  The exact global solution of the Einstein equations [Neugebauer & Meinel,
Phys. Rev. Lett. 75 (1995) 3046] describing a rigidly rotating,
self-gravitating disk is discussed. The underlying matter model is a perfect
fluid in the limit of vanishing pressure. The solution represents the
general-relativistic analogue of the classical Maclaurin disk. It was derived
by applying solution techniques from soliton theory to the axisymmetric,
stationary vacuum Einstein equations. In contrast to the Newtonian solution,
there exists an upper limit for the total mass of the disk - if the angular
momentum is fixed. At this limit, a transition to a rotating black hole, i.e.,
to the Kerr solution occurs. Another limiting procedure leads to an interesting
cosmological solution. These results prove conjectures formulated by Bardeen
and Wagoner more than twenty-five years ago.
