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Cooling in Cosmic Microwave Background

  1. Sep 7, 2011 #1
    I am a little confused about how exactly the CMB "works". At first hearing, it makes perfect sense that as time goes on and the universe expands radiation would decrese in frequence, but when I think about it a little more deeply I miss something.
    My thought is how exactly does the light "cool", why does a photon, propagating through space, lose frequency or energy.
    I thought of a photon travelling though a box big enough that it did not hit the sides, and imagine the box "expanding", without any heat input, and I couldn't imagine a mechanism that would make the photon lose its frequency, does it in this case? And if so where does the energy go? Or is this where I am going wrong in thinking of CMB as a set number of photon particles?
     
  2. jcsd
  3. Sep 7, 2011 #2
    As the universe expands the average density of photons contained within that area spread out as there is less overall density decreases. Therefore the overall energy also spreads out resulting in cooling. Temperature being an average energy level in a volume.

    Also the area between particles at the time have more space to move around resulting in less average energy being released as the matter/antimatter annihilations become less frequent the quantity of matter and antimatter in that form also decresed to maintain those reactions.
     
    Last edited: Sep 7, 2011
  4. Sep 7, 2011 #3

    phyzguy

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    In your analogy of the box, only the walls of the box are getting further apart. In our universe, all of space is expanding, not just the boundaries. So the distance between two peaks of the EM field in a light wave are getting further apart as time moves forward. Since the wavelength of the light is increasing with time, the frequency and energy are decreasing with time.
     
  5. Sep 7, 2011 #4

    russ_watters

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    You're not looking at a box, but rather boxes within boxes, all expanding together. So if you look now, you'll see one wall, but if you look again later, you'll see a different wall -- one further away and therefore moving faster and more redshifted.
     
  6. Sep 7, 2011 #5

    russ_watters

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    That's not how temperature works for EM radiation. Only frequency matters, not photon density.
     
  7. Sep 7, 2011 #6

    xts

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    I may give you two pictures:

    1. (from the light's perspective) - the whole universe is expanding. All the distances are increasing. Also distances between wave maxima. Thus, the wavelength rises -> frequency lowers.

    2. (from Earth observer perspective). CMB had been radiated pretty long time ago, so it comes from pretty far parts of the universe. We observe (Hubble's law) that the light coming from distant sources gets shifted towards red (it is an implication of the expanse of the Universe, see http://en.wikipedia.org/wiki/Hubble's_law)
    CMB travelled so long (comes from so distant part of the Universe) that it got shifted really far towards "red" - to the microwaves.
     
    Last edited: Sep 7, 2011
  8. Sep 7, 2011 #7
    Your correct were not measuring Photons I was trying to relate it to the OP's question and should have been more exact. However the case still applies in that all EM forms do have a particle including radiation. Gamma rays are high energy photon rays, microwaves are photon rays with frequency 10^8 to 10^12 beta rays are electrons, alpha rays are the nuclei of of helium atoms,
    Without those particles interactions and frequencies there would be no radiation.
     
  9. Sep 7, 2011 #8
    As a few of the other replies have said, space itself has expanded, stretching the wavelength of those first photons.
     
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