Abstract

How did the universe evolve prior to the creation of the cosmic microwave background? In the standard picture, inflation was immediately followed by a radiation-dominated era that persisted uninterrupted for 51,000 years before giving way to matter domination. However, there are no direct observational probes of the universe’s expansion history prior to the onset of Big Bang nucleosynthesis (BBN), and numerous theories predict deviations from radiation domination during the universe’s first second. Meanwhile, a persistent discrepancy between local and cosmological measurements of the Hubble constant has prompted us to reconsider the evolution of the universe between BBN and recombination. I will show how we can use the dark sector to learn about the expansion history of the early universe. I will discuss how observational constraints on the injection of photons and dark radiation restrict the impact that new unstable massive particles could have on the expansion rate after BBN. I will also demonstrate how the evolution of universe prior to BBN can enhance the abundance and compactness of the smallest dark matter halos. The resulting boost to the dark matter annihilation rate makes it possible to use astronomical observations to learn about the origins of dark matter and the evolution of the universe during its first second.