KiDS-1000 Cosmology: Constraints beyond flat ΛCDM

We present constraints on extensions to the standard cosmological model of a spatially flat Universe governed by general relativity, a cosmological constant (Λ), and cold dark matter (CDM) by varying the spatial curvature ΩK, the sum of the neutrino masses ∑mν, the dark energy equation of state parameter w, and the Hu-Sawicki f(R) gravity fR0 parameter. With the combined 3 × 2 pt measurements of cosmic shear from the Kilo-Degree Survey (KiDS-1000), galaxy clustering from the Baryon Oscillation Spectroscopic Survey (BOSS), and galaxy-galaxy lensing from the overlap between KiDS-1000, BOSS, and the spectroscopic 2-degree Field Lensing Survey, we find results that are fully consistent with a flat ΛCDM model with ΩK = 0.011−0.057+0.054, ∑mν < 1.76 eV (95% CL), and w = −0.99−0.13+0.11. The fR0 parameter is unconstrained in our fully non-linear f(R) cosmic shear analysis. Considering three different model selection criteria, we find no clear preference for either the fiducial flat ΛCDM model or any of the considered extensions. In addition to extensions to the flat ΛCDM parameter space, we also explore restrictions to common subsets of the flat ΛCDM parameter space by fixing the amplitude of the primordial power spectrum to the Planck best-fit value, as well as adding external data from supernovae and lensing of the cosmic microwave background (CMB). Neither the beyond-ΛCDM models nor the imposed restrictions explored in this analysis are able to resolve the ∼3σ tension in S8 between the 3 × 2 pt constraints and the Planck temperature and polarisation data, with the exception of wCDM, where the S8 tension is resolved. The tension in the wCDM case persists, however, when considering the joint S8 − w parameter space. The joint flat ΛCDM CMB lensing and 3 × 2 pt analysis is found to yield tight constraints on Ωm = 0.307−0.013+0.008, σ8 = 0.769−0.010+0.022, and S8 = 0.779−0.013+0.013.