A Classical Calculation Shows That the Critical Mass Density of the Universe

Multiple Choice

A classical calculation shows that the critical mass density of the universe is given by where H is the Hubble constant and G is the gravitation constant. If the density is below this value, the universe will expand forever, whereas if it is above this value the universe will eventually stop expanding and collapse. Approximately how many hydrogen atoms per m³ is the critical density equivalent to?

A) 4
B) 40
C) 400
D) 4000
E) 40,000