The retarded gravitation produced by the matter and energy content of the observable universe is formulated and shown how this cosmological gravity gives rise to inertial forces in accelerated frames of reference. The model is developed for spatially flat Friedman-Robertson-Walker (FRW) universes. The retarded potential of cosmological gravity is determined first in fundamental frames of reference, in Einstein-de Sitter (ES)and in LambdaCDM models. Despite the expansion, the observable universe gives rise to a static, retarded gravitational potential equal to c2 in ES universe, while gravity is weakening during the expansion in LambdaCDM model.
In the locally Minkowski spacetime occurring in a small domain of the universe, a covariant Lagrangian of a test body moving in the cosmological gravitational field is derived. Inertial forces are determined from a test body’s Euler-Lagrange equation, in Newtonian approximation. The model calculation indicates, that the inertial forces meet exactly the ones observed in ES universe and Mach’s principle holds there. In the present-day standard, LambdaCDM model inertial forces are weaker by 10 % than expected.