Joshi, P. S.Narlikar, J. V.2012-03-242012-03-241982-02-16http://hdl.handle.net/11007/1580The usual definition of a black hole is modified to make it applicable in a globally hyperbolic space-time. It is shown that in a closed globally hyperbolic universe the surface area of a black hole must eventually decrease. The implications of this breakdown of the black hole area theorem are discussed in the context of thermodynamics and cosmology. A modified definition of surface gravity is also given for non-stationary universes. The limitations of these concepts are illustrated by the explicit example of the Kerr-Vaidya metric.enCosmologyBlack holesGeneral relativityArticle