Article révisé par les pairs
Résumé : We study conformally invariant fields within the context of semi-classical gravity. We claim that, generically, conformal flatness implies Friedmann-Robertson-Walker behaviour. A proof is presented here for the case in which the Ricci tensor is of the perfect fluid type. We also rewrite the field equations as a quadratic three dimensional autonomous system of ordinary differential equations, the critical points of which are Minkowski space and de Sitter space. Both these critical points are unstable in the linear as well as in the non-linear theory. © 1991 Plenum Publishing Corporation.