 These models describe an expanding universe, characterized fully by the behavior of the scale factor \(R(t)\). The worldlines of “fundamental observers”, defined as at rest with respect to matter, are orthogonal to these surfaces, and the cosmic time corresponds to the proper time measured by the fundamental observers. The scale factor \(R(t)\) represents the spatial distance in \(\Sigma\) between nearby fundamental observers as a function of cosmic time. The evolution of these models is described by a simple set of equations governing \(R(t)\), implied by Einstein’s field equations (EFE): the Friedmann  equation,[7] 