Carroll's question is provocative, but his answer is compellingly well-reasoned and thoughtful. It helps that he sensibly circumscribes the problem space:
Some questions science has more or less answered: "What happens when something catches on fire?" But "Where did the universe come from?" is not one of these questions. So we are not faced with a matter of judging the merits of a mature and compelling scientific theory. Rather, we are trying to predict the future: will there ever be a time when a conventional scientific model provides a complete understanding of the origin of the universe? Or, alternatively, do we already know enough to conclude that God definitely helps us explain the universe we see, in ways that a non-theistic approach can never hope to match?A brief and quite comprehensible explanation of the state of our cosmological knowledge follows. Along the way, Carroll clears up misconceptions that have arisen around concepts and terms of art that have seeped into the wider public consciousness, like "the Big Bang."
Then he surveys the various theories purporting to explain how the universe came to be. It is in these theories that science and religion contend with one another, and what becomes clear in Carroll's survey is that which theory -- or which type of theory -- one believes is very much a philosophical rather than scientific question.
First, Carroll notes the attractiveness for the theologically inclined of the scientific observation that the universe's physical characteristics are exceptionally well-suited for our form of life. Certain physical parameters could have taken many different values, resulting in many different types of universe; even small differences from what we have measured could have resulted in a universe in which life on Earth could not have existed (at least, not in its current form). Since we seem to exist by virtue of a long shot, probabilistically speaking, the idea that there must have been an entity "setting" these particular values is attractive at first blush. However, Carroll points out possible alternative explanations:
- Other values for those parameters might have led to other forms of life emerging, forms whose characteristics we can't imagine.
- We got extremely lucky that the universe came into being in this way.
- Different parts of the universe have different values for the physical parameters, and we happen to live in one of the parts where life is possible.
Life may be very fragile, but for all we know it may be ubiquitous (in parameter space); we have a great deal of trouble even defining "life" or for that matter "complexity," not to mention "intelligence." At the least, the tentative nature of our current understanding of these issues should make us reluctant to draw grand conclusions about the nature of reality from the fact that our universe allows for the existence of life.(That's one problem I've always had with the anthropic principle: it always has struck me as unjustifiably specific to humans.)
The "we got lucky" explanation is certainly possible, but from the standpoint of a cosmologist it doesn't lend itself to the imaginative flights of fancy that the alternative explanations do. (That's my opinion. Carroll dismisses "we got lucky" on the basis that it is highly improbable, which is consistent with his assignment of probabilities to each of these alternative explanations.)
The third alternative explanation requires introducing the concept of the "multiverse." A universe is a region with a single set of values for fundamental physical parameters, and therefore a particular set of physical laws. If universes with different values for the same physical parameters exist, the whole collection of universes is called the multiverse.
There are different theories for how a multiverse could have arisen, but the bottom-line question for Carroll is the likelihood of a multiverse coming into existence versus the likelihood that a (or rather, the) universe was created by a deity.
One popular objection to the multiverse is that it is highly non-parsimonious; is it really worth invoking an enormous number of universes just to account for a few physical parameters? As Swinburne says:It's here that we arrive at the question of what is philosophically preferable in a theory. Simpler theories are better, but what is the measure of simplicity? Carroll's answer is, the theory that can be expressed most compactly (for a technical definition of "compact") is the one scientists prefer.
To postulate a trillion trillion other universes, rather than one God in order to explain the orderliness of our universe, seems the height of irrationality.
... The physics of a universe containing 10^88 particles that all belong to just a handful of types, each particle behaving precisely according to the characteristics of its type, is much simpler than that of a universe containing only a thousand particles, each behaving completely differently.There are legitimate objections to the notion of a multiverse. However, whatever challenges arise from positing a multiverse to explain our physical reality, those same challenges, and worse, confront "the God hypothesis." For instance, one fundamental question facing an explanation of the universe as God's handiwork is, why is the universe so much more complex than it needs to be for the purpose of creating man?
Likewise, a multiverse that arises due to the natural dynamical consequences of a relatively simple set of physical laws should not be discounted because there are a lot of universes out there. Multiverse theories certainly pose formidable problems, especially when it comes to making predictions and comparing them with data; for that reason, most scientists would doubtless prefer a theory that directly predicted the parameters we observe in nature over a multiverse ensemble in which our local environment was explained anthropically. But most scientists (for similar reasons) would prefer a theory that was completely free of appeals to supernatural agents.
In my experience, the typical rejoinder to this kind of objection is, "God's reasoning is beyond our comprehension." This assertion is a showstopper: there is no way to convince a nonbeliever that it's true, and no way to convince a believer that it isn't.
What is often overlooked is that there is a corresponding showstopper argument if you come at things from the opposite direction. "God hypothesis" advocates cite various roles God could have played or could be playing in the unfolding of the universe, but they all have the purpose of explaining why things happen. And as Carroll writes:
... the ultimate answer to "We need to understand why the universe exists/continues to exist/exhibits regularities/came to be" is essentially "No we don't."Carroll thinks that some people need to find causes for all things, while others are able to see the universe as a thing beyond the need for causation. It's this fundamental difference in mindset that explains why some people need a "God hypothesis" and others don't.
There is no reason, within anything we currently understand about the ultimate structure of reality, to think of the existence and persistence and regularity of the universe as things that require external explanation. Indeed, for most scientists, adding on another layer of metaphysical structure in order to purportedly explain these nomological facts is an unnecessary complication.It's a terrific read.