Quasi-black holes: definition and general properties

Objects that are on the verge of being extremal black holes but actually are distinct in many ways are called quasi-black holes. Quasi-black holes are defined here and treated in a unified way through the displaying of their properties. The main ones are (i) there are infinite redshift whole regions, (ii) the spacetimes exhibit degenerate, almost singular, features but their curvature invariants remain perfectly regular everywhere, (iii) in the limit under discussion, outer and inner regions become mutually impenetrable and disjoint, although, in contrast to the usual black holes, this separation is of a dynamical nature, rather than purely causal, (iv) for external far away observers the spacetime is virtually indistinguishable from that of extremal black holes. It is shown, in addition, that quasi-black holes must be extremal. Connections with black hole and wormhole physics are also drawn.