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A Could the CBM be Dark Energy?

  1. Dec 24, 2016 #1
    Is it possible that the cosmic microwave background is actually evidence of dark energy, and is driving the accelerating expansion of the universe?

    It seems that CMB has - and dark energy is hypothesised to have - similar properties of being uniform and all-pervasive; perhaps the theoretical implications of the observed CMB have been under-interpreted so far?

    Or not.
  2. jcsd
  3. Dec 24, 2016 #2


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    No. The CMB is a radiation component in the Universe, it has positive pressure. Dark energy by definition has negative pressure. Furthermore, the CMB does not even come close to having a large enough energy density.
  4. Dec 24, 2016 #3
    Doesn't dark energy have a very low energy density?
  5. Dec 24, 2016 #4


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    No, it's over twice as high as the density of all matter (baryonic and dark). Radiation density at this epoch in the evolution of the universe is negligibly small.

    It would be good for you to reexamine where the perception of low DE density came from.
  6. Dec 26, 2016 #5


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    The CMB contributes to our understanding of dark energy, but isn't affected by it directly all that much.

    Dark energy has an energy density that is close to constant with time. But matter dilutes as galaxies move away from one another. Back when the CMB was emitted, the universe's average matter density was about a billion times its current value. Thus the dark energy density at the time was completely inconsequential compared to the matter density.

    However, in the later universe dark energy has a subtle effect on how large structures (like galaxy clusters and larger) evolve over time. This has the effect of increasing the CMB temperature differences between places on the sky separated by large distances (regions of the universe with more matter pick up a small blueshift, while regions of the universe with less pick up a small redshift).

    Dark energy also affects the geometry: the CMB shows a universe that is very nearly spatially flat, but the current expansion rate is too small compared to the current matter density to make the universe spatially flat. Dark energy fills the gap.
  7. Dec 27, 2016 #6
    Quick question. Is it approximately 2.7 times denser? Just checking to make sure my math is close.
  8. Dec 27, 2016 #7


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    Just remember what it means when you say that. This is comparing universal-scale energy densities.
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