Expansion by doppler (red) shift

  1. Hello all,
    so I've always wondered about the expansion of the universe as indicated by the observed doppler (red) shift... i get the observed spectrum change and how this can be credited to the stretching of electromagnetic waves... what doesn't make sense to me is that if this is due to a 'stretching' of space-time, or distance itself is expanding, then this should be undetectable from within the universe. ie. if the wavelength of EMR is say 1m, but then the 'fabric of space' is stretched which in turn stretches the EM wave(and gives the observed increase in wavelength), we should see no change as 1m is still 1m... if the wave is traveling at a constant C, then the effect of 'stretching' distance makes no difference, the wave should still cover the entire 1m length in a given time, otherwise either the wave has slowed to below C, or extra space has been added and you now have more than 1m.
    Im having trouble explaining my thinking in text and can visualize it better kind of like in a warped mirror you see at a carnival, you can only see the warping effect from the outside - inside the mirror all the ratios are the same and you won't notice any amount of stretching/compression.

    can anyone get my meaning and explain it to me?
  2. jcsd
  3. Drakkith

    Staff: Mentor

    It's not that distance itself is expanding, as distance is a measurement between two points in spacetime. It's that the distance between two points in space increases over time. So two points that are initially 1 meter apart gradually separate. Note that solid objects don't expand, as they are bound together by electromagnetic and nuclear forces. The same is true for Galaxy clusters and smaller objects. Gravity binds them together and prevents the expansion of space from affecting them. It is only once you get to the scale of galaxy superclusters (clusters of galaxy clusters) that the distances are large enough and gravity is small enough for expansion to have a noticeable effect.

    Since an EM wave isn't a solid object, the wave is gradually expanded over time as it travels through intergalactic space.
  6. Drakkith

    Staff: Mentor

    I mean that if we say "distance itself is expanding" then that means that the a 1 meter measuring stick gets larger over time. But this isn't what happens. What happens is that two points (or objects) in space get further apart over time. IE the distance between them increases.
  7. ok great thanks - so it is actually space expanding and carrying any matter along... units of distance remain the unchanged - so is current thinking that 'space' is expanding (if so how is this different from the abandoned cosmic ether idea? - what is expanding as it affects distance between physically separate matter?), or is it that more space been created? is there anyway way to distinguish between these two ideas or is there any real difference?
    i guess I'm getting caught on the idea of space acting as an elastic material that all energy/matter exists in...
  8. I've just been thinking more about the explanation above: if the electromagnetic/nuclear forces are great enough to hold matter together and the gravitational force is great enough to hold galaxies together as space expands, there must be some 'force' value associated with expansion - i.e. the gravitational force among relatively close objects (galaxies) is stronger than the expansion, but a much decreased gravitational force between distant clusters is not and increase in separation is observed. So is there a value we could put on the expansion 'force'?
  9. marcus

    marcus 24,229
    Science Advisor
    Gold Member
    2014 Award

    according to me, the expansion of geometry is not "pulling objects apart" so there is no force comparison needed. There is in cosmology a CRITERION OF STILLNESS, and distances between stationary objects are increasing (currently at a rate of 1/144 of one percent per million years.)

    that means that given a solid object like a steel measuring rod you cannot have both ends absolutely stationary.

    There are papers that have been written about the "tethered galaxy problem" where you picture two galaxies somehow harnessed to opposite ends of a long cable. Light the measuring rod I mentioned but longer. It turns out that one or both galaxies have to be moving towards each other for the distance to remain the same.

    Other people understand this differently. that's just my take. ancient light criterion of stillness (I think it is essential to defining the cosmic standard time that the mainstream model runs on)
  10. If the distance between distant objects is increasing then any expansion must be able to overcome the very small gravitational attraction between them ...am i right to think that gravitational attraction never gets to zero no matter how much the objects are seperated? So if there is some attractive force (even a near-zero value ) , then is there some point where that attractive force is not great enough to overcome cosmic expansion and the objects receed from each other? If so then a value could be put on the expansion... then again im probably way off in my thinking here and have more digging to do... :-)
  11. marcus

    marcus 24,229
    Science Advisor
    Gold Member
    2014 Award

    Mike, all the buzz we hear about "expanding universe" is basically people (journalists and others) popularizing the Hubble law. This presupposes a cosmic time parameter and so called proper distances measured between stationary points as of some instant t of universe standard time. People try to popularize it without getting into the underlying definitions, so there is bound to be verbal vagueness and lack of clarity.
    the Hubble law is v(t) = H(t)D(t) and it concerns the distance between a pair of stationary observers (at rest with respect to Background).

    It says at a given instant t, the distance between them D(t) is increasing at speed v(t) and that rate is proportional to the distance by a constant H(t) which is the same everywhere, at that moment.

    Strictly speaking Hubble law is not true for galaxies which have their own random individual motion towards or away from each other. But it is approximately true because as a matter of experience most galaxies are not moving very fast with respect to background. A few hundred km/s this way or that. So the law applies to galaxies approximately.

    But it applies strictly to pairs of objects or ideal observers at rest relative to Background. An idealization, in other words. But extremely useful and widely applicable. The equation that governs the gradual change in H(t) is called "Friedmann equation" and it is the core equation of the standard cosmic model.
  12. thanks marcus, ill read up on that
  13. Drakkith

    Staff: Mentor

    I think it's important to understand that the primary theory dealing with expanding space is General Relativity. GR is a theory primarily about geometry. In other words, it's a theory about the position of objects relative to other objects and the underlying rules that govern this relationship. Unlike your normal geometry you learned (or are learning) in high school, GR deals with differential geometry. That is, geometry that can change. For example, an object with mass alters the geometry of the space surrounding it, which ends up causing the effect of gravity.

    Similarly, the expansion of space is another geometrical effect, but instead of causing objects to move towards each other, it causes them to move away from each other unless overridden by another force.

    Whether this means that spacetime is "really expanding" like an elastic fabric would do is more interpretation than anything else. (At least as far as I know)
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