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Last scatter by e-, light looks hotter, cooler, time, wavelength.

  1. Mar 17, 2014 #1
    Over what approximate time frame does 90% (most) of last scattering occur, minutes, seconds, fraction of a second?

    How does this time frame relate to the period of the gravitational radiation?

    If the period of gravitational radiation were short it would washout b-modes?

    Thanks for any help!
  2. jcsd
  3. Mar 18, 2014 #2


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    What scattering are you talking about?
  4. Mar 18, 2014 #3
    When the Universe became transparent to light from the afterglow, light no longer scattered.
  5. Mar 18, 2014 #4


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    The surface of last scattering is quite thick in time. If I recall correctly, it took around 300,000 years for our universe to transition fully from a plasma to a gas. This is close to the age of our universe at the time this transition began.

    In practical terms, this causes the smaller scales on the CMB to become blurred, which is why the CMB power spectrum gets smaller and smaller at high multipoles.
  6. Mar 18, 2014 #5
    Wiki is a little vague. From wiki, Chronology of the universe, Recombination


    ... As the universe cools down, the electrons get captured by the ions, forming electrically neutral atoms. This process is relatively fast (actually faster for the helium than for the hydrogen) and is known as recombination. ...

    From reading about how B-modes were produced I thought I had an idea about what was going on from the following picture, see also below.




    If we had gravitational radiation effecting hydrogen as it recombined it seemed that for every atom that got the polarization enhanced one way there would be another atom with polarization the other way if the gravitational wave were "waving". It seems like the effect that produced B-modes should all get averaged to zero unless the period of the gravitational wave was longer or comparable to the time for most of the plasma to neutralize?

    I hope my confusion is clear? Thank you for your help!

    Attached Files:

  7. Mar 18, 2014 #6
  8. Mar 18, 2014 #7


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    The B-mode polarization signal peaks at about 2 degrees across the sky. At the time, this was a distance of about 1.5 million light years. So yes, the oscillation period was clearly greater than the time that it took for the plasma to neutralize.
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