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Abstract In cosmologies with hidden sector dark matter, the lightest hidden sector species can come to dominate the energy budget of the universe and cause an early matter-dominated era (EMDE). EMDEs amplify the matter power spectrum on small scales, leading to dense, early-forming microhalos which massively boost the dark matter annihilation signal. We use the Fermi-LAT measurement of the isotropic gamma-ray background to place limits on the parameter space of hidden sector models with EMDEs. We calculate the amplified annihilation signal by sampling the properties of prompt cusps, which reside at the centers of these microhalos and dominate the signal on account of their steepρ∝r^-3/2density profiles. We also include the portions of the parameter space affected by the gravitational heating that arises from the formation and subsequent destruction of nonlinear structure during the EMDE. We are able to rule out significant portions of the parameter space, particularly at high reheat temperatures. Long EMDEs remain poorly constrained despite large structure-induced boosts to the annihilation signal. 

This paper summarizes the discussions which took place during the PITT-PACC Workshop entitled “Non-Standard Cosmological Epochs and Expansion Histories,” held in Pittsburgh, Pennsylvania, Sept. 5–7, 2024. Much like the non-standard cosmological epochs that were the subject of these discussions, the format of this workshop was also non-standard. Rather than consisting of a series of talks from participants, with each person presenting their own work, this workshop was instead organized around free-form discussion blocks, with each centered on a different overall theme and guided by a different set of Discussion Leaders. This document is not intended to serve as a comprehensive review of these topics, but rather as an informal record of the discussions that took place during the workshop, in the hope that the content and free-flowing spirit of these discussions may inspire new ideas and research directions.