IGC Colloquium: The Bardeen-Petterson Effect in MHD or How Twisted Disks Straighten Out
Accretion disks occur in a wide variety of astrophysical contexts, from planet formation to accretion onto black holes. For simplicity, they are generally imagined as thin and flat. However, whenever the disk's angular momentum is oblique to the angular momentum of the central object(s), a torque causes rings within the disk to precess, twisting and warping it. Because the torque weakens rapidly with increasing radius, it has long been thought that some unspecified "friction" will end up bringing the inner portions of such disks into alignment, while the outer parts remain in their original orientation. Nearly all previous work on this topic has assumed that such a disk's internal stresses can be described by an isotropic viscosity, even though it has been known for more than four decades that fluid viscosity is far too weak to be significant, and for two decades that accretion stresses are actually due to anisotropic MHD turbulence. In this talk I'll report on the first work showing how twisted disks align when their mechanics are described only in terms of real forces, including MHD turbulence. The results have interesting implications for the evolution of misaligned disks in systems spanning the range from binary protostars to merging supermassive black holes.