The English used in this article may not be easy for everybody to understand. (December 2011)
Critical density is the value at which the Universe is at balance, and expansion is stopped. This value is estimated as (1-3)×10^-26 kg/m³ 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.
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.