...through inertial mass, namely to explain away the sameness of inertial and gravitational mass? If you assume that only inertial mass is a "real" effect, then gravity would simply become a fictious force arising from inertial mass holding matter back from expanding alongside spacetime during the expansion of the universe. I know this is already achieved for the most part in relativity but it seems to me that it is never explicitly stated that gravity within a universe expanding at an ACCELERATING rate could be viewed as identical to any other fictious force within a non inertial reference frame. I don't know Big Bang cosmology even nearly well enough but it seems to me that the origin of the "motivating" force to cause & drive the expansion is largely unaccounted for. It does not seem very logical to imagine two opposing forces, one eventually running out allowing the other to drive the universe towards a "Big Crunch" when you can just imagine a single force, held back by a known phenomenon. Please excuse my very basic understanding. I'm grateful for any pointers you can give me to satisfy my curiousity about this.