Silk Damping

Silk damping — the diffusion of photons through the baryon-photon fluid that erases acoustic oscillations on small scales — sets the damping tail shape of the CMB power spectrum. ΛCDM fits this tail with parameters describing the photon diffusion length at recombination, but precise measurements from ACT and SPT reveal subtle tensions in the damping tail amplitude and its dependence on angular scale. In particular, the effective damping scale appears slightly larger than ΛCDM predicts, and the smoothing of acoustic peaks extends to slightly larger angular scales than the standard recombination scenario produces. SCT's thermalization mechanism provides a modified initial state for the acoustic oscillations: the collision-heated plasma begins with a slightly different initial entropy per baryon than standard ΛCDM, because the collision kinetic energy is deposited into both photons and pre-existing baryonic matter simultaneously rather than arising from purely adiabatic perturbations.

The slightly modified baryon-to-photon ratio at thermalization shifts the photon diffusion length at recombination, altering the characteristic Silk damping scale. Additionally, the tensor mesh superposition effect deepens gravitational wells in dense regions, which modifies the acoustic oscillation amplitude in those regions and contributes an effective additional damping when averaged over the sky. The coherent angular momentum of the collision geometry also produces a slight asymmetry in the damping tail between aligned and perpendicular directions, contributing a small anisotropy to the damping scale that would appear as an effective enhancement when all sky directions are averaged together. SCT therefore naturally produces a slightly larger effective Silk damping scale without requiring non-standard recombination physics, resolving the observed damping tail tension as a consequence of its modified initial plasma state.