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Baryon acoustic oscillations and sound horizon

  1. Jan 4, 2015 #1
    I'm having some trouble understand this correctly, so I was hoping someone could enlighten me a bit :)

    Okay, so in the early Universe most of the hydrogen and helium was formed, and then kept in equilibrium, and ionized via photons. So we have a plasma of baryonic matter, including dark matter, and photons. Now, due to fluctuations, matter "clumps" up via gravity and eventually the contraction is so large, that photons, that are a part of this plasma, begins to press against the matter, due to radiation pressure. This in turn gives a kind of "sound wave" that propagates outward.

    So, what confuses me is, that the sound horizon is supposed to be the largest "sound wave", that apparently comes from this plasma compresses once, and then until recombination sets in. So when recombination sets in, the outward pressure from the photons stops, and so does the wall of the "sound wave", and then we have dark matter in the middle, and the rest of the matter in the walls, which is the pulled inwards due to the dark matter - and this is then our "sound horizon", which can be seen as the first acoustic peak in the power spectrum of the CMB.

    But does other regions not compress? Or is it only one place in the "cavity" of the Universe at that point?
    I mean. Are there a lot of "sound waves" inside the Universe at this time, or just one?

    And then some extra questions, regarding the same thing. It is said, that if you have a galaxy, then if you travel the distance of the sound horizon, you should see more galaxies that usual, due to the fact, that it was more dense in these regions. But is that for all galaxies? Are every galaxy you encounter one "sound horizon" away, or how does that come about?

    Basically, I understand how the "sound waves" are formed. But not what they do actually. Why is the CMB coming from the large "sound wave" and not other "sound waves". Is that because there only is ONE "sound wave", or...?

    I know, a bit confusing, and a lot of questions. But it's such a weird thing in my mind :)
     
  2. jcsd
  3. Jan 4, 2015 #2
    I find it confusing too, not least because I have worked through so little of it yet. While we wait for more knowledgeable commenters, let me look into some easy references and see what I can respond to.

    Baryon acoustic oscillations are:

    - created around overdense regions in the early universe, and released at decoupling. Therefore the cavities are local (and now gone).

    "... the center of the sound wave, the origin of the overdensity. Before decoupling, the photons and baryons move outwards together. After decoupling the photons are no longer interacting with the baryonic matter so they diffuse away. This relieves the pressure on the system, leaving a shell of baryonic matter at a fixed radius. This radius is often referred to as the sound horizon.[3] Without the photo-baryon pressure driving the system outwards, the only remaining force on the baryons is gravitational. Therefore, the baryons and dark matter (still at the center of the perturbation) form a configuration which includes overdensities of matter both at the original site of the anisotropy and in a shell at the sound horizon."

    [ http://en.wikipedia.org/wiki/Baryon_acoustic_oscillations ]

    - many and so statistical in nature.

    "... the universe is not composed of one sound ripple. ... one can measure this signal statistically by looking at the separations of large numbers of galaxies."

    [Ibid]

    The cosmic microwave background is correlated to the baryon acustic oscillations by them (and damping) deciding its structure.

    "The peaks correspond, roughly, to resonances in which the photons decouple when a particular mode is at its peak amplitude."

    [ http://en.wikipedia.org/wiki/Cosmic_microwave_background ]

    The CMB is a thermal (and polarization) imprint of the initial over- and underdense regions that comes out of quantum fluctuations in the inflation field. For reasons of the inflation expansion overtaking the then horizon the earliest (largest) fluctuations are "frozen in" and later released as inflation stops.

    So we look at these regions as a spatial spectrum. (Observed by looking at the circular patches the regions cover.) The largest mode (peak) has its height decided by the total energy density in the universe, which I believe is inherited from the inflation field. The second mode has its height decided by the baryon densities. The third mode sums both baryon and dark matter densities (which is why it is so large). And then there are more effects as we traverse the spectra.

    As I remember it (and my computer is acting up, so no checks today), the test of the correspondence between BAO observations and the WMAP LCDM model was done with the full model. But Planck has such resolution that they can predict the BAO from spectral features too. (Which is used to improve the overall model.)
     
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