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

No no, he’s got a point, it’s not even a “we are humans so we describe it in a way humans understand”

I get the whole “from our point of view, the universe is x age” but what do we gain from saying? Like, if we find a rock, and we know this rock has existed since the start of the universe, we cant say that its 13.8 billion years old because we dont know the speed at which its travelled for the last 13.8 billion of our years, it could be older if it travelled slower than us, or younger if it travelled faster, and thats ignoring the effect of gravity