When it comes to the apparent “fine-tuning” of the universe for the existence embodied life, naturalists typically appeal to multiple universes (or a multiverse) to explain why the fine-tuning is not really surprising at all.
As the story goes, if there are a virtually infinite number of baby universes inside a multiverse – each with different initial conditions and different values for the constants and variables in the known laws of physics – then one of those baby universes is bound to be hospitable to life, however unlikely this might appear from the perspective of human observers.
My aim in this post is not to explore the empirical merits of the multiverse option (MVO) but to examine what I suspect to be an explanatory weakness in it – insofar as the worldview of naturalism enlists it to increase the probability that this universe would exhibit fine-tuning. The possibility still remains that as quantum physics and cosmology progress, new empirical discoveries will one day confirm the existence of universes other than our own. Therefore, I think it wise to offer a philosophical examination of how the MVO can be enlisted to shore up the explanatory resources of naturalism without trying to forecast whether (or to what extent) future scientific research will support or undermine the MVO.
In my view, the main weakness in using the MVO to provide a naturalistic explanation of fine-tuning is not that it accounts for too little, but rather that it can (ironically?) account for anything we observe! It can explain any physical feature or fact about this universe as unsurprising, even virtually certain, insofar as this universe is merely one among infinitely many others. No matter how improbable some physical feature may be on naturalism, the MVO can come to the rescue by showing how at least one world in the cosmic pool of multiple worlds had to exhibit that feature. But why, you might ask, is this a weakness?
Well, one of the virtues of a good explanation is its ability to operate like a “well aimed bullet” – not only by making certain phenomena more likely (or expected) than its competitors, but also by leading us to expect them independently of which ones we actually observe in this universe. An explanation that gives us reasons to believe the actual universe would look a certain way (independent of how it does look) is better than one that can accommodate virtually anything we happen to find after observing the fact.
How does this apply to the theism vs. naturalism debate?
First, on naturalism + the multiverse, there is no reason to expect that this universe would exhibit fine-tuning, apart from the fact that it does. Had the universe been inhospitable to life-forms, we would, of course, not be around to observe this fact, but the lack of fine-tuning (or the presence of it) would still cry out for explanation, even if no observers were around to wonder about it. What’s worse, because the “naturalism + multiverse” hypothesis fits so easily with any physical fact that could be observed, it seems to lack explanatory power. A view that is compatible with anything explains nothing.
Second, on theism, the likelihood of any universe is somewhat low given that (1) God could have decided not to create anything and that (2) other universes may also be consistent with his creative purposes. At any rate, any universe God did create would likely contain orderliness, aesthetic and moral beauty, as well as finite creatures capable of enjoying valuable states of affairs (including divine love). Such a universe would require a lot of “stage-setting” (or fine-tuning) in order to be exhibit these features.
In other words, if we can imagine a target that consist of points which represent possible universes, theism operates like a well-aimed bullet by narrowing down the space on the target where the bullet is likely to hit. By contrast, naturalism (+ the multiverse) cannot predict beforehand where (on the target) a particular bullet will likely hit. Rather, it predicts that every point on the target will be hit because an infinite number of randomly aimed bullets are being fired!
What kind of explanation do you, the reader, believe is better: a well-aimed bullet or an infinite spread of randomly aimed bullets? How would your answer to this question impact the theism vs. naturalism debate over the fine-tuning of the universe?
 M-Theory currently allows for 10 to the 500th possible worlds within a multiverse, which is a far cry from an actually infinite number of worlds, but that is beside my point in this blog post.