Critical density

The critical density is the average density of matter required for the Universe to just halt its expansion, but only after an infinite time. This value is estimated as $(1\sim 3)\times 10^{-26}\,{\text{kg/m}}^{3}$ and it’s calculated when you take the matter-energy density of the universe and divide it by the matter-energy density of the universe that is required to achieve that balance.

Estimated relative distribution for components of the energy density of the universe.

This is related to the cosmic density parameter in many ways; this term describes the ratio of the actual density value of the Universe to the critical density value. If the ratio is one, then the Universe is at balance, and the Universe is flat and will expand forever. If the ratio is greater than one, then the actual density of the Universe is greater than the critical density, and thus the Universe will eventually become closed and will ultimately end up collapsing in on it self. If the ratio is less than one, then the actual density is less than the critical density, and the Universe is open and expands forever in every direction.