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Why can we see the CMB in every direction we look?

  1. Apr 30, 2014 #1
    Hi guys,

    As you can probably tell this is my first post, so I thought I ask a question that I've had trouble getting my head round. I think I know the reason but I'm not a 100% certain.

    Why is it that we see the CMB radiation from every direction we look? I would really appreciate if someone could explain this to me, even better if there was a diagram to go along with it to help me understand our position in the universe.

    Thanks in advance.
  2. jcsd
  3. Apr 30, 2014 #2


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    Hi cosmictde, welcome to PF!

    Imagine a large volume of space, perhaps even infinite if you can do that. Now, imagine that at a certain point in time each and every point in that space emits a photon in some random direction.

    Wait some time for the photons to travel around in straight lines.

    Now pick a random point in the volume of space. Imagine it's the Earth, and look around.

    There will be photons coming at you from every direction. Wait some more. The photons you saw previously had already passed you by, but you still see other photons that only now had the chance to reach you. It'll go on like that forever, whichever point in space you choose to look from.

    What you should gather from the above, is that the emission of Cosmic Background Radiation was an event that happened everywhere at a single point in time, rather than an even that happened somewhere in particular.
  4. Apr 30, 2014 #3


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  5. Apr 30, 2014 #4
    Thank you so much. That makes things much more clear.
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