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

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 $Ω_K = 0.08^{+0.16}{-0.17}$ (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 $S_8 = σ_8\sqrt{Ω_\mathrm{m}/0.3}$ 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.

💡 Research Summary

The paper presents a comprehensive cosmological analysis that combines the final KiDS‑Legacy weak‑lensing data set with a suite of external probes—including primary CMB temperature and polarization from Planck, CMB lensing from Planck, ACT and SPT‑3G, baryon acoustic oscillations from DESI, red‑shift‑space distortion measurements from eBOSS and other surveys, and Type‑Ia supernova distances from Pantheon+. The authors focus on three minimal extensions to the baseline flat ΛCDM model: (i) a free total neutrino mass Σ mν, (ii) a time‑varying dark‑energy equation of state parameterised as w(a)=w0+wa(1−a), and (iii) a non‑zero spatial curvature ΩK.

The KiDS‑Legacy catalogue contains 41 million galaxies over 967 deg², split into six tomographic redshift bins (z ≤ 2) with an effective number density of 8.79 arcmin⁻². Galaxy shapes are measured with an updated LENSFIT algorithm and calibrated using the SKiLLS simulations. The two‑point shear statistics are compressed into the first six COSEBI modes covering angular scales from 2 to 300 arcmin. The theoretical predictions use the HMcode 2020 non‑linear matter power spectrum, a single baryonic feedback parameter (log₁₀ TAGN), and the NLA‑M intrinsic‑alignment model with two nuisance parameters (AIA, β) informed by external priors.

Parameter inference is performed with CosmoPipe and MontePython, employing uniform priors for the cosmological parameters (e.g., ωcdm∈