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Abstract Dark matter annihilation has the potential to leave an imprint on the properties of the first luminous structures at Cosmic Dawn as well as the overall evolution of the intergalactic medium (IGM).In this work, we employ a semi-analytic method to model dark matter annihilation during Cosmic Dawn (approximately redshiftz= 20 to 40), examining potential modifications to IGM evolution as well as gas collapse, cooling, and star formation in mini-halos. Our analysis takes into account the effects of dark matter-baryon velocity offsets, utilizing the public21cmvFASTcode, and producing predictions for the 21cm global signal.The results from our simplified model suggest that dark matter annihilation can suppress the gas fraction in small halos and alter the molecular cooling process, while the impact on star formation might be positive or negative depending on parameters of the dark matter model as well as the redshift and assumptions about velocity offsets. This underscores the need for more comprehensive simulations of the effects of exotic energy injection at Cosmic Dawn as observational probes are providing us new insights into the process of reionization and the formation of first stars and galaxies. 

This dissertation explores the impact of dark matter on the early universe and cosmological observables, with a focus on dark matter annihilation effects on thermal history and dark matter annihilation at the small scales, including the formation of the first stars and galaxies. Dark matter annihilation, enhanced by cosmic inhomogeneities, reshapes the gas temperature and ionization history of the early universe. Annihilation injects energy into the IGM, raising the gas temperature and ionization fraction. This process can either suppress or accelerate the star formation. This study examines the effects of dark matter annihilation on the minimum cooling mass of halos at different redshifts. Notably, this work presents the first combined calculation of dark matter annihilation and dark matter baryon velocity offsets, which have previously been treated separately. Our detailed calculations reveal the non-trivial effects of interplay between dark matter annihilation and dark matter baryon velocity offsets affects the evolution of structure formation. To explore these effects further, we extend existing models to include both molecular and atomic cooling halos, allowing star formation to occur in lower-mass halos and offering insights into how dark matter annihilation, streaming velocity, and cooling mechanisms shape early observable signals. Our study calculates the sky-averaged brightness temperature of the high-redshift 21cm absorption signal against the cosmic microwave background, also known as the “global 21cm signal”, including the effects of both dark matter annihilation and velocity offsets. These factors create distinct features in the 21cm signal, providing potential observational signatures of dark matter properties. We also examine energy transfer processes within dark matter halos, including inverse Compton scattering, photoionization, and pair production. By applying a refined Monte Carlo energy-transfer calculation code, we link single-particle simulations to energy deposition fractions. These developments will be crucial for connecting small-scale effects with large- scale galaxy formation models and ultimately interpreting observational data from the early universe. 

Abstract Cosmology and astrophysics provide various ways to study the properties of dark matter even if they have negligible non-gravitational interactions with the Standard Model particles and remain hidden. We study a type of hidden dark matter model in which the dark matter is completely decoupled from the Standard Model sector except gravitationally, and consists of a single species with conserved comoving particle number and conserved comoving entropy. This category of hidden dark matter includes models that act as warm dark matter but is more general. In particular, in addition to having an independent temperature from the Standard Model sector, it includes cases in which dark matter is in its own kinetic equilibrium or is free-streaming, obeys fermionic or bosonic statistics, and processes a chemical potential that controls the particle occupation number. While the usual parameterization using the free-streaming scale or the particle mass no longer applies, we show that all cases can be well approximated by a set of functions parameterized by only one parameter as long as the chemical potential is nonpositive: the characteristic scale factor at the time of the relativistic-to-nonrelativistic transition. We study the constraints from Big Bang Nucleosynthesis, the cosmic microwave background, the Lyman-α forest, and the smallest halo mass. We show that the most significant phenomenological impact is the suppression of the small-scale matter power spectrum — a typical feature when the dark matter has a velocity dispersion or pressure at early times. So far, the Lyman-α forest and the small dark matter halo population provide the strongest constraints, limiting the transition redshift to be larger than ∼6.2×10⁷.