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Expansion of space in our neighborhood

  1. May 5, 2015 #1
    I know that expansion of space operates farther away from our Local Group of galaxies. But the reason for not considering expansion in our galactic neighborhood is:

    A. The space expands everywhere, the expansion is just too small close to us so we don't need to talk about it.


    B. The space really doesn't expand AT ALL in our local group. Matter, local galaxies are bound together and as matter influences space, the texture of space is held together by gravitation and space doesn't expand AT ALL in our Local Group.

    Is my question silly? Is it just semantics? Curious to hear your answers.

    Thanks, Virgil.
  2. jcsd
  3. May 5, 2015 #2


    Staff: Mentor

    Yes. "Expansion of space" is just another way of saying "objects in the universe are, on average, moving farther apart". The average has to be taken on a large enough distance scale to eliminate the effects of locally bound systems like galaxies or solar systems or stars or planets.
  4. May 5, 2015 #3


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    I think it is indeed partly semantics, Virgil. Since space is not a "thing" or a "material", what do we mean by "space expands"?
    What is the operational meaning, in terms of measurements we make and results we expect?

    I think "space expands" means that distances between observers at CMB rest grow in size at some fractional rate.

    The concept of at rest is very important because one cannot even be clear about distances at some given time, or growth at a given rate, unless one has a concept of universe standard time. The Hubble law, the apparent pattern of expansion, only has meaning if one pictures a network of stationary observers (at rest wrt the ancient light from when the ancient matter was more or less evenly spread). These stationary observers can all synchronize their clocks because they are at rest. To be contemporaneous means to measure the same temperature of the CMB and estimate the same age of universe expansion. Geometry is the web of geometric relations between them. The distances between them are steadily increasing.

    To the best of mortal knowledge all the galaxies and gas clouds etc live in a soup of ancient light which is remarkably uniform and is what provides us with our criterion of rest. I forget how many CMB photons there are per liter of volume, at the present time. Some known number.

    Redbelly, a PF member, said something perceptive--I remember it from years back. In effect it is not "space" that expands, it is the CMB that expands.
    It is the soup of ancient light that expands.
    It is the criterion for being at rest in the universe that expands.
    And so OF COURSE the distances between stationary observers increase at some fractional or percent rate per unit time.

    When people study the expansion and make increasingly careful measurements of the rate, and the "redshift distance relation" etc, they correct for the motion of the solar system relative to CMB. they convert all their measurements so as to be from the standpoint of an observer at rest.
    The solar system is not at CMB rest, it is moving 370 km/s in the direction of constellation Leo. The speed and coordinates have been determined much more precisely, but I don't remember them. That motion is subtracted out of the data. Distance expansion is something observed to happen between observers at rest.

    Of course the observers at rest in the Virgo cluster are hypothetical. :smile:PeterDonis makes an important point when he says "ON AVERAGE" moving apart. We expect that the random individual motions of things in their surrounding space will sort of average out, at least over some large scale. And they do seem to.

    It would be nice to know some people in the Virgo cluster who could measure their individual motion wrt CMB the way we do, and converse with us and tell us their motion, that they correct for. So we could be really precise about this. But we don't and it would take such a long time if we did. So we have to rely on individual random motions averaging out, and take a large sample and do "least squares" etc. Statistics.
    Last edited: May 5, 2015
  5. May 5, 2015 #4
    Thank you PeterDonis and Marcus. I was trying to picture some kind of interaction matter - space like the ubiquitous "matter curves space" from GR, thinking that maybe matter within bound systems "keeps hold" of the space within. But these kind of descriptions seem unnecessary, maybe like not being able to describe quantum mechanics using normal macroscopic intuition / verbalization.

    Thanks for your feedback,
  6. May 5, 2015 #5


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    I think you are right! I think it does "keep hold". I'm thinking of the geometry close to a massive body or black hole. Let's try to sort this out.

    Maybe our stationary observers can't be deep in anybody's gravity well. That would throw their clocks off, for one thing. And it is hard to see how they would measure the CMB dipole. Maybe they have to be freely drifting in interstellar/intergalactic space.

    The themes PeterDonis emphasized, like on large scale and on average, do seem important.

    I think your initial question was good, and also your intuition about matter and geometry being interlocked somehow. It doesn't hurt to try to sort this out. There does seem to be an essential vagueness about the scale at which these ideas apply. I'm not an expert, just an interested bystander. I don't know a clear neat resolution.
    Last edited: May 5, 2015
  7. May 5, 2015 #6


    Staff: Mentor

    The FRW model of the universe as a whole has no "gravity wells" in it; it assumes a constant density of stress-energy everywhere. Obviously this is only true when averaged over large distance scales.

    If you want to look at things on smaller distance scales, the key question is, what determines the dynamics of objects? More precisely, is there any contribution to those dynamics from "expansion of the universe"? The answer to that is, no, there isn't. (Strictly speaking, there is a very, very small contribution from dark energy; but it's easier to see the distinction I'm making if we assume zero dark energy for this discussion.) For example, if we look at the dynamics of stars in the Milky Way galaxy, that dynamics is the same as it would be if spacetime were flat and the universe were not expanding; the stars do not "know" that they are in an expanding universe. Their dynamics is purely determined by the overall gravity well of the galaxy.

    Similarly, if we look at the dynamics of galaxies in a cluster, say the Virgo cluster, the galaxies do not "know" that they are in an expanding universe; their dynamics is determined by the overall gravity well of the cluster. And if we look at another cluster a few billion light-years away, the dynamics of the galaxies in that cluster is purely determined by the overall gravity well of that cluster. But, if we look at the relative motion of the two clusters (say, of their respective centers of mass), we find that they are moving apart. That sort of relative motion is what we call the "expansion of the universe"; but as far as the clusters are concerned, it's just relative motion, not any kind of extra dynamic effect.

    As far as the idealized "stationary observers" are concerned, it is true that the closest approximation to such an observer would be someone who was out in the middle of empty space between galaxy clusters, and who was moving in such a way that they see the CMB as isotropic. Proper time for such an observer would be the same as the "comoving time" that appears in the FRW metric (or at least, it would be closer than any other observer we could imagine). An observer who was in a gravity well, say someone near the center of a galaxy cluster, but still in empty space between galaxies, could still see the CMB as isotropic, but their proper time would tick more slowly than the proper time of the first observer above, because of being in the gravity well. And, of course, someone very deep in a gravity well, say by hovering near the horizon of a black hole, could have a proper time ticking much more slowly still, and also could not see the CMB as isotropic because of the bending of light by the hole's gravity.
  8. May 5, 2015 #7
    I have found some interesting ideas in Lineweaver and Davis, "Misconceptions about the Big Bang", it's on the 10th page (the article has 11 pages)

    I quote:

    "... Expansion by itself - that is, a coasting expansion neither accelerating nor decelerating -
    produces no force... A changing rate of expansion does add a new force to the mix, but even this
    new force does not make objects expand or contract.

    In fact, in our universe the expansion is accelerating, and that exerts a gentle outward force on bodies. Consequently, bound objects are slightly larger than they would be in a non accelerating universe, because the equilibrium among forces is reached at a slightly larger size. At Earth’s surface, the outward acceleration away from the planet’s center equals a tiny fraction (10–30) of the normal inward gravitational acceleration. If this acceleration is constant, it does not make Earth expand; rather the planet simply settles into a static equilibrium size slightly larger than the size it would have attained."

    So it would seem that there is some interaction. Of course it is exceedingly small.

  9. May 5, 2015 #8


    Staff: Mentor

    Yes, this is the tiny effect from dark energy that I left out in post #6.
  10. May 5, 2015 #9
    Thanks for that quote. Utterly fascinating. As one who has been pondering a possible inflow of spacetime for a very long time, such snippets are very valuable when trying to bring things into focus.

    It was on this forum many years ago that a moderator (I think) pointed me to the review of a paper on Gravitational Inflow. The paper required a very specific 'Fabric of Space' and I'd always assumed it was such an all-pervading field that was expanding at a very precise rate - later to learn that rate was changing. Is it conceivable that such a field is flowing into any object with mass, while at the same time expanding?

    As an aside, Gorge Smoot's use of a U2 as one of his tools to measure the CMB was clever, but the use of the CMB itself to 'accurize' the autopilot, was nothing short of astounding.
  11. May 5, 2015 #10
    Great, I'm happy then, I have my answer. Thank you all.
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