MG11 
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Perturbations of Dark Sectors from the CMB

The kinetic properties of dark cosmological sectors are imprinted on the CMB through the gravitational impact of dark sectors' perturbations. The impact occurs primarily during the horizon entry, when general relativity plays a key role. By requiring that formalisms for the perturbation evolution relate the apparent evolution to physical interactions within the local Hubble volume, we find a class of natural formalisms which alleviate gauge artifacts of the traditional approaches and are technically simpler. Our analysis shows that perturbations of cold dark matter suppress large-angle CMB anisotropy by an order of magnitude stronger than conventionally stated, while a textbook phenomenon of CMB ``radiation driving'' is a gauge artifact. The dark sectors are constrained by the CMB fluctuations on either small angles, where the signatures of the dark perturbations are non-degenerate; and large angles, where non-minimal dynamics of dark perturbations could interfere with the significant CDM suppression of the CMB power.