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Higher peaks and damping tail

  1. Sep 30, 2015 #1
    Hello,can someone tell me the physical process ,which occurs before recombination, which is giving the higher peaks and the damping tail at cmb power spectrum?
  2. jcsd
  3. Sep 30, 2015 #2


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    The first (longest-wavelength) peak is a distance scale where matter had just enough time to fall into a gravitational potential well of that length scale. The second (shorter wavelength) peak is matter that had enough time to fall in and then, due to pressure, bounce back out. The third is matter that had the time to fall in, bounce out, then fall back in again.

    Dark matter only contributes to the odd-numbered peaks, as it doesn't experience pressure.

    The damping tail is due to a different effect: the CMB wasn't emitted instantaneously. It took time for the plasma in the early universe to transition to a gas, and this causes our image of the CMB to be blurry. Instead of seeing a crisp slice of the early-universe plasma, we see a thick, cloudy surface. This manifests by suppressing the short-wavelength signal.
  4. Sep 30, 2015 #3
    why shorter wavelengths in the damping tail have so low contribution in the power spectrum?the amplitude of temprature variations of second and third peak is reducing because gravity and pressure is out of phase?
  5. Sep 30, 2015 #4


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    About even and odd peaks, Wayne Hu (U Chicago) has a webpage
    Remember what happens when you add mass to a spring and let it fall in the gravitational field of the Earth. With more mass loading the spring, it falls further before pulled back by the spring. On the other hand, it rebounds to the same position it started from.

    Since the odd numbered (first, third, fifth...) acoustic peaks are associated with how far the plasma "falls" into gravitational potential wells (how much the plasma compresses), they are enhanced by an increase in the amount of baryons in the universe. The even numbered peaks (second, fourth, sixth) are associated with how far the plasma "rebounds" (how much the plasma rarefies). Thus with the addition of baryons the odd peaks are enhanced over the even peaks. For example, baryons make the first acoustic peak much larger than the second. The more baryons the more the second peak is relatively suppressed.
  6. Sep 30, 2015 #5


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    As I said above, the reason the shorter-wavelength peaks have smaller amplitudes is because our image of the surface of last scattering is blurry (the surface of last scattering is the matter that emitted the CMB).
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