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

For the same reason ELI5 exists… Going into a complex explanation every time discussing the subject will confuse most people. If someone asks “how old is the universe?” the answer “13.8 billion years” is sufficient for the huge majority of conversations. From Earth’s perspective, that’s how old the universe appears (or at least that’s our best prediction to date).