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

Cosmologist here.

You are right! Every point in the universe has aged differently. I’ll give two levels of explanation:

Easy: 13.8 by is actually the average over all the different points

Hard: Have you ever heard of the expansion of the universe? Everything is moving away from everything else due to the expansion of space itself. This is called the Hubble Flow. If you don’t move but just go with the flow , ie only see the distances of galaxies change because space is expanding and not because you are moving in it, you are said to be a comoving observer.
So 13.8 by is the time as measured from someone who has been moving with the hubble flow since the big bang.