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More about light energy loss

  1. Apr 15, 2005 #1
    could the lost energy from light cause the expansion of space?
    i know we have no evidence that light can lose energy but over a long distance and i mean like a Mpc or more. TO me this almost seems like a chicken and and egg question.
     
  2. jcsd
  3. Apr 15, 2005 #2

    Garth

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    No, in the Friedmann GR dust model, i.e. one with no pressure, the universe expands perfectly well without any light or CMB within it. In fact, as the present density of the CMB is so low compared with the density of matter in the universe, the present universe is a pretty good approximation to that dust model.

    Garth
     
  4. Apr 17, 2005 #3

    SpaceTiger

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    Not to mention the fact that the presence of radiation slows the expansion.
     
  5. Apr 18, 2005 #4
    how does the pressence of radiation slow the expansion?
    or what papers of expeiments have showen this?
     
  6. Apr 18, 2005 #5
    i guess i also ment the acceleration of the expansion. the light energy is the force pushing the universe.
     
  7. Apr 18, 2005 #6
    Is the light energy really the pushing force of the universe? Does anyone have any evidence to show radiation slows the expansion of space?
     
  8. Apr 18, 2005 #7

    ohwilleke

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  9. Apr 18, 2005 #8
    Evidence enough for me. I don't know about the rest of you, but the source looked pretty credible to me.
     
  10. Apr 19, 2005 #9

    Garth

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    Radiation is a form of energy. Energy has a mass equivalent that generates a gravitational field. The gravitational field slows the expansion of the universe. Apparent acceleration of that expansion requires negative energy, hence Dark Energy, if you can stomach it!

    Garth
     
  11. Apr 20, 2005 #10
    no i think dark energy is a very bad idea it is made up just to satisfiy one condition if this energy was so dominent then it would affect every aspect of the universe yet thie expansion is only seen were gravity is not abundtant. if you show me dark energy ill belive
    it
    fish
     
  12. Apr 20, 2005 #11

    SpaceTiger

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    That was what many people thought when dark matter was proposed, but now all observations seem to support its existence (despite the lack of a direct detection). I agree that dark energy is fairly ad hoc and I think very few scientists would disregard intelligent discourse on alternative explanations for the acceleration (in fact, they have been entertaining such notions of late).

    Also, keep in mind that "dark energy" is a fairly vague description and there are many vastly different theoretical phenomena that could be said to fall into that category. I'd say it's currently little more than a parameterization for our ignorance.
     
  13. Apr 20, 2005 #12
    Since the main evidence for dark matter comes from the shape of galactic rotation curves, other explanations could be that gravity does not work as we think or that gravity is not the only force determining the dynamics of the galaxy.

    Axions and neutralinos are the most promising but magnetic fields and filamentation cannot be ruled out.
     
  14. Apr 20, 2005 #13

    SpaceTiger

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    No, that was the first evidence for dark matter. Your information is about 20 years out of date. Since then, the CDM paradigm has been able to explain large scale structure, the power spectrum, lensing results, and the CMB. These things are all explained naturally with dark matter, while MOND has been struggling to restructure itself with each new observation. Until dark matter is directly detected, we won't be sure, but the case is getting stronger and stronger with time.
     
  15. Apr 20, 2005 #14
    So basically what you are saying, Tiger, is that there is credible evidence to support the theory of dark matter. Right?
     
  16. Apr 20, 2005 #15
    The evidence is indeed gravitational. The most promising present candidates are axions, though there's a probability that gravity does not work as we think it does.

    Here's one great paper about the topic.
     
    Last edited: Apr 20, 2005
  17. Apr 20, 2005 #16
    Can I ask what axions are?
     
  18. Apr 20, 2005 #17
    Axions are hypothetical elementary particles proposed to explain the absence of an electrical dipole moment for the neutron. Axions has no electric charge, no spin, and interact with ordinary matter (electrons, photons, quarks, etc.) only very weakly.
     
  19. Apr 20, 2005 #18

    SpaceTiger

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    The evidence is gravitational, but that doesn't mean that it's consistent with another gravitational law. That's an important distinction. MOND can be tweaked to explain the observations, but it's very ad hoc.
     
  20. Apr 20, 2005 #19

    SpaceTiger

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    That's right.
     
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