Critical density

Definition

Critical density is the theoretical density of matter needed for the universe to have a flat geometry. It determines whether the universe will expand forever, collapse back on itself, or reach a stable size.

5 Must Know Facts For Your Next Test

1. The critical density is approximately $1 \times 10^{-26} \, \text{kg/m}^3$.
2. If the actual density of the universe is greater than the critical density, the universe 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 critical density depends on both the Hubble constant and gravitational constant.
5. Dark matter and dark energy play significant roles in determining if our universe's actual density meets or exceeds this critical threshold.

Review Questions

• What happens to the universe if its actual density is less than the critical density?
• How does dark matter influence whether our universe reaches or exceeds critical density?
• What are two constants that affect the calculation of critical density?

Related terms

Hubble Constant: A measure of the rate at which the universe is expanding, typically denoted as $H_0$.

Dark Matter: A form of matter that does not emit light or energy, making it invisible yet detectable through its gravitational effects.

Cosmological Constant: $\Lambda$ represents a term in Einstein's field equations of general relativity that accounts for dark energy and accelerated expansion of the universe.