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Thread: Why would the uniform Expansion of space produce redshift?

  1. #1 Why would the uniform Expansion of space produce redshift? 
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    I have two questions:


    1. If I understand redshift properly,
    it is the result of expanding space "stretching out" a wavelength of light along its axis of propogation.
    Why wouldn't the wavelength of light be stretched out uniformly along all its axes?

    2. If space is expanding uniformly, how could this expansion be observable from within a closed system?
    When I build a 3D model of a set of objects,
    and then scale them all uniformly,
    their size:distance relationships do not change.


    Were I to be inside the model and scale myself along with the rest of the model,
    I would not perceive any change had occurred,
    and be unable to measure the effect from within.


    Can you shed any light on these questions?


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  3. #2  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Aiden View Post
    1. If I understand redshift properly,
    it is the result of expanding space "stretching out" a wavelength of light along its axis of propogation.
    Why wouldn't the wavelength of light be stretched out uniformly along all its axes?
    This is not a completely accurate description, although it is one that is used quite often. It would be slightly more accurate to say that red-shift is due to the light being detected in a location with a different "scale factor" (a different amount of expansion) than where it was emitted.

    I'm not sure I understand the question, though. There is just one wavelength. It increases. I'm not sure what it would mean for " light [to] be stretched out uniformly along all its axes".

    2. If space is expanding uniformly, how could this expansion be observable from within a closed system?
    When I build a 3D model of a set of objects,
    and then scale them all uniformly,
    their size:distance relationships do not change.
    The space between things (galaxies, or more accurately galaxy clusters) is expanding. The galaxies and stars are not expanding.

    Were I to be inside the model and scale myself along with the rest of the model,
    I would not perceive any change had occurred,
    and be unable to measure the effect from within.
    But if you didn't scale yourself, just the distance between you and all the other objects (which also don't scale), then you could tell it was happening.

    One analogy that is sometimes used (I have never liked it) is a raisin loaf cooking: the dough (space) expands moving all the raisins apart from one another. The raisins don't get bigger though.

    Can you shed any light on these questions?
    Did I?


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  4. #3  
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    Your reply gets at the nature of my concern - if space is expanding uniformly, then so are all points in space, including - us. We get bigger too - if space itself is getting bigger.
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  5. #4  
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    Strange,

    in effect I am asking, "why don't the raisins get bigger?"

    Dimensional space is the fabric that underlies everything,
    including mass objects "raisins"
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    If the "raisins" stay the same size, then this means that the expansion of space is not uniform,
    but expands at a greater rate where there is less mass.

    Is the universe's expansion, in fact, not uniform then, but influenced by mass?
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  7. #6  
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    Quote Originally Posted by Aiden View Post
    Strange,

    in effect I am asking, "why don't the raisins get bigger?"
    The simple answer is that raisins (and we) are held together by electromagnetic forces which overcome the expansion. On a large scale, galaxies, and even galaxy clusters, are held together by gravity and so they don't expand. In fact, within our local cluster at least one galaxy is heading straight for us.

    Again, that is a bit of a simplification (everything but the math is a simplification!). What the math says (I think; much of it is over my head) is that expansion is only inevitable when the distribution of mass is homogeneous (even/smooth/same all over). That is true of the universe on very large scales. It is very much not true of galaxies and people. So there is nothing forcing us to expand. And so the forces holding us together continue to ... hold us together.
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  8. #7  
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    Quote Originally Posted by Aiden View Post
    If the "raisins" stay the same size, then this means that the expansion of space is not uniform,
    but expands at a greater rate where there is less mass.

    Is the universe's expansion, in fact, not uniform then, but influenced by mass?
    Bingo.
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  9. #8  
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    Thanks!
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    As an almost uneducated country bumkin I find it hard to understand how undoubtedly better educated people find this concept so difficult to grasp.........
    Take eleven footballs, place them I metre apart in a straight line, then increase the distance between each football by ten centimeters, if it had taken one minute for this period of expansion to take place then viewing this event from the first football the second football would have moved away from it by ten centimeters per minute, the third would have moved away from the first by twenty centimeters per minute and the last would have moved away by one hundred and ten centimetres. If however you were to view this event from the football at the centre of the line then footballs five and six would have moved away from you at ten centimeters per minute while one and eleven would move away from you at fifty centimetres per minute. In essence although the rate of expansion is constant its incremental, so the more distant the object the faster it appears to be moving away from the point you are viewing it from, hence the red shift.
    The reason for the expansion is another story altogether, I have my own opinion on what is happening but will leave that for another post.
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  11. #10  
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    Quote Originally Posted by Aiden View Post
    I have two questions:

    1. If I understand redshift properly,
    it is the result of expanding space "stretching out" a wavelength of light along its axis of propogation.
    Why wouldn't the wavelength of light be stretched out uniformly along all its axes?

    2. If space is expanding uniformly, how could this expansion be observable from within a closed system?
    When I build a 3D model of a set of objects,
    and then scale them all uniformly,
    their size:distance relationships do not change.

    Were I to be inside the model and scale myself along with the rest of the model,
    I would not perceive any change had occurred,
    and be unable to measure the effect from within.

    Can you shed any light on these questions?
    Galactic redshifts defined: Redshift - Wikipedia, the free encyclopedia

    The presence of at least some atomic elements can be observed and identified via their absorption and emission lines as explained by the above link. The shifting of these lines toward the red-end of the EM spectrum is called the redshift within the spectra, or simply galactic redshifts.

    As to your first question concerning redshifts: Why wouldn't the wavelength of light be stretched out uniformly along all its axes?

    Redshifts are not characteristics of galaxies, you might call these redshifts an optical allusion of the observer. According to the present standard interpretation, redshifts are due to the apparent relative motion away from us of galaxies. This is equivalent to a Doppler shift whether the galaxies are actually moving or whether space were expanding between galaxy clusters, as is the present model. This is the standard model meaning of the words cosmological redshifts. Redshifts accordingly would be different for each possible observer depending upon their distance away from the observed galaxy.

    The answer to your first question
    therefore is that redshifts are solely an observer phenomena based upon the relative motion and distance of the observer from the galaxy(s) they are observing.

    As to your second question: how could this expansion be observable from within a closed system?

    Were I to be inside the model and scale myself along with the rest of the model,
    I would not perceive any change had occurred,
    and be unable to measure the effect from within.
    If you would scale yourself to start with you could accordingly perceive through observation, the application of theory, and calculations, that the space between galaxy clusters was accordingly growing/ expanding relative to your own size.

    Expansion of the universe is not fact but is presently well-accepted theory. Besides the Big Bang model most other cosmological models, including the more well-known Steady State models, also propose the explanation for the observed galactic redshifts as being caused by the expansion or the universe, even though there have been a number of other lesser- known proposed explanations for these observed galactic redshifts.

    The standard explanation to your second question is directly related to the answer to your first question. That the universe is expanding as a whole is present theory based upon the interpretation that the space between galaxies is expanding at the galaxy-cluster scale. You could accordingly tell the changes occurring concerning scale in that the space between galaxies is supposedly becoming larger while the galaxies themselves are supposedly remaining the same size excepting for galaxy growth and mergers. As in your first question, the expansion of universe is accordingly attributed to the observable redshift of galaxies which determines distance by way of calculations. Using the Hubble formula one can accordingly calculate distances based upon the Doppler shift assumption, but secondarily distance can be more roughly calculated by Luminosity Distance whereby the apparent brightness of a galaxy decreases according to its distance via the inverse square law of light. Thirdly distances to galaxies can be estimated by their Visual Angle. The farther away a galaxy, for example, the lesser its apparent visual angle/ angular diameter witch is a telescopic measurement of the apparent angle of the galaxy measured across its diameter.

    Angular diameter - Wikipedia, the free encyclopedia

    The answer to your second question
    (#2 above), according to theory, is very similar to the answer of your first question: the supposed expansion of the universe is determined/ inferred via the observed redshifts of galactic EM radiation. This is based upon the ongoing relative changes in the size of matter to the observed size of the space it occupies as interpreted by the observed redshift of light and other EM radiation such as radio-wave telescopes, infra-red scopes, etc.

    The reason for the supposed expansion of the universe is debatable. According to the standard model such explanations and hypothesis are usually hypothesized as being a function(s) of the following: the original BB itself, Inflation, dark energy, a cosmological constant, dark matter, and other proposals.
    Last edited by forrest noble; July 28th, 2012 at 11:05 AM.
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  12. #11  
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    Quote Originally Posted by forrest noble View Post
    The answer to your first question[/B] therefore is that [B]redshifts are solely an observer phenomena based upon the relative motion and distance of the observer from the galaxy(s) they are observing.
    Attributing the red shift to relative motion is even less accurate than the description in the OP (light being stretched by the expansion of space). Work out the numbers using Doppler shift based on recession velocity: I think you'll get the wrong answer.

    Besides the Big Bang model most other cosmological models, including the more well-known Steady State models
    I doubt steady state theories are better known (especially to anyone under 50). They have also been proven wrong. Why do you keep bringing this nonsense up - are you deliberately trying to cause confusion?

    The reason for the supposed expansion of the universe is debatable.
    You mean apart from being an inevitable consequence of General Relativity? (Which, as I'm sure you know, predicted it before it was detected experimentally.)
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  13. #12  
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by forrest noble View Post
    The answer to your first question[/B] therefore is that [B]redshifts are solely an observer phenomena based upon the relative motion and distance of the observer from the galaxy(s) they are observing.
    Attributing the red shift to relative motion is even less accurate than the description in the OP (light being stretched by the expansion of space). Work out the numbers using Doppler shift based on recession velocity: I think you'll get the wrong answer.
    Relative velocities is the basis of the Hubble formula. The definition of relative motion is: The continuous change of position of a body with respect to a second body or to a reference point.

    (my quote)
    Besides the Big Bang model most other cosmological models, including the more well-known Steady State models
    I doubt steady state theories are better known (especially to anyone under 50). They have also been proven wrong. Why do you keep bringing this nonsense up - are you deliberately trying to cause confusion?
    Here I am referring to alternatives proposed to the Big Bang model such as the Steady State models and other lesser known models, the SS models being better known than other models such as Plasma Cosmology and my own model for instance. Sorry for any ambiguous wording.


    (my quote)
    The reason for the supposed expansion of the universe is debatable.
    You mean apart from being an inevitable consequence of General Relativity? (Which, as I'm sure you know, predicted it before it was detected experimentally.)
    No, the expansion of the universe is not an inevitable consequence of GR. It is a possible solution along with contraction solutions. The expansion solution(s) to Einstein's cosmological equations were first discovered and independently published by Friedman and Lemaitre. With the inclusion of a cosmological constant, a steady state condition of a non-expanding universe was a solution according to Einstein.

    Here I am referring to Steady State models being known alternatives to the BB model(s), better known than other models such as Plasma Cosmology and my own model for instance. Sorry for any ambiguity.

    ....They have also been proven wrong
    Steady State cosmology has not been disproved or "proven wrong."

    The correct wording would normally be expressed this way:

    The steady state model is now largely discredited, as the observational evidence points to a Big Bang-type cosmology and a finite age of the universe.
    Steady State theory - Wikipedia, the free encyclopedia
    Last edited by forrest noble; July 28th, 2012 at 11:09 AM.
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  14. #13  
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    I think the OP's question would be better stated as: "Why is only the wavelength of light changed by expansion, rather than both the wavelength and amplitude?" And that is of course, to do with the nature of photons having their frequency related to their energy by What enforces that proportionality is an interesting question to ask.

    Another possible question would be "Why doesn't expansion cause the light to be both dimmer and redshifted?" And I'm not sure it doesn't. So I might as well ask: Does expansion change the apparent brightness of distant stars (in particular "Standard Candle" stars we use to measure distance) by way of causing the photons emitted to be more spread out over a wider horizontal distance as they travel further and further through space? I'm thinking that as a beam of light travels, it gets dimmer according to the inverse square law because the photons keep spreading out over a wider area. But also as they travel through expanding space, the lateral aspect of that expansion should be causing them to spread out even more.


    Indeed, the two possibilities would yield opposite results. If a photon were like any other kind of wave, and its amplitude were increased by expansion, we should expect that it would have more energy due to that, and so light from distant sources would be brighter than normal. But if the photons are being spread out laterally as they travel through space (by which I mean the space between photons in increasing), that would make light from distant sources appear dimmer instead.
    Some clocks are only right twice a day, but they are still right when they are right.
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    kojax,

    Why doesn't expansion cause the light to be both dimmer and redshifted? And I'm not sure it doesn't.
    You are right, the light becomes dimmer when redshifted. Dimness of light is related to how much energy the light wave has. As light is redshifted and its wavelength increases, its energy per wave decreases.

    An interesting point might be that for supernova events for the redshifting of supernova light, like the galaxies that contain the supernova, the energy emitted per time period is less because of the redshifting and not just because of the increased distances. This is compensated for, however, by our observation of the supernova event as lasting longer. If you could do a consecutive count of the number of waves in a type 1a supernova event you might realize that the number of possible consecutive waves in the total event would be about the same. This means that although each light wave is dimmer and has less energy, the duration of the event from our perspective lasts longer so that the total energy we receive is about the same after adjustments for the inverse square law of EM radiation.
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