In a cover story of Newsweek (May 21, 2012), it reports Professor Brian Greene’s new physics, and it is carried by The Daily Beast with the article “Welcome to the Multiverse, http://www.thedailybeast.com/newsweek/2012/05/20/brian-greene-welcome-to-the-multiverse.html “.
In the conclusion, Greene said, “... because the proposal is unquestionably tentative, ... the multiverse can be a cop-out that diverts scientists from seeking deeper explanations. On the other hand, failure to consider the multiverse can place scientists on a Keplerian treadmill in which they furiously chase answers to unanswerable questions.”
In the article, Greene gave three points to support the proposal of the Multiverse.
1. The observed dark energy of “this” universe is much different from the theoretical calculation. He used the Keplerian treadmill analogy to kill the issue. That is, if this universe is not the only one (similar to the Earth is not the only planet of this solar system), then the dark energy issue will become a nonissue in a Multiverse.
2. Inflationary cosmology makes accurate predictions about microwave background radiation, and it allows the multiple-big-bangs. Thus, the Multiverse proposal does have scientific foundation.
3. The String theory has multiple solutions which allow the manifestation of the Multiverse.
Seemingly, this Multiverse proposal is not coming out from any wild imaginations, but Greene did admit that the string theory remains hypothetical.
Semantically, the term of “universe” is commonly defined as the "totality" of everything that exists. Thus, even if the multiple-big-bangs were allowed by the inflationary cosmology or there were more than one Cosmos, they are still parts of the universe.
If we define the “universe” as only the part that is observable by us, then there is, of course, something beyond this universe, as there is indeed an event horizon. Then, this multiverse exists without the need of Greene’s three supporting points.
In terms of physics, only if the other universes are having the different physics laws than ours, then we are in a multiverse. The many manifestations of other cosmos with the same physics laws are still parts of this universe both in physics and in linguistics.
If the multiverse physics is searching for a new set of physics laws which are not parts of this universe, then it is a genuine science. Otherwise, it is just a semantic gimmick.