Geometry Transition in Spinfoams

We show how the fixed-spin asymptotics of the EPRL model can be used to perform the spin-sum for spin foam amplitudes defined on fixed two-complexes without interior faces and contracted with coherent spin-network states peaked on a discrete simplicial geometry with macroscopic areas. We work in the representation given in Ref. 1. We first rederive the latter in a different way suitable for our purposes. We then extend this representation to 2-complexes with a boundary and derive its relation to the coherent state representation. We give the measure providing the resolution of the identity for Thiemann's state in the twisted geometry parametrization. The above then permit us to put everything together with other results in the literature and show how the spin sum can be performed analytically for the regime of interest here. These results are relevant to analytic investigations regarding the transition of a black hole to a white hole geometry. In particular, this work gives detailed technique that was the basis of estimate for the black to white bounce appeared in Ref. 2. These results may also be relevant for applications of spinfoams to investigate the possibility of a 'big bounce'.