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Boundary of the universe

  1. Feb 14, 2009 #1
    the universe is supposedly isotropic. it is also supposedly finite. if i were on a galaxy farthest from the earth, i would experience nonIsotropy (forgive the made up word). because if i look in the direction of the earth i would see a appropriately populated neighborhood with galaxies moving away with their speed proportional to their distance from me. but if i were to look in the opposite direction (with the earth behind me), i would see no other galaxies as i am on the farthest galaxy from the earth. if i were to see a galaxy, i wouldn't be the farthest, that one would be. how then is the universe truly isotropic (even at the boundary)? help!

    p.s. forgive me if i'm in the wrong subForum and direct me to the right one
     
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  3. Feb 14, 2009 #2

    marcus

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    You are in just the right forum!
    What puzzles me is why you imagine that such a place exists: a "galaxy farthest from earth".
    Modern cosmology does not assume any boundary.
    Matter and space are assumed to be co-extensive----more or less the same density everywhere----no surrounding empty space.

    You can help us by letting us know where you are coming from. Where did you get the idea that the universe is "supposedly finite"?
    It could indeed be finite! But it could also be infinite. I mean the standard model homogeneous isotropic universe---the basic spacetime region that we see. More has been learned about this---we may be able to decide this question before long. But for now we can't say for sure.

    The most important thing to realize for starters is that even if space is finite it can still be boundaryless. It does not have to have any surrounding more empty space. It can have a finite volume and nevertheless have no edge and no center. It might help you imagine this if you took a look at the balloon "sticky" thread---and also if you watched Ned Wright's short animated balloon-model movies. Their links are in the sticky thread.
     
    Last edited: Feb 14, 2009
  4. Feb 14, 2009 #3
    thanks for the reply, marcus.
    if i were to take off on a spaceShip from earth and move in a direction such that i'm always moving away from earth. i make sure i'm not moving towards the earth. the following could happen. 1, i reach a physical boundary (quite unlikely as i've read about the balloon model a bit). 2, i don't reach a physical boundary but continue to explore new galaxies forever in an infinite universe. 3, i essentially return to the earth or at least a neighborhood of the earth thus completing a loop (which i think is allowed in the balloon model). this seems like the likeliest option. i know a bit (not mathematically) about the hypersphere and other theories which present a 3D balloon surface in a 4D environment (analogous to the 2D balloon balloon surface in a 3D environment). i watched some animated balloon-model movies. i find it very satisfactory. i am trying to visualise what a journey around the universe would be like when taken by entities that exist within the universe and are bound by it's rules (not having an outside 4D perspective). i'm assuming in this journey, i am fast enough and live long enough to explore the entire span of the universe (ie it isn't growing faster than i can travel). it's unrealistic but i don't think it violates the validity of the question. will i finally reach the earth again after going around the balloon?
     
  5. Feb 14, 2009 #4

    gel

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    you should add 4) you are eventually destroyed in the http://en.wikipedia.org/wiki/Big_Crunch" [Broken], which is what I think would happen in a finite universe. If it wasn't for that inconvenience, you would eventually arrive back at earth.
     
    Last edited by a moderator: May 4, 2017
  6. Feb 14, 2009 #5
    what do you think, marcus? gel here seems to agree with the "return to earth" notion. i really want to know what you think (you come highly recommended :approve:). i found your previous reply very helpful
     
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  7. Feb 14, 2009 #6

    marcus

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    I agree with the general idea of what gel said except about the big crunch. Until 1998 cosmologists would often tell people that a finite volume universe implied an eventual big crunch (like gel says). But that was wrong and they corrected it after 1998, when accelerated expansion was measured.
    Now we can't say that finite implies crunch. As far as we know the universe could have finite spatial volume and yet keep on expanding indefinitely.

    But I agree more or less with what gel says about getting back to the place you started. at least if navigation worked all right and you weren't swerved off course by random bumps dimples black holes.
    You mentioned making sure it wasn't expanding too fast. Since it's an unrealistic scenario in the first place, one clear way to state the assumptions is to say we freeze the expansion process, while you make your circumnavigation.
     
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  8. Feb 15, 2009 #7

    Chronos

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    A wrap aroune universe is possible, but, unlikely. M
     
  9. Feb 15, 2009 #8

    Chalnoth

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    I don't see why it's unlikely.

    Anyway, with a wrap-around universe, the fact of the matter is that unless the accelerated expansion halts at some point in the future, we could never actually travel around the universe and get back to our starting point. The reason is just that no matter how far or fast we travel, there is a future event horizon that we simply cannot cross: the expansion of the universe will always outpace our travel beyond this point. If the accelerated expansion is caused by a cosmological constant, this distance is about 16 billion light years away, if my approximations are to be believed. That is to say, if we look at an object that is currently more than 16 billion light years away, we know that no matter how fast we travel, we could never reach that object, because the expansion rate will keep it moving faster away from us than we can travel towards it.
     
  10. Feb 15, 2009 #9
    I think the universe does have a boundary. Cosmologists don't like that because that would imply that there is something (or nothing) beyond it which they can't imagine or wrap their mind around, so they wrap the universe around instead, then they don't have to think about what's beyond it. I think that it is nothing that is beyond the boundary, the only thing that can actually be infinite by it's very nature.
     
  11. Feb 15, 2009 #10

    Chalnoth

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    It would be good to learn about something before criticizing it. Based upon our current knowledge, it is possible that our universe has a boundary. It is possible that it does not. This is a purely empirical statement: we see no boundary out to as far as we can see, but we know there's more stuff out there beyond what we can see. Therefore, there may or may not be a boundary somewhere beyond what we can see.

    Perhaps one day we will discover a strongly-evidenced theory for the beginnings of our own region of the universe which unambiguously predicts that there either is or is not a boundary. But as of right now, we just don't have enough evidence.
     
  12. Feb 15, 2009 #11
    There is also (5) You eventually find yourself alone as everything else crosses your cosmological event horizon due to the accelerated expansion of the universe and (6) You get torn apart in a 'Big Rip'

    I would say that given the current understanding of the universe the most likely outcome is (5)
     
    Last edited by a moderator: May 4, 2017
  13. Feb 15, 2009 #12

    Chalnoth

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    Well, as far as (5) is concerned, in a universe dominated by a cosmological constant, gravitationally-bound objects like our galaxy still remain together.
     
  14. Feb 17, 2009 #13
    thanks marcus. that 'freeze time' tool is very useful in understanding this thing. i have a hang of what this boundaryLess universe is like.
    thanks everyone else who replied. you guys have some interesting ideas
     
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