Covariant approach to the conformal dynamical equivalence in astrophysics

We use covariant techniques to examine the implications of the dynamical equivalence between geodesic motions and adiabatic hydrodynamic flows. Assuming that the metrics of a geodesically and a non-geodesically moving fluid are conformally related, we calculate and compare their mass densities. The density difference is then expressed in terms of the fundamental physical quantities of the fluid, such as its energy and isotropic pressure. Both the relativistic and the non-relativistic case are examined and their differences identified. Our analysis suggests that observational determinations of astrophysical masses based on purely Keplerian motions could underestimate the available amount of matter.