I’m still trying to figure out (not being an astrophysicist but wanting to know) what actually the “Doppler Peaks” in CMB are. If I’ve understood, at the moment of the transition from the opaque to the visible Universe there were different velocities and directions of stuff. But what happens then? Why some write “Doppler peaks” and some “actually sound peaks” and what is the actual nature of the peaks, what causes the “sound” there (obviously it’s some vibration trough the matter, not air, but what is actually understood under to be that “sound”), and where is the Doppler effect actually involved?