The Importance of Being Symmetric: Flat Rotation Curves from Exact Axisymmetric Static Vacuum Spacetimes

Abstract: Starting from the vacuum Einstein Field Equations and a static axisymmetric ansatz, we find two new exact solutions describing an axisymmetric static vacuum spacetime with cylindrical symmetry: One of this exhibits an additional symmetry in $z$-direction and the other has $z$-coordinate dependent coefficients. In analogy to the Schwarzschild solution, these metrics describe a static vacuum spacetime and apply in similar settings except for the changed symmetry conditions. Furthermore, we consider an approximate solution built on the cylinder solution which asymptotically approaches the Minkowski metric. Analyzing the low-velocity limit corresponding to the Newtonian approximation of the Schwarzschild metric, we find an effective logarithmic potential. This yields flat rotation curves for test particles undergoing rotational motion within the spacetime described by the line elements, in contrast to Newtonian rotation curves. The analysis highlights how important the symmetry assumptions are for deriving general relativistic solutions. One example of physical objects that are generally described in the static vacuum low-velocity limit (reducing to Newtonian gravity in the spherically symmetric case) and exhibit axial symmetry are disk galaxies. We show that symmetries and appropriate line elements that respect them are crucial to consider in such settings. In particular, the solutions presented here result in flat rotation curves without any need for dark matter. While these solutions are limited to static vacuum spacetimes, their application to physical galaxies relies on appropriate approximations. Nonetheless, they offer valuable insights into explanations for flat rotation curves in galaxies and their implications for dark matter.