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I was reading the book of Barbara Ryden "Introduction to cosmology":

In the chapter number 9, in the page 203. She says:

"The photons, electrons, and protons together make a single photon-baryon fluid, whose energy density is only about a third that of the dark matter. Thus, the photon-baryon fluid moves primarily under the gravitational influence of the dark matter, rather than under its own self-gravity. [...]

"If the photon-baryon fluid finds itself in a potential well of the dark matter, it will fall to the center of the well. As the photon-baryon fluid is compressed by gravity, however, its pressure starts to rise. Eventually, the pressure is sufficient to cause the fluid to expand outward. As the expansion continues, the pressure drops until gravity causes the photon-baryon fluid to fall inward again."

Question

As far as I understand the Dark matter don't interact with the electromagnetic radiation, so my intuition say that the " Dark matter potential well" will increase his size as time pases. So, every time the photon-baryon photon fluid enters to the "potential well", it will take more time to get out ?

I mean the Baryonic acoustic oscillations are not some kind of harmonic oscillaton, they are more like some kind of damped oscillation?

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    $\begingroup$ The size of the wells grows with the universal expansion, and the dark matter also evolves under its own gravitation. I think the overall effect is that the wells get gradually deeper. $\endgroup$
    – Andrew Steane
    Commented Jun 16, 2020 at 17:28

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