The CMB contains information about baryon acoustic oscillations in which baryons (I assume protons and electrons) and photons form a plasma exhibiting sound waves. How is information about the baryon asymmetry of the universe contain in this?

Why do we not get some combination of baryon and (charged) lepton asymmetries if there are both electrons and protons in the mix?

Why does the dependence not go as their density but rather the asymmetry?

  • $\begingroup$ How is information about the baryon asymmetry of the universe contain in this? As far as I know, baryon acoustic oscillations (BAO) constrain $\Omega_M$, but they don't tell us anything about baryon asymmetry. Maybe I'm just ignorant, but do you have some source of information that says that BAO really do relate to baryon asymmetry? $\endgroup$
    – user4552
    Commented Feb 1, 2019 at 5:20

1 Answer 1


Here is a description:

The acoustic oscillations arise because of a conflict in the photon–baryon plasma in the early universe. The pressure of the photons tends to erase anisotropies, whereas the gravitational attraction of the baryons, moving at speeds much slower than light, makes them tend to collapse to form overdensities. These two effects compete to create acoustic oscillations, which give the microwave background its characteristic peak structure. The peaks correspond, roughly, to resonances in which the photons decouple when a particular mode is at its peak amplitude.

You ask:

How is information about the baryon asymmetry of the universe contain in this?

Please note the correction to the previous version of the answer, as I found a paper where the acoustic oscillations of the CMB are used in order to estimate the baryon asymmetry.

They have a phenomenological model of how more matter and antimatter annihilation gamma rays would affect the acoustic oscillations at photon decoupling to give the CMB


at 380.000 years after the Big Bang, where it says transparency point for light, where the photons of the CMB have their last scatter before reaching our detectors.

These oscillations depend on the density of matter previous to the CMB , according to their model, the existence of an asymmetry can be found from the oscillations. Baryon antibaryon's annihilating would affect by the gamma rays generated the final photon spectrum at photon decoupling time.

The subject is still at a research state.

  • $\begingroup$ As this has gotten negative votes, I wish people left a comment and a link to why it is wrong, I am always ready to learn if there is new research and correct any answer or delete it. $\endgroup$
    – anna v
    Commented Feb 2, 2019 at 5:30
  • $\begingroup$ I found this to be helpful and correct and I like the image which gives a good summary. I think the date in the text should be 380,000 not 280,000 years. $\endgroup$ Commented Mar 14, 2019 at 16:54
  • $\begingroup$ @AndrewSteane thanks,I corrected it and gave the link whence I took it but miss-copied it. $\endgroup$
    – anna v
    Commented Mar 14, 2019 at 17:46

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