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So far my understanding of BAOs is that they are a relic of the old universe formed by the freezing of acoustic density waves in baryonic matter as the universe entered the recombination epoch. These oscillations can be used as a "standard ruler" for length scale, providing means to calculate Hubbles Constant.

I don't understand how this then links to evidence for dark matter.

Any help on this would be very much appreciated. Thanks :)

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  • $\begingroup$ It's much more of a guess than an answer, but perhaps BAOs are related to the wobble conjectured by Davis & Lineweaver (authors of the widely-used diagram of cosmological horizons) as occurring in subatomic particles as a result of spatial expansion: It's mentioned in people.smp.uq.edu.au/TamaraDavis/papers/SciAm_BigBang.pdf , and would seem to be what keeps matter present during spatial expansion, about as much as moving a piece of furniture on earth might faintly reduce the amount of space between here and, say, Alpha Centauri. $\endgroup$
    – Edouard
    Commented Apr 18, 2022 at 15:18
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    $\begingroup$ en.wikipedia.org/wiki/… Check that out. $\endgroup$
    – Allure
    Commented Apr 19, 2022 at 1:40
  • $\begingroup$ The only thing is, its "Baryonic matter" section seems dismissive of any relation between baryons & DM. (Which sorta cancels the relation of my own comment to the OP's question, but I'll leave it up just because the idea of "space" doing anything on its own always gives me a pain....It's like a deity with personality zero....) $\endgroup$
    – Edouard
    Commented Apr 19, 2022 at 4:50

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I don't think there is a direct connection to dark matter. Take this figure from the first observation paper:

BAO

The top three fits (green, red, blue) are models with baryonic matter and dark matter, the bottom fit (magenta) is dark matter only. So, no baryons, no BAO; or if you measure BAO, you can infer the baryonic density.

The connection to dark matter comes only indirectly, in that a consistent picture requires both dark matter and baryonic matter to fit all available data (including the CMB, etc).

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  • $\begingroup$ The caption to this figure ends with the following sentence: “The pure [cold dark matter] model, magenta, is actually close to the best-fit due to the data points on intermediate scales.” I think this says that the overall data fit is best if one assumes only dark matter, but that the baryon acoustic oscillation peak suggests the existence of baryons. Your answer is overall consistent with my guess that the connection between BAOs and dark matter is quantitative, rather than qualitative. $\endgroup$
    – rob ♦
    Commented May 2, 2022 at 17:55
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    $\begingroup$ I don't read it like that. The magenta line is a poor fit, as it does not replicate the BAO bump (which in the meantime has been measured with smaller error bars too). One can use this fit to quantitatively measure the amount of baryonic matter, as indicated by the green/red/blue curves. As far as I know, BAO do not even give a direct qualitative connection to dark matter, but only indirectly through other measurements and the consistency of the $\Lambda$CDM model. $\endgroup$
    – rfl
    Commented May 2, 2022 at 18:41

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