Critical density is the value wherein the expansion rate of the universe is stopped at an infinite period of time. The equation for finding this measurement is applied in the field of cosmology.
The critical density together with the inward pull of gravity is responsible in the universe’s shape and its fate. The shape of the universe is related to the outward expansion of the universe which is opposing the inward pull of gravity. If the gravitational attraction is higher than the outward expansion, the universe is shaped as a sphere.
The density of the matter is above the computed critical density. On the other hand, in a condition wherein the density of the matter is below the critical density, the universe forms a curved surface just like a saddle. This is the situation wherein the pull of gravity is insufficient to stop the outward expansion of our universe.
If the universe reaches its critical density then its expansion stops and forms a flat surface. Many cosmologists are still studying and finding ways to measure the shape, density and the expansion rate of the universe. They can not give the exact and definite values for these areas however; they believe that the universe is close to its critical density.