Some thoughts concerning backgroun radiation

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Discussion Overview

The discussion revolves around the implications of background radiation, specifically the Cosmic Microwave Background (CMB), on the concept of being at rest with respect to the universe's expansion. Participants explore how this relates to special relativity (SR) and the effects of radiation pressure in different contexts of the universe's history.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that in regions of negligible gravitational curvature, special relativity applies, suggesting no preferred inertial motion exists.
  • Others argue that the presence of background radiation complicates this notion, as being at rest with respect to the universe's expansion implies a uniform intensity of radiation.
  • One participant mentions that if there is a directional intensity difference in background radiation, it could act as a form of "intrinsic friction" in the universe.
  • Another participant confirms that one can be at rest with respect to the CMB without violating special relativity.
  • Discussion includes calculations of radiation pressure differences at various temperatures of the CMB, indicating that while these differences exist, they are relatively small.
  • One participant notes that the background radiation was hotter in the past, aligning with predictions from Big Bang Theory (BBT) and supported by observational studies.
  • There is a mention that the validity of some claims may depend on the cosmological constant being zero.
  • A later reply suggests that a participant may be overlooking broader implications of the discussion.

Areas of Agreement / Disagreement

Participants generally agree on the existence of background radiation and its historical temperature variations, but there are competing views on its implications for special relativity and the nature of being at rest with respect to the universe's expansion. The discussion remains unresolved regarding the significance of radiation pressure and the conditions under which special relativity holds.

Contextual Notes

Some claims depend on specific assumptions, such as the state of the cosmological constant. The discussion also reflects varying interpretations of the implications of background radiation on motion and inertial frames.

espen180
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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.
 
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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.
 
espen180 said:
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.

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.
 
Calimero said:
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.

That was clarifying, thanks! So the effect was never very significant.

russ_watters said:
Yes, you can be at rest wrt the CMB. That doesn't violate SR if that's what you're asking.

No, nothing like that. I was just letting out some ideas.
 
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.
 
espen180 said:
Concider a patch of the universe where there is only negligible gravitational curvature. In this patch, SR applies.
Yes but this is only valid if the cosmological constant is zero.
 
You are missing the big picture, passionflower.
 

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