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

Understand

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 $V(|\varphi|^2)$.
• 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 $c\rightarrow +\infty$.