Spacetime as a Feynman diagram: the connection formulation

Spin-foam models are the path-integral counterparts to loop-quantized canonical theories. Over the last few years several spin-foam models of gravity have been proposed, most of which live on finite simplicial lattice spacetime. The lattice truncates the presumably infinite set of gravitational degrees of freedom down to a finite set. Models that can accommodate an infinite set of degrees of freedom and that are independent of any background simplicial structure, or indeed any a priori spacetime topology, can be obtained from the lattice models by summing them over all lattice spacetimes. Here we show that this sum can be realized as the sum over Feynman diagrams of a quantum field theory living on a suitable group manifold, with each Feynman diagram defining a particular lattice spacetime. We give an explicit formula for the action of the field theory corresponding to any given spin-foam model in a wide class which includes several gravity models. Such a field theory was recently found for a particular gravity model. Our work generalizes this result as well as Boulatov's and Ooguri's models of three- and four-dimensional topological field theories, and ultimately the old matrix models of two-dimensional systems with dynamical topology. A first version of our result has appeared in a companion paper: here we present a new and more detailed derivation based on the connection formulation of the spin-foam models.