  The cosmic microwave background (CMB) has been a treasure trove for
cosmology. Over the next decade, current and planned CMB experiments are
expected to exhaust nearly all primary CMB information. To further constrain
cosmological models, there is a great benefit to measuring signals beyond the
primary modes. Rayleigh scattering of the CMB is one source of additional
cosmological information. It is caused by the additional scattering of CMB
photons by neutral species formed during recombination and exhibits a strong
and unique frequency scaling ($\propto \nu^4$). We will show that with
sufficient sensitivity across frequency channels, the Rayleigh scattering
signal should not only be detectable but can significantly improve constraining
power for cosmological parameters, with limited or no additional modifications
to planned experiments. We will provide heuristic explanations for why certain
cosmological parameters benefit from measurement of the Rayleigh scattering
signal, and confirm these intuitions using the Fisher formalism. In particular,
observation of Rayleigh scattering allows significant improvements on
measurements of $N_{\rm eff}$ and $\sum m_\nu$.
