Why do observed disks transport angular momentum faster than simulations?
In plain words
The magnetorotational instability (magnetic stirring of orbiting gas) explains how disks move angular momentum outward, but simulations give about ten times less transport than outbursts of real disks imply.
Precise statement
Outburst decay times of fully ionized disks in dwarf novae and X-ray binaries imply Shakura-Sunyaev $\alpha \sim 0.1 \text{ to } 0.4$, while MHD simulations of magnetorotational turbulence without net vertical magnetic flux give $\alpha \sim 0.01 \text{ to } 0.03$. Determine which physics closes the gap: net vertical flux, convection, magnetized winds removing angular momentum, or spiral shocks. Answer: a mechanism with $\alpha$ as a function of ionization state and net flux that reproduces outburst light curves.
What would settle it
Global simulations with self-consistent thermodynamics reproducing measured dwarf-nova outburst decay times.