Probing the Early Universe with Dark Matter Annihilation
Observations of the oldest light in the Universe and other astronomical measurements indicate that only 5% of the current energy content of the Universe is stored in elements found on the periodic table. The other 95% is composed of dark matter and dark energy: dark matter is responsible for the growth of galaxies, while dark energy shoves galaxies apart at an accelerating rate. I will summarize the observational evidence for dark matter and dark energy, including how we infer that all galaxies are surrounded by vast halos of dark matter. These halos are thought to have formed through the mergers of smaller clumps of dark matter. As remnants of the earliest stages of structure formation, the smallest dark matter halos provide a unique probe of the expansion history prior to Big Bang nucleosynthesis. I will discuss how the evolution of the early Universe can enhance the microhalo population, thereby boosting the dark matter annihilation rate if dark matter is generated by pair production. The amplitude of this boost is highly sensitive to the size of the smallest halos, which provides an additional window into the dynamics and particle content of the early Universe. It is therefore possible to use astronomical observations to learn about the origins of dark matter and the evolution of the Universe during its first second.