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I How do we measure energy density of radiation?

  1. Mar 14, 2017 #1
    I have that the energy density of matter approx 0.3 , so this is measured by galaxy motion / gravitational lensing etc, that's fine.

    I have that the energy density due to the vacuum is 0.7, so this is measured by the expansion rate- the common suspect being dark energy.

    I have radiation contribution approx 10^{-4}. How is this measured? Or is this theoretical?

    Thanks
     
  2. jcsd
  3. Mar 14, 2017 #2

    Chalnoth

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    The radiation density is the energy density of the CMB, and has been measured directly.
     
  4. Mar 14, 2017 #3
    nothing else contributes? like when a star explodes, the EM radiation given off then e.g?
     
  5. Mar 14, 2017 #4

    Chalnoth

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    Not to any significant degree. The CMB comprises nearly all of the radiation ever emitted, by energy density.
     
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