Radio relics and halos are large diffuse synchrotron sources in galaxy clusters: relics in cluster outskirts tracing merger shocks, halos in cluster cores with turbulent radio emission (Feretti 2012; van Weeren 2019; Brunetti & Jones 2014; Nuza 2017). Matching their occurrence rates, morphologies, sizes, and spectral properties across redshift in ΛCDM requires uncertain particle-acceleration efficiency, seed electron populations, and intracluster-magnetic-field assumptions.
The standard model attributes radio relics + halos to electrons accelerated by merger-shock waves and turbulence. The shocks inferred from X-ray data are often too weak (low Mach numbers) to efficiently accelerate thermal electrons to the required relativistic energies. The model has a long-standing electron-acceleration problem.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, radio relics and halos come from collision-shock physics combined with gravitational superposition. The cascade collisions deposited relativistic plasma at energies producing synchrotron emission (P22, P23: collision energy regime gives kinetic energies sufficient to maintain relativistic electron populations through cluster-scale evolution).
The Plasma Equivalence Theorem (P29, P30) preserves these relativistic plasma signatures adiabatically through cluster-scale assembly. Cluster mergers in SCT operate on already-relativistic plasma populations inherited from cascade-stage events, eliminating the ΛCDM electron-acceleration problem: there is no need to accelerate thermal electrons to relativistic energies because the cascade-deposited plasma is already relativistic. Cluster-merger shocks then re-energize and structurally rearrange the inherited population, producing the observed relic and halo morphologies without requiring high-Mach acceleration efficiency.
Gravitational superposition (P50, P51, P52) provides the cluster dynamics that holds the relativistic plasma in the relic and halo configurations. Pre-existing matter (P25) gives the plasma diversity across clusters that produces variation in relic + halo properties. The same M2 framework that resolves the y-distortion deficit (recid 26), the polarization-bump anomaly (recid 39), the ICM entropy floor (recid 126), and the broader cascade-thermodynamic cluster signatures accounts for radio relics + halos as predicted relativistic-plasma cluster features.
If precision SKA + ngVLA + Athena cluster-merger surveys find radio-relic + halo populations fully consistent with high-Mach-shock acceleration of thermal electrons at ΛCDM-predicted efficiency (no cascade-deposited relativistic-plasma inheritance signature), the M2 cascade-relic explanation is refuted. The signature SCT prediction is relic + halo brightness scaling with cascade-deposited plasma inventory rather than only with current shock-Mach-number predictions.