Do Physicists Believe in Multiple Universes?Many physicists do believe that there are multiple universes, in some form or another. However, it's not particularly clear to me that you could necessarily say this is a majority viewpoint. Most physicists really don't deal with this question at all and, because there's no direct evidence one way or another. Many seem to consider it a distraction which interferes with time that could be spent focusing on more important ideas in physics.
However, if you get most of your physics knowledge from television shows - such as PBS' NOVA, The History Channel's The Universe, or pretty much anything on The Science Channel - then you can be forgiven for thinking that this is something that physicists spend a lot of their time focusing on. It makes for great television, that's for sure, and it's fun to write articles on.
Still, the physicists who do believe in multiple universes do so because invoking the idea of multiple universes takes care of some problems in the realms of particle physics and cosmology.
What Is the Reason for Believing in Multiple Universes?The argument in favor of multiple universes tends to go something like this:
First, there's a strong understanding in cosmology of something called the fine-tuning problem, which is basically the idea that as we understand the way our universe is constructed, the more precarious our existence in it is. As physicists have examined the way the universe has changed over time since the big bang, they have come to believe that the early conditions of the universe, had they just been a microscopic bit different, would have caused a universe that is inhospitable to life.
In fact, if a universe spontaneously came into existence, physicists would expect it to spontaneously collapse or possibly to expand so rapidly that particles never really interact with each other. British physicist Martin Reese wrote extensively about this idea in his classic book Just Six Numbers: The Deep Forces That Shape the Universe.
With an understanding of these "finely-tuned" properties in the universe, some argue for the need of a creator (i.e. God). Since physicists seek to explain properties of the universe without reference to a supernatural agency, they would like to understand the properties of the universe without invoking a deity of any kind. (We'll come back to God in a bit.)
The easiest solution would just be to say, "Well, that's how it is," but the problem with that is that it's not really an explanation. It represents a remarkable lucky break that a single universe would come into being and that universe would just happen to have the very precise properties needed to develop life ... or, for that matter, to develop anything. (Remember: most physical properties would result in a universe which collapses into nothingness instantly or expands into a vast sea of nothingness, so this isn't just a matter of trying to explain human beings as we happen to exist, but of explaining the very existence of any sort of universe.)
The alternative that many cosmologists adopt is that there are actually a vast number of universes, which can have different properties. Within that vast multiverse of universes, some subset of them (including our own) would contain properties that allow them to exist for relatively long periods of time, some subset of those (again, including our own universe) would have the properties that allow them to form complex chemicals and, ultimately, life.
String Theory and Multiple UniversesString theory has recently begun to support this idea, because the results make it clear that there are vast number of possible solutions to string theory - in other words, if string theory is correct then there are still many different ways to construct the universe.
In addition, since string theory presents the idea of extra dimensions, it also includes a structure to think about where these other universes could be located. Our universe, which includes 4 dimensions of spacetime, seems to exist in a universe that may contain as many as 11 total dimensions (often called the bulk by string theorists). There's no reason to think that the bulk couldn't contain other universes in addition to our own. (The bulk, presumably, has always existed.)
The God Hypothesis and Multiple UniversesMost physicists believe in multiple dimensions because they don't want to invoke a creator of any kind, which is as it should be because that would be bad physics. (Even physicists who believe in God typically do not invoke God as a causative agent in their theories.) However, what if there is some sort of a god or creator? Would there still be multiple universes? My answer to this question is "Yes," and here's why:
If there were a creator of our universe, then it is presumed to exist outside of our universe in some fashion. Since the creator exists outside of our universe, this means there must be something outside the universe where the creator exists ... in other words, there has to be at least one other universe. (In addition, there's nothing to say that the creator couldn't create other universes anyway, so there could still be other universes in the more conventional sense.) So, even with a creator, there's still a strong indication there are multiple universes.
Note: I believe that there are some philosophical and theological thinkers (and certainly some science fiction authors) who have postulated a circular creation where God ends up creating himself. In these scenarios, which often include time travel in some form, there wouldn't necessarily need to be any "outside of the universe." We'll set these to the side for the purposes of this discussion.