EXERCISE 8

What's the mass density of the observable universe?  

Express your answer in units of kg/m^3, and in units of hydrogen
atoms per cubic meter.  Along with the information in the text,
The following pieces of information may be helpful:

- The observable universe is approximately a sphere with a radius
of 13 billion light-years.

- One parsec is 3.3 light-years;  one parsec is also 3.1 * 10^16 m

- The volume of a sphere is (4/3) * pi * r^3

- The average galaxy in the universe has about fifteen percent (15%)
the mass of the Milky Way galaxy.

- Including dark matter, the average galaxy contains a total mass
about ten times that of the mass of all its stars put together.

- The Milky Way galaxy contains about 100 billion stars, one of which
is the Sun.

- The mass of the Sun is about 2.0 * 10^30 kg.  The mass of a proton
is about 1.67 * 10^(-27) kg.