MOND and the Universal Rotation Curve: Similar Phenomenologies

Modified Newtonian dynamics (MOND) and the universal rotation curve (URC) are two ways to describe the general properties of rotation curves that have very different approaches concerning dark matter and gravity. Phenomenological similarities between the two approaches are studied by looking for properties predicted in one framework that are also reproducible in the other one. First, we looked for the analog of the URC within the MOND framework. Modifying in an observationally based way the baryonic contribution, V_bar, to the rotation curve predicted by the URC, and then applying the MOND formulas to this V_bar, leads to a "MOND URC" whose properties are remarkably similar to the URC. Second, we show that the URC predicts a tight mass discrepancy-acceleration relation, which is a natural outcome of MOND. With the choice of V_bar that minimizes the differences between the URC and the "MOND URC," the relation is almost identical to the observational one. This similarity between the observational properties of MOND and the URC has no implications about the validity of MOND as a theory of gravity, but it shows that MOND can reproduce in detail the phenomenology of disk galaxies' rotation curves, as described by the URC. MOND and the URC, even though they are based on totally different assumptions, are found to have very similar behaviors and to be able to reproduce each other's properties fairly well, even with the simple assumptions made regarding the luminosity dependence of the baryonic contribution to the rotation curve.