Kinetic SZ Null Hint

The kinetic Sunyaev-Zeldovich effect (kSZ) arises from the Doppler shift of CMB photons scattering off moving free electrons in galaxy clusters and the intergalactic medium. The kSZ signal is proportional to the line-of-sight momentum of the free electrons — their peculiar velocity times their optical depth — and is therefore a direct probe of the large-scale velocity field and the distribution of baryons at high redshift. Several analyses using pairwise momentum estimators applied to galaxy cluster samples have found kSZ signals consistent with or slightly below ΛCDM predictions, with some measurements showing a null or weak detection where a significant signal is expected. This kinetic SZ null hint suggests either that the peculiar velocity field is less coherent than ΛCDM predicts, that the baryon fraction in cluster outskirts is lower than assumed, or that the electron optical depth models used in the analysis overestimate the true column densities.

Successive Collision Theory addresses the kinetic SZ null hint through the spatial variation of the effective expansion rate Λ_eff and its effect on the peculiar velocity field. In ΛCDM, peculiar velocities are sourced by density fluctuations growing under gravitational instability with a well-defined growth rate f = Ω_m^0.55(z). In SCT, the effective gravitational growth rate is modified by the locally varying Λ_eff: in underdense regions like the KBC supervoid, the enhanced effective cosmological term suppresses the growth of peculiar velocity amplitudes relative to the ΛCDM prediction, because the enhanced expansion rate partially counteracts the gravitational infall that drives coherent bulk flows. The kSZ pairwise momentum signal, which is proportional to the mean pairwise peculiar velocity, is therefore reduced in the local universe relative to the ΛCDM expectation, producing a systematically lower kSZ detection consistent with the null hints.

Additionally, the angular momentum inheritance mechanism introduces a component of the peculiar velocity field that is curl-dominated — a vorticity field rather than a divergence field — which contributes to the total velocity of galaxy clusters without contributing to the pairwise momentum estimator that the kSZ analyses typically employ. The pairwise momentum estimator is sensitive only to the irrotational (gradient) component of the velocity field; the curl component from angular momentum inheritance cancels in the pairwise statistic. If a significant fraction of peculiar velocity power in the local universe is in rotational modes sourced by angular momentum inheritance, the kSZ pairwise signal would be systematically lower than expected from the total velocity dispersion, while the total velocity field amplitude remains consistent with observations. This decomposition of peculiar velocity into irrotational and rotational components is a unique SCT prediction testable with combined kSZ and peculiar velocity survey data.