Coel Hellier, a professor of Astrophysics at Keele University, has two interesting blog posts. In the first post he explains why the multiverse hypothesis is not ad hoc and why it is a scientific hypothesis. His basic idea is:
If a scientific theory predicts consequences A, B, C and D, and if we then verify that A, B and C are indeed the case, thus giving us confidence in the theory, then we have sound reasons for accepting D even if D cannot be directly verified. Indeed, we would be obliged to accept D unless we can construct another equally good explanation of A, B and C.
Certain predictions of inflationary theory have been verified and some of those models predict a multiverse.
This is not to say the multiverse hypothesis is proven, but only that it is not ad hoc.
To be clear, I am not asserting that the multiverse has been proven true, even on the balance of probability, but I am asserting that it is a serious scientific concept that will eventually be accepted or rejected on scientific grounds.
In a second post he explains why the multiverse hypothesis is parsimonious.
Thus Occam’s razor tells us to adopt the minimum information-content demanded by the evidence, and to dispense with any that is superfluous … The formulation in terms of information explains, for example, why the idea of a multiverse is not a violation of Occam’s razor. The information content of “take one universe, duplicate it a vast number of times, assign physical constants at random” is actually much less than the information needed to take one universe and specify all the physical constants. Thus a multiverse is a highly parsimonious concept.
But I think Neil Manson and Michael Thrush put this point better:
At this point proponents of the design argument typically object that design [D] is vastly simpler than multiverse [M], and so to choose M over D is to violate Ockham’s Razor. Richard Swinburne is perhaps the most notable advocate of the claim that D bests M in the simplicity department, but many others sing the same tune. We shall not address the issue in this paper except to say that we think clarification is needed regarding how to rate M with respect to simplicity. Consider in comparison the Standard Model in particle physics. It postulates only a few types of elementary particles and so is rightly regarded as an elegant theory, even though the raw number of such elementary particles is vast. The case of the Standard Model suggests that simplicity is measured with respect to types rather than tokens…