  Theory testing in the physical sciences has been revolutionized in recent
decades by Bayesian approaches to probability theory. Here, I will consider
Bayesian approaches to theory extensions, that is, theories like inflation
which aim to provide a deeper explanation for some aspect of our models (in
this case, the standard model of cosmology) that seem unnatural or fine-tuned.
In particular, I will consider how cosmologists can test the multiverse using
observations of this universe.
