When the stars don't align: Investigating inconsistencies in binary black hole formation across population synthesis codes
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 COSMIC, the hybrid code METISSE integrated into COSMIC, and the detailed code 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⊙, each code results in ∼ 5,000−−9,000 BBHs that merge within a Hubble time. However, ≲ 14% of progenitors overlap between two codes, and only 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.
Related
- Type Ia supernova population machine – I. A unified cosmological simulation–binary synthesis framework establishing non-universal delay-time distributions and cosmic progenitor-channel dominance crossover
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- GW231123 Formation from Population III Stars: Isolated Binary Evolution
- Evolution of Massive Main-Sequence Stars in Binary Population Synthesis. II. Consequences for Black Hole High-Mass X-ray Binaries and Binary Black Holes
- Bridging the Population Synthesis of Supermassive Binary Black Holes and the Gravitational Wave Background
BibTeX
@misc{guerrero2026when,
title = {{When the stars don't align: Investigating inconsistencies in binary black hole formation across population synthesis codes}},
author = {Alexandra G. Guerrero and Michael Zevin and Duncan B. Maclean and Katelyn Breivik and Carl L. Rodriguez and Max M. Briel and Daniel E. Holz},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2608.21609},
eprint = {2608.21609},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.21609},
}
RIS
TY - UNPB
TI - When the stars don't align: Investigating inconsistencies in binary black hole formation across population synthesis codes
AU - Alexandra G. Guerrero
AU - Michael Zevin
AU - Duncan B. Maclean
AU - Katelyn Breivik
AU - Carl L. Rodriguez
AU - Max M. Briel
AU - Daniel E. Holz
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2608.21609
UR - https://arxiv.org/abs/2608.21609
AB - 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 COSMIC, the hybrid code METISSE integrated into COSMIC, and the detailed code 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_⊙, each code results in ∼ 5,000−−9,000 BBHs that merge within a Hubble time. However, ≲ 14% of progenitors overlap between two codes, and only 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.
ER -