The fine-tuning of the universe for life

When a physicist says that a theory is fine-tuned, they mean that it must make a suspiciously precise assumption in order to explain a certain observation. This is evidence that the theory is deficient or incomplete. The cosmological constant problem, the flatness problem, the big- and little-hierarchy problems of particle physics and the strong charge-parity problem can be framed as fine-tuning problems. One particular case of fine-tuning is particularly striking. The data in question are not the precise measurements of cosmology or particle physics, but a more general feature of our universe: it supports the existence of life. This chapter looks at this in more detail and considers fine-tuning in the context of Bayesian approaches to testing physical theories. Physical theories predict observations, and so a multiverse model should — in principle — be able to predict what kind of observer we would expect to be.