The issue of testability underlies the question of whether multiverse proposals are really scientific. There are observational barriers preventing direct proof of the existence of any expanding domains other than the one we live in.
Furthermore, the supposed underlying physics is untested and may always be untestable. Consequently, ensemble proposals are not scientific in the usual sense. In order to make them so - e.g. by showing they are based on a theory that has gained credibility by unifying or clarifying some other mysteries -one has to search for extended ways of relating them to observations.
In looking at these issues, what faces us is not just showing that a multiverse exists. If this is a scientific proposition, one should be able to show what specific kind of ensemble is involved, and we have seen there are many different kinds. If you cannot show which particular one exists, it is doubtful you can show that any exists at all. If you cannot describe the characteristics of a purported object, then what scientific content can this claim have?
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