## Details

Abstract: Event Horizon Telescope studies prefer Magnetically Arrested Disk (MAD) accretion flows over their more weakly magnetized Standard and Normal Evolution (SANE) counterparts. These models produce efficient jets via the Blandford-Znajek (BZ) mechanism, which extracts the spin-energy of the black hole. In this talk, we explore the cosmological implications of the MAD model. First, using general relativistic radiative magnetohydrodynamics (GRRMHD) simulations, we develop new "sub-grid" formulae for BZ jet feedback and spin evolution, appropriate for both semi-analytic models and cosmological simulations, for arbitrary spins and accretion rates. Then, we implement these formulae in the semi-analytic model Serotina, which evolves a cosmological population of supermassive black holes from the seeding epoch (z~20) to the present day. We demonstrate that spin-down in a MAD universe due to BZ jets has an observable effect moderating SMBH spins across cosmic time. Forward modeling different selection effects, we make predictions for spin measurements via X-ray reflection spectroscopy, extensions to the Event Horizon Telescope, and the Laser Interferometer Space Antenna (LISA).