We develop in this thesis the principles governing the production of our universes 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.
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