Early Universe

Every wavelength of sound in the plasma, short waves to the right, the note's harmonics on the whole numbers: the line is each wave's temperature (dashed, its Doppler part), the bars the harmonics as you hear them. Watch the waves swing through zero, the short ones faster, the crests moving left as the sound horizon grows, until recombination stops them where the microwave background sees its peaks. Below, the history: equality, then recombination.

A patch of the plasma (or, once the light is free, of the sky it leaves) made of the struck key's waves running every way, red hotter and blue colder: the spots begin small and sharp, grow as the waves of longer wavelength swell, and settle at recombination into the size of the microwave background's spots, about a degree on our sky.

Each note starts bright, every harmonic alike, then its upper harmonics swell and fade faster and faster (their waves oscillate more often) and die as the photons diffuse, until recombination leaves the peaks; the odd ones stronger, by the baryons.