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Edge of the universe?

  1. Dec 13, 2009 #1
    So there must be an edge, a point where matter stops and from there on in is a complete vacuum. The universe is supposedly expanding at an estimated 50-100 km/s/Mpc which is slower than light speed. So this means that light from the universe goes into a place (outside of the universe), that technically doesnt exist.

    So....whats up with this?
  2. jcsd
  3. Dec 13, 2009 #2
  4. Dec 13, 2009 #3


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    We are at the temporal edge of the universe, does it look any different in any direction?
  5. Dec 19, 2009 #4
    Still, where has the CMB photons that have already passed us (e.g. the ones a long time in the past)? Where have they gone? Can they travel forever without reaching some kind of border even though they are traveling faster than expansion?
  6. Dec 19, 2009 #5
    They go away from us.
    Yes, they can travel forever.
    They are not traveling "faster then expansion". At first, some objects in the Universe can recede faster then light (yes, surprise, surprise, you can not analyze the expansion from SR point of view, you need to use General relativity). But no expansion is needed for the photons to travel forever. It is possible in infinite universe or even finite (closed) universe (they will run in circles)
  7. Dec 21, 2009 #6
    Dmitry67's point that is depend if the universe is open or closed determines if the photons travel forever or go around in circles forever.

    My question is if the universe is closed does that mean the escape velocity is greater than c? And does that mean the universe is a black hole?

    It is important to make the distinction between how far we can see out at this time (i.e the age of the universe times the speed of light), versus the extent of matter, versus the extend of space. For myself I am unsure if the last two are related.
  8. Dec 21, 2009 #7
    The escape velocity question does not make any sense.
    Nothing under no conditions can 'escape' from the universe
  9. Dec 21, 2009 #8
    If the universe is open can we ask how fast must an object be going to continue on forever?
  10. Dec 21, 2009 #9
    'open' means that universe is infinite
    For example, flat universe (infinite, "newtonian" 3D space) is an example of "open" Universe
    Obviously, there are no problems for any photon to travel forever in infinite universe.
  11. Dec 21, 2009 #10


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    This issue has nothing whatsoever to do with the concept of escape velocity.
  12. Dec 21, 2009 #11
    What is the definition of a universe being closed?
  13. Dec 22, 2009 #12


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    Usage of the terms "closed", "flat", and "open" changed around year 1998 when it was realized that the universe could be spatially closed (in the sense of overall positive spatial curvature, like a hypersphere, or the idea finite spatial volume) and yet even though it was spatially closed it could continue expanding forever.

    Before 1998 when you looked into pop cosmo books you would see a very neat and misleading triple package.

    A:"closed" meant spatially closed and also the (wrong) idea that she has to end in a crunch
    B:"flat" meant spatially flat, zero spatial curvature, and also the connotation that she keeps expanding but at a diminishing rate that is just barely fast enough to avoid re-collapse.
    C:"open" meant negative spatial curvature, typically an infinite spatial volume, and also carried a connotation about future expansion

    The explainers always locked you into associating some future expansion (or contraction) scenario with each separate spatial configuration. But it doesn't have to be that way.

    After 1998 everybody recognized it wasn't that pat and simple---that with a positive cosmological constant, or equivalently with dark energy, you could have an expansion scenario more or less independent of which spatial configuration.

    So closed has come to mean (at least for some of us) spatially closed. And it doesn't mean you have to end in a crunch.

    The typical picture of a closed universe is the hypersphere, the 3d analog of a 2d surface of a sphere. The 2D toy model analogy can be pictured as the surface of an expanding balloon. But all existence is concentrated on the geometrical surface of the balloon. There is no rubber, there is no inside-the-balloon or outside-the-balloon. All that exists is 2d stars and 2d creatures on the 2d surface.
    And then to get the real world you jack that picture up from 2 to 3d.

    Matter is roughly uniformly distributed over the whole of space---back in the toy model case that means over the whole 2d surface of the balloon. There is no outside-the-universe. No edge. No boundary. A finite 3d volume, just like in the 2d toy model there is a finite 2d area (of the surface of the balloon).

    You asked what does "closed universe" mean. I think that is about it. Spatial closure, edgelessness, finite volume, finite circumference. I'd like to hear Dmitry's definition, though. In my experience he's reliably informed clear and concise.

    If you have problems with anything I or anybody have said, keep asking questions.
    Last edited: Dec 22, 2009
  14. Dec 22, 2009 #13
    Last edited: Dec 22, 2009
  15. Dec 22, 2009 #14
    Marcus definition is more accurate.
    I provided an oversimplified version ignoring Dark Energy

    And you see? I was right, there are muc h more serious issues
  16. Dec 22, 2009 #15
    I dont see how anything of that can be "deduced" from what I said
  17. Dec 22, 2009 #16


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    How does any of this make sense if we reside in the 'oldest' region of the universe?
  18. Dec 22, 2009 #17
    If there are other 'finite' universes like ours out there, there must be an 'infinite' number of such universes past and present. Can't be just 2 or 3 now can there? And if the photons can travel unimpeded forever ... then we would detect these photons leaking into our local universe from any direction.

    Unless there was a property of infinite space we are not aware of that would prevent photons from ever making it to the great voids between universes.
  19. Dec 22, 2009 #18
    I'm too tired to cover the rest tonight. But I will tomorrow.
  20. Dec 22, 2009 #19
    You use the assumption that if there are more than 1 infinite universe they must somehow intersect. You assume that these universes share the same physical space. This assumption is wrong. But definition, Universe is mathematically complete, so nothing can 'leak' to or from another universe.

    Mathematically any number of infinite Universes can coexist. If you ask: do they exists in the same space? the answer is N/A
  21. Dec 22, 2009 #20
    I make no such assumption. In my model they CAN'T intersect. Where did you get this idea? The distance between these infinite in number 'finite, local universes' ... like ours ... is equivelent to the distances between materializing sub-planck particles. They are nowhere near each other.

    'THE' universe is SPACE. It is infinite. OUR universe occupies but one tiny, finite 'geographic' location in infinite space.

    I must be a terrible communicator .... sorry.

    Did you actually read my model? You seem to have missed all the important points.

    Oh. I'm sorry again. Forgot which forum I was on. No, you haven't read my model. Sorry for the confusion. It's posted on Science Forums. SFN. Also under pywakit, if you are interested.
    Last edited: Dec 22, 2009
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