It is proposed that the principle of equivalence—which recognizes momentary equivalence between gravitational and inertial force—is cosmologicalized in an expanding spherical distribution of matter, whereby spacetime itself acts as the accelerating frame of reference of the inertial mass of an object on the surface of the sphere, and the inertial force thus generated is momentarily equivalent at a certain radial distance to the gravitational force experienced by the gravitational mass of the object. The spacetime accelerating frame of reference is parametrized by the cosmological constant Λ. The relation between inertial mass of an object and the cosmological constant can be generalized as a correlation between the matter density in the universe and the cosmological constant. Since matter density dilutes in an expanding universe, it is predicted that the Λ-term should be time varying.