This phenomenon of inflation also leads us to expect that other parts of the universe, which we cannot yet see because of the finite speed of light, will be very different to our own visible part. Moreover, the whole process of inflation is self-reproducing: any region that inflates will create the conditions for further inflation of parts of itself. This process of "eternal" inflation has no end and need have no beginning. It changes our answer to the old question, "Did the universe have a beginning?" Our visible part of the entire universe will have had a beginning but the entire "multiverse" of different regions, all inflating at different rates, need not have one.

During the last decade we have steadily gathered evidence that points to a past era of inflation 13.7 billion years ago. But we have also discovered that the universe began accelerating again four billion years ago after it had expanded to about three quarters of its present extent. This change of gear from cosmic deceleration to acceleration is described with tremendous accuracy by one of the solutions to Einstein's equations that were first found by the Belgian cosmologist and priest, Georges Lemaître, in 1927. Yet, although Lemaître's universe is a very accurate description of what we see, again we want to know why our universe changed gear and began accelerating just a few billion years ago. Why then? One fashionable approach is to imagine that the "multiverse" of possible regions that can emerge in the eternal inflationary scenario covers all possibilities and we just happen to inhabit one of those which started accelerating late enough for galaxies, stars, planets and life to have evolved. 

More interesting, potentially, is a new extension of Einstein's theory developed by Douglas Shaw and myself in Cambridge which is just published in the journal, Physical Review Letters. By imposing the restriction of causality on a quantum cosmology, we are able to explain for the first time why the recent acceleration began when it did. We can also predict another very precise feature of the expansion that the Planck satellite's data will be able to determine. So the magic jar of universes is not exhausted yet. Over the next two years, we will see many new pieces of data that will confirm or exclude a range of possible universes, and help us to understand why ours is just the way it is.