Self-similar galaxy dynamics below the de Sitter scale of acceleration

Radial accelerations alpha in galaxy dynamics are now observed over an extended range in redshift that includes model calculations on galactic distributions of cold dark matter (CDM) in lambda cold dark matter (Lambda CDM). In a compilation of data of the Spitzer Photometry and Accurate Rotation Curves (SPARC) catalogue, the recent sample of Genzel et al. (2017) and the McMaster Unbiased Galaxy Simulations 2 (MUGS2), we report on effective self-similarity in the variable zeta = a(N)/a(dS), given by the Newtonian acceleration a(N) based on baryonic matter content over the de Sitter scale of acceleration a(dS) = cH, where c is the velocity of light and H is the Hubble parameter. SPARC, MUG2, and theory satisfy a(N)/alpha similar or equal to 2.1. 12 asymptotically at small accelerations (zeta < < 1). At zeta = 1, however, a(N)/alpha similar or equal to 0.78 in MUGS2 galaxies falls short by 6 sigma of a(N)/alpha similar or equal to 1 in SPARC galaxies in transition to Newtonian gravity (zeta > > 1) that eluded previous studies. This gap poses a novel challenge to CDM in Lambda CDM against the apparent C-0 galaxy dynamics observed in SPARC. We attribute the latter to reduced inertia below the de Sitter scale of acceleration (zeta < 1), based on a causality constraint imposed by the cosmological horizon H.