**5. Spatially flat cosmology and spectator matter**

The one thing in the discussion of the role of inertia in general relativity that is certainly right is that Brans was absolutely correct in asserting that if spectator matter contributes to the rest masses of test particles through its contribution to the total gravitational potential by changing it (as Einstein assumed), then the Equivalence Principle is violated. This *fact*, however, does not necessitate the assumption of Minkowskian spacetime in small regions. It simply requires that the total locally measured Newtonian gravitational potential is an invariant. If the Newtonian potential is everywhere exactly the same in local measurements, that, by itself, precludes charge to mass ratios of elementary particles depending on the local gravitational environment. The question then is, does the spatially flat spacetime of FLRW cosmology have the requisite properties for a spacetime/gravitational field that yields gravitational induction of inertia?

Spatially flat, that is, curvature index "k" = 0, cosmology has several remarkable properties beyond being the transitional case between spherical, closed and
