A new analysis of cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) data finds a mild preference for gravity on the largest scales being a little weaker than Newton’s constant. The authors introduce a one‑parameter “cosmic glitch in gravity” model in which the effective gravitational coupling that drives the expansion of the Universe differs from the locally measured Newtonian coupling. Fitting this model to current Planck+ACT+SPT CMB data together with DESI DR2 BAO measurements gives G_cosmo/G_N = 0.9920 ± 0.0025, a roughly 0.8% weakening and a 3.3σ preference away from the standard value of 1. This preference remains when CMB lensing and recent supernova data are added.

What the researchers did: they compared the standard cosmological model, a common two‑parameter evolving dark energy model (called w0wa or CPL), and the simpler one‑parameter “cosmic glitch” extension. They used primary CMB maps from Planck together with high‑resolution spectra from the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT), and combined these with BAO distance measurements from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2. The analysis samples the usual six cosmological parameters plus the glitch parameter, evolves perturbations with a modified CAMB code using a parameterized post‑Friedmann (PPF) prescription, and tests robustness to choices such as sound speed and foreground modeling.

How it works at a high level: in the glitch model the Friedmann equation that sets the cosmic expansion uses a slightly different gravitational constant than the one measured locally in laboratories and the Solar System. This single extra number shifts the relation between redshift and distance in a predictable way. When the glitch model is fit to the CMB alone, it makes a relatively tight prediction for the BAO distances measured by DESI. That prediction lines up with the DESI DR2 points. By contrast, the two‑parameter evolving dark energy model can fit those data only after it is given low‑redshift information; fitted to CMB alone it leaves a wide range of possible BAO distances and so does not anticipate the DESI measurements.