Explain the concept of a flat universe.

Question

Accepted Answer

A flat universe occurs when the average density of the universe is exactly equal to the critical density. In this situation, the velocity of celestial bodies and the effect of gravity become zero at the same time. The continuously expanding universe gradually slows down and finally stops expanding. The celestial bodies then remain at very large, fixed distances from each other, and since the distances are so great, they are no longer significantly affected by mutual gravitational force. Such a stable, non-expanding, non-collapsing universe is called a flat universe.