The critical density is the mass density of the universe which just stops the expansion of space, but only after after infinite time. As for how to see what the critical density is, we should turn to the Friedmann equation: The density parameter is defined as the ratio of the actual (or observed) density to the critical density of the Friedmann universe. If the density in the universe is greater than critical density then you will have a sphere shaped universe (Omega > 1).

The ‘critical density’ is the average density of matter required for the Universe to just halt its expansion, but only after an infinite time. If it's less than 1, the universe will be open. The relation between the actual density and the critical density determines the overall geometry of the universe; when they are equal, the geometry of the universe is flat (Euclidean). A cosmological model of the Universe with density greater than the critical density Definition will be closed and first expand, then slow down and contract into a Big Cranch

In a flat universe, the universal expansion slows, but it never reverses into a contraction. Ω = actual average density/critical density. In an open universe, the expansion continues forever. So it is the boundary value between open universe models that expand forever and those that recollapse. If omega (Ω) is greater than 1, then the universe will be closed. Lastly, if the density of the universe is less than the critical density, then the universe is 'open.' If it is less than the critical density then you’ll have a saddle shaped universe (Omega < 1). This critical mass density is currently equal to 6e-27 kg/m^3. A universe that has precisely the critical density it is said to be flat or Euclidean. Critical density is the value at which the Universe is at balance, and is stopped. A Universe with the critical density is said to be flat. Curve 1 represents a universe where the density is greater than the critical value; this model predicts that the universe will eventually collapse. If the Universe is at the critical density, then the total mass of the Universe is closer to 1e53 kg, and the number of atoms (assuming that most of the mass is in the form of hydrogen atoms) about 6e79. Einstein called the Cosmological Constant his "Biggest Blunder." It is no longer a Fudge Factor but the Energy Density of Dark Energy in our universe. The critical density is the average density of matter in the universe today that would be needed exactly to halt, at some point in the future, the cosmic expansion. If the density of the universe is greater than the critical density, then …

Then Hubble showed that the universe was expanding. In the third scenario if the density in the universe equals the critical density then you’ll have a flat universe (Omega = 1). Curve 2 represents a universe with a density lower than critical; the universe will continue to expand but at an ever-slower rate. Measuring about 91 billion light years in diameter and being 13.8 billion years old, the Universe encompasses all matter, energy, time and space in existence. The density parameter (Ω 0) is given by: where (ρ) is the actual density of the Universe and (ρ c) the critical density. Now, years later we find that there is a need for a Cosmological Constant to "fine tune" GR to match what we see in the universe. And if it's equal to 1, the universe will be flat.