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Redshift / Expanding Universe

  1. Aug 15, 2009 #1
    Are there any alternative theories/explanations of why light from distant galaxies is redshifted by an amount proportional to the distance away from us, other than the popular universal expansion explanation? Maybe some theory about light losing energy when travelling over such large distances (the further the distance the more energy is lost, the more the redshift)? Or maybe something related to an alternative gravitational theorem which predicts that other mass out there increases the redshift, ....

    If no other theories on this, then I suppose there is more evidence for universal expansion than just the observed redshifts? Could someone send me a link or some keywords so that I can read more about that.

  2. jcsd
  3. Aug 15, 2009 #2


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    Perihelion of Mercury and Eddington's 1919 observation of bending of light both tend to confirm General Relativity (as formulated in 1915). Much other precision data since then have confirmed GR as the correct theory of how geometry behaves in relation to matter and its own past history.

    We need some theory of gravity and GR is the only one that works and is thoroughly tested. It is a theory of active geometry.

    Expansion of distances was already predicted on the basis of GR in 1922, before the pattern of redshifts was detected. Friedman's 1922 paper even estimated the age of the universe to be about 10 billion years, by extrapolating the expansion backwards in time to its beginning.

    So yes, the expanding universe solution is a very natural consequence of GR. Either that or contracting (but it doesn't seem to be contracting.)
    So all the evidence for GR tends to confirm the expanding universe picture.
    And there is a ton of evidence that GR is correct out to like 6 decimal places.

    So if you want to concoct a non-expanding picture you need some theory of how geometry behaves---how gravity works---that is at least as precise in its predictions as GR. You need an alternative. Or you need some exotic physics that nobody has observed. Or some crackpot ideas---there are plenty of them. But it's hard.

    Really hard to get an accurate model of gravity, that agrees with accurate measurements of planet motion, clock behavior, light bending, pulsar timing etc etc, and that doesn't involve an active geometry where largescale distances expand.

    Of course in science theories are never 100% confirmed. You can only falsify by showing it does NOT make the correct prediction. So nobody can tell you that you have to BELIEVE what GR says. It is just a practical question---seems to be the simplest and best way to understand the world that we have at present. Sensible to go with at least provisionally.
  4. Aug 15, 2009 #3
    From what marcus said it sounds like GR implies either expansion or contraction, but it does not necessarily imply expansion at the rate predicted by redshift data on distant galaxies? True? If so, seems like there must be some other, more compelling reason why we believe the redshifts are due to relative motion only, right?

    My confusion is that, if someone tells me how light from distant galaxies is redshifted by an amount proportional to the distance away, I wouldn't immediately conclude that it was due to relative motion (even if I already know that relative motion can cause redshifts, and that GR predicts either expansion or contraction). Of course, relative motion would be in my list of possible explanations, but it wouldn't be a strong 1st place unless there was more evidence. For this reason I assume there is more evidence, I'm just wondering what it is?

    Thanks again
  5. Aug 15, 2009 #4


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    I don't want to leave you with the impression that the only solutions in GR are either expansion or contraction. They are just the most likely. Solutions that hover between are not very stable, and there are funny "lopsided" solutions which are non-uniform, but our universe looks overall on average pretty uniform. What you say is not an absolute implication, but the most likely and straightforward conclusion to draw.

    I think it is better not to consider the expansion of distance as relative motion. The main lesson from GR is that geometry is active.
    It changes dynamically in response both to its own past history and to the flow of matter.

    We have no right to expect that the sum of angles will be 180 degrees (in a large intergalactic-scale triangle).
    We have no right to expect that the distances between stationary objects will remain constant.
    By stationary I mean according to the best available standard---at rest with respect to early universe matter (the light coming from it is all around us and shows a nearly uniform cloud all roughly the same temperature, all parts approximately at rest relative to all other parts).

    Anyway GR teaches us why we can expect the Euclidean geometry we learn in highschool to work as an excellent approximation under the present circumstances (because of the low density of matter in our neighborhood, and the slow rate of expansion). It is the theory that explains why geometry is the way it is. And it explains how geometry changes and is different under different circumstances. And one of the minor consequences is that you can expect distances between two things which are not moving to increase very slowly sometimes under the right circumstances. And it can predict the rate.

    This is not motion as we normally understand it, because it doesn't get anybody anywhere. It doesn't bring our galaxy or any other galaxy closer to some destination or nearer to some target landmark. General expansion of largescale distances is just that---it is not motion.

    Definitely! I wouldn't conclude that either! You can set up inconvenient partially effective toy models in which redshifts correspond relative motion, but most working astronomers do not normally treat redshift on a cosmological scale as the result of relative motion.
    Basically they treat it as changing geometry----the wavelengths get longer in exactly the same way as the distances expand while the light is traveling.

    The present rate is about 1/130 of a percent per million years. In a million years, largescale distances (between galaxies not connected in some gravitationally bound cluster) will have increased by 1/130 percent. And the wavelength of any light traveling in intergalactic space for a million years will be longer by 1/130 percent.

    Because we don't actually live in a fixed Euclidean geometry, and one has no right to expect that a wavelength will remain constant any more than that the distance between two stationary objects will.
    Last edited: Aug 15, 2009
  6. Aug 15, 2009 #5
    This is a little different subject, but is there a difference between (1) saying that objects are stationary but the distance between them is increasing and (2) objects are moving away from one another? I guess if the rate at which the distance changes itself changes then the difference is that in (2) the objects can show/feel the effects of acceleration while in (1) they cannot? If is the distance changes at a constant rate then the 2 statements are equivalent, right?
  7. Aug 16, 2009 #6


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    We are talking about something that is at an intuitive level. To make math'ly rigorous sense of relative motion as a concept you need the two objects to be in the same reference frame, which is typically not the case. So this is purely intuitive---how you think about it.

    Personally I think of an object as moving if there is some destination that it is approaching. It has to be "going somewhere". If there is no definable place that it is getting nearer, then it isn't moving (for me, intuitively).

    You are welcome to have different intuitive notions about it. Intuitive perspective on things is not hard science that we have to argue about:smile:

    So take the balloon example. All existence is concentrated on the 2D surface, there is no surrounding 3D space, no inside or outside of the balloon surface. And there are two dots painted on the surface. The distance between them is increasing but neither dot is going anywhere. For me, in that 2D universe, neither is moving.

    Or take two widely separated galaxies in a curved expanding universe. In GR you can't get one (flat, non-expanding) reference frame to fit the whole thing---it makes too bad a fit. Like trying to dress in clothes made of wooden boards.
    So the two galaxies are in two different reference frames. Each one has a locally defined frame that approximates its immediate neighborhood. But there is no global shared frame. Relative motion is math'ly undefined.

    Perhaps each galaxy is actually moving slightly in its own frame. Galaxies do typically have their own small "peculiar" velocity of a few hundred km/s. But this is negligible compared with typical distance expansion rates which can be several times the speed of light. So let's ignore the socalled "peculiar" velocities and consider them as both stationary.

    Neither one is getting closer to anything else, just like the dots on the balloon. But the distance between is increasing.

    With distance expansion, the rate has changed greatly over the U history, and it is changing now, but nobody ever feels any acceleration. In what direction would the acceleration be?
    Distance expansion is the same in all directions, for everybody. Nobody is going anywhere so there is no acceleration in any direction.

    So anyway, my intuition seems to work OK for me and it says that this is not ordinary motion (and would not be ordinary motion even if it were at constant rate as you suggested). You have to decide what your feeling about it is.

    Mathematically speaking it is rigorously NOT relative motion, as I said, because no global frame. Could be that it is advisable to align intuition with the math, and with the rough astro community consensus in this case.
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