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Difference between open and closed Universes?

  1. Nov 29, 2011 #1
    What is the difference between a closed Universe and an open Universe? Please explain in layman's terms and describe what type our Universe is.
  2. jcsd
  3. Nov 29, 2011 #2


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    Geometrically, an open universe has a negative Gaussian curvature, K = -1. One way to visualize this geometry is as the 2D surface of a saddle in 3D space. A closed universe has positive Gaussian curvature, K = +1. A closed universe is readily visualized as a sphere. It is not currently known what the global geometry of the universe is; however, the observable universe is very close to flat, K = 0.

    Einstein's General Relativity shows us that the geometry of space is determined by its energy/matter content, and so each of the above geometries also has a characteristic energy density: closed universes are overdense, open are underdense, and flat are at critical density -- they're "just right".

    Wikipedia has a good discussion of these ideas: http://en.wikipedia.org/wiki/Shape_of_the_Universe

    I'd recommend looking there for general questions like this first, then come to PF with more specific questions.
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