Multiverses come in many varieties. In this post, I won’t be talking about unrestricted multiverses, in which anything that can possibly happen, actually happens in some universe. Instead, I’ll be talking about the more modest claim that our universe is just one of a vast number of universes with varying physical constants and different laws of nature, and that there is something called a “multiverse-generator” which churns out baby universes. In an influential essay entitled, The Teleological Argument: An Exploration of the Fine-Tuning of the Universe (in The Blackwell Companion to Natural Theology, edited by William Lane Craig and J. P. Moreland, 2009, Blackwell Publishing Ltd.), Dr. Robin Collins argues that a “multiverse-generator” doesn’t eliminate the need for fine-tuning. The analogy he uses is that of a bread machine, which must have the right structure, programs, and ingredients (flour, water, yeast, and gluten) in order to produce decent loaves of bread. Similarly, the problem with a “multiverse-generator”, whether of the inflationary variety or some other type, is that the laws of the multiverse generator must be just right – i.e. fine-tuned – in order for it to (occasionally) produce universes whose constants and initial conditions permit the subsequent emergence of life. Thus invoking some sort of multiverse generator to explain the fine-tuning of our universe merely pushes the fine-tuning up one level: it doesn’t make it go away.
As Dr. Collins puts it (emphases below are mine):
6.3. The inflationary-superstring multiverse explained and criticized
…[B]y far the most commonly advocated version of the restricted multiverse hypothesis is the “multiverse-generator” version that claims that our universe was generated by some physical process that produces an enormous number of universes with different initial conditions, values for the constants of nature, and even lower-level laws. Many scenarios have been proposed – such as the oscillating Big Bang model and Lee Smolin’s claim that many universes are generated via black holes (Smolin 1997). Among these, the one based on inflationary cosmology conjoined with superstring theory is by far the most widely discussed and advocated, since this is the only one that goes beyond mere speculation. According to inflationary cosmology, our universe started from an exceedingly small region of space that underwent enormous expansion due to a hypothesized inflaton field that both caused the expansion and imparted a constant, very large energy density to space as it expanded. The expansion caused the temperature of space to decrease, causing one or more so-called “bubble universes” to form. As each bubble universe is formed, the energy of the inflaton field is converted into a burst of “normal” mass-energy, thereby giving rise to a standard Big Bang expansion of the kind we see in our universe.
In chaotic inflation models – widely considered the most plausible – space expands so rapidly that it becomes a never-ending source of bubble universes. Thus, an enormous number of universes naturally arise from this scenario. In order to get the parameters of physics to vary from universe to universe, however, there must be a further physical mechanism/law to cause the variation. Currently, many argue that this mechanism/law is given by superstring theory or its proposed successor, M-Theory, which are widely considered the only currently feasible candidates for a truly fundamental physical theory…
As a test case, consider the inflationary type multiverse generator. In order for it to explain the fine-tuning of the constants, it must hypothesize one or more “mechanisms” for laws that will do the following [four] things: (i) cause the expansion of a small region of space into a very large region; (ii) generate the very large amount of mass-energy needed for that region to contain matter instead of merely empty space; (iii) convert the mass-energy of inflated space to the sort of mass-energy we find in our universe; and (iv) cause sufficient variations among the constants of physics to explain their fine-tuning.
[T]o achieve (i)–(ii), we effectively have a sort of “conspiracy” between at least two different factors: the inflaton field that gives empty space a positive energy density, and Einstein’s equation… of General Relativity, which dictates that space expand at an enormous rate in the presence of a large near-homogenous positive energy density… Without either factor, there would neither be regions of space that inflate nor would those regions have the mass-energy necessary for a universe to exist. If, for example, the universe obeyed Newton’s theory of gravity instead of Einstein’s, the vacuum energy of the inflaton field would at best simply create a gravitational attraction causing space to contract, not to expand.
The conversion of the energy of the inflaton field to the normal mass-energy of our universe (condition (iii)) is achieved by Einstein’s equivalence of mass and energy, E = mc^2, along with the assumption that there is a coupling between the inflaton field and the matter fields. Finally, the variation in the constants (and to some extent the laws) of nature is typically claimed to be achieved by combining inflationary cosmology with superstring/M-Theory, which purportedly allows for an enormous number (greater than 10^500) possible combinations of values for the constants of physics. The important point here is that the laws underlying the inflationary scenario must be just right in order to cause these variations in the constants of physics from one universe to another. If the underlying laws are those given by superstring/M-Theory, arguably there is enough variation; this is not the case, however, for the typical grand unified theories that have been recently studied.
And the fine-tuning doesn’t stop here, as Dr. Collins explains:
In addition to the four factors listed, the fundamental physical laws underlying a multiverse generator – whether of the inflationary type or some other – must be just right in order for it to produce life-permitting universes, instead of merely dead universes. Specifically, these fundamental laws must be such as to allow the conversion of the mass-energy into material forms that allow for the sort of stable complexity needed for complex intelligent life. For example, … without the Principle of Quantization, all electrons would be sucked into the atomic nuclei, and, hence atoms would be impossible; without the Pauli Exclusion Principle, electrons would occupy the lowest atomic orbit, and hence complex and varied atoms would be impossible; without a universally attractive force between all masses, such as gravity, matter would not be able to form sufficiently large material bodies (such as planets) for life to develop or for long-lived stable energy sources such as stars to exist.
Although some of the laws of physics can vary from universe to universe in superstring/M-Theory, these fundamental laws and principles underlie superstring/M-Theory and therefore cannot be explained as a multiverse selection effect. Further, since the variation among universes would consist of variation of the masses and types of particles, and the form of the forces between them, complex structures would almost certainly be atomlike and stable energy sources would almost certainly require aggregates of matter. Thus, the said fundamental laws seem necessary for there to be life in any of the many universes generated in this scenario, not merely in a universe with our specific types of particles and forces.
In sum, even if an inflationary-superstring multiverse generator exists, it must have just the right combination of laws and fields for the production of life-permitting universes: if one of the components were missing or different, such as Einstein’s equation or the Pauli Exclusion Principle, it is unlikely that any life-permitting universes could be produced. Consequently, at most, this highly speculative scenario would explain the fine-tuning of the constants of physics, but at the cost of postulating additional fine-tuning of the laws of nature.
When, I wonder, will skeptics wake up to the fact that their favorite multiverse requires an Intelligent Designer?