The pulsar timing arrays detected the nanohertz background with Hellings-Downs correlations at strain 2.4 × 10⁻¹⁵ (NANOGrav 15-year), but standard supermassive binary populations built from observed merger rates and scaling relations predict about 1 × 10⁻¹⁵, short by a factor of a few even when every merger forms an efficiently hardening binary. Meeting the data needs masses, rates, and final-parsec efficiency all pushed to their edges at once.
The model's source inventory is fixed by its assembly history: binaries form from hierarchical mergers of galaxies whose black holes obey local scaling relations, capping the background's amplitude. A louder hum than the inventory allows forces either edge-tuned demographics or new physics imported wholesale.
SCT rewrites the source demographics that set the amplitude. Black holes are collision-seeded at 10⁷ to 10⁹ solar masses (P46) and overmassive relative to local scaling relations through cosmic history, the same demographic JWST observes directly, so the chirp-mass budget of the binary population is systematically larger than ΛCDM bookkeeping permits. Sibling seeds condensing from one debris field begin correlated and close (P31, P58-logic at galactic scale), enriching the binary formation channel beyond what galaxy-merger counting captures: more binaries, heavier binaries, earlier binaries, and a louder integrated hum follows from the seeding inventory without pushing any parameter to its edge.
The final-parsec bottleneck also eases: binaries embedded in dense seeded environments harden against the natal gas and debris (P25), and mesh dissipation (P14) adds a slow secular drain on wide orbits, so the population spends its history radiating rather than stalling. The spectrum carries the test: a seeded-overmassive population shifts spectral weight toward lower frequencies relative to the canonical -2/3 slope, the direction of the mild observed strain.
This is the same seeds-before-hosts demographic behind the JWST overmassive ratios and the quasar clustering excess, now audited in gravitational waves. There is no need to invoke cosmic strings or edge-tuned conventional demographics.
The attribution race is the test: SCT requires the background resolved into a binary population that is overmassive and early relative to local scaling relations. Individual-source detections and anisotropy mapping revealing a binary population fully consistent with local relations and standard merger rates, with the amplitude excess dissolving into improved noise modeling, would remove the seeded-demographic signature; equally, a confirmed primordial origin (strings, phase transition) displacing binaries entirely would falsify this reading while raising its own conflict with SCT's no-inflation tensor floor.