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Shape of the Universe

  1. Feb 25, 2005 #1
    Why do some books talk about universe as a torus, a saddle etc?
    The universe cannot be defined without physical things like matter, right? The edge of the universe is measured from one furthest matter to the other, right?

    The universe can take any shape. The shape of the universe is formed due to the interaction between matters via forces, like gravitational force. If the universe is a sphere, it only means that the gravitational force among all the matters are in such a way that, it happens to form a sphere. Our universe's shape can change with time. Right?
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  3. Feb 25, 2005 #2


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    Yes, matter and energy.
    No, the edge of the universe has yet to be measured. It is defined by the furthest light that reaches earth from all directions. Approx 13.5 billion light years of distance.
    The shape of the universe is (theoretically) defined as the amount of matter to the amount of space (density). Gravity (like you mentioned), plays the role in the shape of the universe. If there is not enough matter to space, then by gravitational cause, the universe will have a hyperbolic shape, a saddle. Otherwise, with enough matter, the universe could collapse (this is the theory of the "big crunch") or form a sphere. We don't really know much about the shape of the universe, just alot of theories.
    Remember the universe is expanding and accelerating.
    Check out this link.
    Read about some of the theories, steady-state, big rip etc...really interesting stuff.
  4. Feb 26, 2005 #3


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    Well, we have no enough evidence to decide if the universe is sphere like or not. It looks like this to us as observers, but we still don't know.

    Because of red-shift effect we are certain that the universe is expanding and probably started with Big Bang. About the end, some current theories suggest the happening of Big Crunch or open universe - depending on the critical density of the universe. According to the amount of dark matter present, the big rip - another theory - is one of the suggestions where matter in the universe would rip and goes back to its main constituents; matter into atoms into protons\neutrons into quarks etc.
  5. Feb 26, 2005 #4


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    To talk about the shape of the universe is only an anlogy as what's really being talked about id the geometry and the topology of the universe.
  6. Feb 26, 2005 #5


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    You mean the extra dimensions right?
  7. Feb 26, 2005 #6


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    It involves extra dimensions, but only one extra (i.e. not the 6 or so extra required for string theory).

    If the universe were a torus, or any other closed shape, you could head out in one direction in a straight line and eventually wind up back at your starting point.

    If the universe were saddle-shaped, you could start out in a direction and never return to your starting point.

    It's not so much about literally the shape as it is about the properties and behaviour of things that are constrained by that shape.
  8. Feb 26, 2005 #7


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    Ahh, thanks Dave, I was assuming the 6d in string theory. So basically were talking about 4 spacial dimensions. Up, down, back, forth, right, left, and around?
  9. Feb 26, 2005 #8


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    Well that's not what I was talkingf baout but if twso spaces have a different number of diemnsiosn they are both geometircally and topologically distinct. Basically what I mena by the goemetry of the universe is which path is the shortest distance between two points and by the topology how are the different regions connected to each other
  10. Feb 26, 2005 #9
    the metric of spacetime is likely emergent from the relationships of fundamental processes- degrees of freedom/ geometry/ topology all are derived from the lattice of causal connections- holography is inherent- since the metric of the universe is abstracted from relationships- it's form can be quite a bit more complicated than most cosmologist suggest-

    a good example: a metric emergent from relationships [ie computations] can provide a finite space that is spatially flat like the screen wrap-around in the videogame Asteroids- flatness does not require an infinite space or a tourus
    Last edited: Feb 26, 2005
  11. Feb 27, 2005 #10


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    If space is homogeneous and isotropic, and if there is no preferred or special direction in the sky, then the shape (topology) of space is determined by its geometry. The geometry of space (the motion of the geodesics in space) in a comoving reference frame is determined by its energy density and it can lead either to an infinite topology, to a sphere or a to saddle.

    If the mentioned conditions do not hold, then there are lots of possbile topologies, for example a 3-torus. In case of a 3-torus, space can be homogeneous an isotropic, but it exists at least on direction which is "different" than the others (it allows a shorter path to the same point in space). Thus, in general, the topology of space is constrained by its geometry, but not determined.

    If space is not homogeneous and isotropic (the cosmological principle does not hold), then the topology of the universe may also change with time. However, this scenarios are not the usual ones considered in cosmology.
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