KiDS-Legacy: Constraining dark energy, neutrino mass, and curvature

We constrained minimally extended cosmological models with the cosmic shear analysis of the final data release from the Kilo-Degree Survey (KiDS-Legacy) in combination with external probes. Due to the consistency of the KiDS-Legacy analysis with the cosmic microwave background (CMB), we could combine these datasets reliably for the first time. Additionally, we used CMB lensing, galaxy redshift-space distortions, and baryon acoustic oscillations. We assessed, in turn, the effects of spatial curvature, varying neutrino masses, and an evolving dark energy component on cosmological constraints from KiDS-Legacy alone and from KiDS-Legacy combined with external probes. We find KiDS-Legacy to be consistent with the fiducial flat Lambda-cold dark matter (Lambda CDM) analysis with c(2)Sigma m(nu) <= 1.5 eV, w(0) = -1.0 +/- 0.7, and w(a) = -1.3(-2.0)(+1.9) while Omega(K) = 0.8(-0.17)(+0.16) (1 sigma bounds) with an almost equal goodness of fit. The w(0)w(a)CDM model is not a significant improvement over Lambda CDM when cosmic shear and CMB lensing are combined, yielding a Bayes factor B = 0.07. If all probes are combined, however, B increases to 2.73, corresponding to a 2.6 sigma suspiciousness tension. The constraint on S-8 = sigma(8)root Omega(m)/0.3 is robust to opening up the parameter space for cosmic shear. Adding all external datasets to KiDS-Legacy, we find S-8 = 0.816 +/- 0.006 in Lambda CDM and S-8 = 0.837 +/- 0.008 in w(0)w(a)CDM for all probes combined.