| Summary: | The low energy limit of string theoretical models often predict the existence of many axions and hidden gauge sectors. This dissertation presents a collection of works exploring some of the consequences of this scenario to particle phenomenology and cosmology. We focus on ultralight axions, which are common occurrence on studies of large ensembles of string vacua, and dark gauge sectors without matter. There are many ongoing experiments looking for axions, a majority of which explores their coupling to photons. In this context, we discuss an enhancement of axion-photon couplings in string compactifications with large number of axions, which results in couplings that are within the reach of both current and proposed experimental searches. Axions are also used in many models of dark matter and inflation/reheating. We investigate here a model of axion reheating with many axions and gauged dark sectors, in which we identify classes of axion reheatons that result in asymmetric reheating. In addition, a dark matter model composed of both ultralight axions and dark glueballs (bound states of dark gluons) is constructed, which displays unique phenomenology, given the "fuzzy" nature of the axions and the strong self-interactions of the glueballs. The formation of these dark glueballs often follows from a first-order confining phase transition, which is known to produce a gravitational wave signal. We end this dissertation by describing a phenomenological model of confinement that can estimate the intensity of this signal.--Author's abstract
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