Primordial-amplitude–optical-depth degeneracy
= Primordial-amplitude–optical-depth degeneracy
{title2=$A_s e^{-2\tau_{\rm re}}$}
The primary small-scale <Cosmic microwave background power spectrum> measures the amplitude $A_s e^{-2\tau_{\rm re}}$, because <reionization> damps each temperature amplitude once. The unaffected large-scale temperature modes are few and limited by <cosmic variance>. Polarization and gravitational lensing provide additional information that partly breaks this degeneracy.