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Abstract In cosmologies with hidden sector dark matter, the lightest hidden sector species can come to dominate the energy budget of the universe and cause an early matter-dominated era (EMDE). EMDEs amplify the matter power spectrum on small scales, leading to dense, early-forming microhalos which massively boost the dark matter annihilation signal. We use the Fermi-LAT measurement of the isotropic gamma-ray background to place limits on the parameter space of hidden sector models with EMDEs. We calculate the amplified annihilation signal by sampling the properties of prompt cusps, which reside at the centers of these microhalos and dominate the signal on account of their steepρ∝r^-3/2density profiles. We also include the portions of the parameter space affected by the gravitational heating that arises from the formation and subsequent destruction of nonlinear structure during the EMDE. We are able to rule out significant portions of the parameter space, particularly at high reheat temperatures. Long EMDEs remain poorly constrained despite large structure-induced boosts to the annihilation signal. 

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. 

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. 

Abstract In cosmologies with an early matter-dominated era (EMDE) prior to Big Bang nucleosynthesis, the boosted growth of small-scale matter perturbations during the EMDE leads to microhalo formation long before halos would otherwise begin to form. For a range of models, halos can even form during the EMDE itself. These halos would dissipate at the end of the EMDE, releasing their gravitationally heated dark matter and thereby imprintinga free-streaming cut-off on the matter power spectrum.We conduct the first cosmological N-body simulations of the formation and evaporation of halos during and after an EMDE.We show that in these scenarios, the free-streaming cut-off after the EMDE can be predicted accurately from the linear matter power spectrum. Although the free streaming can erase much of the EMDE-driven boost to density perturbations, we use our findings to show that the (re-)formation of halos after the EMDE nevertheless proceeds before redshift ∼ 1000. Early-forming microhalos are a key observational signature of an EMDE, and our prescription for the impact of gravitational heating will allow studies of the observational status and prospects of EMDE scenarios to cover a much wider range of parameters. 

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 If dark matter resides in a hidden sector minimally coupled to the Standard Model, another particle within the hidden sector might dominate the energy density of the early universe temporarily, causing an early matter-dominated era (EMDE). During an EMDE, matter perturbations grow more rapidly than they would in a period of radiation domination, which leads to the formation of microhalos much earlier than they would form in standard cosmological scenarios. These microhalos boost the dark matter annihilation signal, but this boost is highly sensitive to the small-scale cut-off in the matter power spectrum. If the dark matter is sufficiently cold, this cut-off is set by the relativistic pressure of the particle that dominates the hidden sector. We determine the evolution of dark matter density perturbations in this scenario, obtaining the power spectrum at the end of the EMDE. We analyze the suppression of perturbations due to the relativistic pressure of the dominant hidden sector particle and express the cut-off scale and peak scale for which the matter power spectrum is maximized in terms of the properties of this particle. We also supply transfer functions to relate the matter power spectrum with a small-scale cut-off resulting from the pressure of the dominant hidden sector particle to the matter power spectrum that results from a cold hidden sector. These transfer functions facilitate the quick computation of accurate matter power spectra in EMDE scenarios with initially hot hidden sectors and allow us to identify which models significantly enhance the microhalo abundance.