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How you picture the expansion of space.

  1. As an explosion from a central point?

    5 vote(s)
    35.7%
  2. As the surface of a swelling balloon?

    3 vote(s)
    21.4%
  3. Infinite flat sheet of graph paper, with squares growing in size?

    4 vote(s)
    28.6%
  4. Infinite loaf of bread-dough growing by yeast action?

    2 vote(s)
    14.3%
  5. Humongous donut...

    0 vote(s)
    0.0%
  1. Jul 24, 2003 #1

    marcus

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    Which of these comes closest to how you picture the expansion of space?

    As an explosion from a central point with galaxies flying away from the center of the explosion?

    As the 3D analog of the surface of a balloon?

    As the 3D analog of an infinite flat sheet of graph paper on which the squares are gradually getting bigger? (suggested by PF poster jcsd recently)

    As an infinite loaf of bread-dough expanding by the action of yeast

    As a humongous inflatable donut
     
  2. jcsd
  3. Jul 24, 2003 #2

    Eh

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    I find it easier to imagine geometric things like that in terms of field lines. Instead of picturing an expanding area or volume, it is much easier to visualize 3 sets of lines forming a grid. Expansion then is easy to picture, with the space in between the grid squares getting bigger.
     
  4. Jul 25, 2003 #3

    marcus

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    I imagine what you are describing as an infinite cubical lattice, correct me if I have misunderstood.
    It seems like a good clear image and could also be seen as the 3D analog of the graph paper image suggested by jcsd.

    So, to test your picture out, the CMB is believed to date from z = 1000 (estimated 300 thousand years after expansion started). So for you, space at that time would be the same infinite cubical latticework picture, but with the lines 1000 times closer together. Right?
     
  5. Jul 25, 2003 #4

    wolram

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    i think of it as an orb of high frequency vibations expanding, as it does so the wave lenghs get longer and more diverse up to time x
    when expantion continues but the wave forms stabilize
    but i also think it is expanding into a pre exsisting PROTO space
    "wording not very good".
     
    Last edited: Jul 25, 2003
  6. Jul 25, 2003 #5
    I like the ole "Balloon-A-Verse" as a 2-d spatial analogy of what is happening to 3-d spatial space, because it shows that the Universe does not have to be infinite. I hate infinities .. they are repugnant to me .. I know of nothing within the Universe that is actually infinite (i.e., it is only a mental concept), so I dismiss the idea that the Universe might be infinite.

    And, in trying to return that back to a 3-d spatial view of the Universe expanding, I like to think of it somewhat like a sponge, the fibrousness (is that a word??) throughout the sponge representing a fabric of space. It shows an underlying structure that I believe the Universe has.

    For expansion, imagine pulling apart the sponge from all sides, but as the volume increases, I think of the fibrousness not getting diluted any, or getting pulled apart. Actually, the fibrousness stays constant, because as the Universe is getting pulled apart, new fibrousness is constantly getting created to fill in the gaps. I think that corresponds to what we think of as the false vacumn, and energy, and furthermore that energy being viewed (later) as matter, by some unknown process.

    So, the Universe didn't create all of its matter/energy at the BB .. it was just the most concentrated episode of matter/energy creation. It is continually being created, as a consequence of expansion.
     
  7. Jul 25, 2003 #6

    wolram

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    proto space is akin to but not, a mixture of two gasses that have the property of stabillity ,a deficiency of one of the gasses would allow the other gass to erupt "similar to exposing phosphorus", the eruption would be our universe, as proto space wants to be stable the eruption would disperse to infinity.
    "terrible analogy because the gasses dont exsist" but the two THINGS do. must get back to the institute bye.
     
  8. Jul 25, 2003 #7

    Eh

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    Something like that, though the lines don't have to be that much further/closer. The distance between lines in a graph can represent any arbitrary distance you want, so in the early universe the scale becomes much smaller.
     
  9. Jul 25, 2003 #8

    Eh

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    As for infinities showing up in nature, I'm not so sure it's easy to brush away. How about the infinity of points, lines and planes in any given volume? Even if space is actually a discrete lattice, is it not true those lines would still be a continuum of points? Other infinities show up in nature as well.
     
  10. Jul 25, 2003 #9
    BTW, I like the ole "Balloon-A-Verse" analogy for another reason. It shows (to me at least) that both Einstein and Newton were correct. I know I'm treading on thin ice here! Be kind!

    Einstein is correct because (as I understand GR with my limited knowledge of it) GR is a description of what can be or could be observed in the Universe using things contained only in that Universe. I think it fails to fully answer what mass/inertia is. I mean, I think there are two components of inertia: a component gained by motion within the Universe, but there is also a "base" value of inertia, or call it mass, that GR doesn't account for. Another way to state it is that not all concepts of Mach's Principle (not that I believe MP) are incorporated into GR. I've read where some cosmologists agree that MP can be fully incorporated into GR if the Universe is finite and unbounded.

    Newton is correct too as there is absolute motion. Back to the "Balloon-A-Verse". The surface is used to show that no point on the surface is the center of expansion, and since no point is the center, that all motion has to be relative. But, acceleration and rotation are not relative. But looking at the balloon, there is a center of expansion, or a place to measure things from absolutely .. the center of the balloon. Einstein's GR doesn't consider it as it considers stuff only within the 3 spatial dimensions. This center lays outside of what we are able to reference with our senses, or observation.

    And therein also is the description of that "base" mass/inertia that GR cannot describe fully. It is actually caused by motion, or acceleration maybe, through a 4th spatial dimension .. moving away from that point/center of expansion. Expansion itself is the cause of the base inertia. We're not just "hanging" in 4-d spatial space reference, but moving/accelerating through it. GR only comes about as a correct description because it doesn't consider that "base" mass/inertia/motion.

    And, I think if you're careful, the problems of understanding rotating objects, why they tend to remain rotating on the same axis for one, drops out too. I can't fully invision this, but rotation is absolute to that point of expansion in the 4th spatial dimension, i.e., any way you could orient yourself in 3-d space and rotate still constitutes rotation around that point in the 4th spatial dimension.
     
  11. Jul 25, 2003 #10
    I'm not sure that is so much of a "physical" attribute of nature/the Universe, but a mental constraint imposed on the Universe by our need to try to describe it. I don't think it a clear-cut case of an infinity in nature.

    What other infinities do you say show up in nature?
     
  12. Jul 25, 2003 #11

    Eh

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    Well, we can logically show that points exist in any given volume. So in that sense, they are real in that they exist, and there are an infinite number of them.

    Let's say, the charge of an electron as you get closer and closer to it. That kind of infinity that seems to show up often. As well, there is the wave function in quantum physics that give us an infinite amount of possible states for the universe as a whole. Unless one can throw away wave functions as not being real, that would seem to be another unavoidable infinity.
     
    Last edited: Jul 25, 2003
  13. Jul 25, 2003 #12
    Well, for this, and back to your original point that a volume can be representated by an infinity of points, lines, etc. I think that is akin to the argument/paradox that goes like this:

    Given a distance to cross, you can never reach it because you first have to go 1/2 the distance, then 1/2 of what is remaining, then 1/2 of what is remaining, etc. You should never reach it, but clearly you can and do reach that distance. It's an infinity argument, and the problem I see with it is that it supposes that time and/or space can be infinitely subdivided, and that the infinity holds only as a concept, not in the nature of distances.

    In the end, I think that is basically the same "1/2" argument as above, as the measure of the charge of an electron by other methods does yield a finite value. It's only our trying to calculate the charge of that electron, using QED, that introduces the infinities, and indication of a failure of QED; it not being a complete description.

    A problem with my arguments here is that all we know or can know about the Universe comes from our thought processes. I'm defining "infinity" to originate in and exist only because of our thought processes. I can always fall back on that in denying infinities! ;) But I don't think our thought processes are/should be considered as being separate from the Universe. In that respect, there is a case of infinity in Nature.

    I'm getting philosophical there, but I don't think it can be completely removed from most scientific discussions.
     
  14. Jul 25, 2003 #13
    I'm really glad you brought a
    selection of baked goods. I'll
    have a doughnut please. I don't
    see any reason to rule out a
    great roiling, expanding toroid,
    such as you see at the base of
    nuclear blasts.
     
  15. Jul 25, 2003 #14

    marcus

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    effective use of alliteration and
    assonant dipthongs---roiling toroid
    good image with a good sound
    personally I greatly prefer raisin bread to donuts
     
  16. Jul 25, 2003 #15
    Absolutely. You are a physicist
    and a gentleman. And a convivial
    thread host.
     
  17. Jul 25, 2003 #16

    Eh

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    Yes, it's called Zeno's paradox. It doesn't seem to hold as much water these days. There have been several threads on this on this site, and one is at https://www.physicsforums.com/showthread.php?s=&threadid=3848&highlight=zeno from the math forum, and one from the philosophy forum at https://www.physicsforums.com/showthread.php?s=&threadid=1589&highlight=zeno

    Yes, those seem to arrive with the assumption on continuous space. But even if it is discrete, the same problems with the lines arise, and the infinities of wave functions seem to be intact.
     
  18. Jul 25, 2003 #17
    Hmmmm. I could say that there would be infinities in a wave function only in an infinite Universe, but if I did that would make my whole argument chicken-and-egg (i.e., one reason I say the Universe is finite is I know of no infinities in it, and then the reason this couldn't be a "real" infinity is because I believe the Universe to be finite). So, I'll look elsewhere ...

    I think you (meaning anybody) have to look to the collapse of the wavefunction for the answer. The wave function is predictive up until that point, and only gives you probabilities. Can you assign a "true" (using that term loosely) meaning to the wave function or its collapse? I'm under the belief "no", to either. But, then again, the Universe to our knowledge seems to act out only under possibilities.

    Also, doesn't the wave function deal with "things" that are to be detected? I mean, it doesn't predict nothing will happen, does it -- or it doesn't ever have give an amplitude of zero does it? If it doesn't ever give an amplitude of zero, I don't see how it could represent infinite possibility (probability is 1/amplitude?????, or something like that).

    I'd say you had a "possible" infinity there. I'm not sold yet! ;)
     
  19. Aug 2, 2003 #18

    marcus

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    Ω0 = 1.02 ±0.02

    In the course of watching cosmologists I have gradually become more interested in knowing their current consensus view than in having an opinion myself!

    What they often say in words is that the U is spatially flat. The most natural picture to go with that is what Eh said:

    an expanding 3D grid made of 3 sets of parallel lines
    like infinite 3D graph paper, it starts out infinite and it continues that way but with the lines getting farther apart.

    but I was just looking at the best current summary of cosmology
    which I believe is the Lineweaver article and it said, as so many other articles do,

    Ω0 = 1.02 ±0.02

    that is there is an ERROR BAR
    and just think, if Ω0 = 1.02 then NACHO IS RIGHT and the whole thing does after all look like a 3D EXPANDING BALLOON.

    Ye gods. In that error bar there is room both for the perfectly flat infinite space of Eh with
    Ω0 = 1 (exactly)
    and the positively curved finite space of Nacho with
    Ω0 = 1.02

    The case with Omega > 1 used to be called the "closed" case but that open/closed terminology has become confusing with the discovery of dark energy. They would ordinarily assume a zero cosmol. const. Λ and then Omega > 1 finite was always doomed to collapse in a Big Crunch wherease Omega = 1 flat was able to expand indefinitely. But now with a positive Λ even the balloon model can keep expanding forever.

    Indeed the indications are that this one we are in, whether Omega is 1.00 or 1.02, will expand increasingly rapidly forever.

    Anyway either picture is reasonable because of the error bar and Ned Wright in his most recent "News of the Universe" has a page where he is considering the finite universe Ω = 1.02 case and fitting supernova data to it to see how it looks. He is obviously keeping an open mind. As behooves him as a scientist because if there is an error bar then it could turn out either way.

    What is this Omega-sub-naught?
    It is the present value of the average density of energy in the universe divided by the critical density. the zero subscript means present value since both the real density and the critical density are changing.

    Ω0 = rho0/rhocrit

    The best list of the current specs, with error bars, occurs in several places one of which is Lineweaver page 32

    http://arxiv.org/astro-ph/0305179

    But he is just copying the WMAP report of Bennet et al

    http://arxiv.org/astro-ph/0302207

    and the WMAP satellite is sitting out there a million miles from earth in its own orbit around the sun and telling us what Omega and other stuff is and until they fly a more accurate instrument they wont be able to say whether Nacho picture or Eh picture is the right one
     
    Last edited: Aug 2, 2003
  20. Aug 2, 2003 #19
    marcus,

    Good read, even though I didn't understand much of it. Section 9 was the best. What caught my eye was Omega(tot) being 1 within measurement errors. It hovering around 1 like that is probably the worst thing that could happen for telling us whether the Universe will expand forever or not. Their error bar may get better in time, but I doubt they will ever be able to tells us which it is: 1 side or the other 1, or 1 exactly. That is, with these kind of measurements.

    Theory is gonna have to take over, and make its best case for which one it is. I think the Inflationary theory seems to indicate (by reasoning) that it is exactly 1, as inflation would have stretched it out if it was not exactly 1. That is one reason I say their measurement errors may get better, but they're not going to get good enough (1 part in 10^60, or around there).
     
  21. Aug 3, 2003 #20

    marcus

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    I agree. the error bar most likely will straddle cases around
    omega = 1. it is simplest to just suppose it is exactly one
    especially with the theoretical reasons for that which people have raised

    the role of inflation seems likely to change if the singularity
    at time zero is removed and the model predicts a prior
    contracting phase, but I gather the people involved want to
    keep inflation as part of the story, and merely narrow down the problems it is supposed to solve
    it solves
     
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