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Mechanism of the cosmological redshift

  1. Sep 10, 2012 #1

    I have read that `cosmological redshift` means a photon is being `stretched`by space dilation and thus gets to have `longer wavelength`

    I am unsure if this is true and if yes, why is it true

    Many thanks
  2. jcsd
  3. Sep 10, 2012 #2


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    It is a good description, and "true" and "why" are something for philosophy.
    However, I think it is quite intuitive in case of photons: You can measure the wavelength. Then you stretch all space, including the space within that wavelength. And then you measure again, and the value increased.
  4. Sep 10, 2012 #3


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    These two papers may be of interest:


    If you want to interpret cosmological redshifts instead as kinematic redshifts, you can. There is no way to resolve the ambiguity in interpretation, because GR can't talk about the velocity of one object relative to another distant object. We have a FAQ about this: https://www.physicsforums.com/showthread.php?t=508610 [Broken]
    Last edited by a moderator: May 6, 2017
  5. Sep 10, 2012 #4
    mfb, as I understand your post, one can consider the photon as a wave propagating through space, having a certain wavelength. Space expansion would then mean stretching of the wavelength of the wave, already positioned in space

    Please help me with this - is this view generally accepted? Thank you
  6. Sep 10, 2012 #5
    If a photon wavelength is being stretched, would atomic nuclei be stretched as well... how would such stretching influence our capability to measure spatial distances, as our rulers would be stretched out too

    These are not final statements, I am only trying to see if I am not missing something
  7. Sep 10, 2012 #6
    This can be a confusing subject because 'distance' in cosmology can be defined in different ways....it varies from model to model according to their 'metric'...the calculated distance. When talking about cosmological distances the messiness of curvature in GR is significant. When you read about redshift you are almost always reading about it in the context of the FLRW cosmological model whose metric [distance measure] is an exact solution of Einstein's equations. And writers are assuming observers static with respect to the cosmic microwave background radiation [CMBR]...you can find descriptions in Wikipedia and these forums.

    The same number of wave crests ...wavelength is from one to the next.....reach us as were emitted. The finite speed of light means it takes longer for all of them to get here. The earliest ones get stretched the most. Those are the 'CMBR' I treferred to emitted very in the time of our universe. The expansion of wavelength correlates pretty much exactly with expansion of distance that occurred while light was in transit.

    This refers to the standard FRW metric and standard cosmological model as does this: The so-called recession "velocity" associated with expansion is actually a 'rapidity' which goes beyond the speed of light 'c'.
  8. Sep 10, 2012 #7
    bcrowell, it seems that the photon stretching concept is not widely accepted according to the 2nd linked paper on arxiv
  9. Sep 10, 2012 #8


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    You're oversimplifying. It's not a question of taking a vote or seeing what is more widely accepted. Neither interpretation is wrong. Our desire to create an intuitive picture clashes with the fact that GR doesn't have tools and concepts that map directly onto these intuitive pictures.
  10. Sep 10, 2012 #9
  11. Sep 10, 2012 #10
    This is the part of the confusion I referred to. Over cosmological distances the light from distant galaxies is redshifted with respect to the frequencies we would observe from an identical nearby stationary source; it arrives here at a different color than an identical ar local source....we observe it as redshifted.

    What is happening is that the metric [our agreed upon standard for distance measure] of spacetime between us and the distant galaxies has changed (the universe has expanded) along the path of the photon from here to there, leading to an increase in the wavelength of the light. What causes the redshift is due to calculational models....and different views are ok.
  12. Sep 10, 2012 #11
    generally, no. Not even earth's orbit is stretched:

    Loads more here:
    [Wallace that I quote is a cosmologist who posts here from time to time.]
    Does Expansion of Universe affect Earth Orbit

    Synopsis: No. Expansion results from the [FRW metric] assumptions of the CP [homogeneity and Isotropic] and those does not apply at solar nor galactic [lumpy] scales
    thread # 162727

    “ If anything there is a vanishingly small FRW element to the metric of bound structures. If the FRW metric 'prevail(ed) on all scales and everywhere, even inside gravitationally bound structures or within atoms' then why do galaxies maintain a constant size as the distance between them expands? Commonly we are told that the local mass concentration 'overcomes' the expansion preventing this from occurring. This is one of the worst and most fallacious explanations you could possibly give someone! What really happens then?

    The FRW metric is the inevitable result of the cosmological principle, CP. which is that the universe is homogeneous and isotropic. The metric is only valid if these principles hold. Consider now a galaxy, solar system or planet. Does the CP hold? No. Is it a remotely useful approximation? Not at all! Unsurprisingly then the dynamics of bodies in these systems and on these scales bears no resemblance to the dynamics of galaxies. So for instance, there is no redshift of light due to a(t) when we observe light from the other side of our galaxy, or from say Andromeda. The FRW metric simply is not valid on these scales.

    [and here is areference the the FRW or FLRW model which I mentioned already]
    Wallace: #63
    ….the 'expansion' (which we both definitely agree is a bad term for it!) is a result of the FRW metric, in particular a(t). The metric in the region of bound structure looks nothing like the FRW metric, in particular it has no global time dependence (though will of course evolve). For this reason I stand by the statement that the FRW metric is not valid on scales which are significantly inhomogeneous, since the metric has no component that reflects the global a(t), and hence the FRW picture does not relate to the dynamics of the system.
  13. Sep 10, 2012 #12
    The Kinetmatic interpretation means that CMBR [relic radiation after the big bang] that started out at about 3,000 degrees K early in the universe now arrives at a much cooler 2.7degrees K...it is frequency redshifted meaning it has cooled. Otherwise, particles, atoms,planets,stars and galaxies would not yet have formed....it would be too hot.
  14. Sep 10, 2012 #13
    Naty1 and bcrowell, thank you for the wealth of information.
  15. Sep 11, 2012 #14
    For further insights on cosmological 'expansion' [which leads to redshift of light] :

    phinds of these forums put together a 'balloon analogy summary here

    which helps explain cosmological expansion 'stretching' of space....

    with input from a buinch of Physics forums members.

    Many interesting details in these two discussions:

    from Marcus:
    Effort to get us all on the same page (balloon analogy anyone?)


    and Re: the balloon analogy (please critique)
  16. Sep 11, 2012 #15
    I'm new to much of this. Can you please give me your opinion on these measurements of redshift?

    I have been informed of the measurements by reading some of Santilli's paper on the redshift of light without relative motion.

    Called IsoRedShift as per the papers below that imply lack of expansion of the universe.

    I would appreciate any comments primarily on the validity of the measurements or Santilli mathematics.

    [1] R. M. Santilli, "Experimental Verifications of IsoRedShift with Possible Absence of Universe Expansion, Big Bang, Dark Matter, and Dark Energy," The Open Astronomy Journal {\bf 3}, 124 (2010), http://www.santilli-foundation.org/docs/Santilli-isoredshift.pdf

    [2] G. West and G. Amato, "Experimental Conformation of Santilli's IsoRedShift and IsoBlueShift," Journal of Computational Methods in Sciences and Engineering, Vol. 12, pages 169-188 (2012),http://www.santilli-foundation.org/docs/Confirmation-IRS-IBS.pdf

    [3] R. M. Santilli, G. West and g. Amato. "Experimental Confirmation of the IsoRedShift at Sun at Sunset and Sunrise with Consequential Absence of Universe Expansion and Related Conjectures, " Journal of Computational Methods in Sciences and Engineering, Vol. 12, pages 165-188 (2012).http://www.santilli-foundation.org/docs/Confirmation-sun-IRS.pdf

    If PM is more appropriate, please do so. Thanks!
  17. Sep 11, 2012 #16


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    The Santilli foundation is kooky. Here's a quote from the first paper cited above

  18. Sep 11, 2012 #17
    Please leave out the ridicule. I'm interested in his work and want to know if it is valid.

    Sometimes we serve humanity best by thinking outside the box. He seems to have experimentally verified these measurements.
  19. Sep 11, 2012 #18


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    His experiments are pointless because the theory he claims to be testing is nonsense. It isn't polite to hijack a thread with private theories. Why didn't you start a new thread ?
  20. Sep 11, 2012 #19
    I'm not trying to be impolite. If a new thread is better, then I can do that instead. My apologies.
  21. Sep 11, 2012 #20


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    It is pointless to discuss that. It is not proper physics at all, and I have no idea how that got published (however, the journals seem to be unrelated to special relativity anyway).

    2-4 out of 5, with an ether theory as explicit example.
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