From the Milky Way to the Early Universe: How Metallicity Shapes the Formation of Double Neutron Stars

Double neutron stars are unique among gravitational-wave-detected compact binaries because the Milky Way contains well-characterized local analogs observed as radio pulsars. However, understanding the broader cosmological DNS population is difficult because their progenitor evolution depends strongly on metallicity.

Using the detailed binary population synthesis code POSYDON, the authors model DNS formation across a cosmological range of metallicities. They find that DNSs form predominantly through common envelope evolution, but metallicity-dependent radial expansion and stellar wind mass loss bifurcate this pathway into distinct subchannels. This produces systematically different merging populations at different metallicities.

Their best representative Galactic models predict local merger rates in broad agreement with the latest GWTC-5 constraints, suggesting that DNS populations observed through radio pulsar surveys and through gravitational waves need not be in tension. Contrary to earlier studies, the intrinsic merger efficiency increases toward low metallicities, a trend robust across varying binary evolution assumptions, although the magnitude and the underlying stellar processes driving the enhancement vary across metallicity regimes. When convolved with the cosmic star formation history, the models show that most merging DNSs originate from progenitors between about 0.1 solar metallicity and solar metallicity, with significant supersolar contributions. The authors also find that weaker stellar winds at low metallicities produce DNS mergers with systematically more symmetric mass ratios, q greater than or about 0.9.

Finally, the study assesses detectability within proposed configurations for third-generation gravitational-wave observatories. It provides a framework for interpreting the cosmological DNS population in the upcoming multi-messenger era.