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Some thoughts concerning backgroun radiation

  1. Jul 11, 2010 #1
    Concider a patch of the universe where there is only negligible gravitational curvature. In this patch, SR applies. We know therefore that there is no preferred intertial motion there.

    But with background radiation taken into the picture, that seems not to be the case anymore. I've read the phrase "being at rest wrt the Universe's expansion", which I take to mean a state where the background radiation is identical in intensity everywhere (roughly).

    If we are not in such a state, there is a direction from which the intensity is greater then other directions, and this should act as a sort of "intrinsic friction" of the universe. Of course, today such radiation is negligible, but earlier in the Universe's lifetime it was considerable.
  2. jcsd
  3. Jul 11, 2010 #2


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    Yes, you can be at rest wrt the CMB. That doesn't violate SR if that's what you're asking.
  4. Jul 11, 2010 #3
    Yes Espen, you are quite right, no doubt about it. The only thing is that radiation pressure difference would be small. For example, for body moving at 0.999 c, at times when Tcmb= 100 K, pressure difference would be around 0.8 Micro Pa. Pressure goes up with 4th power of temperature, so for Tcmb=3000 K (very early times) it would be around 0.7 Pa.
  5. Jul 11, 2010 #4
    That was clarifying, thanks! So the effect was never very significant.

    No, nothing like that. I was just letting out some ideas.
  6. Jul 12, 2010 #5


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    The background radiation is hotter in one direction - the past. When the universe was younger, the CMB termperature was greater. This has been observationally confirmed by more than one study [e.g., http://www.spaceref.com/news/viewpr.html?pid=3414]. The temperature difference matches BBT predictions.
  7. Jul 12, 2010 #6
    Yes but this is only valid if the cosmological constant is zero.
  8. Jul 13, 2010 #7


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    You are missing the big picture, passionflower.
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