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Acoustic peaks

  1. Sep 26, 2011 #1
    I am supposed to have a qualitive knowledge of acoustic peaks for my exam, so none of the maths. After reading around I am still left with a few questions.

    Is the first acoustic peak from the acoustic oscillations at last scattering? I imagine there were many oscillations before then but we just won't know about them. The first acoustic peak is about 2 degrees in the sky, which is the same size as the particle horizon since last scattering so it would make sense it happened then. (I used 'particle horizon' meaning the distance photons could have travelled since last scattering, I hope I used it right).

    If this is right: How did the rest of the acoustic peaks get there? The next one is about 0.4 of a degree and that must be waaay past last scattering, so there would be no plasma to oscillate!

    Thanks for any help. And if what I said above doesn't make any sense, don't blame me, blame my useless course text!
    Last edited: Sep 26, 2011
  2. jcsd
  3. Sep 27, 2011 #2


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    Accoustic peaks are radiation shocks in the CMB. Does that help?
  4. Sep 27, 2011 #3
    Nope sorry. Acoustic peaks are due to gravitation vs photon pressure causing plasma oscillations in the early universe (when it was all plasma). Then last scattering happened, so no more plasma to oscillate and and we see the evidence as fluctuations in the CMB (radiation shocks from the oscillating plasma).

    The first peak is 2 degrees in the sky, agreeing with the particle horizon since last scattering. How oh how did the other smaller peaks get there? The second peak covers 0.4 of a degree, anything that happened at last scattering should have spread to 2 degrees in the sky. If it happened after last scattering then what caused the peaks (as there was no plasma to oscillate)?
  5. Sep 27, 2011 #4
    My google searches for answers took me back to my post! That can't be a good sign.

    Thinking further I'm staring to think I'm wrong saying:

    These don't make sense. I doesn't matter what size the particle horizon is SINCE last scattering. It matters what the particle horizon was AT last scattering. So the first peak corresponds to the biggest sound horizon from the earliest oscillation, and the other peaks from later times and their sound horizon won't be as big. Then when last scattering happened they were released at their current size as fluctuations in the CMB.

    Well this makes more sense to me, but it is just me taking a logical informed guess. And it disagree's with some other things I've read. :(
  6. Sep 27, 2011 #5
    Well it seems that these smaller (angle, and hence size) peaks are the harmonics of the big (sound horizon size) oscillations. And their smaller temperature fluctuations is due to diffusion at recombination that thermalized a little which effected the smaller sized peaks more than the bigger.
  7. Sep 27, 2011 #6

    George Jones

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    Right. See, for example,

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