The shape of our four-dimensional universe comes from the relationship between the amount of matter and energy that lives in the cosmos and the rate at which the cosmos is expanding. A convenient mathematical measure of this is omega: Ω, yet another capital Greek letter with a firm grip on the cosmos. If you take the matter-energy density of the universe and divide it by the matter-energy density required to just barely halt the expansion (known as the “critical” density), you get omega.
