Density Parameter

Transcript: Another fundamental quantity of the big bang model is the density parameter. It’s defined as the ratio of the mean density of the universe to the density just needed to overcome the cosmic expansion. The density parameter is denoted by the Greek symbol capital omega with a subscript zero. If omega equals one the universe is flat. If omega is less than one the universe is open, and if omega is greater than one the universe is closed. Unlike the deceleration the density parameter is a purely local measurement. All that’s required is to take a large volume of space, typically fifty to a hundred megaparsecs in distance from the Milky Way, add up all the matter that’s contained, luminous and dark, divide by the volume, and compare to the critical density. The answer gives the sense of whether the universe will expand forever. The best current measurements indicate that omega is about 0.3 or only one-third of the amount of density needed to overcome the expansion. Based on these measurements the universe will expand forever.