A diagrammatic approach to correlation functions in superfluids

Renaud Parentani has given a vast contribution to the development of gravitational analogue models as tools to explore various important aspects of general relativity and of quantum field theory in curved space-time. In these systems, two-point correlation functions are of the utmost importance for the characterization of processes taking place close to the acoustic horizon. In the present paper, dedicated to him, we present a study of path integral methods that allow to determine two-point correlation functions by a perturbative expansion, in a way that -- beyond its generality -- is especially suited to analyze these processes. Our results apply to non-relativistic superfluids, realizable in terrestrial experiments, as well as to relativistic superfluids, relevant for compact stellar objects.