Critical density

Definition

Critical density is the precise density of matter and energy in the universe needed for it to have a flat geometry. It determines whether the universe will expand forever, collapse, or reach a stable size.

5 Must Know Facts For Your Next Test

1. The critical density is approximately 5 protons per cubic meter.
2. If the actual density of the universe is greater than the critical density, it will eventually collapse (closed universe).
3. If the actual density is less than the critical density, the universe will expand forever (open universe).
4. The concept of critical density is pivotal in cosmology for understanding the fate and structure of the universe.
5. Critical density includes contributions from both visible matter and dark matter.

Review Questions

• What role does critical density play in determining the fate of the universe?
• How does actual density compare to critical density in an open universe?
• What approximate value is given for critical density in terms of protons per cubic meter?

Related terms

dark matter:

A form of matter that does not emit light or energy and cannot be directly observed but is thought to make up most of the mass in the universe.

cosmological constant:

A term introduced by Einstein representing an energy density filling space homogeneously used to explain accelerated expansion.

flat universe:

A model of the universe where space expands forever but at a rate approaching zero as time progresses, consistent with having a total density equal to critical density.