Critical density is the precise average mass-energy density of the universe that determines its overall geometry and fate. If the actual density of the universe equals the critical density, the universe is spatially flat, meaning its expansion slows over time but never fully stops, balancing on the edge between eventual collapse and eternal expansion.
- If the universe’s density is greater than the critical density, gravity would eventually overcome expansion, leading to a closed universe that could collapse in a “Big Crunch.”
- If the density is less than the critical density, the universe would be open and expand forever, gradually thinning out.
The concept of critical density is central to cosmology because it links the universe’s composition (ordinary matter, dark matter, and dark energy) with its geometry and long-term evolution. Observations from the CMB, large-scale structure, and supernovae indicate that the universe’s actual density is very close to the critical density, implying a nearly flat universe with an accelerating expansion due to dark energy.
In short, critical density is the cosmic tipping point that dictates the universe’s shape and ultimate fate.