If every part of the universe has aged differently owing to time running differently for each part, why do we say the universe is 13.8 billion years old?

Question

For some parts relative to us, only a billion years would have passed, for others maybe 20?

In: Physics

Answer 1

A guy by the name of hubble figured out that some of the smudges in the nights sky were actually other galaxies and not just nebulea.

In figuring this out he also discovered that the ones which were further away from us were also moving away from us the fastest. Through plotting these galaxies on a graph of distance and speed he determined the rate of expansion called hubble’s constant.

Through a little mathematics (1/hubbles constant) we can determine how long the universe has been expanding for, approximately 14.4 billion years.