Sunyaev-Zeldovich Power Excess

While the tSZ power spectrum from resolved massive clusters is lower than ΛCDM predictions as described in the previous tension, a complementary excess appears at small angular scales in CMB power spectrum measurements that is attributed to unresolved tSZ signal from low-mass groups and proto-clusters below individual detection thresholds. The amplitude of this small-scale tSZ contribution, characterized by the parameter y₀ in the one-halo SZ power spectrum, exceeds ΛCDM predictions from semi-analytic group catalogs when the ICM pressure profiles of low-mass halos are modeled without strong AGN feedback suppression. Some analyses report a tSZ power excess at arcminute scales at the level of 20–40 percent above predictions, while others find consistency — the tension is sensitive to the assumed feedback prescription and the treatment of gas fractions in low-mass halos.

Successive Collision Theory explains the small-scale tSZ power excess through the pre-existing gas reservoirs and the angular momentum inheritance mechanism applied to low-mass halos. In SCT, galaxy groups and proto-clusters at z ~ 1–3 inherited both baryonic gas from pre-existing pocket material and organized angular momentum that maintained the gas in relatively extended, slowly cooling configurations. This pre-existing gas contribution elevates the gas fractions in low-mass halos above what post-collision stellar feedback models predict, since the pre-existing gas was not subject to the early supernova feedback that standard models invoke to expel gas from small halos at high redshift. The result is higher ICM electron pressures in low-mass groups than ΛCDM AGN feedback models generate, boosting the one-halo tSZ contribution at small angular scales.

The gravitational superposition from overlapping frames also contributes to the small-scale tSZ excess by effectively increasing the gravitational depth of low-mass halos. In SCT, a low-mass group that would be classified as a sub-threshold halo in CDM is embedded within the gravitational fields of surrounding parent frames that contribute additional confining pressure on the ICM. This superposition confinement suppresses gas expulsion from small halos, maintaining higher gas fractions and ICM pressures than feedback-only models predict. The net effect is a systematically elevated tSZ signal from the low-mass halo population at small angular scales, with the excess most pronounced for halos in dense large-scale environments where the frame superposition depth is greatest — predicting a correlation between the small-scale tSZ excess amplitude and large-scale density environment testable with combined SZ and galaxy density maps.