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B Does red-shifted light cause expansion of space?

  1. May 26, 2017 #1
    Light can get red-shifted due to expansion of space, this leads to a loss of energy.
    Would it be possible that this energy is what's causing the universe to expand in the first place?
     
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  3. May 26, 2017 #2

    Drakkith

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    It is not. The high energy density of the very early universe worked to stop expansion, slowing it down. The reduction of this energy density allows the expansion to accelerate (since normal energy density counteracts dark energy), but it is not its cause.
     
  4. May 26, 2017 #3
    As I understand it: The high energy density holds the universe from expanding. We've crossed a line at which the density is not enough to hold the universe from expanding and thus it expands and cause the energy density to become even less. Correct?
     
    Last edited: May 26, 2017
  5. May 26, 2017 #4

    phinds

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    It is possible that you are laboring under the misconception that because the light, as it redshifts, loses energy, this energy has to GO somewhere due to "conservation of energy". The reason this is a misconception is that there there IS no "conservation of energy" on cosmological scales. This seems counter-intuitive when you first encounter it but it is a fundamental part of cosmology.
     
  6. May 26, 2017 #5
    Is it really proven? I mean, how would you know?
     
  7. May 26, 2017 #6

    phinds

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    I don't think "proven" even applies here. How would you "prove" a negative? Energy on cosmological scales is frame dependent. That means that an increase in energy in one frame could be seen as a decrease of energy in another frame. What would "conservation" even mean in those circumstances?

    http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
     
  8. May 26, 2017 #7

    Drakkith

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    The universe is already expanding and has been for 13 billion years. What I think you're talking about is whether or not the universe will continue to expand forever, or whether it will eventually collapse. Evidence currently points towards the former.
     
  9. May 26, 2017 #8

    phinds

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    @Addez123 you clearly have many misconceptions about cosmology. That's perfectly normal but the best way to fix that is to get a beginner's book on basic cosmology and read it. Asking random questions on an internet forum is a poor way to learn what needs to be approached systematically.
     
  10. May 26, 2017 #9
    I would if I had time. I find these things interessting but I don't have time to study it atm. Top prio of studying, outside school, would be quantum physics tho.
     
  11. May 26, 2017 #10

    PAllen

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    In some sense, the situation is even worse. For cosmology, you cannot even consistently define total energy. In GR, you need asymptotic flatness for a global definition of total energy, and cosmological solutions do not have this property. Thus, you don't even get to ask about conservation, because you can't define total energy in the first place.
     
  12. May 26, 2017 #11

    PeterDonis

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    Technically, asymptotic flatness is what you need to define the ADM energy or the Bondi energy. There is also the Komar energy, which requires only that the spacetime be stationary. But cosmological solutions don't have that property either.
     
  13. May 27, 2017 #12

    Orodruin

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    Translation to B-level: Spacetime has some properties that make it difficult to even define what "total energy" means. There are several options that all place different requirements on the spacetime in order for the "total energy" to be defined. The current model that we have for cosmology does not satisfy any of those requirements.
     
  14. May 27, 2017 #13
    E = mc^2 and E = hc/λ isn't valid in cosmology?
     
  15. May 27, 2017 #14

    Drakkith

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    Neither of those give you the total energy of the entire universe.
     
  16. May 27, 2017 #15

    Vanadium 50

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    So your position is that because you don't want to put in the time to read a book, we have to more of spend out time correcting your misconceptions rewriting what is in those books? Just how much less valuable do you think our time is than yours? People have been a lot more polite to you than I would have expected, given this lack of respect for their time.

    Phinds gave you some very good advice. You will learn more by taking it than ignoring it.
     
  17. May 27, 2017 #16

    Nugatory

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    They are both valid in cosmology.

    Much of this is explained in the link that Drakkith provided in post #7 of this thread.
     
  18. May 27, 2017 #17

    Drakkith

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    You mean the link phinds provided in post #6?
     
  19. May 27, 2017 #18

    phinds

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    What a horrible thought that people can't tell us apart ?:)o0)
     
  20. May 27, 2017 #19

    Nugatory

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    :) Sorry about that
     
  21. May 28, 2017 #20
    One last question then.
    Red-shifting occurs when universe expand, this leads to "loss" of energy. But if the universe were to contract the light would be blue-shifted and no energy be lost. So isn't it obeying the conseration of energy then?
     
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