Black Holes to Dark Matter and Back Again

Abstract

This thesis explores the connections which may exist between dark matter and black holes. First, we demonstrate that black holes that form from energetic collisions during a hot big bang could evaporate, producing the observed dark matter relic abundance. For these microscopic black holes to form, the Universe would have to have begun with a plasma temperature T > 10^{17} GeV. We then introduce the concept of Large Extra Dimensions (LEDs) which reduce the true scale of quantum gravity. In the presence of LEDs, black holes can form at temperatures as low as T ~ 100 GeV. Some of these immediately evaporate while others grow to macroscopic sizes and have longer lifetimes. We compare the role both scenarios could play in producing dark matter and demonstrate that the immediate evaporation of microscopic black holes is more efficient. Additionally, we study the LED black holes that grow in the early Universe to a sufficient size such that they survive into the late Universe. We derive constraints on long lived LED black holes based on their evaporation to an isotropic X-ray and gamma-ray signal. We then present a complete set of constraints on LED black holes from recent astrophysical observations. In doing so, we show that in the presence of two LEDs, black holes would have a lifetime longer than the age of the Universe and could comprise all the observed dark matter. Finally, we show that dark matter could also have a role in the formation of supermassive black holes. Dark matter with a sub-keV mass annihilating within a primordial gas cloud's halo would produce a UV background, which suppresses cooling via molecular hydrogen and would allow the gas to collapse into a large black hole seed.