01
The acoustic peaks in the CMB power spectrum are fossils of sound-like oscillations in the tightly coupled photon-baryon plasma before recombination.
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02
A perturbation begins falling gravitationally, compressing the photon-baryon fluid until pressure reverses the motion.
supported03
WMAP and Planck measured multiple acoustic peaks with positions and amplitudes closely matching cosmological models based on adiabatic primordial perturbations and standard plasma physics.