This book had its origins in a coffee-time discussion in the QMW common room in 1993, in the course of which we discussed many issues pertaining to the material contained here. At the time we had this discussion, there was a prevailing view, particularly among cosmologists working on inflationary models, that the issue of the density of matter in the Universe was more-or-less settled in favour of a result very near the critical density required for closure. Neither of us found the arguments made in this direction to be especially convincing, so we determined at that time to compile a dossier of the arguments – theoretical and observational, for and against – in order to come to a more balanced view. This resulted in a somewhat polemical preprint, ‘The Case for an Open Universe’, which contained many of the arguments we now present at greater length in this book, and which was published in a much abridged form as a review article in Nature (Coles & Ellis 1994).

The format of a review article did not, however, permit us to expand on the technical aspects of some of the arguments, nor did it permit us to foray into the philosophical and methodological issues that inevitably arise when one addresses questions such as the origin and ultimate fate of the Universe, and which form an important foundation for the conclusions attained. The need for such a treatment of this question was our primary motivation for writing this book.