In one episode of “The Big Bang Theory,” CalTech string theorist Sheldon Cooper is invited to join his mother on a Christian cruise: the “born-again boat ride.” “Uh, well, Mom, if I did, it would be conclusive proof that your God can work miracles,” Sheldon replies.
Sheldon’s response reflects a popular conception of a scientific attitude toward faith. The concept of a divine law giver, which motivated the pioneers of science to expect law and regularity in nature, has largely been forgotten. Instead, many see theism as an outdated model that will eventually collapse in the face of scientific progress. And the multiverse, motivated by string theory, is sometimes seen as the weapon that will deal God his death blow.
So are God and the multiverse like Harry and Lord Voldemort? Can one not live while the other survives?
Our universe is incredibly improbable. If the physical constants that govern it were changed by as little as 1 part in 10^5, 1 part in 10^11, 1 part in 10^120, etc., our universe would have been unable to produce life. With those odds, it is safe to conclude that our life-permitting universe was not produced by random chance. In principle, a clever new theory could solve these fine-tuning problems, but it is highly improbable that such a theory would have evaded experimental detection thus far. If our universe is somehow not fine-tuned, it was assembled in a such a way as to fool us into thinking that it was.
So, how does the non-theist go about explaining this fine-tuning without resorting to a cosmic designer? The answer given by almost all of my non-theistic colleagues is the multiverse hypothesis.
If the physical constants that govern it were changed by as little as 1 part in 10^5, 1 part in 10^11, 1 part in 10^120, etc., our universe would have been unable to produce life.
The multiverse hypothesis holds that there exist an enormous, possibly infinite, number of universes. In this vast sea of universes, the reasoning goes, some universe’s constants permit life, and because we can only observe a universe if we are alive in it, we necessarily find ourselves living in a life-permitting universe.
If this sounds more like science fiction than science to you, you are not alone. Perhaps the biggest problem facing the multiverse theory is that it is completely untestable. For the non-theist who wants to deny the existence of God because of a perceived scarcity of experimental evidence, the multiverse should be tough to swallow. Belief in the multiverse requires a leap of faith beyond science and into the realm of metaphysics.
Aside from experimental evidence, the next best thing a multiverse proponent could hope for would be a multiverse-creating mechanism that follows from known laws of physics. However, contrary to what some physicists might be claiming, even this is currently a fantasy: no theory that directly implies a multiverse will be testable in the foreseeable future. But if we are willing to consider more speculative theories, things get much more interesting. And this is where string theory comes in: string theory predicts that there are something like 10^500 possible universes, each with slightly different physics. This collection of universes is referred to as the “string landscape.”
Belief in the multiverse requires a leap of faith beyond science and into the realm of metaphysics.
Arguments for a Multiverse
Does the string landscape entail a multiverse? Here, we must be careful. The hypothetical universes of the string landscape are mere possibilities. They need not be realized in any physical way. However, there are several ways in which the string landscape suggests a multiverse. First, string theory has shown that there are plenty of theoretically consistent possible universes besides our own, which is just the sort of thing we would expect to find if a multiverse does exist. In the absence of a cosmic designer who preferred a life-permitting universe, it is difficult to imagine why the physical constants in our universe should be the only ones in the landscape that are actually realized.
Second, one theory of the early universe called “eternal inflation” predicts a scenario in which some regions of space expand indefinitely, growing into “bubble universes” that would each realize a member of the landscape. Given both eternal inflation and string theory, we would indeed expect a multiverse to exist.
So, there are good reasons to believe in a multiverse, and it is not as crazy as it might seem at first. Nevertheless, the multiverse warrants a healthy dose of skepticism.
In the absence of a cosmic designer who preferred a life-permitting universe, it is difficult to imagine why the physical constants in our universe should be the only ones in the landscape that are actually realized.
Arguments Against a Multiverse
First, all versions of the multiverse rely on speculative theories. As a general paradigm—or what we physicists call an “effective theory”—inflation has been extremely successful at explaining observational data. But the effective theory of inflation says nothing of multiverses. Only certain models of inflation give rise to multiverse-producing eternal inflation, and every such model faces problems.
Further, the mechanism for eternal inflation will not by itself give rise to different laws of physics in each of the bubble universes it creates. This means that, without an additional speculative theory (i.e. string theory), eternal inflation cannot explain why the laws of physics will vary from one universe to another, and it is not clear that any universe will permit life.
Finally, and most importantly, the theory of the multiverse is beset with paradoxes. In an eternal inflation multiverse, anything that can happen will happen an infinite number of times, rendering the theory incapable of predicting anything. Attempts to solve this so-called “measure problem” face challenges of their own, making predictions that disagree with observations or lead to bizarre conclusions (e.g. our universe should be much younger than it is, or we should be “Boltzmann brains” that have fluctuated into existence quantum-mechanically rather than biologically-evolved life forms).
In light of these difficulties, it is not clear to what extent the multiverse can be said to solve the problems of fine-tuning—even our best multiverse theories seem to introduce fine-tuning problems of their own.
It would not surprise me if our God, who imagined galaxies and black holes and oceans and epigenetics and breathed them all into existence, would create other universes.
God and the Multiverse
God and the multiverse are not like Harry and Lord Voldemort. Nowhere does the Bible say that God has created only one universe. At best, a successful multiverse scenario could help refute the fine-tuning argument for God’s existence. But as atheistic philosopher Kai Nielsen pointed out, “To show that an argument is invalid or unsound is not to show that the conclusion of the argument is false…. All the proofs of God’s existence may fail, but it still may be the case that God exists.” God may well have created a multiverse. Indeed, when I read chapter 38 of the book of Job, I see an image of a God who is enchanted with the world he has created. It would not surprise me if our God, who imagined galaxies and black holes and oceans and epigenetics and breathed them all into existence, would create other universes.
Nevertheless, a question necessarily arises in the mind of the skeptic: why has God not given us a perfect scientific or logical proof of his existence? John 1:14 says that when God entered the world in the person of Jesus Christ, “The Word became flesh and dwelt among us.” The Greek word John uses for “Word” here is the Greek word logos, and from it we get the English word, “logic.” God has not given us a perfect proof for his existence, but rather he has given us a perfect person in Jesus Christ. Jesus lived the life we should have lived and died the death we were owed for our sin. And with his Resurrection, he conquered death, adopting all those who would receive him into the family of God. God has primarily chosen to reveal himself not by philosophical argument or scientific proof, but by personal relationship. And that relationship, unlike string theory, is something you can test for yourself.
 This phenomenon is famously discussed in cosmologist Martin Rees’ book, Just Six Numbers, and more recently in A Fortunate Universe by astrophysicists Geraint Lewis and Luke Barnes.
 See, for instance, Stephen Hawking’s recent book, The Grand Design, p. 164.
 C.S. Lewis handled the issue of life on other planets in his essay “Religion and Rocketry,” and his arguments apply equally well to the possibility of life in other universes.
 This argument comes from Tim Keller’s sermon, “The Word Made Flesh.”