July 23, 2007
Multiverse and the Design Argument
How do you answer sceptics who say that our universe doesn’t need a designer because it’s just a part of a bigger multiverse which is composed of all kinds of universes? No matter how improbable our universe looks, the chances are that there will be some just like it somewhere in the multiverse. If you deal the cards enough times, eventually every hand will come up sooner or later.
The idea that our universe is just a part of a wider multiverse is an expression of what I call the Many Worlds Hypothesis (MWH). This hypothesis is intimately connected with the so-called Anthropic Principle, which states that our own existence acts as a selection principle determining which properties of the universe we can observe. That is to say, any observed properties of the universe which may at first seem to be astonishingly improbable can only be seen in their true perspective after we realize that other properties couldn’t be observed by us, since we can only observe properties of the universe which are compatible with our existence. The Anthropic Principle implies that observers who have evolved within a universe must observe its constants and quantities to be fine-tuned for their existence, for otherwise they wouldn’t exist to observe them. The Anthropic Principle is used by some people to try to show why we shouldn’t be surprised at the astonishingly improbable fine-tuning of the universe for intelligent life.
Theorists now recognize that the Anthropic Principle can only legitimately be employed to explain away our observation of fine-tuning when it is conjoined to MWH, according to which an ensemble of concrete universes exists, actualizing a wide range of possibilities. MWH is essentially an effort on the part of partisans of chance to multiply their probabilistic resources in order to reduce the improbability of the occurrence of fine-tuning. As you put it, “if you deal the cards enough times, eventually every hand will come up.” The very fact that otherwise sober scientists must resort to such a remarkable hypothesis is a sort of backhanded compliment to the design hypothesis. It shows that the fine-tuning does cry out for explanation. But is MWH as plausible as the design hypothesis?
If MWH is to commend itself as a plausible hypothesis, then some plausible mechanism for generating the many worlds needs to be to be explained. The best shot at providing a plausible mechanism comes from inflationary cosmology, which is often employed to defend the view that our universe is but one domain (or “pocket universe”) within a vastly larger universe, or multiverse. Alexander Vilenkin is one who vigorously champions the idea that we live in a multiverse (Many Worlds in One: The Search for Other Universes [Hill and Wang, 2006]). At the heart of Vilenkin’s vision of the world is the theory of future-eternal, or everlasting, inflation (Vilenkin misleadingly calls it eternal inflation, even though he proves that the inflationary multiverse has only a finite past). According to generic inflationary theory, our universe exists in a true vacuum state with an energy density that is nearly zero, but earlier it existed in a false vacuum state with a very high energy density. The energy density of the false vacuum overwhelmed even the intense gravitational attraction generated by the high matter density of the early universe, causing a super-rapid, or inflationary, expansion, during which the universe grew from atomic proportions to a size larger than the observable universe in less than a microsecond.
But Vilenkin needs more than generic inflationary theory. In order to ensure everlasting inflation, Vilenkin hypothesizes that the scalar fields determining the energy density and evolution of the false vacuum state were characterized by a certain slope which issued in a false vacuum expanding so rapidly that, as it decays into pockets of true vacuum, the “island universes” thereby generated in this sea of false vacuum, though themselves expanding at enormous rates, cannot keep up with the expansion of the false vacuum and so find themselves increasingly separated with time. New pockets of true vacuum will continue to form in the gaps between the island universes and become themselves isolated worlds. Moreover, each island is subdivided into subdomains which Vilenkin calls O-regions, each constituting an observable universe bounded by an event horizon. Despite the fact that the multiverse is finite and geometrically closed, Vilenkin claims that the false vacuum will go on expanding forever, constantly generating new worlds.
At this point Vilenkin executes a nifty piece of legerdemain. As the island universes expand, their central regions eventually grow dark and barren in accordance with the second law of thermodynamics, while stars are constantly forming at their ever-expanding perimeters. We should think of the decay of false vacuum to true vacuum going on at the islands’ expanding perimeters as multiple Big Bangs. From the global perspective of the inflating multiverse, these Big Bangs occur successively over time, as the island boundaries grow with time. In the global time of the multiverse, each island is at any time finite in extent though growing.
Now comes the sleight of hand. When we consider the internal, cosmic time of each observable universe, each can be traced back to an initial Big Bang event. We can now string together these various Big Bang events as occurring simultaneously. Big Bangs which will occur in the global future are now to be regarded as present. As a result, the infinite, temporal series of successive Big Bangs is converted into an infinite, spatial array of simultaneous Big Bangs. Hence, from the internal point of view there now exists an infinity of universes. As Vilenkin puts it, “The infinity of time in one view is thus transformed into the infinity of space in the other” (p. 99).
Vilenkin’s deft transformation seems to presuppose a static theory of time or, as it is sometimes called, four-dimensionalism or spacetime realism, according to which all spacetime points, whether past, present, or future, are equally real. For if temporal becoming is an objective feature of reality, as I have argued in my Time and Eternity (Crossway, 2001), then the global future is potentially infinite only, and future Big Bangs do not in any sense exist. If there is a global tide of becoming, then there is no actually infinite collection of Big Bangs after all. Internal observers, unaware of the global perspective, are simply mistaken in their taking the successive Big Bang events to be occurring simultaneously. This is a good illustration of how issues in the philosophy of time impinge crucially on scientific debates.
By postulating many worlds, Vilenkin can find purchase for the Anthropic Principle in order to explain away the fine-tuning of the universe. Quantum fluctuations in the scalar fields determine what sort of vacuum will decay out of the false vacuum, each associated with a different set of values for the constants of nature. By postulating an infinite array of island universes, randomly varying in their constants, Vilenkin can then appeal to the Anthropic Principle to explain away the observed fine-tuning: we can observe only a universe which is fine-tuned for our existence.
But if an infinite ensemble of simultaneous universes does not actually exist, Vilenkin’s attempt to explain away the fine-tuning of the universe for intelligent life collapses. For if, in fact, an infinite array of universes does not yet exist, if most of them lie in the potentially infinite future and are therefore unreal, then there actually exist only as many observable universes as can have formed since any island’s origin in the finite past. Moreover, since Vilenkin himself has shown that the multiverse cannot be extended into the infinite past but must have had a beginning, there can be only as many island universes now in existence as have formed in the false vacuum since the multiverse’s beginning. Given the incomprehensible improbability of the constants’ all falling randomly into the life-permitting range, it may well be highly improbable that a life-permitting island universe should have decayed this soon out of the false vacuum. In that case the sting of fine-tuning has not been removed.
Vilenkin’s whole multiverse scenario depends in any case on the hypothesis of future-eternal inflation, which in turn is based upon the existence of certain primordial scalar fields which govern inflation. Although Vilenkin observes that “Inflation is eternal in practically all models suggested so far” (p. 214), he also admits, “Another important question is whether or not such scalar fields really exist in nature. Unfortunately, we don’t know. There is no direct evidence for their existence” (p. 61). This lack of evidence ought to temper our confidence in MWH.
Wholly apart from its speculative nature, however, the multiverse hypothesis faces a potentially lethal problem, which Vilenkin doesn’t even mention. Simply stated, if our universe is but one member of an infinite collection of randomly varying universes, then it’s overwhelmingly more probable that we should be observing a much different universe than that which we in fact observe. This same problem proved devastating for Ludwig Boltzmann’s appeal to a multiverse hypothesis in classical physics in order to explain why, if it has existed forever, the universe is not now in a state of thermodynamic equilibrium or heat death. Boltzmann made the bold speculation that the universe as a whole does, in fact, exist in a state of heat death, but that here and there random fluctuations produce pockets of disequilibrium, which Boltzmann referred to as “worlds.” Ours is one of these, and we shouldn’t be surprised to observe our world in such a highly improbable disequilibrium state, since observers cannot exist anywhere else. Boltzmann’s daring MWH has been universally rejected by contemporary physics on the grounds that were our universe but one such world in a multiverse, it is vastly more probable that we should be observing a much smaller region of disequilibrium—even one in which our solar system alone was produced in the twinkling of an eye by a random fluctuation—than what we do observe, since that is incomparably more probable than the whole universe’s being progressively formed by a decline in entropy from an equilibrium state.
Now a similar problem afflicts the contemporary appeal to the multiverse to explain away fine-tuning. Roger Penrose of Oxford University has calculated that the odds of our universe’s low entropy condition obtaining by chance alone are on the order of 1:1010(123), an inconceivable number. If our universe were but one member of a multiverse of randomly ordered worlds, then it is vastly more probable that we should be observing a much smaller universe. For example, the odds of our solar system’s being formed instantly by the random collision of particles is about 1:1010(60), a vast number, but inconceivably smaller than 1010(123). (Penrose calls it “utter chicken feed” by comparison [The Road to Reality (Knopf, 2005), pp. 762-5]). Or again, if our universe is but one member of a multiverse, then we ought to be observing highly extraordinary events, like horses’ popping into and out of existence by random collisions, or perpetual motion machines, since these are vastly more probable than all of nature’s constants and quantities’ falling by chance into the virtually infinitesimal life-permitting range. Observable universes like those strange worlds are simply much more plenteous in the ensemble of universes than worlds like ours and, therefore, ought to be observed by us if the universe were but a random member of a multiverse of worlds. Since we do not have such observations, that fact strongly disconfirms the multiverse hypothesis. On naturalism, at least, it is therefore highly probable that there is no multiverse.
All this has been said, of course, without asking whether the multiverse itself must not exhibit fine-tuning in order to exist. If it does, as some have argued, then it is a non-starter as an alternative to design.