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The universe and its matters: finite or infinite?

  1. Oct 17, 2012 #1
    It's been said that the universe has no edge, it's expanding, it has no center and the big bang was the birth of energy, matters and space-time.

    I also often hear that it's been estimated the universe has approximately 200 billion galaxies or more or much more. Also the number of particles and matters is finite, otherwise we would have infinite amount of event, earth, you, me, star (the sky wouldn't be dark at night???) and scotch whisky (hell yeah!).

    My question is where is the center of that collection of that 200 billion galaxies or any finite amount of particles? I mean if we had only 4 galaxies (or 4 stars) in the whole universe, we could calculate its center or its center of mass with any form of matter distribution and allocation in 3d space, right?

    Many thanks!
  2. jcsd
  3. Oct 17, 2012 #2
    I think the observer is always at the centre no matter if he is on earth or moon . He is the centre of the universe.
    Correct me if i am incorrect :)
  4. Oct 17, 2012 #3
    nemoover, don't mix universe and observable universe
    When you hear: universe has approximately 200 billion galaxies you should read: observable part of the universe has approximately 200 billion galaxies while the total number can be infinite if universe is infinite

    Unfortunately, the word observable is being omitted in many cases.
  5. Oct 17, 2012 #4


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    If you assume Euclidean geometry, then any set of points must have a "center". But in other geometries, that is not true. For example, four points on the surface of a sphere, in three dimensions, may have "center" at the center of the sphere. But thinking of the surface itself as the entire space, they have no center.
  6. Oct 17, 2012 #5


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  7. Oct 18, 2012 #6
    Good point, thanks, Dmitry67!

    If the amount of particles/matters is an infinite number, then space, the universe and the number of light sources are infinite, certainly there's no center, would the sky be dark at night in this case?

    But if the amount of particles/matters is a finite number, I cannot understand why there shouldn't be a center of the whole structure. For example, a Rubik's cube is made of many particles and they occupy a region of 3d space, we can calculate the center of the cube. The same math we can do with earth, solar systems, galaxies and a toilet paper roll (cylinder shape). Suppose the whole universe (not just the observable universe) has x amount of particles, no matter how they distribute in 3d space, should there be a center?

    About the balloon/sphere analogy: Let's say there's a 2d creature live on the surface of that sphere, i understand there's no center on the surface and the the center of the sphere is not reachable (in another dimension...). But please consider this: If that creature choose a direction and keep going straight, eventually it will return to the original point/location. Let's name this point A, this creature also measured the distance it has traveled, call this distance D. So at D/2 there's a point B, now the imaginary straight line AB (just like earth's north and south pole form a straight line) is the diameter of the sphere and center point C of AB is the center of the sphere. C coordinate (though not comprehensible) is calculable. If that analogy also works similarly with our 3d universe, given the Milky Way is pole A, where is pole B?

    One last favor I may ask, could anyone help me with the math that led from WMAP measurements to this: http://en.wikipedia.org/wiki/Shape_of_the_Universe
    Your responses are highly appreciated!
    Last edited: Oct 18, 2012
  8. Oct 18, 2012 #7
    1 Because there is finite time from the Big Bang, light has finite speed, so we can see finite number of objects. The area we can potentially see is called an observable Universe, and as I explained above, it is always finite.

    2 Becase it in only an analogy. Curved 2D space can be embedded in a higher dimension flat space (for 2D it is 3D) but it doesn't mean that such higher dimensional space is real. It helps you imagine the curved space, but it is not required mathematically.
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