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Acoustic wave and CMB

  1. Jun 14, 2013 #1
    Dear all,

    many people refer to the CMB anisotropy picture by WMAP or Plack and call it acoustic waves. I thought there is the anisotropy of the CMB radiation plotted, what exactly can I imagine to be the acoustic wave of it?

    Secondly, what exactly is plotted n the famous plot where the wave number l is on the x-axis and I got those 7 peaks, where observers fit the whole cosmological parameters.

    thank you very much
  2. jcsd
  3. Jun 14, 2013 #2


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    In short, both explanation are correct. The temperature of the CMB is not homogeneous. This is because there were perturbations in the spacial curvature in the early universe that acted like small potential wells. The early universe was filled with baryons and photons, and it was hot enough that the photons were tightly coupled to the baryons: as they baryons tried to fall into the potential wells, the photons would push back out (photons essentially don't like to clump). This back and forth -- gravity pulling the baryons in, photons pushing them out -- resulted in compressional waves in the baryon-photon plasma.

    Then the photons decouple when the universe cools a bit below the binding energy of hydrogen. Now free, the photons stream across the universe comprising what we measure today as the CMB. Because these photons are (more or less) undisturbed since way back when they decoupled, they retain the signatures of the oscillations -- photons that were in the middle of a compression when they decoupled are hot, those that were in a rarefaction are cold(er).

    The CMB arrives on Earth today appearing to have been emitted from the inside surface of a giant sphere with Earth at the center (the surface of last scattering). Due to the inhomogeneities, the temperature of the photons is anisotropic -- hence "temperature anisotropies".

    Now, the CMB is complicated. Indeed, a dominant feature is the acoustic peaks at l > 200 or so. But, there are other things that lead to anisotropies, and it's the job of CMB cosmologists to disentangle all of these effects so that important parameters describing the evolution of the universe can be measured.

    I have to run now, but I'll try to explain more about the famous CMB plot later.
  4. Jun 15, 2013 #3


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    I'll try to drop my own explanation, and try to make it concise: the early universe was a plasma, and sound (acoustic) waves propagated through said plasma. Sound waves are compression waves: some places end up being compressed together, other places stretched apart. The parts that are compressed heat up. The parts that are stretched cool down.

    When we look at the CMB, we see a snapshot of this plasma at the time the plasma cooled into a gas and became transparent. So we see an imprint of just how these acoustic waves had compressed/stretched the very early universe.
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