How binary black holes and neutron stars form and merge is the central question in gravitational-wave astrophysics. A decade after the first direct detection of a binary black hole merger, it remains unclear which gravitational-wave events probe the evolution of isolated massive binary stars, the dynamics of dense environments like star clusters, and active galactic nuclei, or more exotic pathways. One of the main hurdles has been the low-number statistics that prevented statistically robust identification of sub-populations within the observed population of gravitational-waves.
With the full O4 catalogue expected by the end of 2026, the sample is set to grow to several hundred events, ushering gravitational-wave astronomy into a new era of precision population inference. For the first time, this enables robust multi-parameter analyses (masses, spins, eccentricities, and their correlations) which are crucial for breaking the degeneracies that currently prevent us from deciphering the astrophysical origin of compact-object mergers.
The recent O4a and O4b releases have already prompted several interesting (and sometimes competing) claims about the origins of binary black hole subpopulations, including evidence for hierarchical mergers, features associated with the supernova-remnant mass function, and imprints of isolated binary and triple-star evolution. Many of these results remain only partially supported. In parallel, EM observations from (like the Gaia DR4 release) are substantially expanding the sample of dormant compact objects in binaries with luminous companions, providing essential context for interpreting gravitational-wave events.
Taking place just a few weeks after the planned full O4 data release, the workshop will be a timely opportunity to discuss the wealth of new results from the enlarged gravitational-wave catalogue. It will bring together researchers working on astrophysical theory and modelling, numerical relativity, and gravitational-wave data analysis to explore what are the astrophysical lessons that can be robustly drawn from O4.
The workshop will have a very heavy focus on discussions and interactions to promote problem solving structured around three themes:
- Astrophysical modelling: merger signatures expected from different formation channels
- Data analysis: which features in the data are robust
- Falsification: how the new data constrain and refine astrophysical models
Registration deadline: 8 November 2026
The number of participants is limited. We will confirm participation shortly after registration closes.
Organizers
Jakob Stegmann (MPA), Lieke van Son (Radboud University), Davide Gerosa (Milano-Bicocca), Samaya Nissanke (DESY/DZA), and Salvatore Vitale (MIT).