Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Confused about cmb decoupling/recombination

  1. Jan 15, 2010 #1
    I went to wikipedia to look up decoupling and ended up getting a pretty crappy result

    "In physical cosmology, the term decoupling is often used for the moment during recombination when the rate of Compton scattering became slower than the expansion of the universe. At that moment, photons nearly stopped their interactions with charged matter and "decoupled", producing the cosmic microwave background radiation as we know it. The term decoupling is also used to describe the neutrino decoupling which occurred about one second after the Big Bang. Analogous to the decoupling of photons, neutrinos decoupled when the rate of weak interactions between neutrinos and other forms of matter dropped below the rate of expansion of the universe, which produced a cosmic neutrino background."

    could someone answer me this; when it says "photons nearly stopped their interactions with charged matter and "decoupled" " what does it mean nearly,in reference to the cmb,has the cmb completely stopped interacting with matter?is it possible for it to be red/blue shifted enough to be absorbed by any matter at all? do the photons travel undisturbed completely? im just thinking this,if the cmb cant interact with matter in any way then how do we detect it?
    Last edited: Jan 15, 2010
  2. jcsd
  3. Jan 15, 2010 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I studied this a long time ago and have only a vague memory of the math, but I can offer a bit of intuition. Someone else may fill in the math. This is just a preliminary response.
    I think they should have said Thomson scattering.

    For starters just think of a hot expanding cloud of hydrogen, partially ionized. If you like, think about the mean free path of a photon.

    You know that at 4000 kelvin, the cloud would be opaque, like the 4000 kelvin gas at the surface of a star. Stars are not transparent because they are hot, partially ionized.
    You can calculate the mean free path, by knowing the density of the gas and the percentage ionized. Notice that the percentage ionized depends on the temperature.

    Now the cloud expands and cools to 3000 kelvin. At this point (given the estimated density of our universe at that time) the mean free path (MFP) blows up, becomes infinite, by a curious mechanism. If the gas weren't expanding then there would be a finite MFP. Every photon would eventually hit a charged particle. But now, at this critical temperature/density the current MFP is just long enough so that by the time (say 1000 years) the photon would have hit a particle the gas has expanded some more and is less dense and lower temperature and thus less ionized. So the photon gets a new lease on life. Now it has a larger MFP. And by the time that would have run out, expansion and cooling have given it a still longer MFP.

    It is like a guy who hopes to incrementally live forever if he can just make it until medical science improves to the next stage, and then to the next stage....and so on. Successively finding cures for all terminal ailments.
    Only this time, for the photon, it really works.

    Now I just considered a pure hydrogen cloud. But other partially ionized species don't essentiall change things. And when it comes to DUST and STARS and stuff that the photon might hit. Well those things are very sparse. You can sort of neglect the photon having fatal accidents with that stuff. Of course accidents happen, but the MFP is still effectively infinite.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook