Parameterization of temperature and spectral distortions in future CMB   experiments
  CMB spectral distortions are induced by Compton collisions with electrons. We review the various schemes to characterize the anisotropic CMB with a non-Planckian spectrum. We advocate using logarithmically averaged temperature moments as the preferred language to describe these spectral distortions, both for theoretical modeling and observations. Numerical modeling is simpler, the moments are frame-independent, and in terms of scattering the mode truncation is exact. 
Cyril Pitrou, Albert Stebbins
astro-ph.CO gr-qc