Sean Carroll’s excellent book The Big Picture: On the Origins of Life, Meaning, and the Universe Itself contains some great segments and thoughts. This excerpt, from a section on fine-tuning, is no exception. It talks about how the multiverse is not some kind of ad hoc theory that seeks to plug a problematic hole, but a theory that is coherent, predicted and predictive.
But in modern cosmology, the multiverse is not a theory at all. Rather, it is a prediction made by other theories—theories that were invented for completely different purposes. The multiverse wasn’t invented because people thought it was a cool idea; it was forced on us by our best efforts to understand the portion of universe that we do see.
Two theories, in particular, move us to contemplate the multiverse: string theory and inflation. String theory is currently our leading candidate for reconciling gravitation with the rules of quantum mechanics. It naturally predicts more dimensions of space than the three we observe. You might think that this rules out the idea, and we should move on with our lives. But these extra dimensions of space can be curled up into a tiny geometric figure, far too small to be seen in any experiment yet performed. There are many ways to do the curling up—many different shapes the extra dimensions can take. We don’t know the actual number, but physicists like to throw around estimates like 10500 different ways.
Every one of those ways to hide the extra dimensions—what string theorists call a compactification—leads to an effective theory with different observable laws of physics. In string theory, “constants of nature” like the vacuum energy or the masses of the elementary particles are fixed by the exact way in which extra dimensions are curled up in any given region of the universe. Elsewhere, if the extra dimensions are curled up in a different way, anyone who lived there would measure radically different numbers.
String theory, then, allows for the existence of a multiverse. To actually bring it into existence, we turn to inflation. This idea, pioneered by physicist Alan Guth in 1980, posits that the very early universe underwent a period of extremely rapid expansion, powered by a kind of temporary super-dense vacuum energy. This has numerous beneficial aspects, in terms of explaining the universe we see: it predicts a smooth, flat spacetime, but one with small fluctuations in density—exactly the kind that can grow into stars and galaxies through the force of gravity over time. We don’t currently have direct evidence that inflation actually occurred, but it is such a natural and useful idea that many cosmologists have adopted it as a default mechanism for shaping our universe into its present state.
Taking the idea of inflation, and combining it with the uncertainty of quantum mechanics, can lead to a dramatic and unanticipated consequence: in some places the universe stops inflating and starts looking like what we actually observe, while in other places inflation keeps going. This “eternal inflation” creates larger and larger volumes of space. In any particular region, inflation will eventually end—and when it does, we can find ourselves with a completely different compactification of extra dimensions than we have elsewhere. Inflation can create a potentially infinite number of regions, each with its own version of the local laws of physics—each a separate “universe.”
Together, inflation and string theory can plausibly bring the multiverse to life. We don’t need to postulate a multiverse as part of our ultimate physical theory; we postulate string theory and inflation, both of which are simple, robust ideas that were invented for independent reasons, and we get a multiverse for free. Both inflation and string theory are, at present, entirely speculative ideas; we have no direct empirical evidence that they are correct. But as far as we can tell, they are reasonable and promising ideas. Future observations and theoretical developments will, we hope, help us decide once and for all.
What we can say with confidence is that if we get a multiverse in this way, any worries about fine-tuning and the existence of life evaporate. Finding ourselves in a universe that is hospitable to life is no stranger, nor any more informative, than finding ourselves living on Earth: there are many different regions, and this is the one in which we can live.
What should be our credence that there is such a multiverse? It’s difficult to say with our current level of understanding of fundamental physics and cosmology. Some physicists would put the chances at nearly certain, others at practically zero. Perhaps it’s fifty-fifty. For our present discussion, what matters is that there is a simple, robust mechanism under which naturalism can be perfectly compatible with the existence of life, even if the life turns out to be extremely sensitive to the precise values of the physical parameters characterizing our environment.