The Early Universe's Imprint on Dark Matter

Our ignorance of the Universe's evolution prior to the onset of Big Bang Nucleosynthesis profoundly limits our understanding of dark matter: we cannot calculate its relic abundance without knowing when the Universe became radiation dominated. Fortunately, there is another probe of the early Universe that could break this degeneracy. An effectively matter-dominated era prior to the onset of nucleosynthesis can radically enhance the population of microhalos for both thermal and nonthermal dark matter. The resulting abundance of substructure increases the dark matter annihilation rate, which opens up the possibility of using gamma-ray observations to learn about the reheating of the Universe and the origins of dark matter.