A study explores how the choice of background cosmological model affects mock catalogs of standard sirens from the future space-based gravitational-wave detector TianQin. Until now, all TianQin forecasts were generated assuming the ΛCDM model. In this work, two mock catalogs of bright TianQin standard sirens (TQSS1 and TQSS2) are built from massive black hole binary mergers, one generated under ΛCDM and the other under a redshift-binned dark energy reconstruction model (the wbin model). The cosmological implications are then analyzed within the CPL model.

Differences between the two catalogs turn out to be small: the distance-redshift relation differs by at most 1.48%. Taken individually, TQSS1 and TQSS2 yield cosmological constraints similar to those provided by type Ia supernova data, suggesting that this kind of catalogs could complement mainstream cosmological observations. The study also combines TianQin forecasts with baryon acoustic oscillation data from DESI DR2, cosmic microwave background data from Planck 2018, and DESY5 supernova data.

The most notable result is that, when combining either catalog with DESI DR2, CMB and DESY5, the preference for dynamical dark energy persists: TQSS1+DESI DR2+CMB+DESY5 shows a 2.3σ preference, while TQSS2+DESI DR2+CMB+DESY5 raises it to 3.8σ. Adopting different massive black hole seed models does not affect the cosmological implications of the mock catalogs.

The authors stress the importance of taking into account cosmological models other than ΛCDM when generating standard siren forecasts for dark energy. The work paves the way for future studies systematically exploring how TianQin predictions depend on the underlying cosmological model.