The open-Universe signal: A model artifact rather than genuine curvature

Recent late-Universe observations suggest an open Universe. If confirmed, such a departure from spatial flatness would carry profound implications for our understanding of cosmic inflation and the ultimate fate of the Universe. Motivated by this intriguing result and the release of new data, we revisit the question using baryon acoustic oscillation measurements from DESI DR2, multiple Type Ia supernova samples, refined strong gravitational lensing time-delay analyses, and the most up-to-date cosmic chronometer data. We find that within the $\Lambda$ cold dark matter ($\Lambda$CDM) paradigm, the combined data still prefer an open Universe with $\Omega_K=0.049\pm0.037$. However, this preference vanishes in extensions to $\Lambda$CDM, where the data instead favor a flat Universe. The model comparison shows that for $\Lambda$CDM, introducing new physics is preferred over merely allowing spatial curvature, and flat $\Lambda$CDM extensions perform better than their curved counterparts. We therefore argue that the mild open-Universe signal is an artifact of limited model flexibility, rather than a genuine feature of late-Universe observations.