What we have done in this book is to look at those aspects of cosmology in which the density of the universe plays a role as either a prediction or a parameter of a model, and compared them with the data. It is now our task to weigh up the arguments we have described, and try to make a reasoned assessment of their implications. This will be done by a forensic approach: some of the evidence is quite reliable, but some of it is purely circumstantial, some unreliable, and some contradictory. In view of this we shall not adopt the criterion of proof that applies in the criminal court (‘beyond all reasonable doubt’). Rather we look at the ‘balance of the evidence’, as in a civil case. Doing this, we believe that despite some counter-indications, it is possible to discern a strong case for a low-density universe having negatively curved spatial sections, i.e. to conclude on the balance of evidence and argument that we live in an open universe.

The starting point for this conclusion is that, as should be clear on reviewing the various considerations laid out in the previous chapters, there is no convincing observational case to be made for a critical density universe: the strongest motivation for the supposition that Ω0 is very close to unity comes from theory rather than observation. We consider the theoretical arguments most commonly advanced, which rely on presuppositions about physics which are not amenable to direct test, to be inconclusive for reasons discussed at length in Chapter 2.