CMB Constraints on Dark Matter Annihilation following an Early Matter-Dominated Era*

The evolution of the early universe is largely unconstrained: there is a vast gulf between the possible energy scales of inflation and the onset of Big Bang nucleosynthesis. In many theories, oscillating scalar fields or massive particles come to dominate the Universe during this period, before decaying into Standard Model particles. An early matter-dominated era (EMDE) causes dark matter (DM) to cluster into microhalos earlier than in a standard cosmology, increasing the DM annihilation rate for a given annihilation cross section. Additionally, the decay of the species driving the EMDE dilutes the abundance of thermal relics, allowing for smaller DM annihilation cross sections to produce the observed DM density. An EMDE therefore broadens the range of viable DM candidates while also making more of these candidates accessible to indirect-detection methods, which search for energetic particles produced by DM annihilation. The cosmic microwave background (CMB) is particularly sensitive to the injection of energetic particles close to the time of recombination, well before halo formation occurs in the absence of an EMDE. Furthermore, the impact of DM annihilation on the CMB is clearly distinguishable from known astrophysical signals. We demonstrate the constraining power of the CMB for EMDE scenarios and compare to bounds from the isotropic gamma-ray background.