The growth index gamma parameterizes how fast matter perturbations grow: the growth rate is written f(z) = Omega_m(z)^gamma, and general relativity in a flat ΛCDM background predicts gamma = 0.55 almost independently of the other parameters. Measuring gamma is therefore a clean consistency test of the gravity sector. The test is failing: Nguyen, Huterer and Wen (2023, Physical Review Letters; arXiv:2302.01331) combined redshift-space distortions, weak lensing, galaxy clustering, and CMB data and found gamma = 0.633 +0.025/-0.024, excluding the GR value at 3.7 sigma. Adding the Planck fsigma8 information sharpens the result to gamma = 0.639 +/- 0.025, a 4.2 sigma exclusion. A growth index above 0.55 means the growth of cosmic structure during the dark-energy era is suppressed relative to the ΛCDM prediction.
The result is uncomfortable for ΛCDM in two directions at once. The model has no parameter that can move gamma: once the background is fixed, GR fixes the growth history, so a 4 sigma excursion is either new gravitational physics at late times or a coherent systematic spanning several independent probes. And the excursion is not isolated; it is the same suppressed-growth signature seen in the S8 tension and its emerging low-redshift concentration, and Nguyen et al. note that letting gamma float also largely removes the Planck lensing-amplitude anomaly, tying the growth excess and the lensing excess together as one pattern. Standard escape routes do not fit the shape: massive neutrinos suppress growth but nearly uniformly in redshift and are now capped by cosmological mass bounds, while most viable modified-gravity models predict enhanced rather than suppressed late-time growth.
The standing is serious and active: the measurement uses well-understood probes, the significance is at the discovery threshold's edge, and the same direction of effect appears in independent analyses of cluster counts, CMB-lensing cross-correlations, and cosmic shear. DESI full-shape clustering, Euclid, and Rubin-LSST will tighten gamma by factors of a few this decade, making the growth index one of the sharpest near-term stress tests of the concordance model's gravity sector.