  We develop in this thesis the principles governing the production of our
universe's primordial inhomogeneities during its early phase of inflation. As a
guiding thread we ask what physics during inflation can lead to perturbations
so large that they form black holes in sufficient abundance to be the dark
matter. We start with the simplest single-field slow-roll paradigm for
inflation, which cannot produce primordial black hole dark matter, and then
gradually relieve its assumptions. After developing the effective field theory
of inflation, we highlight the importance of the single-clock condition in
controlling the inhomogeneities. Going beyond single-clock inflation takes us
first to a qualitatively different inflationary scenario known as ultra-slow
roll and finally to understanding the physics of the Higgs field during
inflation.
