The CMB blackbody spectrum is the most perfect blackbody in nature with COBE/FIRAS limits |y| < 1.5 × 10⁻⁵ and |μ| < 9 × 10⁻⁵ (Fixsen 1996). ΛCDM predicts small but non-zero distortions from Silk damping + reionization that remain undetected. ARCADE 2 detected radio background excess at low frequencies that cannot be explained by standard sources, creating tension between perfect blackbody at peak + significant excess in radio tail (Seiffert 2011).
The standard model expects predicted spectral distortions from standard processes (Silk damping, reionization, etc.) at detectable levels. The undetected predicted distortions plus the unexpected radio-tail excess force the model to either revise distortion-source models or invoke unaccounted exotic mechanisms.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, the SCT prediction is FIRAS-clean blackbody spectrum exactly. Cascade termination well before t ≈ 1 second (P40) ensures no SCT-specific energy injection between cascade end and recombination. The Plasma Equivalence Theorem (P29, P30) ensures the cascade-thermalized plasma evolves identically to ΛCDM after thermalization, preserving blackbody shape across all observable redshifts.
Mesh dissipation (P14, P15, P16) and dynamical Λ_eff (P17) do NOT inject thermal energy into the photon bath; they manifest as apparent expansion + redshift, which drains rather than adds photon energy. The cumulative cascade-epoch contribution to FIRAS y-distortion is ~2 × 10⁻⁶, four orders below the FIRAS bound. Cascade-epoch μ-distortion is similarly negligible (~10⁻⁸).
The ARCADE 2 radio excess (recid 159) is sourced by cascade-seeded compact-object + pre-existing radio-source populations (M11), not by spectral distortions. The FIRAS blackbody perfection at peak frequencies + the radio excess at low frequencies are both consistent with SCT: blackbody is preserved by the Plasma Equivalence Theorem, while the radio excess comes from cascade-seeded source populations distinct from spectral-distortion mechanisms. The same M2 framework that resolves the y-distortion deficit (recid 26), the heated-CMB term (recid 40), and the broader cascade-thermodynamic CMB signatures accounts for the perfect blackbody spectrum.
Detection of |y| or |μ| > 10⁻⁶ from PIXIE or future spectral-distortion missions that cannot be attributed to standard astrophysical sources (tSZ, kSZ, post-collision reheating) would refute the M2 FIRAS-clean prediction. Equivalently, if the observed CMB blackbody perfection is reconciled with ΛCDM-predicted distortions through revised systematics, both frameworks face the same situation.