7. High-temperature recombination is an exotic solution that would require something like
time variation of the fine structure constant.
Constraints in the CDM+ alpha model space have been studied.
The constraints on the allowed variation of the ne structure constant are substantially
tighter than the variation that would be required to reduce the sound horizon by 7%.
Although this is a specific model, we expect the result is general:
o the damping tail, which is highly sensitive to the history of recombination,
o and quite precisely measured, is not likely to allow solutions of this type.