A key scientific discovery within the last hundred years is that the universe is highly structured. It has precisely defined parameters - qualities such as its age, mass, curvature, temperature, density, and rate of expansion. Calculations and models show that, for many of these parameters, the slightest change to its numerical value would result in a universe in which life could not have arisen. As a result, our universe is said to be "fine-tuned for life." Instead of attributing this cosmic fine-tuning for life to blind luck, many seek a deeper explanation. One explanation offered by some scientists, philosophers, and theologians is that the universe was designed by something or someone outside of it - God. The argument for this conclusion is called "the fine-tuning argument for the existence of God," or just "the fine-tuning argument" for short. An alternative naturalistic explanation for cosmic fine-tuning is that our universe is but one of a vast number of universes, all with values for the cosmic parameters assigned randomly. The existence of at least one life-permitting universe within this vast ensemble is therefore not massively improbable, but rather to be expected. This "multiverse hypothesis" is the primary rival to the design hypothesis.
The fine-tuning argument raises a host of issues spanning metaphysics, epistemology, probability theory, value theory, and philosophical theology. The entries in this bibliographical tool attest to its continuing fascination for philosophers.