Chemical Evolution of the Universe and its Consequences for Gravitational-Wave Astrophysics

Gravitational waves (GW) emitted by merging black holes (BH) and neutron stars are now routinely detected. Those are the afterlives of massive stars that formed all across the Universe-at different cosmic times and with different metallicities. Birth metallicity plays an important role in the evolution of massive stars. Consequently, the population properties of mergers are sensitive to the metallicity dependent cosmic star formation history (f(SFR)(Z,z)). In particular, within the isolated formation scenarios (the focus of this paper), a strong low metallicity preference of the formation of BH mergers is found. The origin of this dependence and its consequences are discussed. Most importantly, uncertainty in the f(SFR)(Z,z) (substantial even at low redshifts) cannot be ignored in the models. This poses a challenge for the interpretation of the observed GW source population properties. Possible improvements and the role of future GW detectors are considered. Recent efforts to determine f(SFR)(Z,z) and the factors that dominate its uncertainty are summarized. Many of those factors stem from the uncertain properties of faint and distant galaxies. The fact that they leave imprint on the redshift-dependent properties of mergers makes GW a promising (and complementary to electromagnetic observations) tool to study galaxy chemical evolution.

Automated summary

This article discusses the impact of metallicity on the population properties of black hole mergers and emphasizes the preference for low metallicity in the formation of these mergers. The article highlights the challenge posed by uncertainty in interpreting the observed gravitational wave source population properties and discusses possible improvements and the role of future gravitational wave detectors. Additionally, recent research on metallicity and its dominant sources of uncertainty are summarized, with the understanding that uncertain properties of distant galaxies have an impact on the redshift properties of mergers, making gravitational waves a promising tool for studying galaxy chemical evolution.