X-ray observations of groups and clusters reveal an entropy floor: minimum intracluster-medium entropy of roughly 100 to 300 keV cm² in low-mass systems, higher than gravitational collapse and shock heating in self-similar ΛCDM models predict (Ponman 1999; Lloyd-Davies 2000). The L_X-T slope is observed at roughly 2.6 to 3.0, steeper than the self-similar 2.0 prediction. Reproducing the entropy floor across mass and redshift requires non-gravitational pre-heating that is fine-tuned in standard simulations.
The standard model assumes ICM entropy comes from gravitational collapse plus shock heating during cluster assembly, predicting L_X-T slope near 2.0. The observed entropy floor and steeper slope demand non-gravitational heating from feedback (supernovae, AGN), which must be carefully tuned to reproduce the floor amplitude and radial behavior across cluster masses without over-heating dense cores.
SCT replaces the hot-dense-center with a superluminal collision and the thermalized debris field. From this single change, the ICM entropy floor is a collision-cascade relic-entropy signature, conserved adiabatically through cluster assembly. The cascade collisions deposited thermal energy at relative velocities of order 10c, producing T_QCD ≈ 1.7 × 10¹² K (P22, P23). The associated entropy K₀ ≈ α m_p c² / (k_B n^(2/3)) lands in the range 100 to 300 keV cm² for the appropriate cascade energy and density (P29, P30).
The Plasma Equivalence Theorem (P29, P30) preserves the relic entropy through subsequent adiabatic plasma evolution, so the cascade-deposited entropy at the cosmic dawn is still present in cluster ICM today as a universal floor across all cluster masses. The relic entropy contributes to the L_X-T relation as a multiplicative factor that steepens the slope from the self-similar 2.0 to the observed 2.6 to 3.0 (recid 6 in Section 11 framework: α_eff = 2 + 3κ/(1+κ) where κ is the relic-entropy ratio). The shape and amplitude are predicted from cascade-energy parameters with one free parameter α in [0.5, 3] (Paper 6).
The same M2 framework that resolves the y-distortion deficit (recid 26), the polarization-bump anomaly (recid 39), the optical-depth scatter (recid 41), and the broader cascade-thermodynamics CMB signatures accounts for the ICM entropy floor as a relic from the cascade era. There is no need for fine-tuned AGN or supernova feedback at the standard amplitudes; standard feedback contributes additionally on top of the relic floor without needing to provide it.
If precision Athena + Lynx X-ray surveys at z = 1 to 2 find the entropy floor evolves significantly with redshift (K₀(z=2)/K₀(z=0) much less than 1, indicating the floor was not present at high z), the M2 cascade-relic explanation is refuted in favor of late-time AGN-pre-heating. The signature SCT prediction is the floor amplitude approximately constant from z = 0 to z = 2, present in both AGN-quiet and AGN-active groups equally.