  The cosmological principle continuous to play an important  role in cosmological modelling to this day. However, Einstein’s  second assumption that the universe is static was in conflict with  his field equations, which permitted models of the universe that were homogeneous and isotropic, but not static. In this regard,  Einstein’s difficulties were essentially the same that Newton had  faced: A static Newtonian model involving an infinite container with  an infinite number of stars was unstable; that is, local regions  would collapse under gravity. Because Einstein was committed to  Mach’s Principle he faced a problem concerning the boundary  conditions for infinite space containing finite  amount of   matter.[77]  Einstein recognized that it was impossible to choose boundary  conditions such that the ten potentials of the metric  \(g_{ij}\) are completely determined by the  energy-momentum tensor \(T_{ij}\), as required by  Mach’s Principle. That is, the boundary conditions “flat at  infinity” entail a global inertial frame that is tied to empty flat space at infinity, and hence is  unrelated to the mass-energy content of space, contrary to Mach’s  Principle, according to which only mass-energy can influence inertia. 