The thermal Sunyaev-Zeldovich effect is a census of all the hot gas in the universe: CMB photons inverse-Compton scattered by intracluster electrons leave a spectral distortion whose angular power spectrum sums the pressure of every cluster and group along every sightline. Planck, ACT, and SPT measure that power consistently below the ΛCDM prediction calibrated from the primary CMB, with the deficit most pronounced at high multipoles beyond l of 2000, the scales dominated by lower-mass halos (Planck Collaboration 2016; Bolliet et al. 2025).
The repair menu is the growth-tension menu. A lower sigma-8 closes the tSZ gap while widening the S8 tension it mirrors; a hot-gas deficit in low-mass halos requires AGN feedback ejecting baryons far beyond current models, energy budgets that strain the engines; and the gas-physics nuisance parameters that absorb the deficit in joint fits do so by assuming exactly what should be measured. The pattern matches the rest of the cluster sector: every probe weighted toward massive collapsed structures, counts, lensing calibrations, and now integrated pressure, reads lower than the primary CMB extrapolation, a coherent shortfall no single gas-physics patch explains.
The standing sharpens as SPT-3G and the Simons Observatory push the tSZ spectrum to higher multipoles and finer precision, closing the parameter space where feedback alone can hide the missing power.