Most scientists concede that there are features of the observed Universe which appear contrived or ingeniously and felicitously arranged in their relationship to the existence of biological organisms in general and intelligent observers in particular. Often these features involve so-called fine-tuning in certain parameters, such as particle masses or coupling constants, or in the cosmic initial conditions, without which life (at least life as we know it) would be either impossible or very improbable. I term this state of affairs bio-friendliness or biophilicity. Examples of such fine-tuning have been thoroughly reviewed elsewhere  and in this volume, so I will not list them here.
It is normally remarked that cosmic bio-friendliness has two possible explanations (discounting sheer luck). One is that the Universe has been designed by a pre-existing creator with life in mind. The other, which is often motivated explicitly or implicitly by a reaction to supernatural explanations, is the multiverse. According to the latter explanation, what we call 'the Universe' is but a small component in a vastly larger assemblage of 'universes', or cosmic regions, among which all manner of different physical laws and conditions are somewhere instantiated. Only in those 'Goldilocks' regions where, by accident, the numbers come out just right will observers like ourselves arise and marvel at the ingenious arrangement of things. Thus the reason why we observe a universe so suspiciously contrived for life is because we obviously cannot observe one that is inimical to life. This is the so-called anthropic, or biophilic selection, principle .
Before reviewing the pros and cons of the multiverse explanation, I should like to make a general point. All cosmological models are constructed by augmenting the results of observations by some sort of philosophical principle. Two examples from modern scientific cosmology are the principle
Universe or Multiverse?, ed. Bernard Carr. Published by Cambridge University Press. © Cambridge University Press 2007.
of mediocrity, sometimes known as the Copernican principle, and the biophilic selection principle. The principle of mediocrity states that the portion of the Universe we observe is not special or privileged, but is representative of the whole. Ever since Copernicus demonstrated that Earth does not lie at the centre of the Universe, the principle of mediocrity has been the default assumption; indeed, it is normally referred to as simply the 'cosmo-logical principle'. It underpins the standard Friedmann-Robertson-Walker cosmological models.
In recent years, however, an increasing number of cosmologists have stressed the inherent limitations of the principle of mediocrity. Scientific observations necessarily involve observer selection effects, especially in astronomy. One unavoidable selection effect is that our location in the Universe must be consistent with the existence of observers. In the case of humans at least, observers imply life. (There is no reason why non-living observers could not exist, and indeed we may conjecture that advanced technological communities may create them. However, it is normally assumed that the emergence of life and intelligence is a precursor to the creation of non-living sentient beings, although there is no logical impediment to abiological sentence arising de novo.) Stated this way - that the Universe we observe must be consistent with the existence of observers - the biophilic principle seems to be merely a tautology. However, it carries non-trivial meaning when we drop the tacit assumption that the Universe, and the laws of nature, necessarily assume the form that we observe. If the Universe and its laws could have been otherwise, then one explanation for why they are as they are might be that we (the observers) have selected it from a large ensemble of alternatives.
This biophilic selection principle becomes more concrete when combined with the assumption that what we have hitherto regarded as absolute and universal laws of physics are, in fact, more like local by-laws: they are valid in our particular cosmic patch, but they might be different in other regions of space and/or time . This general concept of 'variable laws' has been given explicit expression through certain recent theories of cosmology and particle physics, especially by combining string/M-theory with inflationary cosmology. There is little observational evidence for a domain structure of the Universe within the scale of a Hubble volume, but on a much larger scale there could exist domains in which the coupling constants and particle masses in the Standard Model may be inconsistent with life. It would then be no surprise that we find ourselves located in a (possibly atypical) life-encouraging domain, as we could obviously not be located where life was impossible.
Once it is conceded that the Universe could have been otherwise - that the laws of physics and the cosmic initial conditions did not have to assume the form we observe - then a second philosophical issue arises. The multiverse will contain a set of 'universes' that serve as instantiations for certain laws and initial conditions. What, then, determines the selection of universes on offer? Or to express it more graphically, using Stephen Hawking's words : 'What is it that breathes fire into the equations and makes a universe for them to govern?'
Only two 'natural' states of affairs commend themselves in this regard. The first is that nothing exists; the second is that everything exists. The former we may rule out on observational grounds. So might it be the case that everything that can exist, does exist? That is indeed the hypothesis proposed by some cosmologists, most notably Max Tegmark . At first sight this hypothesis appears extravagant. The problem, however, for those who would reject it is that, if less than everything exists, then there must be some rule that divides those things that actually exist from those that are merely possible but are in fact non-existent. One is bound to ask: What would this rule be? Where would it come from? And why that rule rather than some other?
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