Coherence and Sakharov Oscillations in the Microwave Sky

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I discuss the origin of the ``Sakharov oscillations'' (or ``secondary Doppler peaks'') in standard angular power spectra of the Cosmic Microwave Background anisotropies calculated for inflationary models. I contrast this with the production of CMB anisotropies in `active' models and argue that the presence or absence of Sakharov oscillations will reveal fundamental properties of the primordial perturbations.