The momentum-space Feynman rules are expressed in terms of vertices and propagators as depicted in Figure 1. In the figure, space-time indices are denoted by and while the momenta are denoted by or . In contrast to gauge theory, gravity has an infinite set of ever more complicated interaction vertices; the three- and four-point ones are displayed in the figure. The diagrams for describing scattering of gravitons from each other are built out of these propagators and vertices. Other particles can be included in this framework by adding new propagators and vertices associated with each particle type. (For the case of fermions coupled to gravity the Lagrangian needs to be expressed in terms of the vierbein instead of the metric before the Feynman rules can be constructed.)
According to the Feynman rules, each leg or vertex represents a specific algebraic expression depending on the choice of field variables and gauges. For example, the graviton Feynman propagator in the commonly used de Donder gauge is:[44, 45] or in Veltman’s lectures . For simplicity, only a few of the terms of the three-vertex are displayed: i.e., the two indices of graviton are .
The loop expansion of Feynman diagrams provide a systematic quantum mechanical expansion in Planck’s constant . The tree-level diagrams such as those in Figure 2 are interpreted as (semi)classical scattering processes while the diagrams with loops are the true quantum mechanical effects: Each loop carries with it a power of . According to the Feynman rules, each loop represents an integral over the momenta of the intermediate particles. The behavior of these loop integrals is the key for understanding the divergences of quantum gravity.
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