If the actual density of the universe is less than the critical density, it is predicted that the universe will forever expand, as there isn’t enough matter to stop it.

If it is greater than one, our universe … If the actual density of the universe is less than the critical density, then there is not enough matter to stop the expansion of the universe, and it will expand forever. The luminosity density of the Universe is about 190 million solar luminosities per cubic Megaparsec (Mpc) at visual wavelengths for H o = 71. The mass density at the current time was assessed by WMAP to be Ω m,0 = 0.27 +/- 0.04.

A flat universe is a universe with a density equal to the critical density.

It is the ratio between the actual density of the universe and the critical density. This selects one of three possible geometries depending on whether Ω {\displaystyle \Omega } is equal to, less than, or greater than 1 {\displaystyle 1} . However, if the mass density is 10 times greater than it appears to be, as suggested by the missing mass mystery, then the universe may be closed after all.

For a given rate of expansion, there is a critical density—the mass per unit volume that will be just enough to slow the expansion to zero at some time infinitely far in the future.

The critical density is the mass density of the universe which just stops the expansion of space, but only after after infinite time. Comparing the critical density to the actual density can help scientists to understand the cosmos. What we can measure is something called the density parameter, Ω. Ω = actual average density/critical density. The universe is expanding, which means that the universe undergoes a change in scale with time; space stretches and distances grow larger by the same factor everywhere at a given time. The resulting shape is curved like the surface of a saddle. Current estimates indicate that the mass density of the universe falls short of the critical density by a factor of 10 or more, implying that the [162] universe will expand forever. 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. So it is the boundary value between open universe models that expand forever and those that recollapse. If the actual density of the universe is less than the critical density, it is predicted that the universe will forever expand, as there isn’t enough matter to stop it.

An important parameter in fate of the universe theory is the density parameter, omega ( ), defined as the average matter density of the universe divided by a critical value of that density. Since the critical density is 140 billion solar masses per cubic Mpc, the mass-to-luminosity ratio of the Universe needs to be 700 solar if the Universe has the critical density. If Ω is less than one, we live in an open universe. If the actual density is higher than this critical density, then the expansion will ultimately reverse and the universe will begin to contract. If omega (Ω) is greater than 1, then the universe will be closed. And if it's equal to 1, the universe will be flat. If it's less than 1, the universe will be open. Observations show that the mass density of the universe is less than the critical density.