"More specifically: The multiverse is not a 'theory'. If it were, it would be perfectly fair to criticize it on the basis of our difficulty in coming up with possible experimental tests. The correct way to think about the multiverse is as a prediction. The theory - such as it is, in its current underdeveloped state - is the marriage of the principles behind quantum field theory to our basic understanding of how curved spacetime works. Starting from those inputs, we don't simply theorize that the universe could have undergone an early period of superfast acceleration; we predict that inflation should occur. If a quantum inflaton field with the right properties finds itself in the right state. Likewise, we don't simply say, "Would't it be cool if there were an infinite number of different universes?" Rather, we predict on the basis of reasonable extrapolations of gravity and quantum field theory that a multiverse really should exist.

The prediction that we live in a multiverse is, as far as we can tell, untestable. (Although, who knows? Scientists have come up with remarkably clever ideas before.) But that misses the point. The multiverse is part of a larger, more comprehensive structure. The question should be not "How can we test whether there is a multiverse?" but "How can we test the theories that predict the multiverse should exist?" Right now we don't know how to se those theories to make a falsifiable prediction. But there's no reason to think that we can't, in principle, do so."
p. 372