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B Why does expansion redshift light if...

  1. Feb 19, 2017 #1
    isn't expansion essentially making more space rather than moveing us in it? So why does it redshif light?

    Or is that just a super facile explanation of expansion that they explain people like me who wouldn't get it?...
     
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  3. Feb 19, 2017 #2

    phinds

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    No.

    It carries photons away from us (slightly) as they are traveling towards us. Since they always travel at c, they are traveling at c when they reach us but because of the expansion's effect, they are red shifted.

    No, it is an exact description of what is happening.

    EDIT: One of the things that will presumably occur to you eventually, if it has not already, is the following. The photons start out at c at a certain frequency. They get to us still traveling at c but at a lower frequency. This CLEARLY says that they lost energy somewhere along the way. OK, since energy has to be conserved, where does that energy GO ???

    The answer is that is doesn't "go" anywhere, it just disappears, and this is OK because "conservation of energy" is a local effect and does not apply on a cosmological scale. This is one of the many things that made me tear my hair out and say bad words when I first encountered it.
     
  4. Feb 19, 2017 #3
    Ok.. I'm grabing clumps of hair ready to tear. Sooo is it essentially, literally streatching the photons' wave frequency as space is added/streached(?)?

    Was that just a totaly nonsense question?
     
  5. Feb 19, 2017 #4

    PeterDonis

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    No. The redshift of photons with the expansion of the universe is due to spacetime curvature. In a curved spacetime, photons can redshift (or in some cases blueshift) because of the curvature. More precisely, a photon traveling between two observers in a curved spacetime can redshift (or blueshift) depending on the relationship between the worldlines and the geometry of the curved spacetime.
     
  6. Feb 19, 2017 #5
    riiight.. So it's a bit like those depictions of gravity where the grid stretches down in to a gravity well except.. Everywhere at once, and not 2D, and it's the bending of those lines that shifts the frequency.. Am i anywhere close with that?
     
  7. Feb 19, 2017 #6

    PeterDonis

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    Not really, because the "grid lines" you are thinking of are coordinate dependent, and the redshift/blueshift of photons is not; it is there regardless of the coordinates you choose (which means regardless of how you draw "grid lines" in spacetime).
     
  8. Feb 19, 2017 #7
    that's what i meant by "everywhere at once".. I think..?
    anyway, wouldn't that just result in a uniformed shift that wouldn't tell us anything?
     
  9. Feb 19, 2017 #8

    PeterDonis

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    I don't see how.

    I don't understand what you are referring to here. The redshift/blueshift of photons being independent of your choice of coordinates does not mean it's the same for all photons and all observers. It just means that for a given photon and a given observer, the redshift/blueshift is the same regardless of what coordinates you choose to describe it. But your description in terms of "space stretching" only applies in a particular set of coordinates.
     
  10. Feb 20, 2017 #9

    m k

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    Is your "more space" more spacetime, distance or something else?

    Redshift is a name of something that is observated, just like (cosmological)expansion and (energy)conservation.
    For now they don't add up.
     
  11. Feb 20, 2017 #10

    phinds

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    What do you mean "for now they don't add up" ? They will NEVER add up since it is known that energy conservation does not apply on cosmological scales.
     
  12. Feb 20, 2017 #11

    m k

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    As I understand "known" I wouldn't say so.
     
  13. Feb 20, 2017 #12

    phinds

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    Well, then you should study some cosmology.
     
  14. Feb 20, 2017 #13
    Yes.
    Because the speed of light is constant, while wavelength depends on gravity and/or dark energy ... *insert math stuff here*
     
    Last edited: Feb 20, 2017
  15. Feb 22, 2017 #14
    If the light wave is traveling through space, and you stretch out space, then you stretch out the wavelength of the wave. The distance between two peaks or troughs has increased.
     
  16. Feb 23, 2017 #15

    m k

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    I have now studied some of it and learned that we know quite a bit of few percents of it and a bit less of the rest.

    Still wouldn't say so.

    Should you now study some definition [can't include maths here] fine tuning?
     
  17. Feb 23, 2017 #16

    phinds

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    Then you still have not come to terms with the fact that while yes indeed there are tons of things we know little about, energy conservation on cosmological scales is not one of them. There is no energy conservation on cosmological scales and if you continue to insist that there is, you are simply ignoring reality.
     
  18. Feb 23, 2017 #17

    PeterDonis

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    This is not a fair statement of our current theories of cosmology.

    Then you have not understood what you have studied.

    In any case, the OP's question has been answered. Thread closed.
     
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