What feature of the universe does the critical density refer to?

The critical density is a crucial concept in cosmology that determines the geometry of the universe. It represents the precise density at which the universe can be geometrically flat, meaning that the expansion rate of the universe is exactly balanced by the gravitational pull of its matter and energy. If the actual density of the universe is equal to this critical density, the universe will continue to expand forever at a decreasing rate, settling into a flat geometry.

This is essential in understanding the fate of the universe and plays a pivotal role in the Big Bang theory and models of cosmic evolution. The relationship between the universe's density and its geometric curvature informs us whether the universe is open (less than critical density), closed (greater than critical density), or flat (equal to critical density). This concept is foundational in cosmological studies and helps scientists explore the overall composition and future of the universe.