If the density of the universe is at the critical density, this means that:
A)the universe is expanding at a rate equal to the escape speed of the universe.
B)the universe is expanding at a rate less than the escape speed of the universe.
C)the universe is expanding at a rate greater than the escape speed of the universe.
D)the universe is expanding at a rate equal to the speed of light.
E)the universe is expanding at a rate greater than the speed of light.

Question

Quizplus · Accepted Answer

The critical density is the density required for the universe's expansion to eventually slow to a stop (i.e. the escape speed equals the rate of expansion). Therefore, option A is the correct choice. Options B, C, D, and E are incorrect as they describe rates of expansion either less or greater than the escape speed or the speed of light, which are not related to the critical density. At the critical density, the universe is experiencing the exact amount of gravitational force necessary to eventually stop its expansion. This means that the expansion rate is equal to the escape speed of the universe, which is the speed necessary to break free from its gravity and expand indefinitely.

If the Density of the Universe Is at the Critical