Entropy Floors

Galaxy clusters and groups show a characteristic minimum entropy level in their intracluster gas — a so-called entropy floor — that cannot be explained by gravitational heating alone. Gas that falls into cluster potential wells and is shock-heated should have an entropy profile that scales as a power law from the center outward, reaching the lowest values at the center. Instead, observed clusters show a central entropy floor: the gas entropy does not continue to decrease toward the center but levels off at a minimum value of approximately 10–30 keV cm^2, even in the absence of active cooling. Standard ΛCDM attributes this floor to non-gravitational heating processes — primarily AGN feedback and early supernova feedback — that pre-heated the intergalactic gas before it accreted into the cluster. However, the required pre-heating energy, the epoch at which it must have occurred, and its spatial homogeneity across different cluster environments are difficult to simultaneously satisfy with AGN feedback models.

Successive Collision Theory explains the entropy floor through two reinforcing mechanisms. First, the collision thermalization itself deposited a minimum specific entropy into all gas in the thermalized volume — the entropy deposited by the collision front sweeping through the pockets' matter establishes a baseline from which no subsequent purely gravitational process can reduce the entropy (by the second law of thermodynamics). Gas that accretes into cluster potentials carries this pre-existing minimum entropy, setting a floor that gravitational compression cannot cool below without additional radiative losses. Second, the gravitational superposition of the nested comoving frame hierarchy contributes effective pressure support in cluster cores through the coherent superposition of parent-frame gravitational wells. This additional effective pressure reduces the net compression of gas at the cluster center below what the local potential depth alone would produce, maintaining the observed entropy floor without requiring AGN feedback of precisely tuned energy output. The combination of the collision's thermodynamic legacy and the frame superposition pressure provides a physically grounded and spatially homogeneous entropy floor that matches observations across cluster masses and redshifts.