Light from a distant galaxy travels across the universe, passing through vast clumps of dark and ordinary matter. Their gravity slightly bends the path of the rays, like a river's current flowing around invisible boulders. In images, this shows up as a barely noticeable elongation of galaxy shapes, as if they were stretched on an invisible frame. Astronomers call this weak gravitational lensing (the bending of light by gravity) and use it to weigh the universe.
Previously, the counting was done in pairs: they took two random patches of sky and looked at how similarly galaxies were stretched there. This is a reliable but coarse method: it is mainly sensitive to the combination of matter density and clumpiness, and these quantities remained elusive separately. In the new work, based on data from the final KiDS-Legacy survey (which covered 1006 square degrees of sky), scientists for the first time added triple coincidences. Now they looked at how three distant galaxies are simultaneously distorted by the same clump. It's like not just listening to an echo, but catching its cross-reflections from canyon walls: you hear much more detail.
The key result — the value of the cosmological parameter $S_8$—is $0.806^{+0.025}_{-0.023}$, and the matter density $\Omega_m = 0.297^{+0.056}_{-0.040}$. The most interesting part: these numbers agree perfectly with data from the Planck satellite, which looks at the cosmic microwave background (the oldest light left over from the Big Bang). Previously, a crack was emerging between different methods — the so-called '$S_8$ tension', a hint of new physics. Now, with the triple measurements, the discrepancy has disappeared. The universe has become a bit more understandable again.
as written by the authors arXiv:2606.12389
Weak lensing by large-scale structure is a powerful cosmological probe. While most analyses rely on second-order correlations, these are primarily sensitive to the parameter combination $$S_8 = \sigma_8 (\Omega_m/0.3)^{0.5}$$, limiting their ability to constrain $$\Omega_m$$ and other cosmological parameters independently. Higher-order statistics capture non-Gaussian features of the density field and can therefore break parameter degeneracies and extract more cosmological information from weak lensing surveys. We present a joint analysis of second- and third-order cosmic shear in the final data release of the Kilo-Degree Survey (KiDS-Legacy). We combine COSEBIs (Complete Orthogonal Sets of E-/B-mode Integrals) at scales between 2' and 300' with third-order aperture mass moments at scales between 4' and 32' to perform a joint analysis of second- and third-order statistics. Compared to previous KiDS analyses, we implement several methodological advances: an intrinsic alignment model with redshift and mass dependence, a baryon correction model validated on multiple hydrodynamical simulations, and corrections for reduced shear and source clustering. Combining COSEBIs with third-order aperture mass statistics in KiDS-Legacy yields $$\Omega_m = 0.297^{+0.056}_{-0.040}$$ and $$S_8 = 0.806^{+0.025}_{-0.023}$$, significantly tightening the $$\Omega_m$$ constraints and more than doubling the figure of merit in the $$\Omega_m$$--$$S_8$$ plane compared to the two-point analysis alone. The third-order measurements pass stringent internal consistency tests, are fully compatible with the KiDS-Legacy 2-point constraints, other 2+3-point lensing results and with Planck CMB measurements within $$1\sigma$$, providing no evidence for an $$S_8$$ tension and demonstrating the maturity of 3-point cosmic shear as a key probe for forthcoming surveys.
Dark matter is about five times more abundant in the universe than ordinary matter — the stuff that makes up stars, planets, and us. But it cannot be seen; it can only be sensed through its gravitational pull.
In the novel 'The Light of Distant Stars' (K. S. Robinson), characters use gravitational lensing for interstellar navigation, like a map of invisible currents.