Cosmological models in a conformally invariant gravitational theory-I : The Friedmann model

Abstract

The present paper discusses the formulation of the Friedmann cosmological models in terms of a conformally invariant ravitational theory. This theory is Machian in the sense that the mass of a particle arises from the interaction of the particle with a mass field m(X) generated by other particles. In cosmology the mass field m(X) at any particular space-time point X arises predominantly from particles at great distance from X. The Friedmann models are usually discussed in terms of the RobertsonWalker line element. It is known that this line element is conformal to the Minkowski line element ds² = dr²-d² -r² (d0 ² +sin² 0 dᵩ²). Cosmological space-time can therefore be transformed to Minkowski space-time by a suitable conformal transformation. It is not possible in the usual expositions to take advantage of this geometrical simplification because Einstein's gravitational equations are not conformally invariant. However, the present theory is conformally invariant so that transformation to Minkowski space is possible not only for the geometry but also for the physics. The three Friedmann cases k = 0, ± I are discussed in detail from this point of view. Although the cases k = ± I are spatially homogeneous in the Robertson-Walker frame they are not similarly homogeneous in the Minkowski frame, where they can be seen to represent only local clouds that happen to be symmetrically distributed with respect to an observer at r = o. This lack of homogeneity is not shared by the k = 0 case, which emerges from the analysis as the only model consistent with homogeneity in both frames, Robertson-Walker and Minkowski. The conformal transformation function between these two frames is singular at T = o. It is this mathematical breakdown of the transformation function which introduces the well-known singularity of the Friedmann models with respect to the Robertson-Walker frame--the singularity usually referred to as the origin of the Universe. From the present point of view this so-called origin does not arise physically at all. It turns out that the Universe possesses an opposite half, T < 0 in the Minkowski frame, which connects smoothly with ' our' half, T > o. Both halves of the Universe contribute to the mass function m(X), and are therefore connected physically. Indeed the appropriate form for m(X) appears to demand that both halves of the Universe be present. The half T < 0 is missed when the Robertson-Walker frame is used.