Cardoso, Katy Clough, William East, Joseph Silk, Takahiro Tanaka, Yue-Liang Wu.

Local Organising Committee

Haipeng An, Yifan Chen, Xiaoyong Chu, Huaike Guo, Da Huang, Jing Liu, Teng Ma, Andrew Miller, Shi Pi, Jing Ren, Yong Tang, Yu Tian, Shao-Jiang Wang, Huan Yang, Jun Zhang (Chair), Yunlong Zhang, Yue Zhao.

 

REGISTRATIONS and ABSTRACT SUBMISSION ARE NOW OPEN

The registration and abstract submission deadline is 24 July, 2026.

 

Previous edition:

1st New Horizons for Psi School & Workshop, Lisbon, 2024

poster.jpg

Sunday 23 August

14:00 → 20:00 Registration

Monday 24 August

08:00 → 08:45 Registration

08:45 → 09:00 Plenary Session: Opening

09:00 → 10:00 Plenary Session

09:00 TBD

TBD

Speaker: Enrico Barausse

09:30 Extreme mass-ratio inspirals in scalar cloud environments

Future space-based gravitational-wave observatories will open a unique avenue to study the environments surrounding black holes, in particular through the detection of extreme mass-ratio inspirals. In this seminar I will discuss the dynamics and gravitational-wave signatures of extreme mass-ratio inspirals evolving in scalar cloud environments — bosonic condensates that can grow around spinning black holes through superradiance, and which also serve as a useful proxy for modelling more complex environments. I will give an overview of recent efforts to model these systems through fully relativistic perturbative calculations, highlighting some of the interesting phenomena that have emerged from these studies.

Speaker: Richard Brito (CENTRA, Instituto Superior Técnico, Lisbon)

10:00 → 10:30 Break

10:30 → 11:30 Plenary Session

10:30 Discovering gravitational atoms with binary systems

Ultralight bosons can be produced by rotating black holes through superradiance, forming so-called "gravitational atoms". When these are part of a binary system, they give rise to a rich phenomenology. In this talk, I focus on two distinct scenarios. First, for an inspiralling compact binary, I show that the combination of infinitely many resonances and the "ionization jumps" creates a smooth evolution of the binary and its GWs, which is tricky to correctly capture numerically. Second, I show that when a gravitational atom is in a nuclear star cluster, such as the Sgr A* cluster, it induces a measurable precession of the stellar orbits, and drives some of them to decay into the supermassive black hole in a few Myr.

Speaker: Giovanni Maria Tomaselli

11:00 Matters of gravity: the interplay between environmental and beyond-GR effects in asymmetric binaries

Asymmetric binaries, such as extreme- and intermediate-mass-ratio inspirals, are prime targets of future gravitational-wave detectors and offer a unique opportunity to probe the astrophysical environments in which such binaries evolve, as well as to test possible deviations from general relativity. To date, these two classes of effects — beyond-vacuum environmental physics and beyond-GR modifications — have largely been modelled independently, as small perturbations to the baseline vacuum-GR dynamics. In realistic astrophysical scenarios, however, both effects can act simultaneously, jointly shaping the binary's evolution and the resulting parameter estimation. In this talk, I will present a rigorous framework for describing the interplay between (scalar) fields associated with beyond-vacuum environments and with modifications to gravity, built on a perturbative approach inspired by self-force calculations in vacuum GR. Focusing on specific examples, I will then show how these two effects become correlated, and how this correlation impacts the detectability of both environmental and gravitational-modification signatures.

Speaker: Andrea Maselli (Gran Sasso Science Institute)

11:30 → 12:00 Plenary Session: Panel discussion

14:00 → 15:30 Parallel Session A

14:00 Searching for Scalar Ultralight Bosons through High-Frequency X-ray Variability

Scalar ultralight boson clouds around rotating black holes can generate long-lived, non-axisymmetric gravitational perturbations capable of exciting coherent dynamical responses in nearby accretion flows. Focusing on black hole X-ray binaries, we investigate whether this forcing can excite global oscillations of a thin accretion disk and produce observable quasi-periodic variability. Using a linear-response framework for a simplified hydrodynamic disk model, we compare the characteristic frequency and spatial structure of the boson-cloud forcing with the intrinsic modes of the disk, while accounting for resonance structure and damping. We then connect the resulting disk perturbations to possible observational signatures through relativistic ray tracing. I will discuss the physical conditions under which the response may be amplified and the prospects for probing the resulting high-frequency variability with current or future X-ray timing observations.

Speaker: Zhiren Wang (Perimeter Institute & University of Waterloo)

14:15 Observation of Dwarf Galaxies and Fuzzy Dark Matter

As an alternative theory to cold dark matter (CDM), fuzzy dark matter (FDM) has recently attracted widespread attention. FDM consists of ultra-light bosons with masses around 10^{-22} eV. At typical galactic velocities, their de Broglie wavelength can reach kiloparsec (kpc) scales, thereby exhibiting unique wave-like behavior on galactic scales. This leads to complex evolutionary dynamics and observational effects, which may help address several small-scale challenges the CDM paradigm faces. In this presentation, I will introduce some of our recent work on FDM simulations, including the simulations of the dynamical heating effect, tidal stripping, and evolution of compact stellar systems in FDM halos.

Speaker: Yu-Ming Yang (IHEP)

14:30 Probing the wave nature of light degrees of freedom

Many well-motivated UV theories contain bonsonic light degrees of freedom, which can exhibit various wave-like behaviors at low energy. This leads to many new ideas and novel observables in recent years to probe new physics in astrophysical and laboratory setups alike. In this talk, I will briefly review a few examples in leveraging the wave-like features to test axions and ultralight dark matter. These include the axion-induced supernova remnant radio echo, the soliton-imprinted galaxy rotation curves, axion-induced neutron star X-ray signals. I will end with a new axion-photon resonant conversion mechanism induced by spatially varying magnetic field background and show its phenomenological consequences to the LSTW experiments and solar axion searches.

Speaker: Chen Sun (HIAS-UCAS)

14:45 Heating Up the Black Hole X-ray Binary Accretion Disk by Superradiance

The existence of a superradiant axion cloud around a black hole X-ray binary can heat its accretion disk and be detected in a thermal X-ray spectrum. We consider a derivative coupling between axions and fermions and calculate the emissivity of the inverse bremsstrahlung process, which results in a temperature fluctuation of the disk. The energy injection rate per baryon by axion heating is sizable $\sim0.1(|g_{ae}|/10^{-12})^2\,{\rm eV/s}$ if the accretion disk's densest region overlaps with the cloud's Bohr radius, where $g_{ae}$ is the axion-electron coupling. Based on the Novikov-Thorne thin-disk model and the multicolor disk model, we derive the thermal X-ray spectrum with axion heating. A single bump hunting search of the axion heating signature in the X-ray thermal spectrum can already derive a competitive constraint on axion-electron coupling.

Speaker: Fengwei Yang (University of Notre Dame)

15:00 probing black hole environments with gravitational waves

Gravitational waves open a new window onto black holes and their surrounding environments. Realistic environments such as ultralight bosonic fields and accretion environments can leave observable signatures in gravitational wave signals. This talk explores how such environmental effects can be used to probe fundamental physics and astrophysics. Topics include gravitational waves from superradiant ultralight dark matter clouds around black holes, the resulting constraints on ultralight bosons, and the impact of accretion environments on black hole spin evolution and gravitational wave emission. These examples show how gravitational wave observations can probe and constrain ultralight dark matter, accretion physics, and strong-field gravity.

Speaker: Chen Yuan (Shanghai University)

15:15 Black hole superradiance of interacting multifield

We investigate black hole superradiance evolution of the interacting multiple fields. We consider a model of two scalar fields interacting with a cubic coupling, and study the superradiant evolution of the cloud. We demonstrate that superradiance is typically suppressed when the superradiant field couples to another field, even with a very weak coupling strength. This implies that the constraints on dark particles derived from single-field analyses can be revised in the presence of interactions. Moreover, we find that the multifield superradiant evolution and its corresponding observational signatures can be different across parameter spaces, which makes black hole superradiance an even more powerful probe of the dark sector in particle physics.

Speaker: Zhi-Qing Zhu (ICTP-AP)

14:00 → 15:30 Parallel Session B

14:00 Long-living Rings and direct wave related with black hole horizon

Near-extremal Kerr black holes support zero-damped modes (ZDMs), whose small time-domain damping rates make them long-lived probes of the near-horizon region. We show that bound extreme-mass-ratio inspirals (EMRIs) can resonantly drive this response in vacuum general relativity.
Using frequency-domain Teukolsky amplitudes for eccentric-inclined Kerr geodesics, we identify a source-supported orbital harmonic whose real frequency falls within one pole half-width of the fundamental gravitational ZDM. In the complex response, the pole contribution is enhanced by this small half-width, while complex-response tomography recovers the independently computed Kerr pole from real-frequency orbital data. After subtracting the smooth non-pole component, the residual exhibits the phase winding of a coherent simple pole, with a pole contribution comparable to the smooth non-pole part of the EMRI-sourced Teukolsky amplitude. The driven branch also lies in the superradiant regime and carries negative horizon flux. These results establish a pole-resolved, resonantly driven ZDM response by EMRIs and make the recovered pole half-width a route to measuring the horizon surface gravity.

Speaker: Wenbiao Han (SHAO)

14:15 Instability and backreaction of massive spin-2 fields around black holes

Ultralight bosonic fields are compelling dark matter candidates. In particular, massive spin-2 fields can not only extract energy from rotating black holes via superradiant instabilities but also trigger a unique monopolar instability—specifically, the axisymmetric (azimuthal number m = 0) instability—which can even affect non-rotating black holes. This instability dominates in most of the parameter space and could potentially influence all black holes, from stellar-mass to supermassive. A thorough understanding of its nonlinear evolution and final state is therefore of great importance. In this work, based on ghost-free massive gravity theories (dRGT and the Hassan–Rosen bigravity theory), we employ time-domain evolution methods to systematically investigate the nonlinear dynamics of this instability in a spherically symmetric black hole background. Our results show that the final outcome of the evolution depends on the model parameters: the system can either evolve into a stable hairy black hole or experience a breakdown of the theory itself due to nonlinear effects. This study provides important insights into the nonlinear dynamics of massive gravity theories and paves the way for testing ultralight spin-2 dark matter models through astronomical observations.

Speaker: Zhen Zhong

14:30 Nonlinear Tidal Modes in Neutron Stars

Tidal interactions in compact binaries excite neutron-star oscillations and reshape the late inspiral. Semi-analytic studies predict that, as merger approaches, the growing tidal deformation couples stellar modes and shifts their frequencies. We compare nonlinear hydrodynamic evolutions with a coupled-mode treatment of the quadrupolar f-mode. For moderate tides, the measured shift follows the quadratic amplitude dependence predicted by second-order perturbation theory. At larger amplitudes it grows beyond that prediction, showing that the f-mode has entered a strongly nonlinear regime in which the frequency shift is no longer a small correction to the linear spectrum.

Speaker: Guangzhou Guo (Tsung-Dao Lee Institute)

14:45 Does a conformal transformation change the quasinormal mode spectrum?

Conformally related black hole metrics have been reported to exhibit both conformal factor dependent and independent quasinormal mode (QNM) spectra. We show that these two outcomes correspond to different boundary value problems and resolve the apparent discrepancy in four dimensions. We reduce the Bach perturbation equations to two coupled radial equations of second order. For the Schwarzschild seed metric, the Bach spectrum consists of Regge-Wheeler and spin-1 branches. When the conformal factor and its inverse are nonsingular throughout the black hole exterior, the boundary conditions and the normalization of the time coordinate are preserved, the spectra is invariant under the conformal transformation. The same approach can also reduce perturbation equations of similar form in other theories with higher derivatives to a finite set of radial equations of lower order and test how other symmetries are reflected in QNM spectra.

Speaker: Han-Wen Hu (ITP-CAS)

15:00 Random Environmental Perturbations of Quasinormal Modes: Perturbative and Numerical Analyses in the Pöschl–Teller Model

Environmental effects can modify quasinormal-mode spectra and affect their physical interpretation. We model such effects by adding a localized Gaussian random perturbation to the analytically solvable Pöschl–Teller potential. Using logarithmic perturbation theory and the generalized QNM norm, we derive the first- and second-order frequency corrections to the fundamental mode while treating the Pöschl–Teller exterior exactly through outgoing boundary conditions. Independently, we solve the full perturbed wave equation using shooting and Wronskian-matching methods and track the fundamental mode by numerical continuation. Comparing the perturbative and direct numerical results provides a controlled test of the accuracy of QNM perturbation theory under random environmental effects.

Speaker: jianing Chen (HIAS, UCAS.)

15:15 Schwarzschild–de Sitter 时空中奇异致密天体的虚吸收模

We present a study of virtual absorption modes (VAMs) in Schwarzschild-de Sitter (SdS) spacetime under semi-open boundary conditions, where the VAMs correspond to total transmission modes (TTMs) with the reflection amplitude being vanishing. Our numerical analysis reveals that as the reflectivity $|\mathcal{K}|$ decreases, the VAM spectra migrate systematically toward regions of less negative imaginary parts, with each overtone exhibiting a critical reflectivity at which $\text{Im}(\omega_{\text{VAM}})=0$. Using simulations based on spectral collocation methods, we demonstrate that excitation precisely at a VAM spectrum leads to a virtual absorption process. These results establish VAMs as the spectral signatures of virtual absorption processes for exotic compact objects (ECOs).

Speaker: Liang-Bi Wu (HIAS UCAS)

15:30 → 16:00 Break

16:00 → 17:30 Parallel Session A

16:00 Gravitational Atomic Spectra: Dynamics, Emission, and the Axion Potential

Axion fields can form exponentially growing gravitational clouds around compact objects through self-interaction–driven relaxation of ambient axion waves. As the field amplitude approaches the axion decay constant, nonlinear effects become important. We identify two distinct regimes of late-time evolution, determined by the gravitational fine-structure constant and the cloud growth rate: a Bosenova regime, characterized by collapse accompanied by explosive axion bursts, and a saturation regime, in which self-interaction–induced axion emission balances accretion. In the latter regime, the emitted axion radiation exhibits stable discrete spectral lines at odd multiples of the bound-state energy, directly encoding the structure of the axion self-interaction potential. We show that single-cosine potentials and QCD axion-like potentials predict distinct emission spectra, opening the possibility of probing the underlying axion self-interaction structure and its ultraviolet completion through terrestrial detection of relativistic axion fluxes from compact objects.

Speaker: Ximeng Li (TDLI)

16:15 Background-Induced Forces from Quadratically Coupled Ultralight Dark Matter

Quadratically coupled ultralight scalar dark matter behaves as a coherent classical field whose interactions with matter can induce a composition-dependent force through the dark matter background. We present a complete calculation of this background-induced force beyond the spherically symmetric approximation. Using a partial-wave treatment of dark-matter scattering, we determine its angular dependence and derive an analytic description valid even when the dark-matter wavelength is much smaller than the Earth's radius. We show for the first time that Earth screening generates a characteristic frequency-band structure, splitting the signal into multiple sidebands that provide a distinctive experimental signature. We further show that the relative amplitudes of these sidebands vary annually due to the Earth's motion through the dark-matter halo, enabling the construction of a complete signal template. As an application of these results, we re-evaluate constraints from the MICROSCOPE mission, which currently provides the strongest laboratory limits on equivalence-principle violations from ultralight dark matter. We further show that proposed space-based equivalence-principle experiments, such as Galileo Galilei and STE-QUEST, can significantly enhance their sensitivity to ultralight scalar dark matter by incorporating the full frequency-band information.

Speaker: Dr Hailin Xu (Tsung-Dao Lee Institute)

16:30 Constraining axion quadratic couplings with the Hulse-Taylor binary system

The orbital evolution of the Hulse-Taylor binary neutron star system is described with high precision by General Relativity, in which gravity is the only long-range force and gravitational waves provide the dominant energy-loss channel. We use this precision test of relativistic binary dynamics to derive new constraints on axion couplings to stable neutron-star constituents: neutrons, electrons, and muons. In the presence of exotic long-range forces and additional radiation channels, the measured orbital decay constrains the strength of quadratic axion-fermion interactions.
Axions enjoy a perturbative shift symmetry that is broken by non-perturbative effects responsible for the axion mass. Such shift symmetry breaking can induce quadratic couplings between axions and fermions. In an ambient axion dark-matter background, these couplings mediate enhanced long-range, spin-independent forces, while also allowing binary systems to lose energy through dipole and quadrupole emission of axion waves. For light QCD axions and for models in which non-perturbative effects correct lepton masses, our bounds can be recast as limits on the axion decay constant.

Speaker: Ziwen Yin (Tsung-Dao Lee Institute & SJTU)

16:45 Dynamical gravitational-wave lensing by gravitational atoms

Ultralight bosons around spinning black holes can form macroscopic gravitational atoms through superradiance. We show that attractive self-interactions can turn such systems into periodically varying lenses for gravitational waves. During the long-lived 211–322 quasi-equilibrium, coherent interference between the dominant 211 cloud and the self-interaction-populated 322 level produces an oscillating gravitational potential. We formulate the lensing response of this oscillating cloud as a discrete sideband expansion: the periodic lens redistributes each incident frequency component among discrete sidebands separated by integer multiples of the cloud’s beat frequency. Applying this framework to stellar-mass binary-black-hole inspirals lensed by supermassive black holes hosting boson clouds, we compute the dynamically lensed waveforms and compare them with their static-lens counterparts. A waveform-mismatch analysis identifies broad regions of parameter space where the dynamical modulation produces substantial, distinguishable deviations from static lensing. These signatures offer a potential probe of coherent level mixing and self-interactions in the ultralight-boson sector.

Speaker: Zhen-Hong Lyu (Institute of Theoretical Physics, Chinese Academy of Sciences)

17:00 Illuminating Black Hole Shadows with Dark Matter Annihilation

The Event Horizon Telescope (EHT) has significantly advanced our ability to study black holes, achieving unprecedented spatial resolution and revealing horizon-scale structures. Notably, these observations feature a distinctive dark shadow—primarily arising from faint jet emissions—surrounded by a bright photon ring. Anticipated upgrades of the EHT promise substantial improvements in dynamic range, enabling deeper exploration of low-background regions, particularly the inner shadow defined by the lensed equatorial horizon. Our analysis shows that observations of these regions transform supermassive black holes into powerful probes for annihilating dark matter, which is expected to accumulate densely in their vicinity. By analyzing the black hole image morphology and performing electron-positron propagation calculations in realistic plasma backgrounds derived from general relativistic magnetohydrodynamic simulations, we set stringent constraints on dark matter annihilation, requiring contributions below the astrophysical emission. These constraints, derived from both current EHT observations and projections for future upgraded arrays, exclude a substantial region of previously unexplored parameter space and remain robust against astrophysical uncertainties, including black hole spin and plasma temperature variations.

Speaker: Yanjie Zeng (Peking University)

17:15 Constraints on Symmetric Dark Matter from Neutron Star Capture and Collapse

Neutron stars are known to constrain asymmetric dark matter because accumulated particles can trigger collapse. I will show that these constraints extend much further: even when dark matter is produced with equal particle and antiparticle abundances, the capture process inside a neutron star can become intrinsically asymmetric if the two species scatter off nucleons with slightly different strengths. This difference arises naturally from interference between interaction terms with specific charge-conjugation properties, and turns out to be generic across the full set of allowed dark matter–nucleon couplings. As a result, neutron stars can exclude extremely small scattering rates over a wide mass range, and the constraints remain effective even when the capture asymmetry is tiny. Neutron stars probe not only abundance asymmetries but also the microscopic structure of dark matter interactions, significantly broadening their impact on dark matter theory.

Speaker: Yuxin Liu

16:00 → 17:30 Parallel Session B

16:00 Gravitational-Wave Echoes from Layered Compact Objects: A Double-Shell Model

Layering is an ubiquitous feature of astrophysical objects. Motivated by the fact that physical black holes retain such structures in the time definition all outside probes are selected and monitored, we investigate linear perturbations and gravitational-wave echoes of a compact object composed of two concentric thin shells. Compared with the single-shell configuration, the double shell structure introduces an extra peak in the effective potential and partitions the spacetime into multiple resonant cavities. As the mass distribution between the two shells varies, the echo waveform and its spectrum undergo systematic evolution. In particular, we identify a direct correspondence between cavity lengths and the migration of resonance frequencies, and uncover a nontrivial branch permutation of spectral peaks arising from the coupling among multiple cavities. These results reveal generic wave-propagation signatures in the layered compact objects and suggest that gravitational-wave echoes may encode information about their internal structure.

Speaker: Qi Su (Beijing University of Technology)

16:15 Self-force on a static scalar charge in traversable wormholes

The self-force acting on a charged particle is sensitive to the global structure of curved spacetime and can serve as a probe of geometry beyond local curvature. We compute the static scalar self-force on a point charge in the two-parameter family of spherically symmetric wormholes introduced by Konoplya and Zhidenko, members of the broader Morris–Thorne class of traversable wormholes. Using mode-sum regularization, we analyze its dependence on the shape exponent $q$, which controls the throat geometry, and the redshift parameter $p$, which determines the redshift function and tidal strength. We find that the self-force is generally not unidirectional: it can change sign with radial distance from the throat, with up to two distinct zero crossings depending on $(p,q)$. We provide a systematic characterization of how both the direction and large-distance falloff depend on the wormhole parameters. For sufficiently large $p$, the force can decay at a slower rate than the canonical $\sim r^{-3}$ behavior typical of isolated-body spacetimes, with stronger flaring (more negative $q$) leading to more rapid decay. In the combined limit $p \to \infty$ and $q \to -\infty$, the asymptotic falloff approaches that of the static scalar self-force in the Ellis wormhole.

Speaker: Jerome Mecca ((1) University of Northern Philippines, (2) National Institute of Physics, University of the Philippines Diliman)

16:30 Shaving off soft hairs and black hole image memory effect

Soft hairs of black holes are the Noether charges associated with the generalized Bondi-Metzner-Sachs symmetries. In this work, the images of soft-haired Kerr black holes are studied. For an eternal black hole, the image is rotated, dilated, and drifting compared to that of the bald counterpart in the celestial plane. The rotation and the dilation are independent of time, while the drifting occurs at a constant speed and in a fixed direction. These effects all depend on angular directions. The soft hair of an astronomical black hole can change due to the emission of gravitational or electromagnetic waves from various physical processes occurring in the vicinity of the horizon. Then, the image roams in the observer’s view, causing the image memory effect, the smoking gun for the existence of soft hair. The magnitude of the image memory effect of a huge, spinning black hole accompanied by a much smaller one is estimated. It turns out that this effect is proportional to the mass of the large black hole, increases with its spin, but decreases with the mass ratio. Due to the limited angular resolution of current and future detectors, this effect is hard to detect if the impact of cosmological expansion is ignored.

Speaker: Shaoqi Hou

16:45 Extending Kerr–Bertotti–Robinson Spacetime Through Infinity: A Wormhole Connecting a Black Hole’s Exterior and Interior

The surface $r=\infty$ in the Kerr--Bertotti--Robinson (KBR) spacetime is known not to be a physical boundary. We explicitly construct a natural extension across it, obtaining a wormhole that connects the exterior of one KBR black hole to the interior of a neighboring one. Depending on the parameters, the extended spacetime exhibits several distinct global causal structures and can contain a naked singularity. We further study a test scalar field and compute its quasinormal-mode spectrum, which suggests the presence of echoes naturally associated with the wormhole geometry. Unstable modes also emerge near extremality.

Speaker: Yu-Sen Zhou (University of Science and Technology of China)

17:00 XOB Reveals Physics inside Black Holes

XOB provides an eXact One-Body approach to the conservative part of binary dynamics in General Relativity. We will report that dynamics of this approach under the weak-field-low-speed expansion is equivalent to the post-newtonian approximation at all orders, thus making it a truly GR-version one-body formulation of two-body problems. Using this approach and a banana-shape-deformation modulated quadrupole formula, we calculate the gravitational waveform of black hole binary merger process and get results matching numeric relativity better than 99\%. As a fully analytical and whole-process applicable one-body approach, XOB not only produces the gravitational waveform of binary merger process rapidly, but also links its feature to the inner structure of merger bodies transparently.

Speaker: Dingfang Zeng (Beijing University of Technology)

17:15 Causal Green function decomposition for quantum black hole seismology

The growing sensitivity of gravitational-wave detectors enables increasingly precise tests of black hole (BH) ringdown spectroscopy. Yet BH quasinormal modes (QNMs) are spectrally unstable: small near horizon modifications can produce a drastically different QNM spectrum, while causality requires the prompt ringdown to remain BH-like until the reflected signal returns. Quantum BHs with potentially large interior reflection provide a natural setting for this tension. The relation between their time-domain waveform and different QNM spectra, although repeatedly discussed, still lacks a consistent treatment. In this work, we systematically examine the time-domain Green function for quantum BHs, considering sources located outside and inside the light-ring potential barrier. By decomposing the Green function into causally distinct components and choosing the corresponding inverse-Laplace contours consistently, we clarify how the response is built from different sets of QNMs. We find that the quantum BH QNM reconstruction provides a faithful description once the curved-spacetime region is probed, but its practical efficiency depends strongly on the evolutionary stage. Before interior reflection becomes relevant, we prove that this basis is formally equivalent to the BH QNM and tail expansions, although its convergence properties depend on the source location. At late times, the long-lived modes always provide an efficient basis. These results are confirmed by time-domain simulations and provide a unified causal picture of BH spectroscopy and quantum BH seismology.

Speaker: Xi-Li Zhang (IHEP,CAS)

Tuesday 25 August

09:00 → 10:00 Plenary Session

09:00 TBD

TBD

Speaker: Gilad Perez

09:30 GUEST: a mission to fill the microHz gap in GW searches

I will present GUEST, a current proposal showing how satellite laser ranging of two tests masses in properly chosen orbits may be sensitive to GWs backgrounds/events in the microHz band at levels where one expect signals from different phenomena.

Speaker: Diego Blas (ICREA/IFAE)

10:00 → 10:30 Break

10:30 → 11:30 Plenary Session

11:00 Evidence and Constraints on Ultralight Dark Matter

Ultralight dark matter (ULDM) has attracted significant attention as a compelling alternative to cold dark matter, yet it faces increasingly stringent constraints from observations of compact stellar systems and satellite galaxies. In this talk, I present a series of numerical simulation studies from our group that systematically reexamine these constraints. We first show how the almost-dark galaxy Nube with extremely diffuse stellar distribution can be naturally explained by ULDM dynamical heating. I then show that incorporating the realistic tidal potential of the Milky Way can substantially suppresses ULDM heating in satellite galaxies, significantly relaxing the tension for $m_a \sim 10^{-22}$ eV. Then N-body simulations also reveal that internal two-body relaxation competes with ULDM heating, invalidating simple heating-only limits for very compact systems below the de Broglie wavelength. These results indicate that previous idealized bounds may have been overly restrictive, and that ultralight dark matter remains a viable and well-motivated candidate.

Speaker: Xiaojun Bi (Institute of High Energy Physics, Chinese Academy of Sciences)

11:30 → 12:00 Plenary Session: Panel discussion

14:00 → 15:30 Parallel Session A

14:00 Dr

Numerical-relativity data are not generic arrays to be enhanced, but
discretized geometric fields constrained by the Einstein equations. We
introduce AI(\times)NR-Pathfinder, a constraint-aware learning framework
that applies small, stencil-consistent correction operators to
(3+1) numerical-relativity slices while preserving the algebraic
structure of the conformal variables. The framework is designed not as a
replacement for elliptic initial-data solvers, but as a controlled method
for moving an existing numerical-relativity state toward a nearby
lower-constraint configuration within a restricted correction subspace. We
first establish the method in the instance-adapted limit, where a single
binary boson-star slice is optimized to convergence and used to construct
a corrected numerical-relativity state. In this setting, the corrected
state reduces Hamiltonian- and momentum-constraint residuals by
(82)--(93\%) on the evaluated physical domain, remains well behaved
under short-time evolution, and preserves the relevant field-level
structure.

The central nontrivial result emerges from a core-localized comparison
with two directly generated fine-grid reference states: a plain
superposition construction and a physics-motivated remedy designed to
reduce superposition-induced core distortions. The model is not trained
to match either reference state. Nevertheless, the corrected state
suppresses the core-localized constraint residuals to levels substantially
below both fine-grid references in all reported core-window diagnostics.
This demonstrates that the learned update is capable of correcting the
most physically critical regions of the initial data: the boson-star
cores, where superposition artefacts are expected to have the largest
dynamical impact. The result is therefore not merely a reduction of a
domain-averaged loss, but a targeted improvement in the regions most
relevant to the subsequent evolution.

We then train the same correction framework on a family of binary
boson-star configurations and test it on held-out configurations, finding
systematic reductions of both Hamiltonian- and momentum-constraint
residuals across the test set. A preliminary vacuum-GR check on perturbed
Bowen--York data further suggests that the same correction framework is
not tied to the binary boson-star setting, but can also be applied to
vacuum initial data. Together, these results demonstrate that
constraint-aware learning can identify physically meaningful correction
directions in constrained geometric data. AI(\times)NR-Pathfinder
therefore provides a concrete step toward physically grounded
machine-learning assistance for Einstein-constraint control in numerical
relativity.

Speaker: Bo-Xuan Ge

14:15 Black-hole - neutron-star mergers: new numerical-relativity simulations and multipolar effective-one-body model with spin precession and eccentricity

We present 52 new numerical-relativity (NR) simulations of black-hole-neutron-star merger (BHNS) mergers and employ the data to inform a multipolar effective-one-body model for complete waveforms from arbitrary mergers. Our simulations target quasicircular mergers and the parameter space region characterized by significant tidal disruption of the star. Convergent gravitational waveforms are produced with a detailed error budget after extensive numerical tests. We study in detail the multipolar amplitude hierarchy and identify a characteristic tidal signature in the (l,m)=(2,0), and (3,0) modes. We also develop the most updated NR-informed model for the remnant black hole and a new model for the recoil velocity. The numerical data is then used to inform next-to-quasicircular corrections and the ringdown of TEOBResumS-Dali for BHNS. We show an overall order of magnitude improvement in the waveform's amplitude at merger and more consistent multipoles over our older TEOBResumS-GIOTTO for BHNS. TEOBResumS-Dali is further validated with a new 12 orbit precessing simulation, showing phase and relative amplitude differences below 0.5 (rad) throughout the inspiral. The computed mismatches including all the modes lie at the one percent level for low inclinations. Finally, we demonstrate for the first time that TEOBResumS-Dali can produce robust waveforms with both eccentricity and precession, and use the model to identify the most urgent BHNS to simulate for waveform development.

Speaker: Alejandra Gonzalez (University of the Balearic Islands)

14:30 Following massive black hole binaries and triplets to coalescence in live galaxy simulations: gravitational wave source predictions with RAMCOAL

Low-frequency gravitational-wave surveys need not just how often massive black holes (MBHs) merge, but the parameters carried to coalescence: masses, mass ratio, spins, eccentricity, and the delay since galaxy assembly. These depend on dynamical regimes that cosmological galaxy simulations cannot resolve. We extend RAMCOAL, a subgrid model in the RAMSES hydrodynamical code, to follow MBH binaries and now triplets — from galactic scales down to coalescence during the simulation. Black holes evolve from sink particles through a dynamical-friction phase into bound binaries hardened by stellar scattering, gas torques, circumbinary-disc coupling, and gravitational-wave emission, all within the live, evolving galaxy. When a later merger delivers a third MBH that drives the system chaotic, the model maps the interaction onto a library of three-body outcomes and updates the surviving binary, allowing exchanges, recoils, and ejections. Using isolated-galaxy tests, we show that the encounter geometry alone can change which pair finally coalesces, the eccentricity carried into the gravitational-wave band, and the merger delay (~0.3 to ~3.5 Gyr in our two configurations). By keeping coalescence, accretion, spin, and recoil coupled to the host galaxy, RAMCOAL aims to provide pre-coalescence source properties and electromagnetic context for multimessenger predictions.

Speaker: Kunyang Li (Center for Computational Astrophysics, Flatiron Institute)

14:45 Boson star-black hole binaries: initial data and head-on collisions

We present a numerical-relativity study of comparable-mass boson star-black hole (BS-BH) head-on collisions, focusing on both initial-data construction and gravitational-wave (GW) phenomenology. We show that plain superposition can strongly perturb the BS core, leading to large constraint violations and unphysical radial oscillations. To remedy this problem, we introduce a one-body conformal-factor correction and find that it robustly suppresses these artifacts. Using the improved initial data, we analyze GW emission from equal- and unequal-mass BS-BH binaries and compare with matched BS-BS and BH-BH baselines. For equal masses, the BS-BH radiated energy increases with BS compactness and approaches the BH-BH limit for highly compact stars. For unequal masses, the dominant (2,0) mode often remains close to the BH-BH morphology, whereas the subdominant (3,0) mode provides clear discriminatory power when the BH is the heavier companion. Our results identify higher multipoles as a key observable for distinguishing mixed BS-BH mergers from pure BH binaries.

Speaker: 专 宁

15:00 A deep learning framework for amplitude generation of general EMRIs

The major challenge for the fast generation of such waveforms is the generation of the Teukolsky amplitudes for generic (eccentric and inclined) Kerr orbits. The requirement forthe modeling of ∼ 10^5 harmonic modes across a four-dimensional parameter space makes traditional approaches, including direct computation or dense interpolation, computationally prohibitive. To overcome this issue, we introduce a convolutional encoder-decoder architecture for a fast and end-to-end global fitting of the Teukolsky amplitudes. We also adopt a transfer learning strategy to reduce the size of the training dataset, and the model is trained gradually from the simplest Schwarzschild circular orbits to generic Kerr orbits step by step. Within this framework, we obtain a surrogate model based on a semi-analytical Post-Newtonian dataset, and the full harmonic amplitudes can be generated within milliseconds, while the median mode-distribution error for generic orbits is approximately ∼ 10^{−3} . This result indicates that the framework is viable for constructing efficient waveform models for EMRIs.

Speaker: Jian-dong Zhang (Sun Yat-sen Univeristy)

15:15 Relativistic treatment of extreme mass ratio inspirals crossing accretion disks

A small body orbiting around an accreting massive object and periodically crossing its accretion disk is a
common configuration in astrophysics. In this work, we study the secular evolution of extreme mass-ratio
inspirals (EMRIs), where a stellar-mass object (SMO)—such as a star or a stellar-mass black hole (sBH)—
collides with the accretion disk surrounding a central supermassive black hole (SMBH). Using a
perturbation method applied to EMRI geodesics, we find the following: (1) the disk tends to align the
SMO regardless of its initial inclination ι relative to the disk, (2) the final orbital eccentricity of an SMO
captured by the disk is low, although the eccentricity can temporarily increase when the initial inclination ι is
large and the SMO is an sBH, (3) through collisions with the accretion disk alone, only a small fraction of
sBHs that initially lie close to both the SMBH and the disk can be captured within the typical disk lifetime of
active galactic nuclei. Two-body scatterings among SMOs in the nuclear stellar cluster play an essential role,
randomly kicking sBHs toward the disk and significantly boosting the capture rate.

Speaker: Yuhe Zeng (Tsung-Dao Lee Institute, Shanghai Jiao Tong University)

14:00 → 15:30 Parallel Session B

14:00 Primordial black holes as cosmic accelerators of light dark matter

Current multi-tonne-scale dark matter (DM) detectors are largely incapable of detecting light dark matter from the Galactic halo due to the energy threshold limitations of their recoil measurements. However, primordial black holes (PBHs) can evaporate via Hawking radiation to particles whose energies are set by the black hole temperature. Consequently, weakly interacting light dark matter (or dark radiation) particles produced in this manner can reach the Earth with sufficient flux and kinetic energy above the experimental thresholds. This opens up a novel avenue to probe the light dark sector in terrestrial experiments. In this work, we explore this possibility by considering fermionic DM produced through PBH evaporation and investigating its electron recoil signatures in direct detection experiments. We analyze both energy independent (constant) and energy dependent (scalar and vector mediated) DM-electron interactions, highlighting the strong dependence of the recoil spectra on the underlying Lorentz structure of the interaction. In addition, we also account for the attenuation effects due to the loss of kinetic energy while DM traverses through Earth’s crust, which can significantly modify the incoming DM flux. Incorporating these effects carefully, we place constraints on light DM using the electron recoil data from XENONnT, LZ, and PandaX-4T. Finally, we also discuss the detection prospects of such dark matter in current and future generation neutrino detectors, such as Super-Kamiokande and Hyper-Kamiokande.

Speaker: Sk Jeesun

14:15 SM-Charged Portals for Dark Photons Protected by Dark Charge Conjugation

We identify the Standard Model–charged portal multiplets that can connect the visible sector to a massive dark photon while preserving an exact dark charge conjugation that protects it from decay. Requiring the Standard Model gauge fields to remain invariant under this symmetry forces the portal to carry zero hypercharge and to transform in self-conjugate representations of (SU(3)_c\times SU(2)_L). We classify the allowed fermionic and scalar portals and consider, within an effective field theory framework, the minimal colored realization containing a vectorlike color-adjoint fermion. If this fermion thermalizes in the early Universe, its freeze-out leaves stable colored bound states subject to stringent cosmological and direct-detection constraints. We therefore introduce a charge-two dark Higgs whose renormalizable Majorana interactions split the Dirac fermion into two Majorana states. Heavier-state cascades and efficient QCD annihilations can then reduce the colored relic abundance by several orders of magnitude. Charge-two Higgsing nevertheless leaves a residual gauge parity that stabilizes the lightest colored state. Charge-one Higgsing removes this remnant and allows dark-charge-conjugation-preserving higher-dimensional operators to split the fermion and induce its decay. The dark photon can then constitute dark matter if it is the lightest accessible state odd under dark charge conjugation.

Speaker: Qiyuan Gao (ICTP-AP & University of Chinese Academy of Sciences)

14:30 Particle productions during collisions of highly boosted bubble walls

In this talk, we studied heavy particle production from bubble collisions, and we show that by analyzing the trapping equation, the efficiency factor in the high-frequency regime behaves as $f(\chi) \sim \chi^{-4}$, a result we then generalize to $3+1$ dimensions while confirming the same power-law suppression.

Speaker: Hongyi Jiang (Tsinghua University)

14:45 Mechanism of kinetic equilibrium between SIMP and radiation

Strongly Interacting Massive Particles (SIMPs) provide an intriguing alternative to conventional collisionless dark matter (CDM) scenarios, particularly in light of tensions between CDM predictions and observations on small astrophysical scales. In SIMP models, the observed dark matter relic abundance is generated through self-annihilation processes such as $3 \to 2$ or $4 \to 2$, involving only dark sector particles. However, these processes can overheat the dark sector and jeopardise structure formation. This issue can be resolved if the DM particles are in kinetic equilibrium with lighter particles, which redshifts similar to radiation during DM freezeout, either in the SM or the dark sector. In conventional SIMP models, the interactions responsible for kinetic equilibrium are often related by crossing symmetry to pair-annihilation processes, inevitably inducing sizable dark matter annihilation into radiation. This leads to strong constraints on the allowed parameter space. In this talk, I will introduce a novel mechanism in which dark matter–radiation scattering and annihilation are governed by different combinations of parameters. I will show that it can be realized in new physics models, and then discuss their potential detections.

Speaker: Ajay Kaladharan (ICTP-AP)

15:00 Rethinking Neutron-Star Structure with Continuous Gravitational-Wave Constraints

Every rapidly spinning neutron star with slight asymmetry would broadcast a continuous hum of gravitational waves (GW). None of these signals has been detected yet, but the silence itself is measurable: searches now set upper limits on the GW amplitude of dozens of known pulsars, and for a growing subset those limits dig below the energy budget available from the stars' own spin-down. We ask what this kind of data, can say about the interior of a neutron star. The amplitude of the continous signal depends on how much matter the star carries in its deformation and how that matter is distributed, its moment of inertia, which in turn is fixed by the unknown equation of state of dense matter. This opens a route to the neutron star mass-radius relation that is fully independent of the X-ray pulse-profile and binary-merger measurements that anchor current constraints. We build a hierarchial Bayesian pipeline that propagates simulated amplitude limits, treated consistently as non-detections, through stellar-structure calculations to the mass-radius plane, marginalizing over the unknown deformations of the pulsar population. Rather than claiming a new measurement, we chart the sensitivity frontier: how deep continuous-wave searches must go before they begin to weigh in on dense matter on their own.

Speaker: Ayush Hazarika (International Center for Theoretical Physics Asia-Pacific (ICTP-AP), University of Chinese Academy of Sciences)

15:15 Quantum geometry of the Standard Model and Majorana neutrinos

Noncommutative/quantum geometry is the program of studying geometry on noncommutative algebras (modeling quantum coordinates and spacetime functions) endowed with some extra structure (Dirac operator and Hilbert space) giving rise to a spectral triple. It is well known that a special noncommutative geometry, via the spectral action principle, uniquely leads to the classical Standard Model Lagrangian, with all of the correct field representations and symmetry breaking mechanism, coupled to gravity (and optionally right handed Majorana neutrinos).

This internal noncommutative geometry can be viewed as the product of smooth spacetime manifold with a finite internal space. Recently in [1] it was shown that this internal quantum geometry behind the Standard Model definitely carries nonzero torsion tensor and that its components are proportional to the Yukawa couplings. Additionally, Ricci (scalar and tensor) curvatures, along with the Einstein tensor, were computed for this internal geometry and it was found that the results depend on the Majorana mass matrix $\Upsilon_R$, offering perhaps a quantum-geometric test of the existence of Majorana neutrinos and their mass.

[1] - L. Dabrowski, S. Mukhopadhyay and F. Pozar, Spectral torsion of the internal noncommutative geometry of the Standard Model,
[arXiv:2511.08159 [hep-th]].

Speaker: Filip Pozar (Rudjer Boskovic Institute)

15:30 → 16:00 Break

16:00 → 17:45 Parallel Session A

16:00 Gravitational Wave Data Analysis using the F-statistic method

Gravitational-wave observations offer a powerful way to probe compact binary coalescences and test strong-field gravity. In this talk, I will present a preliminary study on gravitational-wave data analysis using the F-statistic, focusing on its applications to black-hole ringdown and full inspiral–merger–ringdown analyses. By analytically maximizing over amplitude-related parameters, the method reduces the dimensionality of the parameter space and improves sampling efficiency. Specifically, the parameter estimation efficiency for ringdown analysis is improved by at least a factor of 100, while for the full inspiral–merger–ringdown analysis, the improvement in parameter estimation efficiency reaches up to approximately a factor of 5. Injection studies and applications to events such as GW150914, GW231028, GW190412, and GW190814 show that the F-statistic can recover source parameters efficiently and help assess evidence for sub-dominant ringdown modes. These results suggest that the F-statistic is a promising tool for future gravitational-wave data analyses.

Speaker: Haitian Wang (Dalian University of Technology)

16:15 Efficiency of current gravitational wave microlensing searches using isolated compact object lenses

Gravitational-wave (GW) microlensing occurs when an intervening compact object diffracts the GW in the wave-optics regime, imprinting characteristic frequency-dependent modulations on the observed signal. Current searches for microlensing signatures in the detected GW events assume an isolated point mass lens (PML) model. However, this assumption is overly simplistic. Astrophysical microlenses are typically embedded within more complex structures, such as galaxies, whose potentials can significantly perturb the lensing signal. In this work, we extend the standard PML model to incorporate external galactic influences by considering microlenses perturbed by galaxy potentials. We demonstrate that neglecting host galaxy effects can systematically reduce sensitivity in microlensing searches, particularly for high microlens masses and at high signal-to-noise ratios.

Speaker: Uddeepta Deka (Shanghai Astronomical Observatory - Chinese Academy of Sciences)

16:30 Resolving individual supermassive black hole binaries in pulsar timing arrays: a harmonic-space framework

Pulsar timing arrays are becoming sensitive to individual supermassive black hole binaries above the nanohertz gravitational-wave background. I will present a harmonic-space framework that consistently incorporates irregular pulsar sampling, non-uniform noise, source confusion, and pulsar terms. By marginalizing over unknown pulsar phases, we obtain a parameter-dependent covariance and derive the corresponding likelihood, detection statistic, and Fisher information. Combined with a Poisson population model, the framework yields semi-analytic predictions for the number and probability of resolvable binaries and clarifies how observation time determine the continuous-wave discovery potential of current and future pulsar timing arrays observations. Following MCMC tests further demonstrate at sufficiently high SNR ratio, the proposed likelihood accurately recovers the single source parameters.

Speaker: Hanyu Cheng

16:45 Searches for the Isotropic Gravitational-Wave Backgrounds with LIGO-Virgo-KAGRA detectors.

The stochastic gravitational-wave backgrounds are expected to arise from the superposition of many individually unresolved sources throughout cosmic history, carrying information about compact-binary populations, gravity beyond general relativity, and possible processes in the early Universe. In this talk, I will present recent progress in the LIGO-Virgo-KAGRA search for an isotropic gravitational-wave backgrounds using cross-correlation measurements between ground-based detectors. I will first describe updates to the analysis, with particular emphasis on new frequency-domain cuts designed to identify and remove narrowband instrumental artifacts while preserving sensitivity to broadband stochastic signals.

I will then discuss the search for a power-law gravitational-wave background with spectral index 2/3, as predicted for a background dominated by inspiralling compact binaries, and compare the observational constraints with predicted compact-binary coalescence backgrounds based on GWTC population models. Beyond the standard tensor-polarized background, I will also summarize searches for non-GR polarization components, including possible scalar and vector modes, and their implications for alternative theories of gravity. Since correlated magnetic noise can mimic a stochastic signal, I will further review the magnetic-noise budget and its impact on the interpretation of the search.

Finally, following the collaboration results, I will introduce recent work on inflation-era phase transitions as a potential cosmological source of stochastic gravitational waves. This provides a complementary perspective on how current gravitational-wave background searches can constrain early-Universe physics and highlights the discovery potential of future ground-based detectors.

Speaker: 迈 乔 (International Centre for Theoretical Physics Asia-Pacific, University of Chinese Academy of Sciences)

17:00 SPRING: A Spectral-Level Framework for Strong-Field Tests with Gravitational-Wave Ringdowns

Gravitational-wave ringdowns encode the characteristic oscillation spectrum of the compact remnant and provide a direct probe of strong-field gravity. Yet conventional analyses usually infer remnant properties with theory-specific waveform templates, entangling the information extracted from the data with the assumed model. Ringdown inference is also often treated in isolation, leaving aside valuable pre-merger information about the progenitor binary and, consequently, the expected excitation of the remnant modes.

We present SPRING (short for Spectral-level Pre-merger-informed RINGdown inference), a framework that separates data-driven spectral inference from theory-specific interpretation. The ringdown signal is first used to reconstruct the observable mode spectrum without assuming a theoretical model. Information from the pre-merger signal is then used to guide the expected mode excitation. SPRING also retains the full joint spectral likelihood, including correlations between the mode spectral parameters, rather than relying only on low-dimensional marginalized posteriors. The same spectral likelihood can then be tested against Kerr and beyond-Kerr predictions.

This viewpoint also suggests a geometric interpretation: each theoretical model defines a lower-dimensional manifold in the observable spectral space, and the theory-agnostic information extracted from the ringdown signal can be compared directly with the corresponding manifolds. Such a formulation may make parameter degeneracies and deviations that can be partially absorbed by shifts in remnant parameters more transparent. We illustrate the framework using GW250114 and controlled injections, and discuss its potential as a common interface between gravitational-wave observations, black-hole spectroscopy, and fundamental theories of compact objects.

Speaker: Mr Shitong Guo (Nankai University)

17:15 Accounting for Dark-Dresses in Searches for Gravitational waves from Inspiraling Primordial Black Holes using Pattern-Recognition Techniques

Primordial black holes (PBHs) are compelling dark matter (DM) candidates; however, current observational constraints indicate that they can not account for the entirety of the DM abundance. This motivates scenarios in which additional DM components coexist with PBHs and form dense structures, often referred to as ``dark dresses,'' around them. Such environments induce dynamical friction, accelerating the inspiral rate of PBH binaries relative to vacuum evolution. Neglecting such environmental effects in gravitational-wave searches can lead to substantial losses in sensitivity. In particular, for third-generation detectors such as the Einstein Telescope, searching for inspirals affected by DM environments using templates that neglect DM effects can lead to signal-to-noise ratio (SNR) losses of up to $\sim70\%$ in certain regions of parameter space, especially for binaries with highly asymmetric mass ratios ($q \sim 10^{-3}$). We demonstrate that a search strategy originally developed for vacuum PBH inspirals can remain effective in the presence of dark dresses with only minimal modifications. Specifically, we apply the generalized frequency-Hough (GFH) method, a pattern-recognition technique that maps time--frequency tracks in detector's plane to lines in source parameter space. Using simulated signals embedded in Gaussian noise, we show that the GFH method successfully recovers inspirals affected by DM environments. These results highlight the potential of DM aware, non-matched-filtering approaches for future gravitational-wave searches and provide a robust framework for probing environmental effects around compact binaries.

Speaker: Charchit Kumar Sethi (International Center for Theoretical Physics, Asia Pacific (ICTP-AP)/ University Chinese Academy of Sciences (UCAS,中国科学院大学))

17:30 Accelerated Bayesian noise estimation for multivariate and non-stationary LISA data

Accurate and scalable noise modelling will be essential for extracting weak gravitational-wave signals and signatures of new physics from LISA data. Realistic time-delay interferometry channels may contain frequency-dependent correlations, while the detector noise may also evolve over the mission.

I will present a Bayesian framework for estimating the full spectral density matrix of multivariate LISA data. The model uses penalised splines to learn both auto-spectra and cross-spectra, while preserving a valid covariance matrix at every frequency. A differentiable, coarse-grained Whittle likelihood enables efficient inference using the No-U-Turn Hamiltonian Monte Carlo sampler, initialised with variational inference.

Applied to simulated LISA data, the multivariate method accurately recovers the full correlated noise structure. For an idealised symmetric detector, it agrees with the standard assumption of diagonal noise in the A, E, and T channels. However, under realistic instrumental asymmetries, this simpler univariate (diagonal) approximation breaks down and produces errors more than an order of magnitude larger than the full multivariate model presented here.

I will also discuss ongoing work extending the framework to non-stationary noise using the Wilson–Daubechies–Meyer time-frequency transform, enabling time-varying structure and joint inference of evolving LISA noise and gravitational-wave signals.

Speaker: Avi Vajpeyi (University of Auckland)

16:00 → 17:45 Parallel Session B

16:00 Measuring the Hubble constant with bright standard sriens formed in active galactic nuclei

Standard sirens - gravitational wave (GW) sources with an electromagnetic (EM) counterpart - can be used to measure the Hubble constant directly and thus will help to ease the Hubble tension. However, systems that emit detectable GWs and EM radiation at the same time are rare. Therefore, binary black hole (BBH) mergers inside active galactic nuclei (AGNs) are gaining more and more attention as potential standard sirens. At the same time, these systems require more delicate modelling as they differ significantly from standard field binaries - in particular, as the supermassive black hole in the center of the AGN can lead to significant Doppler and gravitational shifts of the GWs frequency. In this talk, I discuss the modeling of these sources and two recent papers. In the first paper, 18 GW events from LIGO-Virgo-KAGRA observing runs O3 to O4b paired with 28 candidate AGN counterparts are analyzed. Of the 28 candidate pairs, 21 are positive-to-strong-favoured and from the 21 positive-to-strong-favoured BBH-AGN pairs, 13 unique associations can be identified as the preferred ones. In the second paper, the Hubble constant $H_0$ is measured using the 13 BBH mergers associated with AGN flares. We find $H_0=70.50^{+3.37}_{-2.89} ({\rm stat})\pm1.56 ({\rm cal})$ km s$^{-1}$ Mpc$^{-1}$ ($4.4\%$ precision), consistent with both Planck 2018 ($0.98\sigma$) and SH0ES 2024 ($0.76\sigma$), with no significant preference between the two. Combining with the binary neutron star merger GW170817 sharpens the constraint to $H_0=70.31^{+3.00}_{-2.85} ({\rm stat})\pm1.55 ({\rm cal})$ km s$^{-1}$ Mpc$^{-1}$ ($4.2\%$ precision), and further combining with an independent dark-and-bright-siren sample tightens it to $H_0=69.71^{+2.55}_{-2.40} ({\rm stat})\pm1.54 ({\rm cal})$ km s$^{-1}$ Mpc$^{-1}$ ($3.5\%$ precision).

Speaker: Alejandro Torres-Orjuela (Beijing Institute of Mathematical Sciences and Applications (BIMSA))

16:15 Multimessenger Tests of the Strong Equivalence Principle and Hidden Extra Dimensions with Gravitational-Wave Observations

Multimessenger gravitational-wave observations provide a powerful probe of fundamental gravity in the strong-field and dynamical regime. In this work, we test possible deviations from general relativity from two complementary perspectives: a temporal variation of the gravitational constant, which is closely related to the strong equivalence principle, and the existence of hidden compact extra dimensions inspired by the Kaluza-Klein formulism. We first develop a gravitational-wave waveform model with a slowly varying gravitational constant, including its effects on both compact binary dynamics and wave propagation in an expanding universe. Applying this framework to GW170817, together with electromagnetic information from GRB 170817A, we obtain no evidence for a varying gravitational constant and constrain \dot{G}/G to [-3.36 × 10^{-9}, 5.34 × 10^{-10}] yr^{-1}. We further use GW170817 to place bounds on hidden extra dimensions and forecast the capability of the future Taiji detector. Representative multimessenger supermassive black hole binaries observed by Taiji are expected to provide significantly stronger constraints, demonstrating the potential of future space-based gravitational-wave astronomy for testing gravity beyond four dimensions.

Speaker: Hanlin Song (Institute of Mechanics, Chinese Academy of Sciences)

16:30 Constraints on modified gravity theories of dark energy with the latest LVK data

Cosmic acceleration may arise from modifications to gravity that become significant on large scales. In many scalar–tensor and effective-field-theory models, an evolving gravitational coupling changes the friction experienced by gravitational waves (GWs), causing their luminosity distance to differ from the electromagnetic prediction. In this talk, I will present constraints on dark-energy-related modified gravity using the latest GW catalog from the LIGO–Virgo–KAGRA O4 run. We test this effect with a hierarchical standard-siren analysis that jointly models cosmology and compact-binary population properties in associate with galaxy-catalog information. I will present constraints on some phenomenological parameterization based on scalar-tensor gravitational theories and higher-dimensional gravitational theories, as well as a general ($\Xi_0-n$) parametrization. While the resulting constraints remain consistent with general relativity, they demonstrate that GW distances provide an independent cosmological test of gravity across large distances and establish analysis requirements for future LVK catalogs and next-generation observatories.

Speaker: Anson Chen (International Centre for Theoretical Physics - Asia Pacific)

16:45 Parameter Estimation of Subsolar-Mass Compact Objects with Mini Extreme Mass-Ratio Inspirals

Subsolar-mass exotic compact objects (ECOs) are a possible clue to new physics beyond conventional astrophysics. When such an object orbits a stellar-mass black hole or neutron star, the resulting binary is a mini extreme mass-ratio inspiral (mini-EMRI) system. Gravitational waves emitted by mini-EMRI systems can last from hours to years in LIGO's frequency band, with characteristics closely resembling those of continuous gravitational waves. Such sources can therefore be searched for with ground-based detectors using continuous-wave detection methods, potentially enabling the detection of subsolar-mass ECOs. We perform parameter estimation for mini-EMRI systems using the Fisher information matrix to study how precisely ground-based detectors can measure the mass of subsolar-mass ECOs. For primary masses between $5$ and $50\, M\odot$ and secondary masses between $10^{-5}$ and $10^{-3}\, M_\odot$, we obtain the fractional errors of the secondary mass for different mass combinations. We further examine how the fractional error depends on the evolution time prior to the end of the inspiral and on the source distance for a fixed mass combination. Finally, we find that at a distance of $1\,\mathrm{Mpc}$, as long as the signal can be detected, the secondary mass of a mini-EMRI system can be constrained to the subsolar-mass range.

Speaker: Qimin Song (International Centre for Theoretical Physics-Asia Pacific (ICTP-AP))

17:00 Third-Body Detection in LISA Massive Black Hole Triples via Acceleration

We investigate the detectability of line-of-sight acceleration (LOSA) in gravitational-wave signals from massive binary black holes (MBHBs) observable by LISA. A constant LOSA introduces a $-4$PN correction to the inspiral phase, which we derive and implement in the \texttt{LISAbeta} analysis pipeline.

Our Bayesian analysis reveals that LOSA measurement precision depends strongly on total mass and observation duration. Lower-mass systems ($10^4$--$10^5\,M_\odot$) benefit from extended low-frequency inspiral, with uncertainty decreasing steadily as more pre-merger data are included. High-mass systems ($10^7\,M_\odot$) are insensitive to early inspiral data, as most signal-to-noise accumulates in the final months.

Neglecting a genuine LOSA induces severe systematic biases in chirp mass and mass ratio. We validate the constant-acceleration approximation against time-varying jerk, and map the detectability of third-body perturbers ($\gtrsim 10^8\,M_\odot$) out to $\sim 1\,$pc, establishing LOSA as a novel probe of MBHB environments.

Speaker: Mr zheng WU (L2IT)

17:15 Effects of dark dipole radiation on eccentric supermassive black hole binary inspirals

The final-parsec problem has long posed a central challenge in understanding the merger of supermassive black hole binaries. In this paper, we investigate a scenario in which a dark scalar or vector field is sourced by eccentric binaries, leading to accelerated mergers through additional dipole radiation and thereby extending the range of masses for which the binary can merge within a Hubble time. The Newtonian-order radiation fluxes from an eccentric charged Keplerian binary are derived using general results for localized periodic sources in flat spacetime. We find that dipole radiation, although insufficient to fully resolve the final-parsec problem, can alter the low-frequency spectrum of the stochastic gravitational wave background from supermassive black hole binary inspirals. We construct a simplified model for the spectrum and perform a Bayesian analysis using the current pulsar timing array data.

Speaker: Muchun Chen (TDLI)

17:30 Testing Dipole Radiation with Eccentric Multiband Binary Black Holes

Additional fundamental fields can modify compact-binary radiation, with dipole-radiation effects entering most strongly during the early inspiral. Although binary black holes do not emit leading-order dipole radiation in many commonly studied theories, more general scenarios can endow them with effective charges or additional radiation channels. Space-ground multiband observations therefore provide a valuable opportunity for theory-agnostic tests of such effects. However, residual orbital eccentricity is also most prominent at low frequencies and can introduce important parameter degeneracies.
In this talk, I will present a Bayesian multiband analysis that simultaneously includes eccentricity and a theory-agnostic -1PN dipole-radiation correction. We find strong correlations among the dipole parameter, chirp mass, and eccentricity. Allowing for eccentricity broadens the dipole-radiation posterior, showing that neglecting it may lead to overly optimistic constraints. Building on our previous work on eccentric archival searches and ground-informed inference, we analyze a GW231123-like binary in noisy TianQin and LISA data. With one year of space-based observation and priors informed by a next-generation ground-based detector network, the dipole parameter can still be constrained to approximately $|b|\lesssim \mathcal{O}(10^{-7})$. These results demonstrate both the potential of multiband binary black holes for tests of gravity and the importance of complete waveform modeling.

Speaker: Han Wang (Peking University)

Wednesday 26 August

09:00 → 10:00 Plenary Session

09:00 Can the Universe actually Inflate?

TBD

Speaker: Eugene Lim

09:30 Hubble delayed vacuum decay and their gravitational wave signals

I will discuss the vacuum decay processses that may happen in the early universe and the properties of the corresponding gravitational wave signals.

Speaker: Haipeng An (Tsinghua University)

10:00 → 10:30 Break

10:30 → 11:00 Plenary Session

10:30 FoPTs, PBHs, SIGWs, and SWs from early Universe

In this talk, I will introduce our recent progress on cosmological first-order phase transitions, primordial black holes, scalar-induced gravitational waves, and sound waves from gravitational instability.

Speaker: Shao-Jiang Wang (Institute of Theoretical Physics, Chinese Academy of Sciences)

11:00 → 11:30 Plenary Session: Panel discussion

14:00 → 17:00 Free discussion

Thursday 27 August

09:00 → 10:00 Plenary Session

09:00 Wave Dark Matter: Stochastic Fluctuation and Gravitational Probes

In this talk, I will discuss stochastic fluctuations of wave dark matter. I will discuss what it means and how we can utilize such stochastic density fluctuations of wave dark matter to probe its existence only via gravitational interaction. I will discuss a few specific observational and experimental setup, including inteferometry, pulsar timing, astrometry, and interplanetary range measurements.

Speaker: Hyungjin Kim

09:30 TBD

TBD

Speaker: Arianna Foschi

10:00 → 10:30 Break

10:30 → 11:30 Plenary Session

10:30 Weighing Dark Matter with Gravitational Waves

Gravitational-wave (GW) observations allow us to reconstruct the orbital dynamics of compact binaries with remarkable precision. When these systems evolve in dense environments—such as active galactic nucleus discs, nuclear star clusters, or dark matter structures—their surroundings can leave characteristic imprints on the GW phase evolution. These imprints open a new avenue for probing not only the distribution of dark matter, but also its microscopic properties.

In this talk, I will discuss how gravitational waves can be used to probe dark matter microphysics, including the mass and spin of its fundamental constituents.

I will present results from the first dedicated search for signatures of light scalar fields in the orbital dynamics of black hole binaries using LIGO–Virgo–KAGRA data. In particular, I will discuss the intriguing case of GW190728, whose signal shows features consistent with those expected from the presence of a light scalar field with a mass of order 10^{-12} eV, and assess the significance and interpretation of this result.

Finally, I will show how extreme-mass-ratio inspirals observed by LISA could measure the mass of ultralight dark matter particles, providing a purely gravitational probe of dark matter microphysics.

Speaker: Rodrigo Luís Lourenço Vicente

11:00 Why Dark Matter May Have Self-Interactions

TBD

Speaker: Hai-Bo Yu

11:30 → 12:00 Plenary Session: Panel discussion

14:00 → 15:30 Parallel Session A

14:00 Resonances in extreme mass ratio inspirals within scalar environments

Extreme mass ratio inspirals (EMRIs) are among the key targets for future gravitational wave observations. While the study of EMRIs in vacuum has been extensively developed over the past decades, environmental effects arising from matter surrounding black holes have also attracted significant attention in recent years. A proper understanding of such effects may open up the possibility of probing black hole environments through gravitational wave observations. In this work, we investigate the dynamics of EMRIs in the presence of a scalar field environment. A well-motivated example is a superradiant cloud of ultralight scalar fields, which are motivated by high energy physics and are also viable dark matter candidates. In contrast to previous studies focusing on the resonant transitions of the scalar cloud by the secondary object, we consider resonances induced in the secondary’s orbit by the scalar environment. Understanding these orbital resonances is an essential step toward constructing EMRI waveforms that incorporate scalar environmental effects.

Speaker: Takuya Takahashi (The University of Tokyo)

14:15 Relativistic signatures of scalar clouds in extreme-mass-ratio inspirals

Through black hole superradiance, ultralight bosonic fields may form macroscopic clouds around rotating black holes. Extreme-mass-ratio inspirals (EMRIs), among the primary targets of future space-based gravitational-wave detectors, provide a unique opportunity to probe such environments. We extend the relativistic perturbative framework for EMRIs within scalar clouds from circular, equatorial motion to eccentric and inclined orbits. Working on a Schwarzschild black-hole background, we compute the scalar energy and angular-momentum scattered off to infinity and absorbed at the event horizon. We show that eccentricity induces a dense sequence of strong-field resonances near the last stable orbit, while orbital inclination significantly modifies the fluxes and resonant structure. By evolving the orbits adiabatically, we show that resonant transitions substantially enhance the exchange of energy and angular momentum between the orbit and the cloud, amplifying the accumulated gravitational-wave dephasing relative to circular motion. Our results highlight the importance of extending environmental calculations beyond circular, equatorial motion when modelling realistic EMRIs.

Speaker: Qi-Xuan Xu (Instituto Superior Técnico)

14:30 Constraining Ultralight Scalar Dark Matter in the Galactic Center with the S2 Orbit

The dense environment of our Galactic Center (GC) offers a unique laboratory for probing ultralight dark matter (ULDM). We explore the prospect of detecting a scalar ULDM field through its effects on the orbital dynamics of S-stars around the supermassive black hole in the GC, Sgr A$^*$. We consider both linear and quadratic couplings between the real scalar field $\phi$ and Standard Model particles, and analyze two representative ULDM structures: the scalar gravitational atom and the spherical soliton. We find that quadratic coupling induces a non-oscillatory perturbation, leading to a long-term secular orbital evolution. We use the observed periastron precession rate of S2 star to put stringent constraints on the total ULDM mass in the GC and the quadratic coupling constant. For the gravitational atom $|211\rangle$ state, we constrain the mass ratio of ULDM to Sgr A$^*$ to $\beta \lesssim 10^{-3}$ at $m \sim 10^{-18}$ eV, and for the spherical soliton which extends to $\sim 0.2\,$pc, the mass ratio is limited to $\beta \lesssim 1$ at $m \sim 3\times10^{-20}$ eV. Notably, the resulting limits on the quadratic coupling constant surpass current bounds in the mass range $10^{-20} \,\text{eV} \lesssim m \lesssim 10^{-18}$ eV.

Speaker: 江川 于 (北京大学)

14:45 Evolution of ultralight boson clouds in binary black hole systems

Ultralight bosons, such as axion-like particles, are promising candidates for physics beyond the Standard Model. Rotating black holes can amplify these fields through superradiance, forming macroscopic bosonic clouds known as gravitational atoms. In this work, we investigate the evolution of ultralight boson clouds in binary black hole systems, considering their full evolutionary history from large orbital separations to the late inspiral stage. We show that the common-envelope phase in astrophysical binary evolution can significantly enhance cloud survival by suppressing resonant depletion, expanding the parameter space accessible to future space-based gravitational-wave detectors. We further identify a novel mass-transfer mechanism of boson clouds driven by binary tidal interactions, which arises from the wave nature of the bosonic field and does not require superradiance around the companion object. At small separations, we demonstrate the transition from gravitational atoms to bosonic common envelopes and show that their backreaction can excite binary eccentricity to observable levels. These results provide a unified picture of boson cloud evolution in binary systems and reveal new possibilities for probing ultralight particles through gravitational-wave observations.

Speaker: Ao Guo (International Centre for Theoretical Physics Asia-Pacific, University of Chinese Academy of Sciences, 100190 Beijing, China)

15:00 Ultralight Boson Ionization from Comparable-Mass Binaries

Detection of gravitational waves enables probes of environmental effects around compact binaries. Ultralight bosons, well motivated in particle physics and capable of forming core-like dark matter structures, induce environmental dynamics that differ qualitatively from
those produced by stars or particle dark matter. For comparable-mass binaries, such bosons can form gravitationally bound states analogous to molecules once the binary separation falls below the characteristic wavelength of the bound states, with an inner region co-moving with the binary. We combine numerical simulations and a semi-analytic framework to characterize the structure and ionization of these gravitational molecules. We determine the extent of the co-moving region and compute the ionization flux driven by orbital motion over a range of eccentricities. Using these results, we estimate the backreaction on the binary orbital evolution and identify a new environmental effect: eccentricity-induced ionization of the co-moving component leads to efficient circularization. We further show that this molecular phase can be astrophysically viable and significantly modify the stochastic gravitational wave background from supermassive black hole binaries.

Speaker: Yuhao Guo

14:00 → 15:30 Parallel Session B

14:00 Searching for Scalar Ultralight Bosons through High-Frequency X-ray Variability

Scalar ultralight boson clouds around rotating black holes can generate long-lived, non-axisymmetric gravitational perturbations capable of exciting coherent dynamical responses in nearby accretion flows. Focusing on black hole X-ray binaries, we investigate whether this forcing can excite global oscillations of a thin accretion disk and produce observable quasi-periodic variability. Using a linear-response framework for a simplified hydrodynamic disk model, we compare the characteristic frequency and spatial structure of the boson-cloud forcing with the intrinsic modes of the disk, while accounting for resonance structure and damping. We then connect the resulting disk perturbations to possible observational signatures through relativistic ray tracing. I will discuss the physical conditions under which the response may be amplified and the prospects for probing the resulting high-frequency variability with current or future X-ray timing observations.

Speaker: Zhiren Wang (Perimeter Institute & University of Waterloo)

14:15 Parameter estimation of generalized Ghosh black holes using shadow observables and Event Horizon Telescope Constraints

The Event Horizon Telescope images of M87 and Sgr A give us a unique chance to test gravity in the strong field regime and assess deviations from the Kerr solution. We study the shadows of rotating nonsingular black holes—namely, generalized Ghosh black holes (GGBHs), given by the mass function $m(r) = M e^{-(k/r)^n}$, which reduces to the Kerr black hole ($k \to 0$) and to the original Ghosh model ($n = 1$). We calculate the photon's trajectory via the Hamilton-Jacobi method and construct the shadow seen by a distant observer. For a high spin $a/M = 0.9$, increasing $k/M$ from $0$ to $0.5$ reduces the shadow size by about $13\%$ and the shadow area by about $24\%$, while making the shape more distorted (up to $\sim 20\%$). For larger values $k/M \sim 1.0$, the area can decrease by nearly $30\%$. However, increasing $n$ reduces these effects, and for $n \geq 3$ the shadow becomes very close to the Kerr case (differences below $\sim 5\%$). To compare with observations, we use two simple shadow quantities: the area $A$ and the oblateness $D$. Using EHT observational data, we find that for M87 ($17^\circ$ inclination, $37.8\ \mu\text{as}$ diameter), the parameter is constrained as $k/M \lesssim 0.133$ for $n=1$. For Sgr A ($50^\circ$ inclination, $48.7\ \mu\text{as}$ diameter), the bound is $\mathbf{k/M \lesssim 0.09162}$. These limits become weaker for larger $n$. Overall, the GGBHs are a \textbf{viable candidate} to the Kerr black hole, and future observations may help to detect or further constrain such deviations.

Speaker: Himanshi Gulia (Netaji Subhas University of Technology)

14:30 Probing Hairy Black Holes through Eikonal Ringdown and Null-Geodesic Diagnostics

Black-hole ringdown provides a promising way to test deviations from the Kerr geometry, especially when compact objects are surrounded by matter fields or effective hair. In this work, we study how small deviations from Schwarzschild and Kerr spacetimes modify the eikonal quasinormal-mode spectrum through the properties of unstable circular null geodesics. We derive first-order shifts in the orbital frequency and Lyapunov exponent associated with the photon ring, allowing the real and imaginary parts of the ringdown frequency to be connected directly to perturbations of the background geometry.

We apply this framework to representative regular and matter-supported black-hole models, including Bardeen, Hayward, and Kiselev-type geometries, and compare the perturbative predictions with numerical geodesic calculations. In the rotating case, we analyse the equatorial co-rotating and counter-rotating sectors, where the azimuthal harmonic number selects the corresponding null orbit. We also discuss the scope of the eikonal correspondence in non-vacuum spacetimes and its limitations for interpreting ringdown observables.

This approach provides a compact diagnostic for identifying matter-induced deviations in black-hole ringdown. It offers a bridge between geodesic observables, photon-ring structure, and gravitational-wave tests of compact objects beyond Kerr.

Speaker: Ariadna Uxue Palomino Ylla (Nagoya University)

14:45 3D Summation-by-Parts scheme on Hyperboloidal Slices

This talk summarises our recent work on a fully 3D Summation-by-Parts scheme for a class of linear wave equations on hyperboloidal slices. The scheme is derived in spherical polar coordinates on a Minkowski background, and allows having grid points at the origin and on the z-axis, despite coordinate singularities, and at infinity, by introducing compactification followed by rescaling, and is proved to be stable. Kreiss-Oliger dissipation operators are generalized to curvilinear coordinates and are defined everywhere in the domain, including at the boundary points, such that they satisfy the dissipative property in the energy norms. Promising results are obtained, giving hope for application to fully nonlinear systems, like the Einstein Field Equations, and extracting the resulting gravitational waves free of systematic errors or gauge ambiguities.

Speaker: Shalabh Gautam (Beijing Institute of Mathematical Sciences and Applications (BIMSA))

15:00 Dyonic hairy black holes in $U(1)$ gauge-invariant scalar-vector-tensor theories: Third- and fourth-order sectors

We construct and classify asymptotically flat, static, and spherically symmetric hairy black hole solutions in $U(1)$ gauge-invariant scalar-vector-tensor (SVT) theories [1] carrying both electric and magnetic charges. Extending previous analyses beyond the second-order SVT interaction [2], we include the third- and fourth-order SVT interactions in dyonic backgrounds [3]. We show that the fourth-order SVT interaction generically gives rise to higher-derivative terms in the field equations and derive the condition required to eliminate them, thereby ensuring that the field equations remain second order.
We classify the obtained solutions based on their symmetry properties: shift-symmetric couplings yield secondary hair governed by the Noether current, whereas $\phi$-dependent interactions generate primary hair. Crucially, our analysis reveals that the magnetic charge plays a key role in activating specific interaction sectors such as the third-order SVT interaction $\tilde{f}_3$, which does not appear in the field equations in purely electric configurations. We also clarify the behavior of the solutions in the vanishing monopole limit ($P\to0$). Depending on the interaction sector and the solution branch, the solutions either continuously reduce to the Reissner-Nordstr\"om solution or retain nontrivial scalar hair in the purely electric limit. The scalar field exhibits interaction-dependent asymptotic falloff rates, which may lead to distinct phenomenology. For the branches admitting global solutions, we connect the near-horizon and asymptotic solutions through numerical integration and confirm that the fields remain finite and smooth throughout the exterior region.

[1] L. Heisenberg, JCAP 10 (2018), 054, arXiv:1801.01523 [gr-qc].
[2] K. Taniguchi, S. Takagishi, and R. Kase, Phys. Rev. D 110, 044006 (2024), arXiv:2403.17484 [gr-qc].
[3] M. Kitagawa, N. Tsukamoto, and R. Kase, arXiv:2603.04884 [gr-qc].

Speaker: Masaki Kitagawa (Department of Physics, Faculty of Science, Tokyo University of Science)

15:15 Linear stability of dyonic black holes in U(1) gauge-invariant scalar-vector-tensor theories

We investigate the linear stability of static, spherically symmetric dyonic black hole solutions carrying both electric and magnetic charges in the U(1) gauge-invariant scalar-vector-tensor theory [1]. In the limit of vanishing magnetic charge, our framework reproduces the results of previous studies [2,3]. With appropriate choices of the interaction functions, it also encompasses the theoretical setup considered in Ref. [4]. On the other hand, the recently discovered dyonic hairy black hole solutions [5] are not covered by the frameworks developed in the previous studies and can therefore be analyzed for the first time within our framework. In particular, we focus on higher-order interactions and clarify the effects of the magnetic charge on the stability conditions.

[1] L. Heisenberg, JCAP 10, 054 (2018), arXiv:1801.01523 [gr-qc].
[2] L. Heisenberg, R. Kase, and S. Tsujikawa, Phys. Rev. D 97, 124043 (2018), arXiv:1804.00535 [gr-qc].
[3] C. Zhang and R. Kase, Phys. Rev. D 110, 044047 (2024), arXiv:2404.11910 [gr-qc].
[4] K. Taniguchi, S. Nishimura, N. Tsukamoto, and R. Kase, Phys. Rev. D 112, 124043 (2025), arXiv:2504.21279 [gr-qc].
[5] M. Kitagawa, N. Tsukamoto, and R. Kase, arXiv:2603.04884 [gr-qc].

Speaker: Mr Shunta Nishimura (Department of Physics, Faculty of Science Tokyo University of Science, Japan)

15:30 → 16:00 Break

16:00 → 17:00 Plenary Session: Miniworkshop: How Feasible Is Gravitational Detection of Dark Matter?

18:00 → 20:00 Banquet

Friday 28 August

09:00 → 10:00 Plenary Session

09:00 Probing ultralight axion-like dark matter with pulsar arrays

Ultralight axion‑like dark matter (ALDM) is a well‑motivated candidate whose wave‑like nature is manifest on astronomical scales. Pulsar timing arrays (PTAs) and pulsar polarization arrays (PPAs) offer complementary probes: PTAs search for ALDM imprints on pulse arrival times, while PPAs target linear polarization angle rotations induced by the axion–photon Chern–Simons coupling. In this talk, I will review the status of ALDM searches with PTAs and PPAs, stressing in particular the importance of accounting for the characteristic spatial correlations of the ALDM field. I will also discuss recent efforts to combine timing and polarization data, with the aim of enhancing the ability of pulsar arrays to identify ALDM signals in real observations.

Speaker: Jing Ren

09:30 Quantum Evolution of a Superradiant Cloud

In recent years, black hole superradiance, extraction of energy and angular momentum from a rotating black hole, has attracted considerable attention as a probe of light bosonic particles.
For a massive bosonic field, superradiance can lead to an instability and the growth of a macroscopic boson cloud, which may subsequently emit gravitational waves.
The cloud is often said to be seeded by vacuum fluctuations, which are amplified by the superradiant instability into a macroscopically occupied state.
Nevertheless, the instability is usually described using a classical field, leaving unclear how this quantum growth gives rise to the classical cloud and whether any quantum signatures remain observable.
In this talk, I will discuss the quantization of a massive scalar field around a rotating black hole and its quantum evolution.
I will then explain how gravitational wave emission induces decoherence, providing a route from the initial quantum fluctuations to a classical boson cloud.

Speaker: Hidetoshi Omiya

10:00 → 10:30 Break

10:30 → 11:30 Plenary Session

10:30 Black hole superradiance and beyond

Gravitational and electromagnetic signatures of black hole
superradiance are a unique probe of ultralight particles that
are weakly-coupled to ordinary matter. I will highlight the importance
of such non-minimal couplings as well as self-interactions
on observational signatures and present recent constraints on these
particles obtained from gravitational wave data.

Speaker: Nils Siemonsen

11:00 Rotating Real Scalar Condensate

An ultralight real scalar field could form a localized condensate under gravity, either with an external gravitational field such as the boson cloud around a black hole, or with self-gravity such as a boson star. Different from the complex scalar field, the real scalar cannot form vortices. Then how does the localized condensate take angular momentum? In this talk, I will explain the properties of rotating real scalar condensates and the possible observation signatures.

Speaker: 宏 Hong 张 Zhang (山东大学 Shandong University)

11:30 → 12:00 Plenary Session: Panel discussion

12:00 → 12:10 Plenary Session: Closing remarks