2025ApJ...980..136R

Using the semianalytic model SEROTINA, we investigate the cosmic spin evolution of supermassive black holes (SMBHs) incorporating recent results from general-relativistic magnetohydrodynamics simulations of spin-down from relativistic jets. We compare several variations of our model with compiled black hole (BH) spin measurements derived from X-ray reflection spectroscopy, correcting for a bias arising from the spin-dependent radiative efficiency of accretion flows. We show that the observed spin distribution is in agreement with a model that includes jet-driven spin-down, a key mechanism that acts to modulate spins across cosmic time at both high and very low specific accretion rates. The data also clearly prefer models with coherent accretion over models in which accretion disks rapidly switch from prograde to retrograde. We further predict spin distributions accessible via spatially resolved event horizons by the next-generation Event Horizon Telescope and Black Hole Explorer, as well as gravitational waves by the Laser Interferometer Space Antenna (LISA), each of which offers unique and distinct windows into the population of spinning BHs. Jet-driven spin-down is most strongly imprinted on the abundance of very highly spinning objects in our model. In addition, we show that the spin distribution sampled by LISA events may contain a signature of the natal spin distribution of heavy seeds, but not of light seeds, offering additional discrimination between these seeding pathways. Spin distributions from these future observed samples can be used to constrain the detailed physical properties of the accretion flow on horizon scales around SMBHs.