Title: Observational signatures of early matter domination
The Universe's early thermal history is poorly constrained, and it may have undergone a period of early matter domination (EMD) driven by a heavy particle that decayed into radiation before the onset of Big Bang nucleosynthesis. An EMD epoch can profoundly alter certain physical processes occurring therein, such as dark matter production or baryogenesis. Fortunately, clustering of the early matter species during EMD boosts density variations, providing an avenue for EMD's detection. I will discuss observational signatures of EMD that arise from these boosted density variations and prospects for their detection. In particular, enhanced density fluctuations can result in most of the dark matter becoming bound into early-forming, highly dense sub-Earth-mass microhalos. These microhalos can be detected by pulsar timing arrays when their presence perturbs the motions of tracked pulsars, and an EMD epoch could be detected by near-future pulsar timing experiments with about 20 years of observing time. If the dark matter is a thermal relic, these microhalos also greatly boost its annihilation rate. While the entropy injected by the decaying early matter species allows thermal relics far below the canonical annihilation cross section to achieve the observed dark matter abundance, this annihilation boost could bring these thermal relics back into the view of indirect-detection experiments.