Infrared regularization of quantum gravity

The problem of a smooth massless limit of massive spin-2 field theory is re-examined in the light of modern advances in the understanding of the role of ghosts in gauge theories. It seems that some progress in this direction would be required for a satisfactory formulation of the infrared problem in quantum gravity, and it is more than likely that the infrared difficulties are related to the lack of renormalizability in the ultraviolet. The definition of smoothness is more restrictive than earlier versions. With the help of auxiliary ghost fields with spin-1 and 0 one can avoid all singularities at zero mass, in the Lagrangian and in the propagators. The number of degrees of freedom is preserved, separately in the physical and in the ghost sectors. The massless limit contains one free one-particle mode of each of the helicities ±2, ±1 and 0. The free Lagrangian is invariant under vectorial as well as under scalar BRST transformations. Interactions can be introduced consistently to low order in the coupling, but we have not succeeded in constructing the complete nonlinear Lagrangian