# Calculating the age of the empty universe from the Hubble's constant

This is from p.120 of Ta-Pei Cheng's Relativity, Gravitation and Cosmology. I cannot understand the way the book calculated the age of the empty universe. The velocity $v$ of expansion is proportional to the distance $d$. That means the expansion velocity has not been constant. But why does the book conclude the age of universe from simply dividing the distance $d$ from the velocity of the current moment? Is there something that I am missing? I need help.

• It is a limit estimation – Alchimista Jan 13 '18 at 10:53
• What does that mean? – Keith Jan 13 '18 at 14:27
• That it is just an estimate that indeed does not fit to observations. They clearly state that in an empty universe H is supposed to be constant in time – Alchimista Jan 13 '18 at 14:29