Introduction to Gravitational-Wave Experiments: Exploring the Universe from LIGO and CHRONOS

• Yuki Inoue

Room: Andromeda Meeting Room

Multimessenger binary black hole localisation

• Otto Hannuksela

Room: Andromeda Meeting Room

Spinning Extreme-Mass-Ratio-Inspirals into an Astrophysical Black Hole: Onset of Chaos at Astrophysically Realistic Spin

• Surajit Das (University of Science and Technology of China, Hefei, Anhui 230026, China)

Room: Andromeda Meeting Room The forthcoming space-based gravitational wave detectors such as LISA, Tianqin and Taiji will unlock the low-frequency gravitational-wave sky, offering an unprecedented probe into the fundamental dynamics of gravity. A prime target for such detectors is Extreme Mass Ratio Inspirals (EMRIs)—the adiabatic inspiral of a stellar-mass compact object into a supermassive black hole. While conventionally modeled as integrable geodesic motion with radiative dissipation, this canonical picture is fundamentally altered when the spin of the secondary compact object is considered. The inclusion of spin-curvature coupling breaks the integrability of the system, opening a portal to prolonged resonances and deterministic chaos. We expose a critical astrophysical ingredient, dark matter halo, that dramatically catalyzes this chaotic transition. We systematically investigate the orbital dynamics of a spinning test particle around a non-spinning supermassive black hole immersed in a Hernquist dark matter profile. Our analysis reveals that the dark matter halo profoundly reshapes the effective potential, acting as a trigger for chaos. For the first time, we demonstrate through high-resolution Poincaré maps that chaotic features in the orbital phase space emerge at spin parameter values significantly lower—and thus far more astrophysically plausible—than the upper bound previously established in the literature. Remarkably, we find that these chaotic signatures are not confined to the near-horizon regime but can manifest at large distances from the central black hole, with their characteristic scale spanning orders of magnitude—from fine-grained structures to gross orbital deviations. This broad spatial distribution and amplified sensitivity to spin fundamentally reframe the detectability of chaos in EMRIs. Our findings suggest that the gravitational-wave fingerprints of nonintegrability, imprinted on signals destined for LISA and others, are not just a mathematical curiosity but a realistic and observable feature of astrophysical systems. This opens a transformative new window for characterizing both the spacetime geometry and the ubiquitous dark matter environments in galactic nuclei.

Physics-Informed Neural Network for Kilonova Light Curves Modeling

• Surojit Saha (Institute of Astronomy, National Tsing Hua University, Taiwan)

Room: Andromeda Meeting Room Physics-informed neural networks (PINNs) have gained considerable importance in recent years in the domain of Astronomy \& Astrophysics, particularly as a potential tool to solve differential equations within the given boundary conditions, not only for making accurate predictions but also for providing an efficient approach for large computations. In this work, we have focused on solving the kilonova equations adopted from a specific kilonova model through the direct implementation of the PINN on the differential equations and the respective boundary conditions provided in the model. The PINN architecture is trained on differential equations conditioned on certain boundary conditions, thus learning the evolution of KNe light curves based on given ranges of physical parameters. To test the performance, after successful training, predictions of the light curve for a known set of physical parameters are given as input, and a comparison is made between the true and predicted light curves. Current results point to stable training with significant recovery of the light curves, showing a low mean squared error between them. Training and inference for the light curves are completed in under 2 hours. The ultimate goal is to develop an accurate PINN-based kilonova model for light-curve generation and low-latency parameter estimation.

Dynamical Formation of Merging Compact Binaries

• Dong Lai

Room: Andromeda Meeting Room

Gravitational-wave lensing in an active galactic nucleus

• Paul Martens (The Chinese University of Hong Kong (CUHK))

Room: Andromeda Meeting Room The dense and dynamic environments that is active galactic nuclei (AGN) accretion disks may be a birthplace for binary black holes (BBHs). It is thus a new possible formation channel for BBHs, which is getting more interest recently. This presentation exhibits recent results aimed at describing the gravitational waves (GWs) lensing of a binary merger event occuring behind an AGN (an AGN-BBH system). Such systems present several interesting features. They have the advantage of being simple enough for constraints to be derived with only few assumptions, to a well controlled precision. Using lensing, their relative proportion in observed GWs events should be strongly constrained by future GWs observatories. In particular, this will eventually require devising some discrimination criteria for attributing GW events to lensing in an AGN-BBH system. Thusly, I shall detail the GWs signatures an AGN-BBH system can imprint on the lensed GWs parameters, such as a Doppler shift, that could realistically be detected in the future.

Hierarchical mergers as tracers of globular cluster formation history

• Andris Dorozsmai (National Astronomical Observatory of Japan)

Room: Andromeda Meeting Room The origin of globular clusters remains one of the most important open questions in astrophysics, with essential implications for our understanding of galaxy formation and the early Universe. Globular clusters are unique sites of hierarchical black hole mergers, where the remnant of a previous merger undergoes a subsequent coalescence, a process only expected in the dense dynamical environments of stellar clusters. These mergers have distinct properties compared to binary black holes formed in isolated environments, making them promising candidates to be clearly identified in future observations. Using population synthesis models of gravitational wave sources in globular clusters, I show that the hierarchical merger rate directly traces the cluster formation history. Adopting a model for the delay time, defined as the time between cluster formation and the eventual black hole merger, I demonstrate that the globular cluster formation history can be accurately inferred from hierarchical merger catalogues of third-generation detectors using a non-parametric Bayesian framework. This will constrain the cluster formation history to unprecedented accuracy, shedding light on the cluster formation efficiency across cosmic time, the role of globular clusters in cosmic reionization, and the physical conditions of high-redshift star formation. Third-generation detectors will therefore be unique probes of globular cluster formation and high-redshift star formation, complementing observatories such as JWST, Euclid, and the ELT.

Supermassive Black Hole Formations via Bondi Accretion of Self-Interacting Dark Matter

• Feng-Li Lin

Room: Andromeda Meeting Room

Continuous gravitational-wave searches for dark matter in O4 LIGO data

• Andrew Miller (ICTP - AP / University Chinese Academy of Sciences)

Room: Andromeda Meeting Room Dark matter could compose ~80% of all matter in the universe, and yet it is completely invisible to us. Despite decades of experiments designed to detect dark matter, and numerous models for potential dark matter particles, no concrete evidence has been put forward to support the existence of beyond standard-model physics. Because of this, it is worth asking whether approaching the detection of dark matter from a different point of view, that is, via gravitational-wave interferometers, could provide some insight into explaining the origin of dark matter. In this talk, I will discuss searches for dark matter in two forms: ultralight particle dark matter and sub-solar mass primordial black holes. While not designed to search for dark matter, gravitational-wave detectors can robustly probe a variety of dark-matter models simultaneously, without affecting their sensitivity to canonical gravitational-wave sources, and put competitive and sometimes even stronger constraints than those from other experiments designed to search for dark matter.

Eccentricity evolution consistency test to distinguish eccentric gravitational-wave signals from eccentricity mimickers

• Sajad Ahmad Bhat (Inter-University Center for Astronomy and Astrophysics (IUCAA), India)

Room: Andromeda Meeting Room Eccentric compact binary coalescences (CBCs) are expected to be observed in current and future gravitational-wave (GW) detector networks. Such detections are especially valuable as they can provide insights on the environments that nurture CBCs. However, it has been recently pointed out that a number of other physical and beyond-GR effects could imitate, or be mimicked by, eccentric CBCs. The standard approach to ascertain that a detected CBC is eccentric is to employ Bayesian model selection, where the eccentric CBC hypothesis is compared against other hypotheses. Such an approach is not only computationally intensive and time-consuming, but could also be misleading if none of the models under consideration represent the true model. In this work, we propose a conceptually simple but powerful method to directly confirm or reject the eccentric hypothesis, without needing to compare the hypothesis with the plethora of other possible hypotheses. The key idea is that while spurious nonzero values of eccentricity, at some reference frequency, could be acquired when a noneccentric CBC with additional physical/beyond-GR effects is recovered with an eccentric CBC waveform model, the evolution of eccentricity with frequency will in general not be mimicked. We accordingly formulate an eccentricity evolution consistency test (EECT). The method compares the eccentricity recovered at some low frequency value (e.g., 10 Hz), evolved to higher frequencies assuming GR, with eccentricities recovered at those same higher frequencies. Discrepancy between the two eccentricities at any reference frequency would violate EECT and indicate the presence of a mimicker. As a proof of concept, assuming a few eccentric CBC systems, quasicircular CBCs with additional physics mimicking eccentricity, and an O4-like three-detector-network configuration, we demonstrate that our proposed method is indeed able to reject mimickers at ≥ 68% confidence, while ensuring that truly eccentric CBCs satisfy EECT

Schwarzschild black hole surrounded by Dehnen dark matter halo

• Supakchai Ponglertsakul (Silpakorn University )

Room: Andromeda Meeting Room

Introduction to GR

• Apimook Watcharangkool (NARIT)

Room: Andromeda Meeting Room

GWs: Basics

• Ian Vega (University of the Philippines)

Room: Andromeda Meeting Room

DA Analysis: Hands On

• Otto A. Hannuksela (The Chinese University of Hong Kong (CUHK))

Room: Andromeda Meeting Room

Principles of LIGO/LISA/PTAs: I

• Achamveedu Gopakumar (TIFR)

Room: Andromeda Meeting Room

GW Instrumentation: I

• Yuki Inoue

Room: Andromeda Meeting Room

Principles of LIGO/LISA/PTAs: II

• Achamveedu Gopakumar (TIFR)

Room: Andromeda Meeting Room

DA Analysis: Hands On

• Siyuan Chen (Shanghai Astronomical Observatory, Chinese Academy of Sciences)

Room: Andromeda Meeting Room

EFT cosmology and GWs

• Vicharit Yingcharoenrat (Chulalongkorn University)

Room: Andromeda Meeting Room

TBA

• Alvin Chua (National University of Singapore)

Room: Andromeda Meeting Room

Gravitational waves from super-radiant axion clouds

• Takahiro Tanaka (Kyoto University)

Room: Andromeda Meeting Room

Towards Global EMRI Search with the f-Function

• Davendra Hassan (National University of Singapore)

Room: Andromeda Meeting Room The Laser Interferometer Space Antenna (LISA) will open a new observational window into the millihertz gravitational wave sky, enabling observations of sources previously inaccessible to ground-based detectors. One of the most anticipated is the detection of Extreme Mass Ratio Inspiral (EMRI) systems, in which a stellar-mass compact object completes at least tens of thousands of orbits around a massive black hole before plunging. The resulting gravitational waveforms carry rich harmonic structure that enables precision studies of black holes and further tests of general relativity. However, EMRIs pose a persistent search challenge: the likelihood surface is plagued with non-local degeneracies, in which physically distinct sources can produce nearly identical waveforms, causing standard search algorithms (such as MCMC) to converge on misleading solutions far from the true parameters. In this work, we investigate the f-function introduced by Chua (2022) as an alternative objective function for EMRI searches, designed to exponentially suppress these false solutions. We report results that show convergence near the injected parameters for a near-threshold 2-year EMRI signal searched over a shorter window.

Characterizing Bias in Multi-Source Gravitational Wave Inference with LISA

• Rishav Agrawal (National University of Singapore)

Room: Andromeda Meeting Room The Laser Interferometer Space Antenna (LISA) will be the first gravitational wave detector able to observe sources that emit in the millihertz regime such as Galactic Binaries (GBs), Massive Black Hole Binaries (MBHBs) or Extreme Mass Ratio Inspirals (EMRIs). Consequently, the data stream sent by LISA will be a superposition of an unknown number of various gravitational wave sources. In theory, up to 10,000 compact binaries, several hundred of EMRIs and about a hundred of MBHBs are expected to be sensed by the instrument. This overlap makes inferring source parameters extremely challenging. This project aims to infer the parameters of EMRIs and MBHBs and investigate the intercorrelation between parameters of different sources and intracorrelation among the parameters of the same source. To study this, we compare two inference strategies: a local-fit approach, where individual signals are iteratively inferred and removed while treating other sources as noise, and a computationally intensive global-fit approach that jointly infers multiple sources using a combined likelihood. Our analysis focuses on quantifying biases introduced during parameter estimation. Results show that EMRI signals exhibit minimal overlap with GBs and MBHBs in the frequency domain, suggesting they can often be analysed independently. We also examine interparameter correlations across different source classes and intraparametric correlations within individual sources, finding negligible cross-source correlations but significant correlations among EMRI parameters. These findings highlight the importance of understanding and mitigating inference bias for accurate population studies in future LISA observations,Our simulations have shown promising results and paves the way for future work on multiple EMRIs characterization.

Cosmological and High Energy Physics implications from gravitational-wave background searches in LIGO-Virgo-KAGRA’s O1-O4a runs

• Mairi Sakellariadou (King's College London)

Room: Andromeda Meeting Room

A High-Precision Annual Parallax Measurement Program for PTA Pulsars with VLBI

• Hideyuki Kobayashi (NARIT)

Room: Andromeda Meeting Room Recent international PTA observations have provided compelling evidence for a primordial background gravitational wave, bringing its direct detection within reach. In light of this progress, we are planning precise measurements of the annual parallaxes of PTA pulsars in order to identify the astrophysical sources responsible for the background gravitational waves. While the detection of the background gravitational waves is being pursued through correlations in pulsar timing residuals, identifying their astrophysical origins requires precise distance measurements to individual pulsars. Accurate pulsar distances enable the determination of the arrival directions of the background gravitational waves. The required accuracy corresponds to the gravitational-wave wavelength scale, on the order of several parsecs. To achieve this goal, we plan high-precision astrometric VLBI observations at 1.4 GHz (L band) and 320 MHz (P band). In particular, VLBI observations using the Square Kilometre Array (SKA), currently under construction, are expected to provide a significant improvement in sensitivity as well as substantial gains in phase-referencing accuracy through multi-beam observing capabilities. In this presentation, we report an overview of the proposed observing strategy and the current status of preparation for these VLBI astrometric observations.

Refining Gravitational Wave and Collider Physics Dialogue via Singlet Scalar Extension

• Van Que Tran (NCTS, National Taiwan University)

Room: Andromeda Meeting Room Employing effective field theory techniques, we advance computations of thermal parameters that enter predictions for the gravitational wave spectra from first-order electroweak phase transitions. Working with the real-singlet-extended Standard Model, we utilize recent lattice simulations to confirm the existence of first-order phase transitions across the free parameter space. For the first time, we account for several important two-loop corrections in the high-temperature expansion for determining thermal parameters, including the bubble wall velocity in the local thermal equilibrium approximation. We find that the requirement of completing bubble nucleation imposes stringent bounds on the new scalar boson mass.Moreover, the prospects for detection by LISA require first-order phase transitions in a two-step phase transition, which display strong sensitivity to the portal coupling between the Higgs and the singlet. Interestingly, signals from di-Higgs boson production at the HL-LHC probe parameter regions that significantly overlap with the LISA-sensitive region, indicating the possibility of accounting for both signals if detected. Conversely, depending on the mixing angle, a null result for di-Higgs production at the HL-LHC could potentially rule out the model as an explanation for gravitational wave observations.

The Wave Nature of Gravitational-Wave Lensing and Its Opportunities

• Han Gil Choi

Room: Andromeda Meeting Room

Studying astrophysics with Pulsar Timing Arrays

• Siyuan Chen (Shanghai Astronomical Observatory, Chinese Academy of Sciences)

Room: Andromeda Meeting Room Pulsar Timing Arrays (PTAs) are entering a new era of nano-Hertz gravitational wave (GW) astronomy with evidence for a GW signal found in several PTA data sets, CPTA, EPTA, InPTA, NANOGrav, PPTA and MPTA. They are working together in the IPTA to combine the data sets and analyze the truely best upcoming global PTA data set, IPTA DR3, to be able to conclusively confirm the detection of nano-Hertz GWs. In addition, the GammaPTA project is also advancing and offers a complementary observing frequency range to radio PTAs. In my presentation I will focus on tackling the challenge of the identification and characterization of the source of the GWs. One of the main targets are super massive black hole binaries (SMBHBs), which are formed when two galaxies merge. Several processes will drive the evolution of the binary orbit. Measurements of a GW background can help to address the question on how SMBHBs evolve over a cosmic time scale. Given that direct observations in the electromagnetic spectrum are difficult, we can turn to cosmological simulations for further tests. Finding an individual SMBHB as a GW source will not only allow for the possibility to distinguish different population models of SMBHBs, but also for multi-messenger astrophysics. I will attempt to review the current status of astrophysical interpretation efforts from the PTA community and how we will move forward into the post-detection era.

Special session Room: Andromeda Meeting Room

A Joint Search for Circular Polarization and Kinematic Dipole Anisotropies in the Stochastic Gravitational-Wave Background

• Sevgi Karadağ (Tamkang University)

Certain early-Universe scenarios predict a stochastic gravitational-wave background (SGWB) with nonzero circular polarization, making its detection a potentially powerful probe of parity-violating physics and the physics of the primordial Universe. In addition to polarization, the SGWB may exhibit anisotropies. A particularly well-motivated contribution is the kinematic dipole induced by the observer’s motion relative to the cosmic rest frame. In this work, we develop a unified Bayesian analysis framework to jointly investigate circular polarization and kinematic-dipole anisotropies in the SGWB. We model the cross-correlated detector response in terms of the circular-polarization parameter $\Pi$, the spectral index $\alpha$, the SGWB energy-density amplitude $\Omega_{ref}$, and the spherical-harmonic dipole components $\Omega^D_{1m}$that encode directional anisotropy. The pipeline is first validated through injection–recovery tests using simulated data for third-generation detector networks consisting of Cosmic Explorer and Einstein Telescope. We further assess the projected sensitivity of these future detector networks. We then apply this framework to the LIGO/Virgo O1–O3 cross-spectral dataset. Finding no significant evidence for an SGWB signal, we derive constraints on the parameters related to dipole anisotropies and circular polarization. These results establish a framework for simultaneously probing circular polarization and Doppler-induced kinematic dipole anisotropies in the SGWB, and highlight the potential of next-generation gravitational-wave observatories to place substantially stronger constraints on parity-violating and anisotropic cosmological backgrounds.

Bright sirens, dark sirens and gravitational-wave cosmography

• Hyung Mok Lee (Seoul National Univ.)

Room: Andromeda Meeting Room Gravitational-wave observations of compact-binary coalescences provide a direct measurement of luminosity distance. If the redshift of the source can also be determined, these events can be used as standard sirens to construct a Hubble diagram and probe the expansion history of the Universe. In practice, however, identifying the host galaxies of gravitational-wave sources is extremely challenging because of their large sky-localization uncertainties. GW170817 was an exceptional multi-messenger event: its electromagnetic counterpart enabled the identification of both the binary neutron-star nature of the source and its host galaxy. Most gravitational-wave events detected so far, however, are binary black-hole mergers, which are not expected to produce detectable electromagnetic radiation. These “dark sirens” therefore lack direct host-galaxy identification, and statistical methods using galaxy catalogs have so far yielded only limited constraints on the Hubble constant. This situation could change dramatically with gravitational-wave detectors sensitive to frequencies lower than those accessible to current ground-based observatories. If inspiraling binary black holes can be observed from frequencies around 0.1 Hz, the signals may remain in band for weeks to months before merger. Over such long observation times, the changing orientation and velocity of the detector due to the Earth’s rotation and orbital motion induce characteristic modulations in the gravitational-wave signal. These modulations can significantly improve measurements of both the luminosity distance and sky position of the source. Several detector concepts targeting the mid-frequency gravitational-wave band have been proposed. We show that such detectors could localize some dark sirens with sufficient accuracy, in both sky position and distance, to uniquely identify their host galaxies. This would enable the construction of an accurate gravitational-wave Hubble diagram independent of traditional electromagnetic distance indicators, opening a new route to measuring the cosmic expansion history.