In section 6.2.5 we introduced the strong cosmic censorship hypothesis, which implies that singularities other than the big bang would be unobservable. If this were literally true, then it might be thought that the considerations of this book were physically irrelevant. We shall see, however, that the situation is more complex than this. The main thrust of this book has been the attempt to establish a relation between the curvature strength of singularities (in the sense of section 6.1) and their ‘genuineness’ - i.e. whether or not there is an extension through them. The arguments for cosmic censorship suggest that only sufficiently strong singularities might be censored, and so the crucial question becomes, whether or not the censored singularities are precisely the genuine ones. In view of the importance of this to the whole study of singularities, I give here a more extended account of the cosmic censorship hypothesis.

The weak hypothesis

The strong cosmic censorship hypothesis was preceded by the weak cosmic censorship hypothesis, first formulated by Penrose (1969), who asked: “does there exist a ‘cosmic censor’ who forbids the appearance of naked singularities, clothing each one in an absolute event horizon?”

This last term was subsequently given more precision in terms of future null infinity J+. A full account of this would take us well beyond the scope of this book. In outline, however, J+ is a boundary attached to a conformal extension of a given space-time (M,g).