Black holes in multimetric gravity

We construct a wide class of black hole solutions to the general theory of ghost-free multimetric gravity in arbitrary spacetime dimension, extending and generalizing the known results in four-dimensional dRGT massive gravity and bigravity. The solutions are split into three generic classes based on whether the metrics can be simultaneously diagonalized—one of which does not exist in dRGT massive gravity nor bigravity, and is only possible when one has more than two interacting metric fields. We also linearize the general multimetric theory to determine the dynamics of the massive spin-2 modes, including examples where this can be done analytically, and use the linear theory to discuss the stability of the four-dimensional multi-Schwarzschild and multi-Kerr solutions. We explain how the instabilities that plague these solutions in dRGT massive gravity and bigravity carry across to the general multimetric theory, touching upon ideas of dimensional deconstruction to make sense of the results. Published by the American Physical Society 2024