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UBC Theses and Dissertations

Exploring and constraining new physics in the dark sector Cyr-Racine, Francis-Yan


The standard cosmological model that has emerged in the last decades describes an acceleratingly expanding universe where the familiar baryonic matter accounts for a very small fraction of the overall energy budget. The vast majority of the energy content of the Universe appears to belong to an elusive dark sector made up of dark matter and dark energy. In this thesis, we explore the cosmological consequences of new physics that could govern this unknown dark sector. We first consider a model where dark matter can annihilate to Standard-Model particles through a Breit-Wigner resonance. We show in this case that the energy released by dark matter annihilating in the first proto-halos is likely substantial. We determine that the bounds on the allowed energy injection into the primordial gas and the energy density of the diffuse gamma-ray background strongly constrain the magnitude of the resonantly-enhanced annihilation cross-section. We then perform a thorough analysis of a dark sector made of atom-like bound states. This so-called Atomic Dark-Matter model predicts novel dark-matter properties on small scales but retains the success of cold dark matter on cosmological scales. We revisit the atomic physics necessary to capture the thermal history of the dark atoms and discuss the required improvements over the hydrogen calculation. To solve the perturbation equations, we develop a second-order tight-coupling approximation and further discuss its implications for the baryon-photon case. We compute the matter power spectrum in this model and show that it displays strong dark-matter acoustic oscillations and a cutoff on small scales. Interestingly, we also identify key cosmic microwave background signatures that distinguish the atomic dark-matter scenario from other dark-matter theories. We determine that astrophysical constraints on this model generally favour dark atoms that are both more massive and have higher binding energies than standard atomic hydrogen. We finally consider how oscillations in the bispectrum of primordial fluctuations affects the clustering of dark-matter halos. We discover that features in the inflaton potential such as oscillations and bumps become imprinted in the mass dependence of the non-Gaussian halo bias. This finding opens the possibility of characterizing the inflationary potential with large-scale-structure surveys.

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