Evolution of Dimensionless Angular Momentum of Central Black Holes of Accretion Disks

The effects of the Blandford-Znajek (BZ) process on the evolution of the central black holes of accretion disks are investigated. It is proved that the dimensionless angular momentum a_* of the central black hole will evolve to a stable value in the case of thin disks, while it will evolve to a stable value in the case of thick disks. These results imply that the central black holes of accretion disks will never evolve to extreme Kerr black holes.