When the stars don't align: Investigating inconsistencies in binary black hole formation across population synthesis codes
Year: 2026
Type: preprint
Abstract: Binary population synthesis (BPS) codes are valuable tools for investigating the end-to-end lives of binary stars as well as a myriad of observed astrophysical phenomena. Many ``rapid" BPS codes rely on semi-analytical single-star evolutionary tracks and disjointed prescriptions for binary physics. Updated ``hybrid" or ``detailed" BPS codes incorporate improved methodologies, but at a steeper computational cost that may limit broad exploration of physical uncertainties. Given the widespread use of BPS in modern astrophysical research, it is imperative to systematically compare BPS codes across the spectrum of computational efficiency, flexibility, and physical realism to gauge their consistency and robustness. In this work, we perform BPS on three single-metallicity populations of identical initial binaries using three modern codes --- the rapid code $\texttt{COSMIC}$, the hybrid code $\texttt{METISSE}$ integrated into $\texttt{COSMIC}$, and the detailed code $\texttt{POSYDON}$ --- ensuring consistent physical parameterizations where possible. We find stark differences in the properties, formation pathways, and progenitors of binary black hole (BBH) merger populations across the three codes. In a population of one million binaries at $0.01 Z_\odot$, each code results in $\sim 5,000\mbox{--}9,000$ BBHs that merge within a Hubble time. However, $\lesssim 14\%$ of progenitors overlap between two codes, and only $\textit{one}$ initial binary becomes a BBH merger across all three codes. Identical progenitors often follow different evolutionary pathways in each code and result in inconsistent BBH properties. Our results highlight the need for a deeper understanding of physical and computational differences between BPS codes and caution against over-interpreting results from any individual BPS code.
Source: arXiv (Cornell University)
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, National Aeronautics and Space Administration, Alfred P. Sloan Foundation, Research Corporation for Science Advancement, North Carolina Space Grant, Charles E. Kaufman Foundation, Strong, Science Foundation Ireland, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, University of North Carolina at Chapel Hill, Kavli Institute for Theoretical Physics, University of California, Santa Barbara
Awards: G1137844791, G1297409055, G1587926220, NSF-Simons AI Institute for the Sky (SkAI Institute), G3398237449, G4689434848, GW-Learn: Deciphering the Gravitation-Wave Universe using the Next-Generation Observatories and Machine Learning, Kavli Institute for Theoretical Physics, Population Explosions and Gravitational-Wave Science, G7513257514, G7611263089
OpenAlex ID: W7204212160