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The Edge of the Universe

  1. Oct 8, 2005 #1
    If the universe is expanding, meaning that space-time itself is expanding, what would you see if you were at the edge of space-time looking out? Would it just be blackness? Would there even be a boundary from our perspective? And if you were at the edge, what would happen if you tried to move beyond the edge?

    This question occurred to me a couple of nights ago and I thought it was intriguing. If there is a boundary between our space-time and the total nothingness on the other side (if you can even think of it being the "other side"), what characteristics would this boundary have?

    Any thoughts?

  2. jcsd
  3. Oct 8, 2005 #2


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    Well that's quite a question. I'm afraid it will be one with no satisfactory answer. We can keep asking can't we. If we define the characteristics of the beyond-the-universe "void", then what's beyond this "void". Oh, well note that space IS "void" in our space-time definitions.

    Normally I would consider the "universe" to be the boundaries of where our laws of physics apply and so predicting what lies beyond our science and physics would be purely philosophical. What is space if not for the concept of length and distance. What is a void if not for the concept of mass and energy. Will these concepts exist beyond our universal boundaries?
  4. Oct 8, 2005 #3


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    Welcome to these Forums Neiby! Keep asking questions - it's how we learn.

    Space is generally taken to be either finite or infinite but in both cases without boundary.

    How can a finite space not have a boundary? Consider the surface of the Earth, it is an example of a spherical space (approximately), finite yet you never come to the 'edge' and fall off! A finite universe is thought to be the 3D equivalent of this.

    I hope this helps,

  5. Oct 8, 2005 #4
    I don't grasp that concept at all. Assuming that the Big Bang is true, then the universe, space-time, is expanding like a huge balloon. I'm wondering what it would be like to be right to the surface of the balloon as it expands. Are you saying that the balloon is expanding, but there is no dimensionality to the surface of the balloon?

    In other words, are you saying that there is no "there" there?
  6. Oct 8, 2005 #5
    As a follow-up, let's assume that there is no boundary from our perspective. Let's also assume that the Big Bang actually happened and that the universe is expanding like a balloon. Based on the motion of celestial bodies, it should be fairly simple to determine the center of the universe and also a calculated (if non-existent) boundary. Actually, it wouldn't be necessary to calculate where the boundary lies if you know where the center of the universe is. If you begin at the center, any direction takes you to the expected boundary.

    Now, if you were travelling in a straight line from the center of the universe toward the expected boundary, where would you end up? If it's like walking on the surface of a sphere, would you end up on the other side of the universe heading back toward the center? :-)
  7. Oct 8, 2005 #6
    Here's an interesting link on this topic that I just found:

    http://www.eadon.com/phil/univedge.php [Broken]

    It points out that the universe is expanding faster than light. Is that true, as far as we know? The other interesting point he makes is that from our perspective there is no center of the universe. No matter where we look, it appears that we're looking toward the center of the big bang. Interesting.
    Last edited by a moderator: May 2, 2017
  8. Oct 8, 2005 #7


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    Yes! Except, taking the surface of the Earth as an analogy, where would you place the centre on the surface of the Earth?

    Space-time does not expand, expansion requires time to pass and time is already accounted for within space-time. If you are going to visulaise space-time then you have to visualise it as a "block" universe, static, within which are many 'slices', or foliations, of 3D space separated by the time dimension.

    Note: how you foliate space-time into individual slices of 'space' is dependent on your frame of reference, mutually moving observers foliate the 'present', separating the 'past' from the 'future', differently - simultaneity is relative to the observer.

    If the universe has more than a critical amount of average density then a slice separating out the space hyper-surface simultaneous with the observer co-moving with the centre-of-momentum of the material in the universe will find their slice to be the 3D analogue of a 2D surface of a sphere. [Assuming the normal conditions of homogeneity and isotropy]

    Such a surface would be finite but unbounded. So if you travelled in a straight line you would end up where you started from, just as you do travelling along the surface of the Earth. If you have difficulty with this concept then you are in the same position as ancient mariners worried that they might sail off the edge of the Earth.

  9. Oct 8, 2005 #8


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    But a closed universe isn't like the 2D surface of a 3D sphere, it's more like the 3D surface of a 4D hypersphere. So no matter which direction you travel in 3D space, you're still on the surface.

    Have you read the book Flatland? Imagine a bunch of two-dimensional beings (who are incapable of moving in the third dimension, or even visualizing it, although flatland mathematicians can understand it mathematically) living on a 2D surface curved into a sphere, and you have a good analogy for what a closed universe is like.
  10. Oct 8, 2005 #9


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    Welcome aboard, Neiby. As others alluded to, the expanding balloon analogy is horribly inadequate, but the closest that a 4-dimensional mind can come to envisioning it. The difficult part to handle (for me, anyhow), is that although the universe is expanding re: galactic recession, there's nothing beyond for it to expand into.
  11. Oct 8, 2005 #10
    unless as foam in a bubble bath with each bubble being a self contained universe, we have information and energy leakage into other bubbles by blackhole/white hole mechanisms

    as one bubble expands another contracts and so on but the whole system maintains equilibrium

    only instead of being on the surface of any bubble we are embedded in the membrane skin
  12. Oct 10, 2005 #11


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    Try this model for size.

    The hyper-sphere of 3D space has a radius that is increasing with time as the universe expands. That radius can be used as a measure of time, in fact this is conformal time in the closed GR universe (Omegatotal >1).

    (If the universe is strictly linearly expanding with time then this radius is atomic time, in which case the expansion of the universe and the passing of time may be thought of as two separate ways of experiencing the same phenomenon. This is the crux of Self Creation Cosmology.)

    Whether the radius is conformal time or atomic time does not matter, we can ask if the universe is such a hyper-sphere what then lies 'within' it and what lies 'outside' it?

    One response would be to say: "Nothing - the question is meaningless and arises from pushing the hyper-sphere analogy too far."

    However with the above understanding of the radial nature of the time dimension in a closed universe then the answer is obvious, inside the present universe hyper-sphere is that of the past, and outside it is that of the future.

    "The universe is expanding from its past and into its future!"

    Just a thought,

    Last edited: Oct 10, 2005
  13. Oct 10, 2005 #12


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    It sounds like your talking about taking successive "moments" (spacelike hypersurfaces) and embedding them in a single 4D space, like the layers of an onion, and then labelling the radial dimension time. But you can't take an arbitrary spacetime, foliate it (slice it into a series of moments/hypersurfaces) and then expect that all the slices can be embedded neatly in 4D space with no overlapping. For example, even in the case of a perfectly homogonous, positively-curved universe, where each slice is a hypersphere, you still have the problem that a positively curved universe should eventually begin to contract after expanding for a certain amount of time, so how do you arrange things so that a slice from x years after the big bang doesn't occupy the same points in your 4D space as a slice from x years before the big crunch? And if you dealt with spacetimes that weren't perfectly homogonous, that had little "dimples" in each hypersphere that were moving around, things would probably get even messier. Finally, I don't think even a single 3D spacelike hypersurface from an arbitrary foliated spacetime can necessarily be embedded in 4D space--as an analogy, think of the Klein Bottle, a curved 2D surface which can't be embedded in 3D space without self-intersecting.
  14. Oct 10, 2005 #13
    Okay, my head hurts now. ;-)
  15. Oct 10, 2005 #14


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    I am embedding the space-like foliations, of spherical k=+1 space, in 4D space-time, taking the succesive spherical space hyper-surfaces and projecting them to be concentric, as you say like the shells of an onion. Locally each point in 3D space relates to the time dimension as in Minkowski space-time, orthogonal and with a metric signature [+,-,-,-], or [-,+,+,+].

    In a GR model each event in the diagram is multiply-connected; in the simple case of a single expansion from a Big Bang and single collapse to a Big Crunch each point in the model is double (one point going out and another coming back in), similar to the projection of a 3D transparent sphere onto a 2D surface.

    However, this visualisation does suggest that each point represent a single event and therefore that the expansion is monotonic. In this case we are talking about a modified GR cosmology, if the radius represents atomic time then we are talking about a model in which R(t) = t, linear expansion and k = +1, rather than the GR Milne empty universe where k = -1.

    My work led me to modify GR to include both Mach's Principle and the Local Conservation of Energy to develop Self Creation Cosmologyand in its Einstein Frame the cosmological solution was indeed R(t) = t, and k = +1!

  16. Oct 10, 2005 #15
    ripples in a spherical pond...

    ...that always brings to mind whether we are the first cause stone that made the ripple or just one of the waves echoing into infinity

    with regards to the edge thing...

    ...from my understanding, space is not expanding from the "edges' of the universe. It is expanding from the inside.

    galaxies are spreading apart faster than lightspeeed as more space is being inputted into the system between the galaxies. This makes me think that "white holes" are planck size hence a hard to detect and that spacetime is a semi permaeable membrane which grows like osmosis...

    ...check this and think of spacetime


    then imagine energy leakage from black holes in our universe seeping into other universes by miniscule white holes...


    ...the expansion could then merely be the swirling of the membrane as the skin of a bubble does depending on where you are looking from as we are embedded in the membrane itself

    So in effect what we might have is a form of steady state that is in equilibrium along with other universes in a foam like situation where it appears it is expanding...

    I like to apply the foam analogy of universe at the macro scale to spacetime at the micro scale with the same thing happening at both ends.

    more foam, more bubbles, more spacetime, more universes and i don't see why it's not possible to have bubbles inside of bubbles.

    here's something i wrote akin to poetry quite a few years ago and at the bottom is a link i just googled not 5 minutes ago

    Imagine, if you will bubbles...
    expanding as they float around
    bumping into other bubbles
    and inside of these bubbles
    is another bubble expanding
    and so on...

    ...and if all these bubbles
    made a musical note,
    as they bumped and merged
    and expanded,
    they created chords and melodies
    and so on...


    gravity in this case would be surface tension and photons would have mass density relative to their psotion in the electromagnetic spectrum


    so everything is made of light...kooky eh ???


    please indulge my fantasy world as all this is conjecture and speculation on my part given I have no formal training in any matter related to this...

    ...as you were
  17. Oct 11, 2005 #16
    Well, let's supose that the universe is like the earth in 4D. The earth do have boundaries in the third dimension, the space. So if we make our way into the forth D we could escape the universe. And here the big question returns What would we see, feel, smell... outside the universe?
  18. Oct 11, 2005 #17


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    As long as we have your brain reeling, it seems only sporting to deliver a hard right hand... We already reside at the edge of the observable universe.

    Everything we can observe from earth happened in our past [due to the finite speed of light]. The further out in space you look, the longer ago it happened. The same thing is true for any other hypothetical observer who happens to exist in our 'now' anywhere in the universe. As a consequence, everyone in the universe is at the edge of their observable universe [which also gives rise to the illusion they are at the center].
  19. Oct 11, 2005 #18


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    Normally when we picture a curved 2D surface like the surface of a sphere, we think of it as sitting in a higher 3D space. But while there's nothing to say it's impossible that curved 3D space couldn't be sitting in some larger 4D space, mathematically there is no need for such a thing--instead of describing the curvature of a surface with reference to a higher-dimensional "embedding space", it is possible to describe curvature using purely intrinsic features that could be observed by a being confined to the surface (like whether the sum of angles of a triangle drawn on the surface is more, less, or equal to 180 degrees), and general relativity uses only such intrinsic features to describe what it means for space to be curved--see this page on differential geometry, the mathematical basis for general relativity, which talks about the difference between intrinsic and extrinsic descriptions of curvature. Also, once you describe curved surfaces in terms of differential geometry, I think it's easy to come up with curved N-dimensional surfaces that can't be embedded in N+1 dimensional space without self-intersecting, like the Klein bottle that I mentioned earlier.

    One final point here is that the universe would only be positively curved like the surface of a hypersphere if the average density of matter/energy throughout space is above a critical density, called "Omega". If the density is equal to Omega, space will have 0 curvature and be "flat"--the 2D analogue here would just be a flat plane rather than the positively-curved surface of a sphere. And if the density is below Omega, the curvature would be negative--the usual 2D analogue people give is the surface of a saddle, although I don't think this is a perfect representation (on an actual saddle I don't think the curvature is the same everywhere). For more info on this, look at Ned Wright's cosmology tutorial, which covers the different types of curvatures in section 3. The current evidence suggests the curvature of space is flat, or at least very close to flat.
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