Scientists differ among themselves regarding the scientific status of multiverse theories. Some, such as George Ellis, don’t think multiverse theories are testable, and hence not scientific. Others, think multiverse models are (or could be) testable, and hence are scientific. Many Christian apologists have sided with Ellis et al and rejected the multiverse as a valid scientific theory on the grounds that it is not testable. Some, including myself, have argued that multiverse theories are not based on the evidence, but ad hoc theories invented by cosmologists to get around the theistic implications of fine-tuning in physics.
Jeff Zweerink from Reasons to Believe wrote a short article addressing the scientific nature of and foundation for multiverse theories. He argues that some multiverse models do make testable predictions (even if we are currently unable to test those predictions empirically), and thus should be “included in the realm of scientific investigation (while stopping short of taking a firm position on the demarcation question –whether multiverse theories qualify as scientific).
More importantly, he argues that at least some multiverse theories are based on other scientific findings, and not invented whole-cloth for the purpose of answering the fine-tuning problem:
[T]he current batch of multiverse models gained popularity primarily because they arose from investigations of other phenomena. Scientists did not simply invent a multiverse in order to explain away the beginning of the universe or to account for its life-friendly fine-tuning. The most popular multiverse model (a level II bubble multiverse filled with level I universes) arises from efforts to find an explanation for how inflation works. Granted the multiverse scenario arises after huge extrapolations of well-tested physical models, but most versions of inflation that produce a universe that looks like ours also produce a multiverse.
Zweerink does note that multiverse theories are based on “huge extrapolations from known physics.” While multiverse theories may not be scientific fairy tales, they remain extremely speculative.
While there may be some basis for proposing the possibility of a multiverse (string theory implies it, coupled with inflation), I still think it’s safe to say that their broad appeal in the scientific community is not based on the level of the evidence, but on their ability to answer the theistic implications of fine-tuning in physics. Surely, if the shoe was on the other foot, and it was the multiverse that pointed to theism rather than a single universe, scientists would be quick to dismiss the theists’ appeal to the possibility of a multiverse on the basis that it was extremely speculative and possibly untestable. But when it fares well for naturalism, suddenly the multiverse is eminently reasonable and scientific, and sufficiently answers the fine-tuning argument for theism.
While I’m prepared to acknowledge that multiverse theories are scientific theories and not ad hoc, I’m still going to call a spade a spade. The reason multiverse models are only the rage in science is not because there is good empirical evidence for the multiverse (or even the theory that implies the multiverse – string theory), but because of the metaphysical work they purportedly do to advance naturalism.