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Measuring space expansion without redshift

  1. Dec 27, 2015 #1
    Hi,

    Apart from the redshift of electromagnetic waves (which we do observe), is there any other evidence of space expansion that is not based on redshift? If not, is there a theoretical experiment that could let us observe and measure space expansion?

    Here I am assuming that space is not just the medium for electromagnetic waves to commute in - there are other things going on (like motion of matter) that must be affected by space expansion.

    Would it be possible, for example, to measure the degree of metric expansion by seeing how an elliptical orbit of a galaxy cluster gets "elongated" when it moves sufficiently far away from its center of attraction?

    (I understand that electromagnetic waves may still be needed for us to observe such an event, and they will be redshifted the usual way, but I hope it can be accounted for. I also understand that the "experiment" may require too much time to be practical).

    Thanks
     
  2. jcsd
  3. Dec 27, 2015 #2

    Chalnoth

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    Sort of. To measure the expansion, we combine redshift with measures of the distances to far-away objects. The redshift is typically best-measured by taking a spectrum of the light coming from the object. To get the distance, however, a wide variety of methods are used. Some objects have known brightness, so we can determine how far away they are by how bright they appear. Some objects have known physical size, so we can determine how far away they are by how big they appear. There are a bunch of other methods as well.

    The main reason why we think that the redshift is just fine is that the wide variety of measures of the expansion of the universe all agree to a high degree of accuracy.
     
  4. Dec 27, 2015 #3

    PeterDonis

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    This won't help because the expansion of the universe does not affect gravitationally bound systems. (This is one reason why the term "expansion of space" is not a good one to use to describe what is going on.) So things like the orbits of galaxies in clusters are not affected by the expansion of the universe.
     
  5. Dec 27, 2015 #4
    This is quite surprising. Are you saying stars and galaxies are not affected by space expansion at all, or not affected enough to measure the rate of expansion reliably?

    I thought that there were 2 phenomena in play simultaneously: 1) metric expansion of space and 2) gravity. I visualized space expansion as continuous division of Planck-size "vacuum cells" - locally and everywhere, and gravity as something that caused mass/energy to commute (switch to an adjacent "cell" at most once every Planck time).

    Gravity is weak for objects that are far away, so space expansion dominates and the amount of space between such objects increases. Objects that are sufficiently close remain bound by gravity. At some distance there has to be an unstable equilibrium between space expansion and gravity: anything closer and gravity would keep the system together, anything farther away and space expansion would rip it apart.

    Is this a wrong assumption? If not, and such a distance (where gravity and space expansion are equally strong) exists, it could present opportunities for measurements.
     
    Last edited: Dec 27, 2015
  6. Dec 27, 2015 #5

    PeterDonis

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    Yes. (There is a small effect due to dark energy, but that can be ignored here.)

    There's no such thing. As I said, the term "expansion of space" is not a good one to use to describe what is going on, because it invites erroneous inferences like the one you are making. The correct statement is that galaxies and galaxy clusters, on average, are moving apart; this is referred to as the universe "expanding", but it in no way involves any "division of space cells" or anything like that.

    No. Once again, the reasoning you are using is erroneous. There is no such thing as "space expansion" in the sense you are using the term here.
     
  7. Dec 27, 2015 #6
    Got it. Gone reading. Thanks!
     
  8. Dec 27, 2015 #7
    Back from this: http://arxiv.org/pdf/0808.1081v2.pdf

    So, if I understood it right, the expansion of the Universe (not considering dark energy) is global, kinematic, about distant objects flying away rather than any local physics of the vacuum, and not producing any special detectable phenomena outside of the well-known kinematic ones (like the Doppler red shift). That non-zero outward-facing "coordinate inertia" is basically the "state of rest" for the flat space. If that is the case, then I am not sure I see how we can measure the expansion rate without involving redshift - back to my original question - and therefore verify/falsify what we know about the redshift itself.

    (I now need to understand what experiment falsifies the notion of the locally-stretching space "fabric", and then figure out how to think about superluminal recession speeds of distant galaxies without thinking of the increasing amounts of vacuum between them, but those would be different questions).
     
  9. Dec 28, 2015 #8

    Drakkith

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    It doesn't need to be falsified because it was never a valid idea to begin with. Space has never, ever been described, professionally, as anything close to a fabric. In General Relativity spacetime is described by something known as a metric tensor, which is a mathematical concept that lets us calculate the topology of spacetime. The space portion of this should not be thought of as a fabric. The "stretching" of space is simply an analogy since the increasing recession velocity of distant objects is similar to how objects recede from each other if placed on a fabric and then that fabric is stretched.

    There is increasing distance, and hence increasing space, in between receding objects, but no "new space" is being created and forcing the two apart.
     
  10. Dec 28, 2015 #9
    Perhaps I used a wrong word. By "fabric" I referred to something physical - i.e. vacuum. If one believes the Universe expansion is accelerating due to the negative pressure of the vacuum, then one already allows local phenomena to accumulate and have cosmological effects, and space can no longer be viewed as abstract geometry. If having more vacuum between objects accelerates the expansion of the Universe, would it be unreasonable to wonder if some local phenomenon (say, a field similar to https://en.wikipedia.org/wiki/Inflaton), as opposed to the mathematical freedom allowed by General Relativity, causes the expansion to begin with, or at least contributes to it? What would be problematic about that assumption - the apparent lack of physical force? Does dark energy impart force on receding objects?

    I do understand that no such phenomenon is known; this article explains why it would be hard to detect expansion locally anyway: http://arxiv.org/pdf/astro-ph/9803097v1.pdf. It can be detectable at larger scales though. If it is physical, it may not end up completely uniform (i.e. there could be "expansion waves"). In any case, measuring the expansion is very interesting, and I do wonder if redshift is all we will ever have to measure it.
     
  11. Dec 28, 2015 #10

    Chalnoth

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    As Drakkith mentioned, there is no effect due to the expansion.

    The most naive method for mathematically describing the expansion of the universe assumes that the universe is perfectly uniform. Obviously it isn't perfectly uniform: there are star systems and galaxies and clusters of galaxies and more. When you weaken the assumption that the universe is uniform, what you find is that overdense regions are stable: they don't change* as the universe evolves. The exact same gravity theory which describes the overall expansion also maintains that gravitationally-bound systems are stable.

    So what you have is a universe where early-on, it's mostly uniform, but some regions are a bit more dense than others. Those regions that are "dense enough" compared to their surroundings stabilize and stop expanding. There are also other effects at smaller scales that cause things to collapse inward (friction and the fact that gravity is a non-linear force). So overall, galaxies tend to become more dense over time.

    *Also as Drakkith mentions, this isn't completely accurate. There is some pushing force due to dark energy.
     
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