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 no expert but as I understand it everything comes down to frame of reference.

We are trying to measure the age of our part of the universe. The assumption is that physics works the same in every part of the universe so if we could instantly teleport to the farthest place we can see and take a measurement there we would get the same answer.

Of course time dilation, univers expansion and the like produced some interesting effects like Methuselah star:

And we’re not even so sure about the 13.8 billion years old part: