A preprint released in April 2026 from the Atacama Cosmology Telescope (ACT) collaboration delivers one of the most direct tests yet of whether Einstein’s general relativity still governs gravity across truly cosmological distances. Rather than relying on distant supernovae or galaxy clustering statistics, the team measured the average motion of galaxy groups themselves using the kinematic Sunyaev-Zeldovich (kSZ) effect: as clusters of galaxies hurtle through space, the electrons in their hot gas scatter cosmic microwave background photons, imparting a tiny Doppler-like shift that reveals their velocity. By stacking signals from hundreds of thousands of galaxy pairs in the Sloan Digital Sky Survey (SDSS) DR15 catalog against ACT’s high-resolution CMB maps, they extracted the mean pairwise velocity on scales of 30–230 megaparsecs. The data favor a gravitational acceleration scaling of g ∝ 1/r^n with n = 2.1 ± 0.3—statistically indistinguishable from the inverse-square law (n=2) demanded by standard ΛCDM cosmology.

This is a preprint, not yet peer-reviewed. The analysis uses an SDSS-selected sample of roughly 50,000 massive halos (the exact effective number after cuts is smaller due to matching and foreground cleaning), cross-correlated with ACT DR6 maps. Limitations include sensitivity to astrophysical uncertainties in the electron pressure profile, possible contamination from thermal SZ residuals, and the fact that the measurement averages over a wide range of redshifts (z≈0.1–0.8). The ±0.3 uncertainty on n still comfortably accommodates some modified-gravity scenarios.

What the original arXiv abstract understates is how sharply this result slices through the current cosmology crisis. For years, tensions have mounted: the Hubble constant measured locally by Riess and collaborators (2022) sits at ~73 km/s/Mpc while Planck CMB data prefer ~67 km/s/Mpc—a >5σ discrepancy. Simultaneously, the S8 parameter describing structure growth is lower than expected in many late-universe probes. Several proposed fixes invoke breakdowns or modifications of general relativity on scales larger than individual galaxies but smaller than the Hubble horizon—precisely the regime this kSZ measurement now probes. Models such as f(R) gravity, symmetron screening, or certain braneworld scenarios often predict an effective force law that flattens (n closer to 1) at these distances to boost or suppress clustering. Ruling out n=1 at only ~3σ today is therefore meaningful; the paper notes that forthcoming data from CMB-S4, Simons Observatory, and DESI Year-5 could reach 10σ exclusion.

The work synthesizes earlier ACT kSZ detections (Schaan et al., Phys. Rev. D 2021, arXiv:2009.05558) that first showed the pairwise signal at 4–5σ with smaller datasets, with more recent DESI baryon-acoustic-oscillation results (DESI Collaboration, 2024, arXiv:2404.03002) that mildly favor dynamical dark energy. Those BAO findings hinted that the expansion history might deviate from a pure cosmological constant—another possible signature of modified gravity. By showing the gravitational force law itself appears standard, the new ACT analysis tightens the knot: if gravity is conventional, the explanation for both H0 and S8 tensions must lie either in undiscovered systematics or exotic early-universe physics before recombination.

This positions kSZ velocity measurements as a powerful new cosmological probe, complementary to weak lensing, redshift-space distortions, and standard sirens. Previous coverage has largely treated kSZ as a tool for mapping baryons; this team reframes it as a precision gravity laboratory. The subtlety others missed is the direct connection to the “growth crisis”: because pairwise velocity is proportional to the gravitational force, the measurement bypasses many assumptions about bias and dark-matter clustering that weaken traditional tests. Yet the moderate precision reminds us we are still in early days; a 2.1 ± 0.3 result is consistent with Einstein but cannot yet distinguish between ΛCDM and certain nonlocal gravity models that deviate only at the 10–20 % level.

Ultimately the preprint strengthens the case that general relativity survives another stress test, but the surrounding cosmological tensions remain unresolved. The crisis is not going away—it is simply being cornered into narrower theoretical territory. Upcoming Stage-IV surveys will either confirm the standard force law to exquisite precision or, more excitingly, finally expose the crack that forces us to rewrite our understanding of gravity at the largest observable distances.