Second-order amplitudes in loop quantum gravity

We explore some second-order amplitudes in loop quantum gravity. In particular, we compute some second-order contributions to diagonal components of the graviton propagator in the large distance limit, using the old version of the Barrett–Crane vertex amplitude. We illustrate the geometry associated with these terms. We find some peculiar phenomena in the large distance behavior of these amplitudes, related to the geometry of the generalized triangulations dual to the Feynman graphs of the corresponding group field theory. In particular, we point out a possible further difficulty with the old Barrett–Crane vertex: it appears to lead to flatness instead of Ricci flatness, at least in some situations. The observation raises the question whether this difficulty remains with the new version of the vertex.

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