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

We measure the age of the universe by looking at the Oldest Light we can perceive: The Cosmic Microwave Background.

Light moves at a constant speed, and the expansion of the universe is also constant. The wavelength of Light traveling over such distance “expands” at a steady rate as a side effect of spatial expansion, producing a “red shift.” By measuring the amount of red shift, we know exactly how long the light has been traveling.

We have measured the age of the universe by measuring the amount of red shift in the oldest light we can see. There’s a possibility that there’s older light beyond that boundary, but it hasn’t reached us yet… and that means that we can say that the universe is *at least* as old as the light from the CMB.