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Infinite universe but finite beginning?

  1. Jul 23, 2014 #1
    As far as i understand the current big bang theory, it started as a extremely dense object, finite in size. But we still think (or well it is very accepted to belive) that the universe is infinite. I know inflation should be though as an expansion everywhere at the same time rather than the ball expanding. But how can it transit from finite to infinite?
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  3. Jul 23, 2014 #2


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    There was no transition. It is believed that the universe started off in a very dense, very hot state. If it was infinite in size at this point, it is still infinite now. If it was finite in size, then it is still finite in size. It is currently unknown whether the universe is infinite in size or not.
  4. Jul 23, 2014 #3
    Hmm okay. Its just that to me it sounds like when people mention it, that its a well known idea that the universe was a little "ball" to begin with. But guess i just misunderstood it
  5. Jul 23, 2014 #4


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    If they don't say otherwise, assume they mean "observable universe", not the whole universe. The observable universe is finite in size and is the result of the finite speed of light and the universe having a finite age.
  6. Jul 23, 2014 #5


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    Sometimes people like to visualize it that way. What they really mean is that it was a sphere or "point" that had infinite density. That's just a way to try to help visualize it, if I understand what you’re talking about correctly.

    Honestly, like Drakkith says, we don't really know whether the universe is finite or infinite. All our measurements to date suggest that the universe is flat, relativity homogenous, and infinite. There are a couple of caveats to this however—space-time could be flat but have a topology like a torus or that small pieces of curves can appear relatively flat —it’s still a quick moving area of research.
    Last edited: Jul 23, 2014
  7. Jul 29, 2014 #6
    now a real fun thought for those who do believe that the universe is finite is what is at the edges, One theory is that the universe is infinite because if you keep going in one direction you will just end up back where you started (kinda like driving around the world). There is no way to prove this because we cant even leave our own solar system in a reasonable amount of time, but it is fun to think about what is out there. If we could "drive around the universe" then that would prove the 4th dimension and show that our universe bends and twists in the 4th dimension without our knowing (like the flatlanders bending and twisting through the 3rd dimension)

    Even if we can't for sure prove any of our theories about the shape of the universe beyond our observable bubble, it is impossible to keep from wondering. There is no way to know for sure yet, so let your imagination run wild.

    "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth." -Arthur Conan
  8. Jul 29, 2014 #7
    If you take seriously the idea that traveling in a straight line eventually brings you back to where you started, it seems that some strange things emerge concerning the structure of the universe... especially the relationship between volume and size (as in, how to fit an infinite volume within a finite size). In an expanding universe it looks like one can fit an infinite number of infinite universes within a finite volume... as concentric shells.

    Let's start by temporarily switching the speed of light to infinity in order to help simplify the geometric description, and once that is clear, we can set it back to finite speed and look at the results.

    The reason for setting the light speed infinite to construct the geometry is because the distance to the first shell is beyond the point at which the rate of expansion approaches light speed. As things (galaxies, clusters, etc.) would be observed near that limit to be shortened in their direction of travel, there becomes plenty of room for more of them to slip into place - their apparent thickness approaches paper thin, then much thinner, leaving eventual room to fit an infinite number of infinity thin objects into the last region before the limit...

    All that happens within the first shell, and that is all we can possibly "see", so the temporary infinite light speed serves to extend the inference to the "other shells" that are unavailable to be observed, but only imagined.

    So instead of thinking of traveling out and returning to the point of origin, let's think of looking out in a line of sight and seeing what we see as we look further and further. With the light speed switched to infinite, and assuming we have a magic telescope so can continue to see further and further, we see the back of our head at some very far distance away (where we have visually completed the first lap around the universe). We will see the back of our head in any direction we look - so we note that we are in the center of a shell whose inner surface shows the back of our head no matter which direction we look, and it is way out there.

    No need to stop at this first shell, for if we continue to look further, we essentially are looking through another light lap (still infinite speed for now) around the universe and we again encounter the back of our head, and this happens again and again an infinite number of times... each describing another shell that comprises "the back of our head", or a projection of "here" being out there over and over...

    Now that we see the shells as geometric locations, we can turn the speed of light back to finite, since the shells locations (radii from "here") are "out there" whether see can ever see them or not.

    First question is about the spacing of the shells. With light speed set to infinite, the geometry is a "snap shot", so the distance from one shell to the next would be constant (the lap length around the universe is the same for each lap because geometrically all the laps occur at once). So each shell's radius (distance from us) would be a simple multiple of the first shell's radius.

    But with a finite speed of light in an expanding universe, the apparent distance between each subsequent pair of shells is decreasing, and the farther shells represent earlier and smaller states of the universe. If you see far enough, the spacing between the shells approach a limit where each "lap" around the universe is smaller than the previous one (you are "seeing" through smaller universes with each shall passing).

    So the father you look (or think), the shorter the lap length around the universe - so the smaller the increase in radius to the next shell. Note that each shell is not just a projection or illusionary image of "here", it is an actual "self seeing" just as real as looking at anything else, and this "self seeing" structure as it relates to the geometry of the universe makes an infinite number of these as concentric shells of "here" but "out there", over and over approaching the limit as the distance between shells decrease with distance from us...

    This leads to a somewhat inside out universe as if the world we experience is like a very low density region within a universe that approaches infinite density with distance away from our local region (our "first" primary shell is the central part of this region with subsequent shells leading off toward the infinite density limit as the shells stack up at the limit).
  9. Jul 29, 2014 #8
    So kind of like when you point a video camera at the TV that is projecting what the camera is recording? where the tunnel gets longer faster and faster and each frame appears to be closer and closer together?
  10. Jul 29, 2014 #9
    Woow quite an explanation you made there ! Its really difficult to wrap my head around but i think its starting to settle and im only in high school so my education level doesnt help me heh.
    Now the thing that has been confusing me for a while (which i felt that you clarified here) is the difference between an infinite universe in the terms of that it has no borders and you would end up basically where you started, like you were talking about. And then, the universe that is infinite in dimensions as in x y z meaning that when we continue wandering out in space we continue out for real as in we dont curve around ending up at the same place. Also as you wrote with the expanding universe, that you could fit infinite universes into finite room, just makes my head blow off haha. But its really just so interresting.
  11. Jul 29, 2014 #10


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    Not if the universe has a finite age (which it does). With a finite age and a finite speed of light, any observer will only ever see a finite number of "shells": they will only see one shell until the universe is old enough for light to have gone around it once; then two shells until it's old enough for light to have gone around twice; etc. (And if the universe is expanding fast enough, it's possible that light can *never* make it around even once, so nobody ever sees even a full single "shell".)
  12. Jul 29, 2014 #11


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    You most likely did NOT misunderstand what you heard. This "well known idea" is in fact what is said most often on pop-sci TV shows, along with a lot of other stuff that is equally just flat wrong.
  13. Jul 30, 2014 #12
    Yes... in my toy universe I made the switch for lightspeed selectable between finite and infinite in order to use the infinite setting only to reveal (really infer) the shell geometry, then switched it back knowing that it was unlikely even much of the first shell could be seen.

    I was lax in distinguishing between what might be "potenitally possibly seen" vs what might be "inferred to be there, but unseen, unseeable"...
  14. Jul 30, 2014 #13


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    But if you can only see a portion of a single "shell", on what basis would you infer that there are multiple "shells"? That is, on what basis would you infer that the universe you are in is "periodic" in space?
  15. Jul 30, 2014 #14
    I have a question: If the universe were actually finite, what would we find at the edge? would it have the same properties as when the universe was created? so that the closer you get to the edge, the younger mater is? but this would cause some trouble for if we could actually see this, because we would eventually be able to see beyond the start of the universe. The fun thing is that due to the expansion rate of the universe being greater than the speed of light, we would never see this process happening unless we were able to travel directly to it and experience it ourselves. we would also die... big bang atomic slurry is not good for space ships...
  16. Jul 30, 2014 #15


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    There is no edge and there is nothing "beyond the universe".

    If the universe is finite, it is unbounded, like the surface of a sphere. No center, no edge.
  17. Jul 30, 2014 #16
    How can you say for sure that there is no edge? can you prove it? can you see it, touch it, taste it, smell it or hear it? Everything in this discussion is theory. all we have to "prove it" is our math which for all we know could be flawed.

    Just have fun with the theories, like i said earlier, "Let your imagination run wild"
    Last edited: Jul 30, 2014
  18. Jul 30, 2014 #17


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    Letting your imagination run wild should not include violating known principles. That ends up being a waste of time.

    If you believe that the Cosmological Principle is not valid (and it would have to be invalid, along with a lot of other physics if there were a center or an edge to the universe), this is not a good forum to explore that belief since the Cosmological Principle is accepted physics and this forum is all about accepted physics. There are plenty of forums on the internet where you CAN challenge accepted physics but this isn't one of them.

    Thinking outside the box is best done if you first learn what is IN the box.
  19. Jul 30, 2014 #18


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    Unknown. It's generally believed that if the universe is finite it is unbound, like Phinds said. There's no way to know for sure.

    You are free to let your imagination run wild as long as you stay within the bounds of PF rules.
  20. Jul 30, 2014 #19
    Well... that's where it gets a little shaky.

    If I use a meter stick to measure the curb width of my front lawn, I lay it down, measure, then pick it up and move it, and keep going... If my neighbor comes over and says that I can't repeat using the same meter stick to make each subsequent measure, but must use a tape measure long enough to span the whole width, I will argue with him about that...

    ...on what basis would you infer that there are multiple "shells"?

    1] Is it true that if one lived in a universe where traveling on one direction ultimately leads back to where you started that with sufficient resolving power and time, one would see ones' observing location? So all directions would form a shell?

    2] Then if so, when one looks farther, will it happen again and produce the appearance of more shells?

    One has to take some liberties, but it looks to me that if 1] is true, 2] should follow. Then it looks like there are at least two perspectives:

    One] There is really only one shell. The shell contains everything and the apparent shell boundary is really just you seeing yourself, if it was possible to do so, through the "long way" around the universe (the longest possible way, I think). Each subsequent shell is another "lap" around, and no more real.
    So all the shells are observational artifacts of a curved universe (illusions).

    The main support for this perspective is that from a particular position, all directions point to an interior point on every shell that represents, as an example, "the back of my head"... every interior point of every shell, same thing... and likewise for any position from which I observe... and likewise for every other creature in the universe from wherever they observe (and whatever the "back of their carapace, or horns, or gelatinous dendritic blob" looks like... the shells' appearance content is observer based... only the geometry of the shells is "invariant".

    Two] The shells are real in spacetime. By invoking a spacetime perspective that offers each event the quality of being real, suggesting that since the events comprising the shells could not share simultaneity, the shells coexist at different epochs(?) in spacetime, or they each represent a different "event set" in spacetime, and so are all independently separably real...

    I'm assuming the main support here is that the reality of events in spacetime are considered "as real or more so" than any partition of a present local "reality"...?
  21. Jul 30, 2014 #20


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    Yes, *if* the universe is old enough that there has been time for light to travel all the way around the universe and back to its starting point. But that's a big "if".

    Yes, *if* the universe is old enough that there has been time for light to travel all the way around the universe and back to its starting point, multiple times. But that's an even bigger "if".

    Only if both "ifs" are in fact true. But, for example, in our universe, they're not; that is, if our universe is in fact finite in volume but unbounded, so light can in principle circumnavigate it given sufficient time, there must not yet have been sufficient time, because we don't see duplicate images of our own galaxy (or any other galaxies, for that matter).

    These are two different models that might possibly make the same observational predictions. Up to now, we have been implicitly assuming the first model; considering the second "perspective" you give requires changing the model. See further comments below.

    This is not a model of a universe with finite volume but no boundary (which is what your first "perspective" describes). It is a model of a universe with infinite volume, which just happens to contain multiple copies of the same finite sub-volume, arranged in such a way that it looks, observationally, exactly like a universe with finite volume but no boundary. This can be modeled mathematically, but of course it's astronomically unlikely physically.
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