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Edge of Space?

  1. Dec 29, 2007 #1
    Hello, recently I was listening to a radio talk given by Professor Brian Greene. I have always been able to understand him in his books, but on this particular occasion, he said something that I couldn't quite understand. He said that there is no edge to space, only an edge to time. If the universe is finite, how can it be that there is no edge to space. Is it that the universe curves over onto itself?
     
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  3. Dec 29, 2007 #2
    I think I remember reading something about this, but I don't remember completely what it was. Could some one please clarify this?
     
  4. Dec 29, 2007 #3

    Garth

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    Yes! If the universe is finite but unbounded it is because " the universe curves over onto itself".

    3D space would be the 2D surface of the Earth, go far enough in a straight line and, like Magellan, you would end up where you started. (In fact the expansion of the universe means this little exercise is impossible in practice.)

    The universe of course might actually be infinite in space, when it would also be unbounded, except in some unorthodox cosmologies.

    Garth
     
  5. Dec 29, 2007 #4
    you can start with simple model:
    Definition: SPASE IS THE SURFACE OF A BALL.
    This space (SURFACE) has no ege. Also, it's not infinite...

    and so on
     
  6. Dec 29, 2007 #5
    OK. Scientists actually have clue. There are models of our world, which is some space between a couple of branes. (see string theory). In this particular model space DOES HAVE edge.

    But in another models there is NO EDGE!
     
  7. Dec 29, 2007 #6
    Most models of space that I commonly see have space being flat, not spherical. They say that the over-all perspective of space is more like the "flatness" of a galaxy than it is of a spherical planet. This makes more logical sense b/c most objects that are expanding stretch out along a single axis (in an x,y,z co-ordinate) rather than along all possible degrees, as would be necessary if space where spherical. Also string theory is still in the realm of science fiction, it has a long way to go before it becomes a legitimate theory... theories must be testable.

    The idea of an edge of space is due to the fact that light is constantly traveling, hence the deductive reasoning that "space must be expanding at the speed of light."

    The idea of space folding on itself is still up in the air as well. Most string theorist and their like tend to show space being in a horshoe shape. This is how they try to explain the possibility of wormholes, they try to say that a strong enough force of gravity will connect to points along the two "arms" of the horseshoe.

    As far as the edge of time vs. the edge of space it will depend greatly on what we figure time out to be. Some believe that time is a physical quanta that permeates all of space. This would raise a paradox b/c all particles are subject to the "universal speed limit," the speed of light in vacuo, but the speed of light needs time to be measured. If this is true then time cannot expand faster than the universe, which means that time itself would have an escape velocity b/c if you travel to the edge of the universe and travel faster than light you therefore escape the universe and everything in it, including time itself.

    I have not read or heard Professor Brian Greene's opinion but in order for there to be an edge to time and not space he must be referring to a universal time, not one that originated with the Big Bang but one that regulates the expansion of the Big Bang.
     
  8. Dec 29, 2007 #7
    In layman term, space itself can expand faster than light.


    A universe can be finite without edge and yet being flat. Consider the flat torus for example. There is no need to involve wormholes here.

    In the standard FLRW universe, time does not expand. Only space expands. This is clear from the metric. Although sometimes sloppily we do say expansion of spacetime. But really we only mean expansion of space.
     
  9. Dec 30, 2007 #8
    I don't understand how space could expand faster than light if there aren't any superluminal entities, superluminal in vacuo that is. I've heard of such things in theory but never with any valid proof existence.




    But how can a universe be finite and still expand. It can be finite in the sense of mass properties such as never gaining or losing mass, or energy, but that doesn't necessarily mean that it is finite in its spacial dimensions.

    I understand the concept of the flat torus on paper but that doesn't necessarily mean that it applies to the universe. A similar setup is that of earth in that if you continue to travel in a straight line you will eventually wind up back where you started. The problem arises with the fact that you can escape the so called "edgeless earth." So can the earth really be edgeless if you can escape it with a certain velocity? The same thought pattern could apply to the universe. You can travel in space in a straight line and theoretically end up where you started, but would that depend on the velocity at which you travel in the same way it does here on earth? If you could escape it then could you really call it edgeless?

    As far as the original post about what professor greene any more info, such as context of statement, would help to give an idea of what he was talking about directly. I searched briefly but could not find anything through the search engine for anything written down.
     
    Last edited by a moderator: Jan 20, 2008
  10. Dec 30, 2007 #9
    Sorry, I don't remember what Professor Greene said word for word, but it was something along those lines. The radio talk I listened to was on youtube, but I think it was taken off. Sorry I can't be of more help as far as confirming what he actually said.
     
  11. Dec 30, 2007 #10

    marcus

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    Yenchin: In layman term, space itself can expand faster than light.

    Becker: I don't understand how space could expand faster than light if there aren't any superluminal entities, superluminal in vacuo that is. I've heard of such things in theory but never with any valid proof existence.

    my two cents: Yenchin is right. what he says is standard cosmology---Hubble's law, going back to 1930s. if v is current recession speed and D is current distance and H is current value of Hubble constant then v = H D. So naturally if you go out far enough you get superlum recession speeds. These are not forbidden by special rel. Special rel does not apply to recession speeds. Nothing exotic here.

    We run into this every week or so, please read the March 2005 Scientific American article by Lineweaver "Misconceptions about the Big Bang" It is online

    ***


    Yenchin: A universe can be finite without edge and yet being flat. Consider the flat torus for example. There is no need to involve wormholes here.

    Becker: But how can a universe be finite and still expand? It can be finite in the sense of mass properties such as never gaining or losing mass, or energy, but that doesn't necessarily mean that it is finite in its spatial dimensions.

    my two cents: Yenchin is right. Becker asks a further question: can an expanding universe have finite present spatial volume? The answer is again YES and it is just standard cosmology. An expanding universe can have either finite or infinite spatial volume. The decisive measurable parameter is called Omega and there is still enough uncertainty so we don't know which case applies---both are consistent with the data.

    ***

    Becker: I understand the concept of the flat torus on paper but that doesn't necessarily mean that it applies to the universe.

    my comment: I don't think anyone was trying to say that the universe is toroidal. That was just an example Yenchin was using---one of many possible spatial finite cases. It could toroidal, the data don't rule it out, but that's a possibility not as often considered as two others. The main two that I encounter most often are
    1. space is roughly a threesphere S3----so it is boundaryless, finite volume, expanding
    2. space is roughly Euclidean R3----so it is boundaryless, infinite volume, expanding

    To tell the difference between a very slight positive spatial curvature, which gets you (1) and essentially zero curvature except for local fluctuations which gets you (2) is a difficult measurement job. Right now the errorbar for Omega is just slightly on the positive side. More precise measurements, reducing the uncertainty, are in the works.
     
    Last edited: Dec 30, 2007
  12. Dec 30, 2007 #11

    marcus

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    When people use the surface of the earth as a pedagogical example to illustrate a finite area edgeless surface that is meant to be an example of the 2D surface of a sphere where all motion is confined to that 2D surface and there is no other space besides that. There is no space inside the sphere or outside it. All there is---the whole universe---is that finite-area 2D surface.

    You have to focus on that, and avoid imagining digging down into the earth or taking a helicopter and hopping up off the surface etc. Mentally experience what it would be like to be a 2D being in a 2D spherical surface where that is the space of the universe

    then you can extend that by analogy and visualize the experience of a 3D being living in the 3D analog-----the finite volume edgeless space called S3---where that is all the space there is. See if you can imagine that, there is no direction you can point in that would take you out of it. Imagine those directions don't exist. No analogs to digging down in or hopping up off. If you can imagine that then you can imagine the standard finite-spatial-volume version of the usual cosmology model. the standard model is called LCDM (lambda cold dark matter) and it comes in various versions and the usual finite spatial version with slight positive curvature is what you are picturing the space of.

    If you can't imagine it, well then you just can't picture living in S3 and something about standard LCDM cosmology escapes you. Not to worry, there are worse things :smile:

    I think we may need some kind of stickythread about superluminal recessionspeeds and S3, and expanding boundaryless space. A "please read before posting" kind of thing.
     
    Last edited: Dec 30, 2007
  13. Dec 31, 2007 #12
    Excellent, just beautifull. I agree completely. Time is a feel to make ourself feel passing through ages, or in simple words to make us feel that we exist by knowing and seeing things changing around and within us. people say travel with the speed of light and there is time dilation, i say what abt a particle at rest, time is also dilated for it. just imagine an infinite series of a very closely spaced (as much separated as u want it to be, separated by using the time lapse/interval u use, depends upon what the unit of time u use, second or milli-second, or even a day or a year) unchanged event re-curring throught the UNIVERSAL time frame....
     
  14. Jan 2, 2008 #13
    I understand what you mean here, but this is applying to a different perspective than what I'm talking about. The superluminal recession speed is referring to the expansion of galaxies and of space from our perspective, not necessarily the universe in its entirety. Because the universe is expanding at the rate that it is we cannot see the true "edge" of the universe. As the article says

    "An accelerating universe, then, resembles a black hole in that it has an event horizon, an edge beyond which we cannot see. The current distance to our cosmic event horizon is 16 billion light-years, well within our observable range. Light emitted from galaxies that are now beyond the event horizon will never be able to reach us; the distance that currently corresponds to 16 billion light-years will expand too quickly. We will still be able to see events that took place in those galaxies before they crossed the horizon, but subsequent events will be forever beyond our view."

    I am speaking of a more theoretical observation, I am speaking of the universe in it's entirety, beyond the observable "universe." Galaxies can recede from each other at superluminal velocities but the galaxy itself cannot expand faster than the speed of light, hence the resemblance to the event horizon mentioned in the article. If it could we wouldn't have an "event horizon" to our observable universe. If a galaxy cannot expand faster than the speed of light then the universe should not be able to either. Space may be able to expand faster than the speed of light but only if it is not carrying any information with it. That being said here is another theoretical question: What do we consider the universe to be, space or information?

    If it is information than it cannot expand faster than light, if it is space, well then what is space. A void? Because a void is nothing.
     
    Last edited by a moderator: Jan 20, 2008
  15. Jan 2, 2008 #14
    GeeSuggest you read Marcus's super posts again. The answers are there.

    I must admit that I am stuck though by your comments about galaxies expanding faster than light:confused: Oh maybe you mean that they separate faster than light?
    Whatever maybe a good idea to reflect on what has been said :cool:
     
    Last edited by a moderator: Jan 20, 2008
  16. Jan 2, 2008 #15

    Hi all, this is something i was thinking, you know the electron/positron pair thing that happens at the event horizon, could this also be happening at the edge of the universe or has the universe no edge.
     
    Last edited by a moderator: Jan 20, 2008
  17. Jan 3, 2008 #16
    I think I have a vague idea of what you are on about. Maybe this will help...
    Get a rubber band. Mark it at 20mm separations. Now stretch the band. None of your marks moved along the band did they? That is no galaxy moved through space.

    However each mark (galaxy) separated at a speed from each other because the band (space) had an expansion in length. Remember each mark (galaxy) has not moved position (galaxies are not travelling THROUGH space).

    Next step: Choose a mark. The adjacent marks (galaxies) have a recessional speed. The next marks further away have a recessional speed but it is twice as fast as the adjacent ones!!
    Each mark further away has a faster recessional speed from your chosen mark. An infinite band with infinite marks would have infinite recessional speeds even though none has exerted any energy to enable it to move along the band.

    Now, you notice that the marks themselves have got wider? I assume this is what you meant by expanding galaxy? Ok, the addition of space, the stretching of the band, is offset by gravitation. Imagine a thin sheet of slightly sticky rubber. Put 5 marbles on it in close proximity (stars). They form a depression (they are gravitationally bound). Now stretch the sheet in all directions slowly. They stay in their position in the depression even though the sticky stretching encourages them to separate.

    You can see that if you increased the stickyness then some marbles (stars)
    might separate and expand the galaxy. It's all a balance between the expansion of space and gravitation.

    You also said "Hi all, this is something i was thinking, you know the electron/positron pair thing that happens at the event horizon, could this also be happening at the edge of the universe or has the universe no edge."

    Very interesting
     
  18. Apr 21, 2008 #17
    Think of it in this way- Ask yourself "where is the edge of today?" Of course there is no "edge" as such, except, that is, the present- the now. Its the same for the universe. As I have suggested in other posts- everything (us, the planets, the universe) exists only in the "now". There is no physical edge. There is just "the present" the "Now"
     
  19. Apr 21, 2008 #18
    Except there is no way to observe what the universe is doing "now". The edge of time is right where YOU are right now. As you look out, you may ONLY observe the past.
     
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