Why do red-giant cores rotate far slower than models predict?
In plain words
Star vibrations measured by the Kepler telescope show that the cores of red giant stars spin slowly. Standard models, which let the core contract and spin up, predict cores that spin about ten times faster or more.
Precise statement
Asteroseismic rotational splittings of mixed modes give core rotation rates of low-mass subgiants and red giants roughly 10 to 100 times below those of models with hydrodynamic transport and also below models with the Tayler-Spruit magnetic dynamo. Identify the angular momentum transport process (magnetic instabilities, internal gravity waves, mixed-mode transport, or another) and its effective viscosity $\nu_{\mathrm{AM}} \sim 1e3 \text{ to } 1e5\,\mathrm{cm}^{2}/\mathrm{s}$ required, as a function of mass and evolutionary stage, so that models reproduce the measured core and envelope rates.
What would settle it
A transport prescription derived from first principles that reproduces Kepler core-rotation rates from the subgiant to the core-helium-burning stage without stage-by-stage tuning.
Status in the literature
Core magnetic fields of 30 to 100 kG were measured asteroseismically in 2022 (Li, Deheuvels, Ballot and Lignieres, Nature, 2022); a revised Tayler-Spruit transport (Fuller, Piro and Jermyn, MNRAS, 2019) fits red-giant cores but not all evolutionary stages.