Speaker: Christopher Dessert, Flatiron Inst., New York
Abstract: The presence of light axions in the infrared is a generic feature of many ultraviolet (UV) scenarios, including string theory, with the number of such axions N naturally O(10-100). Even when these axions interact very weakly with the Standard Model (SM), the presence of N light axions is constrained by the number of relativistic degrees of freedom N_eff. Adopting an effective field theory approach, I compute the freeze-in abundance of Axiverse axions. Under generic assumptions for the structure of the axion-Standard Model couplings, the predicted N_eff may be already excluded by Planck or probed by ongoing cosmic microwave background surveys like Simons Observatory.