07 Ch 2 (11-42)

from both philosophers and physicists, is its use in cosmology to explain the apparent fine-tuning of our universe. “Fine-tuning” refers to the supposed fact that there is a set of cosmological parameters or fundamental physical constants that are such that had they been very slightly different, the universe would have been void of intelligent life. For example, in the classical big bang model, the early expansion speed seems fine-tuned. Had it been very slightly greater, the universe would have expanded too rapidly and no galaxies would have formed. There would only have been a very low density hydrogen gas getting more and more dispersed as time went by. In such a universe, presumably, life could not evolve. Had the early expansion speed been very slightly less, then the universe would have recollapsed very soon after the big bang, and again there would have been no life. Our universe, having just the right conditions for life, appears to be balancing on a knife’s edge (Leslie 1989). A number of other parameters seem fine-tuned in the same sense—e.g. the ratio of the electron mass to the proton mass, the magnitudes of force strengths, the smoothness of the early universe, the neutron-proton mass difference, perhaps even the metric signature of spacetime (Tegmark 1997). Some philosophers and physicists take fine-tuning to be an explanandum that cries out for an explanans. Two possible explanations are usually envisioned: the design hypothesis and the ensemble hypothesis. Although these explanations are compatible, they tend to be viewed as competing. If we knew that one of them were correct, there would be less reason to accept the other. The design hypothesis states that our universe is the result of purposeful design. The “agent” doing the designing need not be a theistic God, although that is of course one archetypal version of the design hypothesis. Other universe-designers have been considered in this context. For examCHAPTER 2 Fine-Tuning in Cosmology