The cosmological constant as a zero action boundary

ABSTRACT The cosmological constant ? is usually interpreted as Dark Energy (DE) or modified gravity (MG). Here, we propose instead that corresponds to a boundary term in the action of classical General Relativity. zero for perfect fluid solution and this fixes average density ? pressure p inside primordial causal boundary: = 4?G <?+3p >. This explains both why observed value related matter today also other contributions ?, such DE MG, do not produce cosmic expansion. Cosmic acceleration results from repulsive force occurs when expansion reaches horizon. universe similar ?CDM universe, except on largest observable scales, where expect departures homogeneity/isotropy, CMB anomalies variations parameters indicated by recent observations.