Definition of Critical Density

Shape of the Universe

• The shape of the universe depends on the density of matter in the universe. More specifically, the shape of the universe is the result of the outward expansion of the universe against the inward attraction of gravity.

Closed Universe

• In a closed universe, the density of the matter is above the critical density. This produces a universe shaped as the surface of a sphere. In a closed universe, the gravitational attraction exceeds the outward expansion, resulting in an eventual collapse of the universe termed the Big Crunch.

Open Universe

• If the density of the matter in the universe is below the critical density, then the universe is said to be open and forms a curved surface like a saddle. In an open universe, the gravitational attraction between particles of matter is insufficient to stop the outward expansion of the universe, leading to the eventual dispersal of matter over an ever-increasing area called the Big Freeze.

Flat Universe

• If the density of the matter in the universe is equal to the critical density, then the universe is flat. It is in this universe that expansion ceases after infinite time.

Expectation of the Universe

• While the shape, density and even the rate of expansion are not yet fully measured, many cosmologists estimate the universe to be close to critical density.