A parameter-free acceleration scale from the cosmic horizon: a test of a0 = cH0/2pi on SPARC rotation curves

A short, self-contained test of the hypothesis that the galactic acceleration scale is fixed, with no free parameter, to the horizon value a0 = cH0/2pi (approximately 1.04e-10 m/s^2). Applied to the 175 galaxies of the public SPARC database via the standard simple interpolating function, the parameter-free scale reproduces the rotation curves as well as empirically-fitted MOND (statistically comparable on the reduced chi-squared metric) and improves on baryons-only Newtonian gravity by roughly a factor of three. Because a0 = cH0/2pi contains no adjustable constant, the SPARC data cannot be used to fit it; they can only prefer or reject it, and they weakly prefer it over MOND's fitted value. The paper is deliberately narrow: it is not a new gravitational field theory and makes no claim about dark matter at cluster or cosmological scales. The analysis protocol was pre-registered prior to data analysis, and all code and per-galaxy outputs are archived openly in the companion deposit (DOI 10.5281/zenodo.21302151).