Curvature Mode Coupling

In a perfectly flat, isotropic, and Gaussian universe, scalar, vector, and tensor perturbation modes evolve independently — they do not couple or mix during linear evolution. Observations hint at anomalous correlations between modes of different character that ΛCDM cannot produce at the linear level; any explanation requires either nonlinear effects, foreground contamination, or beyond-standard-model physics. In SCT, mode coupling arises naturally from the collision event's angular momentum. The impact parameter of the collision deposits a specific angular momentum vector into the thermalized plasma, and this vector breaks the decoupling between scalar density modes and the rotational (vector) and gravitational wave (tensor) sectors. The angular momentum effectively acts as a background that rotates scalar fluctuations into vector perturbations and vice versa, generating a mixing term in the evolution equations that is proportional to the local angular momentum density.

This coupling is strongest on the largest angular scales, where the angular momentum density from the collision is still coherent, and becomes negligible at subhorizon scales where local gravitational dynamics dominate. The mode coupling predicted by SCT involves no new physics: the mixing arises entirely from the GR description of a rotating stress-energy tensor sourcing both scalar and tensor metric perturbations simultaneously. Observationally, SCT predicts that the TB and EB correlation functions in the CMB polarization field should be nonzero at low multipoles — a signal that ΛCDM forbids in the absence of parity violation but that SCT generates through the helical structure of the angular momentum field imprinted at the collision.