What is the difference between a closed, open, and flat universe?

A closed universe has positive curvature, an open universe has negative curvature, and a flat universe has zero curvature.

A closed universe has positive curvature, meaning that space is curved like a sphere. This means that if you were to travel in a straight line, you would eventually end up back where you started. A closed universe is also finite, meaning that it has a limited size. In a closed universe, the density of matter is greater than the critical density, so gravity will eventually cause the universe to collapse in on itself. To understand more about how physical quantities affect the universe, you can read about the foundations of physical quantities.

An open universe has negative curvature, meaning that space is curved like a saddle. This means that if you were to travel in a straight line, you would eventually diverge from your starting point. An open universe is also infinite, meaning that it has no boundaries. In an open universe, the density of matter is less than the critical density, so the universe will continue to expand forever. The principles of gravity play a critical role in shaping these universal models.

A flat universe has zero curvature, meaning that space is flat like a sheet of paper. This means that if you were to travel in a straight line, you would continue in a straight line forever. A flat universe is also infinite, meaning that it has no boundaries. In a flat universe, the density of matter is exactly equal to the critical density, so the universe will continue to expand forever, but at a decreasing rate. Concepts such as wave and tidal energy can be analogously used to understand different energy dynamics in the universe.

A-Level Physics Tutor Summary: In simple terms, a closed universe is curved like a sphere and will eventually collapse due to its finite size and high matter density. An open universe is saddle-shaped, infinite, and expands endlessly because it has less matter density. A flat universe is like a never-ending sheet of paper, infinite and expanding forever at a slowing rate, with its matter density just right.