Graviton propagator from background-independent quantum gravity

TL;DR

This paper computes the graviton propagator within background-independent loop quantum gravity using spinfoam formalism, demonstrating correct spacetime dependence and addressing previous doubts about model viability.

Contribution

It provides a first-order calculation of the graviton propagator in loop quantum gravity with new techniques and clarifies the role of the Regge action in the large distance limit.

Findings

Correct spacetime dependence obtained

Exponential of Regge action dominates at large distances

Suppression of problematic terms by vacuum phase

Abstract

We study the graviton propagator in euclidean loop quantum gravity, using the spinfoam formalism. We use boundary-amplitude and group-field-theory techniques, and compute one component of the propagator to first order, under a number of approximations, obtaining the correct spacetime dependence. In the large distance limit, the only term of the vertex amplitude that contributes is the exponential of the Regge action: the other terms, that have raised doubts on the physical viability of the model, are suppressed by the phase of the vacuum state, which is determined by the extrinsic geometry of the boundary.