UNC-CH Physics and Astronomy PhD Defense

Kayla Redmond

“How an Era of Kination in the Early Universe Impacts Dark Matter Annihilation Signatures”

Our ignorance of the period between the end of inflation and the beginning of Big Bang Nucleosynthesis limits our understanding of the origins and evolution of dark matter. One possibility is that the Universe’s energy density was dominated by a fast-rolling scalar field while the radiation bath was hot enough to thermally produce dark matter. We investigate the evolution of the dark matter density and derive analytic expressions for the dark matter relic abundance generated during such a period of kination. We use observations of dwarf spheroidal galaxies by the Fermi Gamma-Ray Telescope and observations of the Galactic Center by the High Energy Stereoscopic System to place tight constraints on the allowed dark matter mass and the temperature at kinaton-radiation equality for scenarios where dark matter reaches thermal equilibrium during an era of kination. To further constrain these scenarios, we investigate the evolution of small-scale density perturbations during such a period of kination. We determine that once a perturbation mode enters the horizon during kination, the gravitational potential drops sharply and begins to oscillate and decay. Nevertheless, dark matter density perturbations that enter the horizon during an era of kination grow linearly with the scale factor prior to the onset of radiation domination. Consequently, kination leaves a distinctive imprint on the matter power spectrum: scales that enter the horizon during kination have enhanced inhomogeneity. The resulting boost to the small-scale matter power spectrum leads to the formation of enhanced substructure and increases the dark matter annihilation rate. We calculate the minimum boost factor required to push the remaining kination scenarios into tension with observational bounds. Utilizing the minimum boost factor, we are able to establish the maximum allowed temperature at kinetic decoupling that remains consistent with observations for the remaining kination scenarios.