Light from distant galaxies passes through clumps of matter, and the gravity of those clumps slightly bends its path, like a lens. By measuring how much the images of distant galaxies are distorted, we can infer the distribution of matter and how gravity affects light.
The authors of the new study combined several datasets: weak lensing maps from the KiDS-Legacy survey, cosmic microwave background measurements from Planck, ACT, and SPT, baryon acoustic oscillation data from DESI DR2, and supernova observations from DES-Dovekie. They used a parameterization that introduces two numbers: μ (mu) — how much more strongly matter clumps under gravity, and Σ (sigma) — how much more strongly gravity bends light.
This discrepancy is mainly driven by stronger-than-expected gravitational lensing of the cosmic microwave background on large angular scales. If confirmed, it could mean gravity acts on light not exactly as Einstein predicted. However, systematic errors in the data are also possible.
Weak gravitational lensing distorts the shapes of distant galaxies by only fractions of a percent, but statistical analysis of millions of such galaxies can reveal even tiny deviations.
In Liu Cixin's novel "The Three-Body Problem," gravitational light bending is used as a weapon — but there the effects are far stronger than in real observations.