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

DISCLAIMER: I am not a physicist,,, at all.

But iirc they say this because of the “cosmological constant” which is something in math that describes the rate to which the universe expands (because it is always expanding).

And by assuming everything started as 1 point in space (directly prior to the big bang) we can use that constant to determine when most of what we observed would be around the same point.

Again, not a physicist, so I apologize if that’s completely wrong.