1ST-ORDER FORMALISM OF F(R) GRAVITY

The first order formalism is applied to study the field equations of a general Lagrangian density for gravity of the form L(G) = square-root -g f(R). These field equations correspond to theories which are a subclass of conformally metric theories in which the derivative of the metric is proportional to the metric by a Weyl vector field. The resulting geometrical structure is unique, except when f(R) = aR2, in the sense that the Weyl field is identifiable in terms of the trace of the energy-momentum tensor and its derivatives. In the case f(R) = aR2 the metric is only defined up to a conformal factor. We discuss the matter conservation equations which are implied by the invariance of the theories under diffeomorphisms. We apply the results to the case of dust and obtain that in general the dust particles will not follow geodesic lines. We consider the linearized field equations and apply them to obtain the weak field slow motion limit. It is found that the gravitational potential acquires a new term which depends linearly on the mass density. The importance of these new equations is briefly discussed.