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We propose a novel scenario to obtain the correct relic abundance for thermally underproduced dark matter. This scenario utilizes a strongly first-order phase transition at temperature $T_{\mathrm{PT}}$that gives rise to dark matter mass $m$. Freeze-out in the broken phase can yield the desired abundance in the entire region currently allowed by observational bounds and theoretical constraints for ${10}^2{T}_{\mathrm{PT}}\lesssim m\lesssim {10}^4{T}_{\mathrm{PT}}$. We show that the accompanying gravitational waves are strong enough to be detected by many upcoming and proposed experiments. This, in tandem with dark matter indirect searches, provides a multimessenger probe of such models. Positive signals in the future can help reconstruct the potential governing the phase transition and shed light on an underlying particle physics realization. Published by the American Physical Society2024 

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. 

A<sc>bstract</sc> We study the phenomenology of superheavy decaying dark matter with mass around 10¹⁰GeV which can arise in the low-energy limit of string compactifications. Generic features of string theory setups (such as high scale supersymmetry breaking and epochs of early matter domination driven by string moduli) can accommodate superheavy dark matter with the correct relic abundance. In addition, stringy instantons induce tinyR-parity violating couplings which make dark matter unstable with a lifetime well above the age of the Universe. Adopting a model-independent approach, we compute the flux and spectrum of high-energy gamma rays and neutrinos from three-body decays of superheavy dark matter and constrain its mass-lifetime plane with current observations and future experiments. We show that these bounds have only a mild dependence on the exact nature of neutralino dark matter and its decay channels. Applying these constraints to an explicit string model sets an upper bound of$$ \mathcal{O} $$ $O$(0.1) on the string coupling, ensuring that the effective field theory is in the perturbative regime.