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Redshift and Expansion of the Universe

  1. May 21, 2012 #1
    So, IIRC, the big deal about dark energy is that the universe appears to be expanding.
    How do we know the universe is expanding?
    As far as I know (and that's not much), we know because of redshifting of light on the spectrum as it moves away, as per the Doppler effect, which has been observed in distant galaxies and stars.

    If I'm not mistaken, then, the single bit of evidence we have for the expansion of the universe, is that one physical phenomenon. But, what if light redshifts as it travels?

    Would naturally redshifting light as it travels through a vacuum violate any known laws or observations, and would it explain the mysterious apparent acceleration of the expansion of the universe?
  2. jcsd
  3. May 21, 2012 #2


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    There is no physics to have light red shift in a vacuum. Also it would violate conservation of energy. Finally there is plenty of evidence that it doesn't happen.
  4. May 21, 2012 #3


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    The so-called "tired light" theory was roundly discredited years ago.
  5. May 23, 2012 #4
    Ah, alright, those responses were just the thing I was looking for. It was just kinda a thought that was bothering me, thanks for the replies.
  6. May 31, 2012 #5
    Mathman - can you explain this for me?

    Are you saying here that light does not redshift as it moves through space?
  7. May 31, 2012 #6
    Classically, light acts as a wave so due to it's wave like nature it travels at different speeds in different density mediums.Vacuum and space are different things altogether.

    P.S: Space is not a perfect vacuum due to various reasons...btw. can anyone clarify for me the following point: Quantum oscillators have a ZPE state , how about a perfect vacuum (which probably doesn't exist) ?
  8. May 31, 2012 #7


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    Yes, he is saying that light does not just redshift in and of itself when traveling through unperturbed space. That's the "tired light" hypothesis that was discredited years ago.

    It DOES get redshifted as it travels through expanding space, but that was not what the OP was asking about. He was espousing "tired light"
  9. May 31, 2012 #8


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    You are confusing doppler redshift with the redshift caused by expanding distances. They are not the same, although I found that a bit hard to get my head around when I first encountered it. Doppler shift happens when two objects are moving relative to each other in the same frame of reference. The expanding universe means they don't have the same frame of reference, so the effect is not a doppler shift. This is not just semantics, because saying that the red shift is due to a doppler effect is the same as saying that the universe it not expanding.
  10. May 31, 2012 #9
    Cheers all.

    I can kind of see the complexity here :)

    Are all the following statements true (just for my benefit)

    The wavelength of the light emitted from a star is "set" at the stars surface as the photons escape. Its wavelength is caused by the motion of a star in a certain direction. At the "front" of the star (in the direction the star is moving), the wavelength will be shorter than that created at the rear of the star (in the direction from where the star came).

    As we orbit the Sun, we observe different wavelengths from it at different times of the year, because the Sun has motion through space in a direction.

    Is all this true?
  11. May 31, 2012 #10
    What matters is its motion through space relative to us, since it doesn't have motion through space. Neither do we, neither does that little green man in a spaceship traveling at .9c relative to us. A star's light can't be said to be redshifted; it can be said to be redshifted with respect to this observer. So when our component of velocity towards the sun is positive, we see a very tiny negligible blueshift, and when our component of velocity towards the sun is negative, we see a very tiny negligible redshift. It's distant galaxies that matter. The distance between us and any arbitrary distant galaxy appears to be increasing, and if you do the math, an observer on that distant galaxy would be in the exact same situation we are, with the same amount of redshift for galaxies the same distance away from them.

    We have more evidence for the BBT than just redshift, we also have the Cosmic Microwave Background Radiation, and General Relativity alone would be enough to suggest a Big Bang.
  12. May 31, 2012 #11
    "since it doesn't have motion through space"

    I am afraid I dont understand that.

    As I learn - I try to create images in my head if thats ok.

    So in terms of your reply - for which I am grateful, how does this sound?

    I am flying through space away from a star, I observe a redshift in the light it is emitting. This is because the wavelength appears to be "stretching".

    If thats correct - why is this?
  13. May 31, 2012 #12
  14. May 31, 2012 #13
    "Change of wavelength because of motion of the source."
    Definately NOTHING to do with the motion of the observer?

    If we knew the wavelength of light from a motionless source, then we would have a baseline I suppose.
  15. May 31, 2012 #14
    It has nothing to do with two nonexistent values. It has to do with the relative motion between the source and the observer.
  16. May 31, 2012 #15
    Can ANYTHING EVER be stationary?

  17. May 31, 2012 #16

    George Jones

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    In special relativity, a source and observer can be in the same inertial reference frame.
  18. May 31, 2012 #17
    Can you just give me a quick example which explains in laymans terms "frame of reference"?
  19. May 31, 2012 #18

    George Jones

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    See the post
  20. May 31, 2012 #19
    Sorry double post I didnt see that reply.
  21. May 31, 2012 #20
    No, both Newtonian/Galilean and Special Relativity quite clearly state that what you just asked means nothing. If you had asked if anything could be stationary with respect to a certain (inertial) reference frame, then the answer would be yes.
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