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definitions

flat universe

If there is just enough matter in the universe for its gravitational force to bring the expansion associated with the big bang to a stop in an infinitely long time, the universe is said to be flat. The flat universe is the dividing line between an open universe and a closed universe.

Note

Most theorists believe that the universe is flat.
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.