Multiverse is the idea that our space/time is merely one "universe" in a huge limitless number of parallel worlds. Atheists often use this concept to argue against the fine tuning argument by saying with all those universe out there the odds of hitting one that can bare life is not so great. Our life bearing universe is not as improbable as the FTA would have us believe because when we consider that it's just one of a limitless expanse of other worlds then it's not so improbable that one would have life. We just happen to be it, if we weren't we wouldn't know about it. We would not be here. Sometimes they also argue that against the cosmological argument on the grounds that the universe is eternal and infinite and parallel words have been popping up forever. Then there's no way to say "here's the moment of creation."
Atheists have another favorite tactic and that is to argue that Occam's razor rules out God because God is not the simpler idea. There they are confusing it with Parsimony. Occam was priest and he believed in God he didn't think the razor got rid of God. For that reason I've always been somewhat peeved by their use of this argument. Moreover, what the razor really says is no not multiply entities beyond necessity. The thing is you see, atheists assume that since they don't believe in God then is not necessary so God is multiply beyond necessity. That's the argument made by those who at least know the real version of the argument but they don't know what it means. Let's try to understand it first by understanding Occam's nominalism. four senses of nominalism:
(1) Denial of metaphsyical universals: applies to Occam.
(2) reduce one's ontology to bare minimum, streamline categories: applies to Occam.
(3) Nix abstract entities, depending upon what one means here Occam may or may not have been a nominalist in this sense. he did not believe in mathematical entities but he did believe in abstraction such as whiteness, or humanity.
Ockham removes all need for entities in seven of the traditional Aristotelian ten categories; all that remain are entities in the categories of substance and quality, and a few entities in the category of relation, which Ockham thinks are required for theological reasons pertaining to the Trinity, the Incarnation and the Eucharist, even though our natural cognitive powers would see no reason for them at all. As is to be expected, the ultimate success of Ockham's program is a matter of considerable dispute.He was not getting rid of God. Occam's razor never allows us to deny what spade calls "putative entities" which would definitely include God. It merely bids us referain from positing them without good reason. Of course the many choruses of atheist propagadna slgoanizing would have it that this does include God, but with my 52 arguments we know better, don't we? In fact for Occam humans can't really know what is necessary, "For Ockham, the only truly necessary entity is God; everything else, the whole of creation, is radically contingent through and through. In short, Ockham does not accept the Principle of Sufficient Reason.." Wait a minute, not a contradiction because all the reasor says is refrain form multiplying entities without good reason, not rub them out of existence. Note that he includes God as the only truly necesasry entity. Thus atheist are violating Occam's razor in trying to use it on God.
Yet this raises the question of the Multiverse. Is the multiverse necessary? It's a matter of empirical question and there is empirical evidence to support it. Claims have been made of hard data proving Multivese, but when investigated they evaporate. Here's a physicist who opposed string theory and multiverse he argues that his evaluation of the papers finds irresolvable problems.
In recent years there have been many claims made for “evidence” of a multiverse, supposedly found in the CMB data (see for example here). Such claims often came with the remark that the Planck CMB data would convincingly decide the matter. When the Planck data was released two months ago, I looked through the press coverage and through the Planck papers for any sign of news about what the new data said about these multiverse evidence claims. There was very little there; possibly the Planck scientists found these claims to be so outlandish that it wasn’t worth the time to look into what the new data had to say about them. One exception was this paper, where Planck looked for evidence of “dark flow”.If hard evidence turns up for it then we have to deal with that on it's own terms. Until that time Multiverse should be shaved with Occam's razor. We don't need it to explain reality, it's only advanced to keep from having to turn to God. It's naturalistic so it's an arbitrary necessity at best. Arbitrary necessitates are logical impossibilities, contingent things jumped up to the level of necessity to answer a God argument. It's not we are going to disprove the unnecessary entity but we are going refrain from advancing it's existence as an assumption until such a time that real empirical evidence makes it necessary. Therefore, Multiverse should be taken out of the issues of God arguments.
C.K. Brampton, "Nominalism and the Law of Parsimony." The Modern School Men, Volume 41, Issue 3, (March 1964), 273-281.
the sentiment of that slogan "don't multiply entities beyond necessity" is in line with Occam's thinking although he didn't actually say that.
Spade, Paul Vincent and Panaccio, Claude, "William of Ockham", The Stanford Encyclopedia of Philosophy (Fall 2011 Edition), Edward N. Zalta (ed.), URL = . Fall 2011 (substantive content change) [new author(s): Spade, Paul Vincent; Panaccio, Claude]
 Spade, et al, Ibid.
 42 God arguments on Doxa, and 10 more on Religious A prori.
 Peter Woit, Not Even Wrong,May 22, 2013
Woit, Ph.D. particle theory form Princeton, Post doctorte in phsyics and math from Berkeley, tught at Columbia since 1989.