Difference between open and closed Universes?

In summary, a closed universe has a positive Gaussian curvature and can be visualized as a sphere, while an open universe has a negative Gaussian curvature and can be visualized as the surface of a saddle. The observable universe is currently very close to flat, with a Gaussian curvature of 0. This is determined by the energy/matter content of the universe according to Einstein's General Relativity. The Wikipedia article on the shape of the universe provides more in-depth information.
  • #1
Mind Bender
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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.
 
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  • #2
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.
 

1. What is the main difference between an open and closed universe?

An open universe is one in which the expansion of the universe continues forever, while a closed universe is one in which the expansion eventually stops and the universe collapses in on itself.

2. How do we know if the universe is open or closed?

Scientists use various methods to determine the curvature of the universe, including observations of the cosmic microwave background and measurements of the mass and energy density of the universe.

3. What is the role of dark energy in determining the openness or closedness of the universe?

Dark energy is believed to be the force driving the current accelerated expansion of the universe. If the amount of dark energy is high enough, it could indicate that the universe is open and will continue to expand forever.

4. Are there any other types of universes besides open and closed?

Yes, there is also the possibility of a flat universe, in which the expansion rate remains constant and the universe does not collapse or continue to expand indefinitely.

5. What implications do open and closed universes have for the ultimate fate of the universe?

If the universe is open, it will continue to expand and eventually become cold and dark. If it is closed, it will eventually collapse in on itself in a "Big Crunch" event. However, the exact fate of the universe is still unknown and subject to ongoing research and theories.

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