Ultralight vector dark matter from non-slow-roll inflation

Inflation not only provides a promising explanation for the observed structure of our universe; but also reveals a novel and elegant mechanism for the production of dark matter, distinct from the historically considered scenarios. Among the others, inflation naturally generates vector field fluctuations which, after being stretched to super-horizon scales and re-entering the horizon, are viable dark matter candidates. The longitudinal mode of a massive vector boson is especially noteworthy, being abundantly produced during inflation. Unlike scalars and tensors which are typically produced with a nearly scale-invariant spectrum, the longitudinal mode inherits unique dynamics from its inflationary origin, satisfying cosmological constraints and providing a robust dark matter candidate. In this thesis, we propose a mechanism through which ultralight dark matter is generated during inflation. We introduce a brief departure from slow-roll inflation to address the challenge of producing a realistic dark matter candidate and explore its consequences. The violation of slow-roll conditions during inflation induces a rapid change in the vector boson mass, producing significant features in the power spectrum. We show that this rapid variation, occurring during a short non-slow-roll phase, enhances the power spectrum and naturally generates an ultralight dark matter relic with extremely small vector masses, allowing for the inclusion of candidates with masses as low as m ≃ 10^(−19) eV or even smaller. Our results demonstrate that the non-standard evolution of inflationary parameters strongly amplifies the longitudinal mode fluctuations, enabling ultralight bosons to be naturally produced by quantum fluctuations during inflation. This mechanism establishes a theoretically consistent pathway for generating ultralight dark matter and provides a framework to explore a unified inflationary origin for mixed dark matter scenarios.