Cosmic Fine Tuning and the Multiverse Hypothesis

The observable universe is necessarily hospitable for life. There are indications, however, that the laws of physics and cosmological parameters need not take the form and values observed, and if they were slightly different life could not exist. A common approach to this fine tuning problem is to propose a cosmos with an ensemble of domains, mostly inhospitable for life. A Bayesian method is used to show that this hypothesis is more credible than a homogeneous fine tuned universe. This conclusion is straightforward for a finite ensemble, but can be extended to an infinite ensemble by applying a formulation of the Principle of Mediocrity.