In holography, thedimensional reduction of phase space to two dimensions defines a dynamical dark energy of ( ) L =q H 1 2 , associated with the cosmological horizon at aHubble radius of = R c H H , and inertia m of baryonic matter at acceleration α in terms of a thermodynamic potential = U mc 2 of Rindler horizons at x a = c 2 . Here, H is the Hubble parameter with deceleration q and c is the velocity of light. In weak gravity, m drops below Newton's value m 0 as a < a H , when Rindler

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... ons fall beyond the cosmological horizon. The onset to weak gravity across a = a H is sharp by causality. Striking evidence is found in galaxy rotation curves, whose asymptotic dynamics is parameterized by Milgrom's scale of acceleration ( ) p = a cH q 2 1 0 . This onset presents a new challenge for canonical dark matter distributions on galactic scales in ΛCDM. Instead, future galaxy surveys may determine ( ) | = = Q dq z dz z 0 0 , to provide a direct test of dynamical dark energy ( > Q 2.5 0 ) versus ΛCDM ( < Q 1 0 ) and establish a bound of -10 eV 30 on the mass of the putative dark matter particle with clustering limited to galaxy clusters.