Cosmological Aspects of Non-Minimal Dark Sectors

Abstract

Many proposals for physics beyond the Standard Model do not give rise to only a single dark-matter candidate --- they give rise to an entire dark sector consisting of many independent dark degrees of freedom. In this dissertation, we explore some of the cosmological implications of such non-minimal dark sectors. In the first part of this dissertation, we examine the phenomenology of dark sectors in which the density of dark states grows exponentially with mass. Ensembles of such states arise naturally as the ``hadronic'' resonances associated with the confining phase of a strongly-coupled dark sector; they also arise naturally as the gauge-neutral bulk states of Type I string theories. We study the dynamical properties of such ensembles, including their effective equations of state, and investigate some of the immediate model-independent observational (astrophysical and cosmological) constraints on such ensembles that follow. Remarkably, we find that these constraints allow such sectors to exhibit energy scales ranging from the GeV scale all the way to the Planck scale, but that the total present-day cosmological abundance of the dark sector must be spread across an increasing number of different states in the ensemble as these energy scales are dialed from the Planck scale down to the GeV scale. In the second part of this dissertation, by contrast, we examine the possibility of non-trivial dynamics within non-minimal dark sectors, focusing on processes in which heavier constituents within a non-minimal dark sector decay to lighter constituents. We begin by demonstrating that such decays can leave non-trivial imprints on the phase-space distribution of the resulting dark matter. Indeed, as a result of these effects, this phase-space distribution need not be thermal --- it can even be multi-modal, with a non-trivial pattern of peaks and troughs as a function of momentum. We then proceed to study how these features can induce non-trivial changes in the shape of the resulting matter power spectrum. The results of this project therefore provide an interesting way of learning about (and potentially even constraining) non-trivial dynamics in the early universe.