Critical Density Universe

The Critical Density Universe refers to a specific universe with just the right amount of matter and energy to slow its expansion over time but never stop it entirely. If the universe has exactly this "critical" density, it will expand forever at an ever-decreasing rate, approaching a constant expansion rate. If the density is higher, gravity would eventually halt and reverse expansion, causing a "closed" universe. If lower, the universe would expand forever faster, called an "open" universe. Critical density thus marks a balanced point determining the universe's long-term fate.