Spin foam propagator: A new perspective to include a cosmological constant

Understand

In recent years, the calculation of the first non-vanishing order of the metric 2-point function or graviton propagator in a semiclassical limit has evolved as a standard test for the credibility of a proposed spin foam model.

• The existing results of spinfoam graviton propagator rely heavily on the so-called double scaling limit where spins $j$ are large and the Barbero-Immirzi parameter $\gamma$ is small such that the area $A\propto j\gamma$ is approximately constant.
• However, it seems that this double scaling limit is bound to break down in models including a cosmological constant.
• We explore this in detail for the recently proposed model by Haggard, Han, Kaminski and Riello and discuss alternative definitions of a graviton propagator, in which the double scaling limit can be avoided.