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How could inflation interact with recombination radiation?

  1. Mar 18, 2014 #1
    Okay, I will be the one in the class that asks the questions that some of us are afraid to ask but still want to know.

    How could the light from recombination, which occurred about 380K years after the Big Bang, interact with the gravitational waves generated from inflation, which occurred instants after the Big Bang, in order to produce the CMB polarization that was announced?

    There probably is a simple explanation, but all of the new releases and responses I read kind of gloss over that portion of it.
  2. jcsd
  3. Mar 18, 2014 #2
    From the following link, section 2,


    "The amplitude of the gravity wave is proportional to the expansion rate H during inflation, which in turn is proportional to the inflation energy scale squared:

    GW amplitude ∝ H ∝ Einf^2, where Einf~<10^16GeV "

    So after all that time the amplitude died down but did not completely go away?

    See also,

  4. Mar 18, 2014 #3


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    Gravitational waves are generated continuously during inflation. These gravitational waves are created from the vacuum and stretched by the inflating spacetime to extremely large scales. Since this is happening continuously, when inflation ends, you have a spectrum of gravitational waves, existing across a range of wavelengths: those that were created earliest have been stretched the most and are of the longest wavelength, and conversely for relatively young gravitational waves. When inflation ends, it dumps all its energy into radiation and matter -- so-called reheating. Immediately, this radiation can begin interacting with the gravitational waves set up by inflation. It takes a while for gravitational waves to die away: those that were stretched the most have been stretched so large that they have a wavelength larger than the observable universe! Such waves cannot evolve because parts of the wave are acausally separated from other parts of the wave: they must wait for the observable universe (which is growing all the while) to catch up to them. Once they fall within the observable universe, they begin to evolve, redshifting away with the expansion. So it's primarily these "super-horizon" gravitational waves that interact with the baryon-photon plasma before recombination; once the photons are liberated at recombination, they carry away the characteristic B-mode polarization -- proof that they once rubbed elbows with the elusive gravitational waves.

    (Incidentally, since the gravitational waves redshift once they "fall" within the horizon of the observable universe, we don't see B-mode polarization on scales corresponding to and smaller than the horizon size at the time of recombination.)
  5. Mar 18, 2014 #4
    What are typical wavelengths for the gravitational waves that produce the B-modes at the time of recombination?
  6. Mar 18, 2014 #5


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    Wavelengths of order today's horizon size down to the horizon size at decoupling.
  7. Mar 18, 2014 #6
    If recombination is relatively fast does that mean that the gravity waves don't change much and are basically static during recombination?

    Thank you!
  8. Mar 18, 2014 #7


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    Yes. Although, at recombination all the gravity waves that are relevant to the B-mode polarization signal are superhorizon-sized and not evolving anyway!
  9. Mar 18, 2014 #8
    Making more sense, thanks!
  10. Mar 18, 2014 #9
    Wow, not as simple as I thought, but still simple enough to understand. Thanks!
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