Later on, Fred Adams and I and two other colleagues, the pioneering physicist Stephon Alexander at Brown University and Evan Croh, then at the University of Michigan, expanded our investigation into the merits of the anthropic arguments. We inquired whether the fine-tuned values of the constants of nature observed in our universe provided the only possible conditions for life. For life to arise, we need a handful of requirements, including a certain amount of complexity of at least 10¹⁵ particles in the universe and long-lived stars that act as factories to produce heavy elements from lighter elements under the gravitational pressure inside the star's core

The result of our investigation surprised even us: Habitable universes could exist even if we made the strength of gravity a lot weaker or a lot stronger and even if we changed other constants of nature (like the constant that controls the strength of electro-magnetism) by many millions of times from their known values. We concluded that the constants of nature in our universe are not specially selected to allow for habitation. Even worse for the anthropic argument, we found that our universe seemed only borderline habitable based on the anthropic selection rules. There were many other possible universes with very different constants of nature from ours that would be more likely to allow life to arise

A few years ago, I was debating this shift at the annual How The Light Gets In festival at Hay-on-Wye in Great Britain with a conservative opponent of the multiverse. Exasperated by the arguments in favor of the multiverse, he declared to me and the audience: "But half of the physics community does not believe in a multiverse!" He paused briefly in order to let his grave statement settle in, and to lighten the mood, I replied jokingly: "So you are agreeing that the other half of the community does believe in the existence of the multiverse?" Everyone laughed. And in my heart, I knew that we were both right

The German philosopher Arthur Schopenhauer put it best when he said, "Truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident." Today, many scientists view as self-evident the possibility of our cosmos being vaster than a single universe

The final problem is what we might call the
holography/complementarity
problem. As I explained a while ago, thinking about
black hole entropy
has led physicists to propose something called “horizon complementarity” — the idea that one observer can’t sensibly talk about things that are happening outside their horizon. When applied to cosmology, this means we should think locally: talk about one or another pocket universe, but not all of them at the same time. In a very real sense, the implication of complementarity is that things outside our horizon aren’t actually real — all that exists, from our point of view, are degrees of freedom inside the horizon, and on the horizon itself

If something like that is remotely true, the conventional story of eternal inflation is dramatically off track. There isn’t really an infinite ensemble of pocket universes — or at least, not from the point of view of any single observer, which is all that matters. This helps with the measure problem, obviously, since we don’t have to take fractions over infinitely big ensembles. But one would be right to worry that it brings us back to where we started, wondering why inflation really helps us solve naturalness problems at all

. . .

The one I fret about the most is the “unitarity” or “Liouville” problem. This is essentially Penrose’s original critique, updated to eternal inflation. Liouville’s Theorem in classical mechanics states that if you take a certain number of states and evolve them forward in time, you will end up with precisely the same number of states you started with; states aren’t created or destroyed. So imagine that there is some number of states which qualify as “initial conditions for inflation.” Then eternal inflation says we can evolve them forward and get a collection of universes that grows with time. The problem is that, as this collection grows, there is an increasing number of states that look identical to them, but which didn’t begin with a single tiny inflating patch at all. (Just like an ice cube in a glass of water will evolve to a glass of cooler water, but most glasses of cool water didn’t start with an ice cube in them.) So while it might be true that you can generate an infinite number of universes, at the same time the fraction of such states that actually began in a single inflating patch goes to zero just as quickly. It is far from clear that this picture actually increases the probability that a universe like ours started from inflation

There is an obvious way out of this challenge, which is to say that all of these “numbers of states” are simply infinite, and this purported calculation just divides infinity by infinity and gets nonsense. And that’s very plausibly true! But if you reject the argument that universes beginning with inflation are an infinitesimally small fraction of all the universes, you are not allowed to accept the argument that there’s some small probability inflation starts and once it does it makes an infinite number of universes. All you can really do is say “we can’t calculate anything.” Which is fine, but we are left without a firm reason for believing that inflation actually solves the naturalness problems it was intended to solve

The existence of a multiverse is the only physical explanation for the fine-tuning of parameters that leads to our existence

The multiverse is a reasonable theoretical explanation of the fine-tunings, but this does not help in observationally confirming the hypothesis. The issue here is, which is more important in cosmology: theory (explanation) or observations (tests against reality)? The essential proposal is that one should downgrade observational testing in favour of theory — a dangerous road to take. In any case, the major problem with this proposal is that it can explain anything at all, because in a multiverse with an infinite or extremely large variety of universe properties — for example, the 10⁵⁰⁰ possibilities allowed by the landscape of string theory — virtually anything can happen. In that case, the hypothesis does not predict any specific testable fact. The existence of universes with giraffes is certainly predicted by many multiverse proposals, but universes where giraffes do not exist are also predicted. Observing a giraffe neither confirms nor disproves the multiverse