Anthony Aguirre is a physicist at UC Santa Cruz. He wrote a paper in 1995 on the multiverse hypothesis. Here is the paper’s abstract:
The notion that there are many “universes” with different properties is one answer to the question of “why is the universe so hospitable to life?” This notion also naturally follows from current ideas in eternal inflation and string/M theory. But how do we test such a “multiverse” theory: which of the many universes do we compare to ours? This paper enumerates would would seem to be essential ingredients for making testable predictions, outlines different strategies one might take within this framework, then discusses some of the difficulties and dangers inherent in these approaches. Finally, I address the issue of whether there may be some general, qualitative predictions that multiverse theories might share.