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

Abstract: We constrain 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 can combine these data sets reliably for the first time. Additionally, we use CMB lensing, galaxy redshift-space distortions, and baryon acoustic oscillations. We assess, in turn, the effects of spatial curvature, varying neutrino masses, and an evolving dark energy component on cosmological constraints from KiDS-Legacy alone and in combination with external probes. We find KiDS-Legacy to be consistent with the fiducial flat $Λ$CDM analysis with $c^2 \sum m_ν\leq 1.5\,$eV, $w_0 = -1.0\pm 0.7$ and $w_a = -1.3^{+1.9}_{-2.0}$ while $ΩK = 0.08^{+0.16}{-0.17}$ (1$σ$ bounds) with almost equal goodness-of-fit. $w_0w_a$CDM is not a significant improvement over $Λ$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 $2.6σ$ suspicousness tension. The constraint on $S_8 = σ8\sqrt{Ω\mathrm{m}/0.3}$ is robust to opening up the parameter space for cosmic shear. Adding all external datasets to KiDS-Legacy, we find with $S_8 = 0.816 \pm 0.006$ in $Λ$CDM and $S_8 = 0.837 \pm 0.008$ in $w_0 w_a$CDM for all probes combined.