Renormalization of symmetry restricted spin foam models with curvature in the asymptotic regime

Abstract

We study the renormalization group flow of the Euclidean Engle-Pereira-Rovelli-Livine and Freidel-Krasnov (EPRL-FK) spin foam model in the large-$j$-limit. The vertex amplitude is deformed to include a cosmological constant term. The state sum is reduced to describe a foliated spacetime whose spatial slices are flat, isotropic and homogeneous. The model admits a non-vanishing extrinsic curvature whereas the scale factor can expand or contract at successive time steps. The reduction of degrees of freedom allows a numerical evaluation of certain geometric observables on coarser and finer discretizations. Their comparison defines the renormalization group (RG) flow of the model in the parameters $(α,Λ,G)$. We first consider the projection of the RG flow along the $α$ direction, which shows a UV-attractive fixed point. Then, we extend our analysis to two- and three-dimensional parameter spaces. Most notably, we find the indications of a fixed point in the $(α,Λ,G)$ space showing one repulsive and two attractive directions.

24 pages, 13 figures; v2: references added, corrected counting of attractive / repulsive directions in RG flow; v3: matching published version (revised plots and more thorough discussion of background independent RG flow)