I have one question about baryon acoustic oscillation.
I understand why we should have the baryon-photon fluid sound wave before recombination: Suppose we have a spherical overdense region. This region attracts matter gravitationally, however the pressure gradient of photon counteracts this attraction that is the seed of baryon acoustic oscillation. Since then, baryon-photon fluid moves outward with the speed of sound.
I don't understand the last statement. Maybe, I am confused with a silly thing, but as far as I understand, transmission media don't move toward the direction of wave. Media(baryon-photon fluid in this case) just oscillate. In this sense, I would say that baryon-photon fluid should just oscillate, but it really moves outward.
What am I missing? I tried to make an analogy with actual sound wave in air, which was not quite helpful.
Thank you so much.