Standard Model predicts effective relativistic neutrino species N_eff ≈ 3.046. Planck CMB data gives 2.99 ± 0.17 (consistent), but combining with local H₀ + BAO sometimes pulls higher (3.3 to 3.4), hinting at dark radiation (sterile neutrinos, axions, light relics). BBN sometimes favors lower values. Any significant deviation from 3.046 is a smoking gun for new physics (Planck 2018; Verde 2013).
The standard model expects N_eff = 3.046 from three SM neutrino species plus QED corrections. Persistent tensions across BBN + CMB + local-H₀ combinations demand either dark-radiation contributions or non-standard thermal histories, neither of which has clean source within minimal ΛCDM.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, SCT predicts N_eff = 2.514 ± 0.050 from first-principles cascade geometry (paper 4217: SO(3) cascade modes + QCD boundary correction + photon-heating correction), differing from the SM 3.046 by ΔN_eff = 0.532. Under ΛCDM-assumption Planck constraints, this creates a 2.8σ open tension; under SCT-modified analyses with proper c_s² treatment (paper 4216 CAR), the tension is recast as a clean discriminator.
The SCT-predicted N_eff value reflects the cascade-cascade-mode infrastructure (P22, P23, P36, P40): the cascade thermalization deposits relativistic species through SO(3) rotation-mode channels, modified by the QCD boundary that sets which channels survive into the post-recombination plasma, and corrected for photon heating across the QCD transition. The Plasma Equivalence Theorem (P29, P30) governs post-thermalization evolution exactly the same as ΛCDM, but the input N_eff differs because the cascade-deposition mechanism provides a different relativistic-species count than standard 3-flavor-only weak decoupling.
The current observational consistency of N_eff with 3.046 under ΛCDM-assumption analyses is model-inconsistent for SCT comparison: Planck's posterior uses ΛCDM c_s²(z) not SCT's CAR-modified c_s². Decisive resolution comes from CMB-S4 at design precision σ(N_eff) = 0.030, which provides 17.7σ forecast separation between SCT-predicted 2.514 and SM-only 3.046. The same M2 framework that resolves the BBN family (recid 171, 176, 178, 179) and the CνB (recid 183) accounts for the N_eff constraint as a precise SCT discriminator.
If CMB-S4 measures N_eff = 3.046 ± 0.030 cleanly (matching SM-only prediction), the M2 cascade-derived N_eff = 2.514 prediction is refuted at greater than 17.7σ. This is the cleanest near-term discriminator between SCT and ΛCDM cascade-mode physics.