In Nature (19 January 2011), "George Ellis reminds us that Brian Greene's beguiling book on parallel worlds is more theory than fact.
The book is The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (Allen Lane: 2011), and Ellis comments,
... Greene's nine types of multiverse are as follows. First, if space extends forever, an infinite number of domains similar to ours might lie beyond the part of the Universe that we can see. Second, some versions of inflationary theory — the idea that the newborn Universe had a fleeting period of super-fast accelerating expansion — predict the existence of innumerable other universes, with different characteristics from our own. Third, string theory, the pre-eminent theory of quantum gravity, suggests that our Universe might be one of many four-dimensional 'braneworlds' floating in a higher-dimensional space-time.Yes but, in a multiverse, "mutually exclusive" is meaningless by definition.
This option is developed further in the fourth and fifth proposals, which involve cyclic universes, or variations on physical parameters that are possible in the string-theory landscape. The sixth is a quantum mechanics idea that many worlds simultaneously exist as branches of the wave function of the Universe. The seventh suggests that the Universe is a holographic projection. The eighth states that we live in one of a set of artificial universes created as simulations on a super-advanced computer. The ninth argues that it is a philosophical necessity that every possible universe must be realized somewhere, in “the grandest of all multiverses”.
By presenting this plethora of theories, Greene gives the impression that the multiverse is on a sound scientific footing, but these nine arguments are mutually exclusive.