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I Observation of photons from CMB by different observers

  1. Apr 1, 2017 #1
    If two observers on earth in different locations around the globe, were both viewing the CMB with their equipment pointing at the same point in the sky, and charting the fluctuations in it, would they correspond or vary from each other greatly. In other words, if you made a graph of the CMB and overlayed them, would they correspond identically one to the other? (
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  3. Apr 1, 2017 #2

    Vanadium 50

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    Yes. The variations in the CMB are in the sky.
  4. Apr 2, 2017 #3


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    The OP means looking at the same point in the sky.

    They would differ in the same sense that an image of e.g. the Andromeda galaxy differs from one observer to the other due to signal reception delay equal to their separation divided by the speed of light.
    So, let's say you have one observer on the equator, and the other on one of the poles, so that the distance to CMBR differs by ~6Mm. Then the observer on the pole sees the CMBR as it were some 0.02 seconds earlier. Since you're looking at a rather large 'object' - the sphere of CMBR being approx 42 Mly across at emission, each degree of angular size corresponds to ~700 kly - the evolution of fluctuations during those 0.02 seconds is pretty much impossible to measure (even though the speed of sound in the pre-recombination plasma is something like 0.6c).
    Last edited: Apr 2, 2017
  5. Apr 2, 2017 #4
    Are you saying that at the same simultaneous moment (if we checked the time on an atomic clock for example) that they would be different merely because the same signal that's being observed takes longer to get to one observer as opposed to the other? But if we compensate for the time difference, would we then be seeing the same thing? What I am looking for is a fluctuating signal and can be seen from two points widely separated that is reliably present that is not a point source such as a star. Also, it may help to understand that I am not wanting to view an actual single point in the sky, but a region, where both observers are viewing the same region, the center of which would be the same point.
  6. Apr 3, 2017 #5


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    They're not. The separation is negligible when compared to the size of the observed object.
    When you're looking at the CMBR you see fluctuations in density developed over 380 000 years from initial homogeneity.
    Those fluctuations are huge objects - thousands of light years across.

    So let's say you're looking at a single density fluctuation in spherical volume of plasma 100 000 ly in radius (at emission), whose presently observed temperature varies by 50 μK from edge to the centre. Assuming that temperature gradient is a straight slope*, two observers separated radially by 6000 km (~Earth's radius) = ~6E-10 ly comparing their observation made at the same time can see a variation in temperature of 3E-19 K. Completely negligible.

    *it isn't, but we're making order of magnitude calculations so we don't care
  7. Apr 3, 2017 #6
    Thank you for taking the time to answer my questions.
  8. Apr 3, 2017 #7


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    Keep in mind the last scattering surface [LSS] is severely time dilated. Even from a fairly distant galaxy the LSS will not look very much different than it does to us.
  9. Apr 6, 2017 #8


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    The CMB has variations with a typical "size" of 1 degree, or ~700,000 light years in the early universe. This is now stretched to about 800 million light years. If you go that far away, you will see a very different CMB spectrum. You can probably note a different spectrum for two observers 100 million light years apart. But not with two observers 10-9 light years apart.
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