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For decades scientists of all disciplines recognized exquisite fine-tuning in the laws governing the universe in order to permit life. If the individual strengths or ratios of the four fundamental forces of nature observed today--gravity, electromagnetic force, and the strong and weak nuclear forces--were minutely stronger or weaker, life becomes impossible. The same constraint also applies to the fundamental parameters of the universe, such as the mass of the electron, the number of large space dimensions, or the expansion rate. Nontheistic scientists' responses to such fine-tuning generally fall into two categories.
One response is to hope and search for some physical principle that forces these fine-tuned quantities to assume the values they do. For example, it takes fine-tuning to make a long, thin rod stand on its end and not fall over. However, if the same rod is held at the top so that it cannot tip, the appearance of fine-tuning disappears because the position of the rod can only assume one value. To these scientists' dismay, this "solution" to the fine-tuning "problem" seems increasingly unlikely.
The second category of explanations, growing in popularity, invokes the idea of a multiverse. Naturalistic scientists posit that instead of this universe being all that exists, there also exists a multitude of other "universes"--each exhibiting different laws and fundamental parameters. These different universes never contact one another, so it is impossible to detect or to directly measure any of them. However, if this multitude of universes exists, scientists argue that the required fine-tuning for life in this universe could simply be a consequence of life arising in the universe where it is possible. As summarized by the Weak Anthropic Principle (WAP), this universe must appear designed for life or else we would not be here to observe it.
While at first glance the multiverse proposal sounds ad hoc, scientists point to two lines of support. First, recent evidence strongly points to a period of hyperexpansion--dubbed inflation--very early in the universe when the universe grew by a factor of 10[size=1]30 in an infinitesimal fraction of a second. Some speculated theoretical modeling suggests that the physics causing inflation results in many different inflating "bubbles." One bubble contains this universe, but other bubbles could contain other universes.
Second, the two most tested and verified scientific theories, general relativity and quantum mechanics, are mutually exclusive if only four space-time dimensions operate in the early universe. How could both be true? As scientists currently understand the universe, string theory provides a superior model that successfully incorporates both general relativity and quantum mechanics. However, the possible solutions to the equations of string theory are about 10500, each with different laws, dimensionalities, and parameters. Cosmologists point to this plethora of possible solutions as possible universes for inflation to populate. Interestingly, none of the explored solutions match this universe, indicative of the fine-tuning exhibited by the universe.
Part of the favor that multiverse proposals garner derives from the notion that strictly natural theories are inherently more scientific than those which include supernatural causation. However, all multiverse models can be recast in terms of a single universe. Although possible, the multiverse proposal rests completely on speculated theoretical calculations--to date, no experimental or observational data exists that differentiates between a single, finely tuned universe and any of the multiverse speculations.
Talking to many members of the scientific community, one easily gets the impression that the multiverse model is a done deal, the ultimate reality. Undoubtedly the growing popularity of the multiverse idea will spill over into the popular culture.
It is important to realize that the multiverse proposal rests entirely on theoretical calculations. Although theoretical calculations play an integral role in developing successful scientific models, many models that look good on paper end up in the trash bin when confronted with hard experimental and/or observational evidence. Even given the theoretical modeling, the multiverse concept relies on additional speculation.
One currently testable aspect of the multiverse model provides further reason to doubt its validity. In a multiverse, the geometry of this universe will be open. In more technical terms, the total density parameter, Ω, of an open universe will be less than one. However, the best measurements for our universe have Ω total = 1.02 +/- 0.02 (in other words, one or greater).
Multiverse supporters believe that this marginally negative result will disappear as more precise measurements are made, but it is not encouraging when the first tests of a model tend toward falsification. Beyond this one test, no experimental evidence exists that would distinguish a multiverse from a universe. Until such evidence exists, nothing should compel a scientist--or a nonscientist--to accept a multiverse model as the final word.
On a more philosophical note, the reason many in the scientific community gravitate toward multiverse explanations is because they view completely naturalistic models as inherently more favorable than models invoking supernatural causation. This motivation appears to drive multiverse thinking because all multiverse models can be recast as a single universe. However, since multiverse models seem to provide a naturalistic explanation for the tremendous fine-tuning evident in this universe, some scientists tend to favor them over universe models.
Interestingly, it appears that many multiverse models redefine the term natural. Not too long ago the natural realm encompassed all space, time, matter, energy, and the physical laws associated with this universe. Anything beyond, by definition, was considered supernatural. However, all multiverse models assume a reality beyond the space, time, matter, energy, and physical laws of this universe. Thus, while multiverse enthusiasts would characterize their models as naturalistic, these models actually invoke supernatural or metaphysical features.
Those gravitating toward multiverse models commit a form of the gambler's fallacy. Upon seeing the immense fine-tuning of this universe, they assume the existence of a large number of universes beyond ours which exhibit no fine-tuning. However, the scientific community currently has no way of knowing whether these alternate universes exist or if they differ from this universe. A better approach would be to look more closely at this universe and see if the evidence for fine-tuning increases as scientific understanding advances. If so, that increased evidence would argue more powerfully for a Designer who created and maintains this universe. Unfortunately, many people would rather gamble than embrace the evidence.[/size]