Having long assumed that there was a single universe, I’ve been intrigued to read about the speculation among physicists about the possibility of multiple universes (mentioned in recent popular books by Brian Greene and Lee Smolin, for instance). Now, I don’t want to get hung up on a terminology issue here – we could say a collection of universes is just a larger universe. Informally, let me say that a universe defined as distinct from ours would have the following characteristics: it is unobservable from our own universe in the conventional sense – none of its particles or electro-magnetic radiation reach us; it could not be visited by us using technologies of the sort we’re currently familiar. (It may be possible to detect using other means – some speculate gravity waves from another universe could reach us). Such a universe could have different fundamental physical laws from our own. Finally, universes may give rise to each other from a process such as inflation or black hole formation: our universe could have a parent universe, or perhaps spawn children of its own. (For now I am not talking about multiple universes in the sense implied by the “many-worlds” interpretation of quantum physics.)
I find it exciting that a multiverse may exist given the promise that we may be able to extend scientific explanation much further than previously thought. The old idea of a single universe which arose in a Big Bang from a singularity was an abrupt and to me unsatisfying boundary on science. Proposing that the world suddenly appeared from a dimensionless point really didn’t say much at all. Now there is hope we may be able to understand how the universe grew out of a previously existing context.
Of course, the multiverse hypothesis continues the trend of science’s steady progress in making our earthly vantage point less special. Not only is our star system but one in huge sea in our galaxy, while our galaxy is but one of a huge swarm in the universe, but now even our universe may be one of a great many. The problem of how the physical constants governing our universe were “fine-tuned” to permit star-formation and ultimately life on earth is placed in a larger context of multiple universes with variable laws.
Of course while the question of the ultimate origin of existence will continue to recede it will not disappear. As long as people feel they can ask the question -- “why something rather than nothing?” -- a mystery beyond normal natural explanation will persist.