Algebraically special perturbations of the Kerr black hole: a metric formulation

Abstract: Perturbations of the Kerr black hole are notoriously difficult to describe in the metric formalism and are usually studied in terms of perturbations of the Weyl scalars. In this work, we focus on the algebraically special linear perturbations (ASLP) of the Kerr geometry and show how one can describe this subsector of the perturbations solely using the metric formulation. To that end, we consider the most general twisting algebraically special solution space of vacuum General Relativity. By linearizing around the Kerr solution, we obtain two coupled partial differential wave equations describing the dynamics of the Kerr ASLP. We provide an algorithm to solve them analytically in the small spin approximation up to third order, providing the first exact solution of this kind in the metric formulation. Then, we use this framework to study the zero modes of the Kerr geometry. We present the exact analytical form of the shifts in mass and spin together with the required change of coordinates needed to identify them. Finally, we also provide for the first time closed expressions for the solution-generating perturbations generating the NUT and acceleration charges, thus deforming the Kerr solution to the linearized Kerr-NUT and spinning C-metric. These results provide a first concrete and rare example of perturbations of the Kerr black hole which can be treated entirely in the metric formulation. They can serve as a useful testbed to search for hidden symmetries of the Kerr perturbations.