[How black hole spin affects disks. Image: NASA/JPL/Chandra/NuSTAR.]
Spinning black holes are the only laboratories where we can explore gravitation in the strong field limit. Spin sets the innermost stable circular orbit (ISCO) for orbiting gas, with higher spin permitting tighter orbits. Using distortions to atomic lines produced in the innermost accretion disk, we can measure the spin of the black hole. Building on work that started in 2002, the Miller group has measured more spins than any other (for a review see this paper). We continue to pursue the spins of stellar-mass black holes, with an eye to comparisons with spins in gravitational wave-producing binaries. We are pursuing model-independent measures of spin through reverberation mapping, the spins of distant black holes through gravitational lenses, and the demographics of black hole spin in complete, volume-limited surveys as a window on black hole growth and evolution.
