Final answer:
Open, flat, and closed describe the curvature of space in relation to the development of the universe. An open universe has negative curvature, allowing unlimited travel in any direction. A closed universe has positive curvature, causing eventual return to the starting point. A flat universe has zero curvature and is supported by recent discoveries.
Step-by-step explanation:
When describing the development of the universe, the terms open, flat, and closed refer to the curvature of space. In an open universe, the space is negatively curved, meaning that you can travel an unlimited distance in any direction. In a closed universe, the space is positively curved, and if you travel far enough in any direction, you will return to your starting point. A flat universe has zero curvature.
The recent discovery of the cosmological constant has shown that the universe is very close to flat and will expand forever. Theorists feel the universe is flat because it is a part of the inflationary scenario, which helps explain the flatness of the microwave background. General relativity also implies that matter creates the space in which it exists, resulting in a special symmetry to a flat universe.