The universe was supposed to be all there was - the whole of existence. For centuries the size of the universe and whether or not it is finite, fascinated scientists and philosophers. But however big the universe may be, there being only one gave astronomers and cosmologists a headache. Existence as we know it is contingent on so many constants in nature which, if altered only slightly, would make life impossible. This seems so unlikely to happen by chance. What if, instead, there is in fact a vast range of different universes, with many variants of the universal contents, making ours just...