  Rayleigh scattering from neutral hydrogen during and shortly after
recombination causes the CMB anisotropies to be significantly frequency
dependent at high frequencies. This may be detectable with Planck, and would be
a strong signal in any future space-based CMB missions. The later peak of the
Rayleigh visibility compared to Thomson scattering gives an increased
large-scale CMB polarization signal that is a greater than 4% effect for
observed frequencies greater than 500GHz. There is a similar magnitude
suppression on small scales from additional damping. Due to strong correlation
between the Rayleigh and primary signal, measurement of the Rayleigh component
is limited by noise and foregrounds, not cosmic variance of the primary CMB,
and should observable over a wide range of angular scales at frequencies
between roughly 200GHz and 800GHz. I give new numerical calculations of the
temperature and polarization power spectra, and show that future CMB missions
could measure the temperature Rayleigh cross-spectrum at high precision, detect
the polarization from Rayleigh scattering, and also accurately determine the
cross-spectra between the Rayleigh temperature signal and primary polarization.
The Rayleigh scattering signal may provide a powerful consistency check on
recombination physics. In principle it can be used to measure additional
horizon-scale primordial perturbation modes at recombination, and distinguish a
significant tensor mode B-polarization signal from gravitational lensing at the
power spectrum level.
