Kinematic SZ Dipoles

The kinematic Sunyaev-Zeldovich effect produces a dipolar pattern in the CMB temperature field when free electrons in galaxy clusters are moving coherently in a preferred direction: the leading hemisphere of electrons scatters CMB photons to higher temperatures while the trailing hemisphere scatters them to lower temperatures, producing a cluster-scale CMB temperature dipole aligned with the cluster's bulk velocity. Statistical analyses of these cluster-scale kSZ dipoles across large cluster samples probe the velocity field correlation function and bulk flow amplitudes on scales of hundreds to thousands of megaparsecs. Several analyses have reported bulk flow amplitudes on scales of 300–500 Mpc that exceed ΛCDM predictions by factors of two to three, sometimes at two-to-three sigma significance, indicating more coherent large-scale motion than the standard concordance model predicts.

Successive Collision Theory naturally predicts enhanced large-scale bulk flows through two mechanisms. First, the KBC supervoid creates a systematic infall toward the denser surrounding walls and filaments — a bulk flow component on the 300 Mpc scale that is driven by the enhanced Λ_eff gradient within the void. This void-driven bulk flow adds to the gravitational infall predicted by ΛCDM and amplifies the kSZ dipole signal in the direction of the supervoid's bulk motion toward surrounding overdensities. Second, the initial pocket collision imprinted a large-scale momentum asymmetry on the debris field through the collision's center-of-mass velocity — if the two colliding pockets had different total masses or velocities, the thermalized debris field retains a residual bulk momentum that manifests as a large-scale coherent flow on the scale of the observable patch. This residual bulk velocity from the collision kinematics contributes to the kinematic SZ dipole pattern at the largest angular scales.

The combination of the void-driven infall and the collision-inherited bulk momentum in SCT produces a kSZ dipole signal that is larger than ΛCDM predicts on scales of 200–500 Mpc, consistent with the observed excess bulk flow amplitudes. SCT makes the specific prediction that the direction of the excess bulk flow should be aligned with both the CMB kinematic dipole — which in part reflects the local void infall — and the collision axis encoded in the CMB quadrupole-octupole alignment. The coincidence of these directional alignments — bulk flow direction, CMB dipole, quadrupole-octupole axis — would be expected if all three trace the same underlying collision geometry and angular momentum field, rather than being independent random vectors as ΛCDM would predict.