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Thread: Red Shift - Cosmological or Doppler or both?

  1. #1
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    Default Red Shift - Cosmological or Doppler or both?



    Hello!

    Recently I have realized that my understanding of such fundamental thing as Red Shift probably needs more brooding.

    It has been looking obvious to me that we see the light from distant objects like quasars shifted into red part of spectra and it is due to expansion of the Universe.

    Fine. As it could be found on many Internet sources: ~ "photons are traveling through cosmic space and as it gets stretched they are getting redshifted as well". The obvious question that would raise: if the space itself expands - why the "regular" matter (as our Milky Way stars, etc) are not expanded too? So far I found following explanations: as "regular" matter is bound by gravity - it doesn't get stretched out ... but photons - as they travel through "empty" (voids between clusters of galaxies?) space - they are dropping energy and getting more and more into red side of spectra...

    In the same time Red Shift implies the source of these photons - such a distant quasar - is also receding from observer at high speed (I mean at the moment of emitting photon). Say, quasar emits photon at moment zero (T0) and at that moment it had velocity v0=Hz (Hubble law), so v0 for z > 1 (say) is already high enough to make a significant Doppler effect _at_the_moment_of_ emitting_.

    (Here is also a bit unclear moment - does Hubble Law allows to calculate velocity of emitting object at the moment of emitting (I reckon so) or it actually calculates the velocity of object NOW - e.g. at the moment of registration?)

    So the question is: is Red Shift is due to Cosmological expansion OR due to just Doppler effect (yet still as a part of Expansion) or may be due to both?

    Thank for possible explanation.
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    Default Re: Red Shift - Cosmological or Doppler or both?

    Cosmological red shift is not due to the Doppler shift. The motion of a remote galaxy, in its own part of space may be zero, yet it is still receding from us due to the expansion of space. So, even though its motion is zero, its light will be red-shifted. This is the cosmological red shift.

    Now a galaxy could be moving locally in its own part of space, and this will add or subtract a Doppler component to the red shift. However, the amount of the Doppler shift would be small in comparison to the cosmological red shift.
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    Default Re: Red Shift - Cosmological or Doppler or both?

    Thank for reply.

    This element:

    However, the amount of the Doppler shift would be small in comparison to the cosmological red shift.
    Is not clear to me. For example (according Relativistic Doppler effect - Wikipedia, the free encyclopedia) for given velocity V of emitting object (e.g. distant quasar):

    z= sqrt((1+B)/(1-B))-1; where B=V/c.

    For example, quasar receding with V=c/2 would have:

    z=sqrt((1+1/2))/(1-1/2))-1=sqrt(3)-1~1.7-1~0.7 (1) (Doppler shift, relativistic)

    The same quasar according to Hubble Law:

    z~ V/c (approximation - valid for "small" V) ~ 0.5 (here it is probably more correct refer to "recessional velocity"?).

    So they are approximately of the same magnitude - ?
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    Default Re: Red Shift - Cosmological or Doppler or both?

    The red shift from very distant sources results primarily from the expansion of space. The Doppler part of that is small. The Doppler shift from close stars is much larger than the shift due to space expansion. Compare the distance in ly's to see which one should dominate.
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    Default Re: Red Shift - Cosmological or Doppler or both?

    Quote Originally Posted by i555i666wHeretic View Post
    For example, quasar receding with V=c/2 would have:

    z~ V/c (approximation - valid for "small" V) ~ 0.5 (here it is probably more correct refer to "recessional velocity"?).

    So they are approximately of the same magnitude - ?
    No. V = c/2 would not be considered a "small" velocity.
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    Default Re: Red Shift - Cosmological or Doppler or both?

    Quote Originally Posted by i555i666wHeretic View Post
    Thank for reply.

    This element:



    Is not clear to me. For example (according Relativistic Doppler effect - Wikipedia, the free encyclopedia) for given velocity V of emitting object (e.g. distant quasar):

    z= sqrt((1+B)/(1-B))-1; where B=V/c.

    For example, quasar receding with V=c/2 would have:

    z=sqrt((1+1/2))/(1-1/2))-1=sqrt(3)-1~1.7-1~0.7 (1) (Doppler shift, relativistic)

    The same quasar according to Hubble Law:

    z~ V/c (approximation - valid for "small" V) ~ 0.5 (here it is probably more correct refer to "recessional velocity"?).

    So they are approximately of the same magnitude - ?
    Your math may work, but I'm not aware of any quasars that are traveling away from us at that speed. On the other hand the expansion of the universe is supported by several observations.
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    Default Re: Red Shift - Cosmological or Doppler or both?

    Quote Originally Posted by kencrowder View Post
    Your math may work, but I'm not aware of any quasars that are traveling away from us at that speed. On the other hand the expansion of the universe is supported by several observations.
    Ok, found some supportive paper (http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf - "Baffled by the expansion of the universe? - You’re not alone. Even astronomers frequently get it wrong" By Charles H. Lineweaver and Tamara M. Davis) to that opinion:

    ... Galaxies hardly move through space, so they emit light with nearly the same wavelength in all directions (top). The wavelength gets longer during the journey, because space is expanding. Thus, the light gradually reddens (middle and bottom ). The amount of redshift differs from what a Doppler shift would produce. ...
    So it implies - without doubt - there is no any significant velocities (peculiar) for galaxies to produce any significant redshift via Doppler mechanism.

    To support that authors (above) refer to following observation:

    ... Scientists first proposed this hypothesis some 75 years ago, and like any good model, it makes predictions that can be tested. But like any bad model, its predictions do not fit the observations. For example, when a star explodes as a supernova, it brightens and then dims
    — a process that takes about two weeks for the type of supernova that astronomers have been using to map out space. During these two weeks, the supernova emits a train of photons. The tired-light hypothesis predicts that these photons lose energy as they propagate but that the observer always sees a train that lasts two weeks. In expanding space, however, not only do individual photons get stretched (thereby losing energy) but the entire train of photons also gets stretched. Thus, it takes longer than two weeks for all the photons to arrive on Earth. Recent observations confirm this effect. A supernova in a galaxy of redshift 0.5 appears to last three weeks; one in a galaxy of redshift 1, four weeks ...
    Well, I am still brooding ;-) First at all - the time dilatation is nothing special for relativistic object: simple Lorenz t=t0/sqrt(1-(v/c)^2) for given supernova lifespan t0 in moving (from us) distant galaxy with velocity v would produce the same effect as expansion of Universe, wouldn't it? (I will be grateful for explanation for that point).

    Second problem (to me) still persists - why ONLY expansion of Universe contributes to Red Shift? E.g. I can imagine the Red Shift as a superposition of Expansion AND Doppler shift.

    And last one - there are many different books/papers/etc with just opposite opinion, for example (http://en.wikipedia.org/wiki/Hubble%27s_law):

    Hubble's law is the name for the theory in physical cosmology (proven by observation) that: (1) all objects observed in deep space (intergalactic space) are found to have a Doppler shift observable relative velocity to Earth ...
    Or another example (http://www.astro.uni-bonn.de/~porciani/igm/igm.pdf):

    The spectrum of a quasar consists of a power-law continuum (typically ...) and broad emission lines (with a Doppler parameter of
    several thousand km s^-1). If the spectrum contains more than one emission line, it might be possible to identify the atomic transitions to
    which they correspond (using the ratio of their wavelengths and also their strengths). This allows us to measure the redshift of the quasar ...
    Actually assumption of quasi-zero peculiar velocities of galaxies (e.g. their V is not enough to produce serious Doppler shift) implies no impulse for matter after Big Bang - only stretching the space in voids ...

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    Default Re: Red Shift - Cosmological or Doppler or both?

    You may be focused on one piece of the puzzle and trying to imagine the whole picture. There are many pieces. Consider the CMB. Consider the observations of galaxies and clusters of galaxies. Consider the evidence from the red shift from super nova type 1a that appear to be near the time of the big bang.

    The one model that seems to explain all the observations is the big bang occurred about 13.7 billion years ago. A plasma existed which then cooled to produce matter and fields which evolved into what we observe today. That evolution is quite complex and some of the details are still in discussion. But the basic concept seems to work.

    We think the basic laws of thermodynamics should still work. We think GR is reliable. Any alternative model should satisfy those laws or explain how they have changed. The conservation of energy should apply. That imposes significant constraints on any alternative model.

    If one only looks at the red shift, you are right. One can't separate motion from the expansion of space. But it is hard to imagine how the structure we observe of galaxies and clusters of galaxies came about.
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    Default Re: Red Shift - Cosmological or Doppler or both?

    Quote Originally Posted by i555i666wHeretic View Post
    Second problem (to me) still persists - why ONLY expansion of Universe contributes to Red Shift? E.g. I can imagine the Red Shift as a superposition of Expansion AND Doppler shift.
    You are being too "logical" and insufficiently quantitative in your reasoning. The individual peculiar velocities of quasars is simply so small compared to the expansion of the universe that they don't matter. In fact sine the spectral lines of quasars tend to be broad, it is difficult to measure the velocities precisely enough to show the Doppler component, which, again, is small compared to the expansion of the universe.
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    Default Re: Red Shift - Cosmological or Doppler or both?

    Quote Originally Posted by i555i666wHeretic View Post
    Ok, found some supportive paper (http://www.mso.anu.edu.au/~charley/p...DavisSciAm.pdf - "Baffled by the expansion of the universe? - You’re not alone. Even astronomers frequently get it wrong" By Charles H. Lineweaver and Tamara M. Davis) to that opinion:
    The Lineweaver and Davis work you corrects some popular misconceptions. To go from that to "even astronomers get it wrong" is, well, wrong. Popular media get it wrong. A few fringe scientists get it wrong. That is all.

    So it implies - without doubt - there is no any significant velocities (peculiar) for galaxies to produce any significant redshift via Doppler mechanism.
    Relatively, insignificantly small is not the same as zero, though!

    Well, I am still brooding ;-) First at all - the time dilatation is nothing special for relativistic object: simple Lorenz t=t0/sqrt(1-(v/c)^2) for given supernova lifespan t0 in moving (from us) distant galaxy with velocity v would produce the same effect as expansion of Universe, wouldn't it? (I will be grateful for explanation for that point).
    Why apply such a formula incorrectly to a situation where we believe General (not Special) Relativity applies? It's a physics mistake even if it works mathematically. The relevant formula is here:
    https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

    Anything Doppler and peculiar would get added to that.

    And last one - there are many different books/papers/etc with just opposite opinion, for example (http://en.wikipedia.org/wiki/Hubble%27s_law):
    Your Hubble Law link is bad. The correct link is:http://en.wikipedia.org/wiki/Hubble%27s_law

    It does not support your position as you seem to think. Early in the 20th century when Hubble did his work, Einstein was pushing a static cosmology. So Hubble "interpreted" the red shift as individual motion. With the Friedmann-Lemaitre-Robertson-Walker metric, we now know that interpreting the redshift as due to galaxy motion is obsolete.

    https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

    Looks like a bad link to me. Maybe they are doing maintenance. If it supports your view then it perhaps is an oversimplification or error.

    Actually assumption of quasi-zero peculiar velocities of galaxies (e.g. their V is not enough to produce serious Doppler shift) implies no impulse for matter after Big Bang - only stretching the space in voids ...
    Yes. That is precisely correct. Voids grow. Electromagnetic and gravitational forces keep the bound stuff bound. No problem.
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