Is BAO a standing or moving wave?

[Doctor Strange]

The sound horizon is the distance that a wave of plasma can move from the end of Inflation to Recombination (roughly 300,000 years). In several papers and talks, this is described as a moving wave (see https://www.youtube.com/watch?v=JSqIBRbQmb0 at the 23 minute mark). The velocity of the wave is given as $v_{sound}= \frac {c} {sqrt {3}}$ . When recombination occurs, the driving pressure disappears and the density of energy is frozen at that location and is observed by us as a slightly higher temperature than average (the sound horizon).

However, other papers I've read (see http://www.quantumfieldtheory.info/CMB.pdf) talk about standing waves where the sound horizon is a function of the fundamental frequency and the second and third peaks are harmonics of that fundamental frequency. How do I resolve the image of a wave moving down the length of a rope vs. a standing wave on the rope? Is the first peak of the Temperature Power Spectrum associated with a shockwave moving outward from the over-density (as described by Eisenstein) or is it a collapse of baryons inward towards the over-density (as described by Klauber)?

 

[Chalnoth]

There's not really any fundamental difference between standing waves and traveling waves. They're just different superpositions of the same underlying waves.

 

[Chronos]

Waves can be represented many different ways. I prefer to think in terms of amplitude and frequency. It makes more sense that way.