In the August 2011 edition of Scientific American, famed cosmologist, George Ellis, wrote an article titled “Does the Multiverse Really Exist?”  Here are some great excerpts from that article:

“Similar claims [about a multiverse] have been made since antiquity by many cultures.  What is new is the assertion that the multiverse is a scientific theory, with all that implies about being mathematically rigorous and experimentally testable.  I am skeptical about this claim.  I do not believe the existence of those other universes has been proved—or ever could be.  Proponents of the multiverse, as well as greatly enlarging our conception of physical reality, are implicitly redefining what is meant by ‘science.’”—pg 39