UHECRs above 10²⁰ eV must come from within the GZK horizon of 50 to 200 Mpc, yet the local volume holds too few sources at the required energetics, the arrival sky is too isotropic for a sparse local population, and the composition runs to heavy nuclei the candidate engines make inefficiently (Alves Batista 2019; Aab 2020). Six decades on, the engines remain unidentified.
The model's accelerator inventory is its visible extreme-environment census: AGN jets, bursts, and starburst winds, each near its theoretical limit at 10²⁰ eV. The particles need engines more capable, more numerous, and more heavy-nuclei-friendly than the census provides, and the census is complete enough that nothing conventional is hiding.
SCT extends the accelerator inventory the same way it extends the emitter inventories: with the cascade's own population. The seeded compact objects distributed along the deposited web (P46, P25), uncounted by the visible-source census, provide acceleration sites throughout the GZK volume rather than at a few cataloged beacons, which is exactly the configuration the isotropy demands: many modest distributed engines rather than few bright clustered ones. The composition follows from the fuel: these engines accelerate the matter around them, and by the cascade's own bookkeeping that matter's nuclei are in-cycle products, since thermalization erased all prior composition (P40) and the front-loaded early stellar generations that collision seeding starts early forged heavy nuclei fast, the same chemistry behind the early maturity sector (recid 113). A heavy-enriched spectrum at the highest energies reflects enriched source environments rather than an engine inefficiency to be excused.
The energetics ride on the same physics that built the population: collision-front shock structures and the seeded engines' interactions tap the cascade's deposited kinetic hierarchy (P22, P23, P29), the energy reservoir that thermalized our patch, of which 10²⁰ eV per particle is an unremarkable tail. This is the fourth window onto the one undercounted demography, alongside the FRBs (recid 153), the FBOTs (recid 149), and the radio background (recid 159): four anomalies, one inventory omission. The seeding mechanics are in Paper 3, From Chaos To Collisothermal Cosmogenesis.
Keystone economy: P46 supplies the engines, the front-loaded in-cycle stellar generations the heavy fuel, P22 the energy scale. The mystery endured because the census was mistaken for complete.
AugerPrime's event-by-event composition carries the kill: if arrival directions at the highest energies resolve into a small set of cataloged local sources, AGN or starbursts carrying the full flux, the distributed seeded population is unnecessary. SCT separately requires the anisotropy to remain mild as exposure grows, tracing the deposited web's smooth geometry rather than sharpening into point-source clustering; a sky that resolves into beacons refutes the many-modest-engines reading.