Metric gravity theories and cosmology: I. Physical interpretation and viability

Abstract

We critically review some concepts underlying current applications of gravity\ntheories with Lagrangians depending on the full Riemann tensor to cosmology. We\nargue that it is impossible to reconstruct the underlying Lagrangian from the\nobservational data: the Robertson-Walker spacetime is so simple and "flexible"\nthat any cosmic evolution may be fitted by infinite number of Lagrangians.\nConfrontation of a solution with the astronomical data is obstructed by the\nexistence of many frames of dynamical variables and the fact that initial data\nfor the gravitational triplet depend on which frame is minimally coupled to\nordinary matter. Prior to any application it is necessary to establish physical\ncontents and viability of a given gravity theory.\n