Cosmological PPN Formalism and Non-Machian Gravitational Theories

By turning to a differential formulation, the post-Newtonian description of metric gravitational theories (PPN formalism) has been extended to include cosmological boundary conditions. The dimensionless expansion parameter is the ratio distance $L$ (measured from the center of a selected space region) to Hubble distance $c/H_0$. The aim was to explore the significance and applicability of a Newtonian cosmology and to clarify to some extent its relation to general-relativistic cosmology. It turns out that up to post-Newtonian order two classes of gravitational theories can be distinguished, here called Machian and non-Machian. In a non-Machian theory like General Relativity the dynamics of cosmic objects within a space region $L \ll c/H_0 $ is described by the usual PPN metric set up for the objects, without introducing time-dependent Newtonian potentials at the origin of the PPN coordinate system. Such potentials of obviously cosmological origin seem to be required for the majority of (by our definition) Machian gravitational theories (including, e.g., Brans-Dicke). Conditions for a theory to be Machian or non-Machian are given in terms of algebraic relations for the PPN parameters.