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Spacetime: Stretching or Growing? Or

  1. Feb 17, 2012 #1
    Let me try to establish two things up front. First, is why I am making this post. And that is fairly straightforward: we have a pool of knowledge here that may provide clarification/refutation on some of the things covered below. Second, why do I even have a notion that the expansion is one of the types in the thread title. This, also, is fairly straightforward. A ballon, while expanding, gets thinner. To put it another way, the ballon is made of no more material when blown-up then when deflated. The material is stretched thinner. An empire which expands, on the otherhand, will do so by acquiring more territory. It has grown.

    Expansion is a general word. It can be applied to two quite different processes and yet say nothing of how a process is occuring - other than via context. And in the context of the expansion of space it would seem to say very little.

    I am, or should more accurately state, was open to either form - or a third if anyone can think of a suitable analogy. I am, in a sense, still open to either, yet also feel that with only a little additional reasoning one can be laid to rest, even if the other remains with a question mark over it.

    Where did I get these ideas from? The ether. I'm sure you'll be pleased to know I'm not talking about that ether. No, it's really just random thoughts colliding. I have reached the point of making this post primarily because of a series of Susskind lectures on cosmology, available on youtube. After an initial post on these forums using a thought experiment, which, while terribly erroneous, still managed to reveal alot, I was still left unsatisied about the mainstream view on how space expands.

    Susskind's lectures add considerable weight to the view of a growing space. He seemed quite clearly to be telling his students that new space forms and this is why the cosmological constant stays constant, not diluting as the universe expands. Multiple times he offered an analogy of a rubberband being stretched, yet never breaking as "an invisible reservoir of atoms" filled in the forming gaps between the bands existing atoms. So how do you view this? I can find nothing wrong on the surface. Space must be expanding in some fashion and the idea of 'growth' does not preclude any of the observed phenomena, such as redshift.

    Redshift, however, is much trickier to explain in a stretching space. Well, the redshifting itself isn't. But as the light enters a 'less-stretched' region of space, you would expect it to begin blueshifting. But by how much? Enough to entirely undo the redshifting (which of course would be contrary to observation)? Does redshifted light in a growing space also experience some amount of gravitational blueshift? This would make objects further away still than they are already calculated to be. As you can see, my inability to provide answers to these questions, is partly what leaves the issue so wide open.

    From here we move into speculation, some mine: some not. Following Susskind's lead, rather than turn every sentence into a mouthful of "if, buts, maybes" and "it is suggested/expected" I will just say it plain and you can take it with as big a pinch of salt as you care to, as your leanings on the question of inflation go.

    During inflation the vacuum energy was considerably higher than it is today. The bottom dropped out after a number of e-foldings had occured. In Susskind's words "the universe went over an edge". As well it did. Most of the VE was turned into thermal energy and other particles. The Vacuum was then left with very little "in the tank", so to speak.

    So, what's this got to do with space? If new space is formed during expansion, does it come into existance containing the post-reheat energy or the inflation period energy? If the former, why? If the latter, it might suggest the energy spreads out around the region leading to the notion that the expansion has steadily been filling the tank back up, a drop at a time. The stretching space scenario seems to have a superficially simpler explanation for the constancy of the vacuum energy, via concepts such as tension increasing the potential energy. This would seem to move toward string theory.

    There are questions about how either idea plays out in a matter dominated region, with stretching space coming out ahead, compressing then in the presence of gravity in accord with the curvature of General Relativity. Growing space, however, has a lurking can of worms labelled with a big question mark of ultra-speculation waiting for us beyond the more straight-forward acceptance that while it may grow, once grown, space simply acts according to GR. Part of this is related to a very simply question: does space stretch, then fill in with new space, or does new space form, pushing out the 'old' space?

    This question provides a neat cut-off between a direct, potentially meaningful querying of current views, versus baseless speculation. I hope I have stayed on the rightside of this line throughout and apologise if any of it goes too far.

    Having said that, one last area does remain. And I feel it shouldn't be over-stepping the mark to make some logical deductions about it. The Big Bang. If space has been expanding through stretching, then it implies there is no more space today than there has ever been. However, the growth of space would seem to lead inevitably to the idea that t=0 also means space=0, such that the undesired singularity is reduced to non-existance. I make no assertion that this explains the existance of the universe, and only reveals another in the series of Russian nesting dolls. Indeed, I'm fairly certain the general idea isn't even original.

    I came here to learn. Not to teach.
  2. jcsd
  3. Feb 17, 2012 #2
    What we call space expansion can as well be viewed as matter contraction.
    As particles and galaxies shrinks wherever they are, they appear to be moving further apart.
    Are space expansion and matter contraction two exactly equivalent ways to describe the same phenomenon?
    Maybe they are, but maybe not.
    Of course I don't know the answer.
    In any case seeing things from another viewpoint can help.

    Just look at a computer simulation showing a comoving volume of space.
    We see matter contracting and coalescing into a 3D spiderweb like network.

  4. Feb 17, 2012 #3
    Here, have a read:


    "Expanding Space: The Root of all Evil?"

    It doesn't matter whether or not you think space is expanding or that we are shrinking, as long as you understand the thoughts behind it from those who actually understand what is going on. After that, you can call it what you want, but there is an excepted terminology for the sake of causing less confusion, which is kind of what is going on here.
  5. Feb 17, 2012 #4


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    I can't see the picture from this computer, but I'm pretty sure a shrinking universe cannot fit the observed evidence without some pretty severe asumptions that break current known laws.
  6. Feb 17, 2012 #5
    It's a really good picture. Insta-saved it.

    As for the paper that was linked. Dubious. They say space has no more a meaningful existance than a model of a magnetic field, then started proposing it only be thought of in terms of cosmological fluid, which is essentially a GR model... In some respects I was on board with what they had to say - space expansion isn't really a well-defined description, but they argue Wheeler's well known description of spacetime can be shortened to simply "matter tells matter what to do".

    It had the merit of being quite straightforward, though, particularly with regard to explaining why an individual galaxy doesn't fly apart. They even mention the formation of new space at one point.
  7. Feb 17, 2012 #6
    I basically agree with your comment.
    Thanks for the reference, it is very interesting.
    It deals directly with the many issues I have trying to understand expanding space.
  8. Feb 17, 2012 #7


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    Ok, I'm at home now and can view this picture. That picture is not of "matter contraction". It is normal gravitational attraction between matter and possible dark matter with the result being the clumping of galaxy clusters into filaments that we see today.
  9. Feb 18, 2012 #8
    In his defence he stated it was images from a computer simulation of a comoving volume of space. I assume his point was to say that with the individual images fixed at one size the matter gathering and appearance of voids cannot readily be discerned as either purely space expanding or matter contracting. Of course, the process being modelled actually covers both, but without the added information that the images are comoving volumes you cannot, by the images alone, say space is expanding. In my defence, I'm not suggesting he's right.

    In another thread in which he posted much the same I inquired if a matter-contraction model actually required the opposite of a big bang: a large volume of existing space within which diffuse matter contracts under gravity, becoming ever denser.

    EDiT: Actually, looking over those images I'm not sure they actually show any spatial expansion - the filaments don't adjust. The basic structure is clear in all 6 images, the rest of the material is just gravitationally drawn in.
    Last edited: Feb 18, 2012
  10. Feb 19, 2012 #9

    The main simulation generated by the CLEF-SSH collaboration is the CLEF hydrodynamics run, which features a flat Omega_m=0.3 cosmology and about 158 million particles (2x(428)^3) of gas (baryons) and dark matter inside a comoving volume of (200 Mpc/h)^3. This simulation was run at the French national parallel computing centre of CINES, and consumed about 66000 cpu hours on 128 (SGI R14k) processors. This corresponds to a total wall-clock time of about one month.

    The CLEF hydrodynamics run is presently one of the largest large scale structure simulations including models that account for the effects of radiative cooling and energy feedback in the formation history of Galaxy Clusters. It was run using a modified version of the parallel code package Gadget II (Springel, Yoshida & White, 2001), with an energy feedback model by (Scott Kay, 2004).
  11. Feb 19, 2012 #10


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    Yes, but that picture is not showing "Matter Contraction" which you said could be what expansion is.
  12. Feb 19, 2012 #11
    I don't get space stretching ( or growing ). Take the Bohr radius for example. It's a constant length, the product of universal constants. How does the space inside the radius know not to expand and the space outside the radius know to expand? This doesn't make sense to me.
  13. Feb 19, 2012 #12


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    Inside of and around most matter the gravity is much too strong for space to expand. This is analogous to how you can pick up one magnet with another in the sense that one force simply dominates over another. The strength of the magnetic field is too much to allow gravity to pull the magnets apart. It's simply a result of the different forces adding up as they do. So space never "knows" when to expand or when not to it simply happens when the strength of gravity drops off enough.
  14. Feb 19, 2012 #13
    So what about a lone atom in a very very void part of space? You say space expansion "simply happens" and yet the Bohr radius is the Bohr radius. The space inside the radius knows not to expand and the space outside the radius knows to expand. I don't get it.
  15. Feb 19, 2012 #14


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    Are you sure space doesn't expand inside the Bohr radius? It is a given that the atom doesn't increase in size, but this is because the electromagnetic force holds the electron to the nucleus. Besides, just because space expands doesn't mean that it simply carries everything with it. If the various forces are strong enough they will hold things together against expansion. On large scales this is why galaxies stay grouped into clusters and superclusters.

    As an analogy, consider the radiation pressure from a star pushing on a planet such as Earth. This pressure doesn't force us from our orbit because gravity simply overwhelms it. The force on the Earth is like 0.0000000001% the strength of the Suns gravity, much to weak to affect us that much. It does, but the effect is so small as to be unnoticeable on everything but the most precise measurements.
  16. Feb 20, 2012 #15
    The Bohr radius is the Bohr radius. It's a forever constant length, the product of universal constants that never change. The space inside the atom knows not to expand and the space outside the atom knows to expand. Is this an unanswerable paradox of nature?
  17. Feb 20, 2012 #16
    The space inside the atom does expand, but the electromagnetic forces holding the electron and proton at the Bohr radius easily overwhelm the new space being formed and maintain a constant distance.

    Just as gravity does to everything closer than 200 million light years.
  18. Feb 20, 2012 #17
    "Overwhelm?" or completely, absolutely, and perfectly negates all traces of space expansion? If the Bohr radius is the Bohr radius, a forever constant length, the product of universal constants that never change, then there's no compromise. The space inside the atom must not expand one bit. Otherwise, the Bohr radius is not the Bohr radius.
  19. Feb 20, 2012 #18
    The electron shell isn't a solid barrier. A spatial expansion within the shell radius might simply flow outward without disturbing the electon. The electron will only move if supplied with the energy to do so.

    You also need to understand that if spatial expansion did cause an atom to grow - let's exaggerate and say it doubles in size - then everything, including you and your measuring device that tells you the Bohr radius is the Bohr radius will grow equally. You will continue to measure that the Bohr radius as the Bohr radius, utterly unaware of what has happened.

    Finally, as others and yourself have point out, the attractive forces of the atom may prevent the space from actually expanding in the first place.

    In all three cases the measure of the Bohr radius is unaffected.
  20. Feb 20, 2012 #19


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    No, not if it doesn't carry the electron with it. To be honest if space is expanding inside the Bohr radius then it might apply a "counter force" and push the electron out a little bit, increasing the Bohr radius. However this increase would be so tiny that we wouldn't be able to measure it. So our constant would still be just as useable either way. Just because we call it a constant doesn't mean that it actually IS a constant.
  21. Feb 20, 2012 #20
    I agree with you. This image doesn't show or prove "Matter Contraction".
    My point is just that "matter contraction" has exactly the same observational effects as "space expansion": redshifted galaxies, the more redshifted the further away they are.
    Of course, if both descriptions are equivalent, then we might as well stick with a standard viewpoint like "space expansion". Nothing wrong with that.

    A comoving volume basically always contains the same portion of the universe (the same galaxies, etc...).
    We have the redshift formula 1 + z = d(0) / d(z).
    d(0) is the distance to a given galaxy today.
    d(z) is the distance to a given galaxy when the light we see was emitted.
    So, in the usual description:
    the comoving volume at z=4.0 is 5 times smaller than at z=0.0.
    the comoving volume at z=3.0 is 4 times smaller than at z=0.0.
    the comoving volume at z=2.0 is 3 times smaller than at z=0.0.
    the comoving volume at z=1.0 is 2 times smaller than at z=0.0.

    Because a comoving volume basically always contains the same portion of the universe,
    I find it is more natural to consider such a comoving volume as dimensionnaly fixed, which also means we have to consider its content (galaxies and atoms) as shrinking.

    What is gain in practice by adopting this viewpoint?
    Really nothing concrete I must say.

    So it's not like "matter contraction" is any better than "space expansion".
    But I still think it is good to be aware of both interpretation.

    We always have to be careful
    1-to accept the observational evidences (of course)
    2-to be critical of our own model build upon them

    For example, the observed redshifts can not be denied.
    What they really mean is another matter, open to debate.

    Stretchy Space? http://www.chronon.org/articles/stretchyspace.html

    Is space expanding? Is space stretching? Is is thinning as it stretched?
    Is matter shrinking?
    (if we could see our bubble universe from some other bubble universe, whatever that may mean,
    then maybe space expansion would be different than matter contraction,
    cause we would have an external reference point)

    Consider this interesting problem, the tethered galaxy problem.
    This is a quite perplexing problem.

    If we could hold a galaxy at a fixed distance for a while, using some strong rope, what would happen if we release the rope?
    Would it stay at the distance it is because it has no recessional velocity to begin with?
    Would it (suddenly?) be dragged away by the underlying expanding space? by some kind of frictional force?
    Try the same problem in the contracting matter interpretation. It's no simpler.

    Clearly I don't really understand space expansion, and the somewhat recent discovery that it is accelerating is not helping.

    I also recommend the reference provided by Spourk in a previous post:
    "Expanding Space: The Root of all Evil?"
    Last edited: Feb 20, 2012
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