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Theories and questions on the expansion of the universe

  1. May 25, 2016 #1
    The universe is expanding at an accelerated rate. Space has no edge. If space is geometrically "round," meaning that a long enough journey could bring you back to where you started, then it could be said that the universe has no outer edge.

    At the moment of the big bang, the universe created space. There may have been space around it, but the universe we know now existed moments after the big bang as a tiny, dense version of what it is now, entirely contained within the totality of space as it was then. Every point in the universe is the center of the universe. Which seems to indicate that space is indeed "round."

    So, as a thought experiment, imagine that another big bang occurred, within our universe. What would it look like? It would be creating a self contained universe, with no edge. Would we even see it? What would happen to the fabric of spacetime in our universe?

    String theory necessitates the idea of curled up dimensions, hidden away within our normal four dimensional perception by being very tiny. A thin enough fiber will appear to be a two dimensional line, but under close enough magnification will reveal itself to be a fully textured three dimensional object.

    Let's also consider the dark energy, and dark matter necessary to keep our galaxies both self contained, and flying apart from each other. If every point in space is expanding, because space itself is expanding, and if every point in space is a tiny new big bang (which has its own completely independent laws of physics, and its own set of dimensions, including time), it is worth questioning if this could account for the missing matter and energy within our known universe.

    But maybe not every point in space is a big bang. Let's consider the apparent shape of the universe. It appears to be like a sponge, or the interior of a loaf of bread. There are huge bubbles of empty space. Stellar wastelands. And there are filaments of galaxies and nebulae stretched throughout, stretching away from each other, as if the loaf were rising.

    What's going on in those big empty spaces? Are they little big bangs?
  2. jcsd
  3. May 25, 2016 #2

    Simon Bridge

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    Welcome to PF;
    Personal theories are not discussed here ... but we do correct common misconceptions of regular physics so:
    The current emeging picture of the large-scale geometry of space-time is that it is flat and infinite ... but there are flat but non-infinite geometries so we can go with this for the sake of argument. You are proposing that the large scale topology of space time is finite and spherical.

    This is a common misconception.
    The "big bang" of big bang cosmology refers to the state of rapid expansion from the pre-existing hot dense state ... thus space-time pre-exists the big bang.

    This is a common misconception: the big bang was a rapid expansion of space itself - it should not be pictured as an expansion into space.
    There is no reason to posit any space "around" the Universe for it to expand into, which is a good-enoough reason not to.

    This does not indicate a spherical topology ... a flat topology is equally indicated.

    It is not clear what that means - you need to be careful to define your terms. By current models a big bang inside an existing space-time is a contradiction in terms.

    ... this is incorrect: a point has no dimensions so it cannot expand. Space expands - so labelled points are getting farther apart.
    ... makes no sense at all.
    The "new" space resulting from the cosmological expansion has the same laws of physics as the space that came before. That's kinda the point!
    No it isn't, because the idea is nonsense and the post degenerates into baseless speculation from here.
    To be fair - you were basing your speculation on common misunderstandings - this is a failure in pop-science communication, so it isn't yours.
    I am sorry you have been mislead like this. Hopefully you can start finding out the real physics instead.
    It's fun: enjoy.
  4. May 25, 2016 #3
    Thank you for responding so thoughtfully.

    I didn't realize that creative thought wasn't allowed here, but I'm glad you read the whole thing.

    I read as much as I can about physics, and i find it exciting. Clearly much is not understood, probably much is understood incorrectly. I always think about the "wheels on wheels" of pre-Copernican astronomy. It seemed to work, with enough tweaks, but there was anunderlying assumption that was wrong. I look at the state of our understanding of the physical universe, and i see wheels on wheels. Dark matter, dark energy, hidden dimensions, the whole inflation thing, etc. I think we might all be missing something really fundamental. I was hoping, not to be right, but to inspire a line of questioning among people who are more educated than myself.

  5. May 25, 2016 #4

    Simon Bridge

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    Steady on there - of course creative thought is allowed. It is possible to think creatively without proposing personal theories.
    The general guidelines can be found here:
    ... scroll down to "general contents guidelines".

    This is a science forum - so we encourage people to construct their ideas through application of the scientific method.
    This is a form of constrained creativity which is useful for making models of reality.

    Unconstrained creative thought is fun, it's called "art".
    Also google, "introduction to philosophy of science".

    That is great. However you should be aware that works intended for the general public are often "for entertainment" and contain many errors and omissions that can be misleading if you try to extrapolate from them. The idea is to get your brain "fizzing", as you seem to have experienced, so you will be encouraged to go to better sources for a further education.

    ... but the statement is not much use as written: what is "not understood", and by whom?
    It is fun to make creative statements that are open to interpretation, but science demands creative statements that can be tested. This means you have to be fairly definite when you talk to scientists.

    Do you suppose that "lines of questioning" are not being pursued, or, perhaps, that this particular one has not already been considered before?

    It is a frustrating fact that many of the things you or I can think of have already been considered and discarded - usually many times over.
    There are, after all, far more ways of being wrong than there are of being right.

    The best use of these forums is to ask questions about areas of physics you find puzzling - especially as you learn more and from better sources.
    You could try:

    If you'd like to see what physicists use, as opposed to a watered down artistic version for the general public, have a look at:
    ... cosmology draws on general relativity. I won't expect you to learn all that, but it gives you an idea of what you are missing.
    You will eventually need it
    Last edited by a moderator: May 7, 2017
  6. May 26, 2016 #5
    I guess what I find puzzling is how spacetime can be flat and infinite, with its center everywhere, but not be "round," or perhaps a better term would be "closed." I always interpreted that, while not actually spherical, the nature of our universe was such that traveling far enough in a straight line would bring you back where you started. It was this understanding that led to my idea that closed bubbles of spacetime separate from our universe might exist within it. I clearly have more reading to do.
  7. May 26, 2016 #6
    Just to clarify, the reason I inferred that the universe is closed is that, while infinite and edgeless, with its center everywhere, it is expanding, and was once much smaller than it is now. So I'm also puzzled by this.

    During the first handful of Planck times, just after the big bang, if you could somehow survive in the heat and chaos, the universe would be very small, but still have no edge, and its center would still be everywhere. Imagining that you were able to survive, and had a space ship that could travel in this hot, dense universe, and could travel at a speed faster than the expansion of the universe, (this is a thought experiment, i realize none of these things is possible) what would happen if you set out in a straight line? There is no edge to meet, but the universe is still very small. Would you not end up back where you started?
  8. May 27, 2016 #7

    Simon Bridge

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    You are trying to understand cosmological expansion by a common analogy: that of an expanding balloon.
    Probably a better way to handle this for an infinite flat space-time is just to think of it as particle density.
    At some point in time, the particle density was very high, now it isn't.
    This happened because the particles moved away from each other.
    They do this because they are stuck on a grid that is expanding.
    If you were one of those particles, you'd see all the others moving away from you - with faster speeds for the ones farther away.

    This is a little harder to imagine but gets you away from things expanding into other things.

    Inifinite-flat is not the only option though.
    It is possible to have a flat closed geometry (toroidal), flat, semi-closed (cylindrical), and it is also possible that the Universe has spherical geometry just with less curvature than we can measure. There are infinite other possible topologies that are consistent with the observations though, these are just the real simple ones and Occam's Razor applies.

    Note: a balloon is a curved 2D surface in an outside 3D space that encloses it, but space-time has no "outside" so we infer intrinsic curvature based on geometry we can measure "on the inside" as it were.
  9. May 27, 2016 #8
    I'm actually fairly comfortable with this interpretation, I think.

    So, if I understand this correctly, the idea that the universe used to be smaller than it is now, or that it is getting bigger, is incorrect? It has always been infinite, just more dense? Now it is getting less dense, but is not necessarily getting larger? If that understanding is correct, it would go a long way toward helping me understand how it could be expanding with its center everywhere and still be flat. Definitely most texts I've read for laypersons have described the early moments of the big bang as being like a point, but perhaps this is just a mechanism for explaining the density. The entirety of the mass and energy (originally just energy) was not contained in an infinitesimally small area, but any given region of the universe was as hot and dense as if it were, yet the actual size of the universe was still completely infinite, and flat (or maybe not, but possibly)? Am I now thinking on the right lines?

    P.S. I really should have looked into this forum and what is about before I posted. I just had what I thought was a great idea, and wanted to post it somewhere people with higher education than myself who have devoted their study to understanding these things might read it, and I did a search and found this forum.
  10. May 27, 2016 #9
    I do want to add one more question.

    If this type of geometry is possible, is there a mechanism for describing this type of closed space existing within our universe, be our universe flat or closed, or is that a mathematical absurdity?
  11. May 27, 2016 #10

    Simon Bridge

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    Yes and no.
    In common terms - yes. If we say that one ball is bigger than another we mean it occupies more of the 3D volume external to itself than the other one does. The Universe has no outside so that concept cannot apply.
    But the Universe is getting bigger in the sense that each chunk of mass inside it occupies more of the volume inside the Universe: i.e. it is getting less dense: there is more empty space for all the matter to be in.
    This is annoying when thinking of infinities ... the infinite volume of space is bigger than the infinite volume of matter it contains because, while all the matter is totally within the space, there are parts of the space that do not have any matter in them. If you got that, look up "Cantor's Hotel".


    You can think of the scale as being the average separation of particles ... the equations that seem a reasonable guess for the Universe contract to a singularity as you go back: this is a special situation where the mean separation is zero.
    Nobody thinks this is literally the case - it's just that there is something wrong with the equation.
    (This is different from the singularity of a black hole - which is a point if it is not rotating.)
    The word "singularity" is a maths term for "woah - what happened?"

    Nearly ... all the energy (mass and energy are the same thing) was distributed equally through the entire infinite universe - just no gaps between bits.
    That's a bit handwavey... kinda suggests there were bits. I'm just getting you to concentrate on the infinity within infinity concept for now.

    Remember: way wayy before you get to the singularity, you get quantum mechanics.
    We currently have no verified unification between quantum mechanics and general relativity.
    This is why the big bang refers to the rapid expansion phase after the singular point and a bit after time started.
  12. May 27, 2016 #11

    Simon Bridge

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    In principle it is possible to tell from the inside if the universe is a cylindrical or toroidal vs infinite and flat, yes.
    i.e. if there is no curvature and you just went on a journey that got you back to where you started then it's a good bet you are in toroidal or cylindrical space-time.
    How long it takes you depends on which direction you travelled in: unlike in spherical space-time.

    Practically you test this by looking for repeated objects in the sky ... i.e. if you see the same galaxy in different directions it suggests the light took different paths around the universe to get to you. On a small scale this is called gravitational lensing. We've seen lots of the small scale example but none of the cosmological scale stuff, hence the assumption of infinite in all directions.

    I liked the spherical-closed one too... and the common posters about the evolution of the Universe shows a closed universe.
  13. May 27, 2016 #12
    Okay. I'm fully aware of the relationship between mass and energy, and the incompatibility of relativity with quantum mechanics. I'm comfortable with non-euclidean geometry, and I'm also aware that a singularity, like an infinity, is a mathematical way of saying "you're asking the question wrong." Those are precisely the wheels on wheels I was speaking about before. But, I also understand that my original post was more along the lines of art than science. I still like my idea, but clearly, I have a long way to go to get my thought experiments to in any way approach anything that could be considered science. I do really appreciate the time you've spent responding.
  14. May 27, 2016 #13

    Simon Bridge

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    Sure: most of what happens in PF is telling people stuff they already know ... so that, in effect they already knew the answer to their question.
    This makes talking to us more like sorting through your ideas.

    Cosmologists have some weird ideas - as an intro, try looking up the question "are we inside a black hole".
    One of the weirder ones - and you need the above to understand it - involves a speculative method for a Universe to give birth to another one: where a section of space-time gets closed off in on itself. There's lots of speculative stuff about this which is better for science fiction - Gregory Benford got a book out of it.
    Thing is: that level of speculation is really for elsewhere - not PF. If you want to understand something you've read about it though
  15. May 27, 2016 #14
    Is there a "lounge" type forum within PF, or another forum you can recommend for purely speculative, but not entirely uninformed discussion. I suppose you might not be permitted to advocate another forum within this one. I will search. Thank you again for your time and for your thoughtful and patient explanations.
  16. May 27, 2016 #15

    Simon Bridge

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    There is a "general discussion", a "science fiction", and a "beyond the standard model".
    Probably others. You can report your own post to a mentor and ask if you like... or report any post you suspect may be a bit borderline and ask the mentor for help improving it.
  17. May 28, 2016 #16


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    Simon gave you some excellent answers. Now I'm afraid I'm going to have to lock this thread.
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