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

I’m not smart enough to offer an answer, but this video on the [Twin Paradox](https://youtu.be/UInlBJ4UnoQ) might offer some additional insights.