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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.

  • $\begingroup$ It is a limit estimation $\endgroup$ – Alchimista Jan 13 '18 at 10:53
  • $\begingroup$ What does that mean? $\endgroup$ – Keith Jan 13 '18 at 14:27
  • $\begingroup$ 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 $\endgroup$ – Alchimista Jan 13 '18 at 14:29

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