Hydrodynamic representation of the Klein-Gordon-Einstein equations in the weak field limit

Using a generalization of the Madelung transformation, we derive the hydrodynamic representation of the Klein-Gordon-Einstein equations in the weak field limit. We consider a complex self-interacting scalar field with an arbitrary potential of the form $V(\mathrm{\mid }\phi {\mathrm{\mid }}^2)$. We compare the results with simplified models in which the gravitational potential is introduced by hand in the Klein-Gordon equation, and assumed to satisfy a (generalized) Poisson equation. Nonrelativistic hydrodynamic equations based on the Schr"odinger-Poisson equations or on the Gross-Pitaevskii-Poisson equations are recovered in the limit $c\to +\mathrm{\infty }$.