Cosmic Dawn in a Fuzzy Universe : Constraining the nature of Dark Matterwith 21 cm Cosmology

The cold dark matter (CDM) paradigm underlying the standard CDM model of cosmology is successful on large scales but faces potential problems on small scales partly related to a seeming overproduction of dwarf galaxies. This could be alleviated in exotic dark matter models that suppresses small-scale structure formation. One such attractive model is known as fuzzy dark matter (FDM). FDM positsthat dark matter is composed of ultra-light bosons with masses eV. With such light particle masses, quantum effects become important. More specifically, a pressure-like term appears in the equations of motion that counteracts gravitational collapse on small scales. Because small galaxies form first in CDM, it follows that the early history ot galaxy formation predicted by FDM should be markedly different. One novel way to probe this effect would be to use the 21 cm line of hydrogen which acts as a sensitive probe of the epoch of reionization (EoR) and Cosmic Dawn — when the first galactic sources of X-rays started to reheat theintergalactic medium (IGM). In this thesis, the evolution of the 21 cm signal have been simulated for both CDM and FDM. These simulations indicate that the fluctuationsin the 21 cm signal amenable to future observations are extremely weak ( 1 mK) — and probably unobservable — for FDM at high redshifts compared to CDM (which tend to yield signals with amplitudes 1 mK). This is mainly due to the delayed galaxy formation in FDM resulting in delayed Lyman- coupling of the 21 cm spin temperature to the kinetic temperature of the IGM. A robust prediction from all FDM scenarios explored in this thesis is that any detection of a signal at would rule out interesting particle masses for FDM, and would be evidence for CDM-like structure formation. Future work that properly models ionization fluctuations during the EoR could also yield strong predictions at lower redshifts