If time is relative, and spacetime is always expanding, how can the age of the universe be so specifically 13.787 billion years? From whose perspective?

Question

If time is relative, and spacetime is always expanding, how can the age of the universe be so specifically 13.787 billion years? From whose perspective?

In: Physics

Answer 1

We are just measuring the distance of the furthest thing we can see and then we’re reversing the expansion of space back until everything we can see is where we are and we’re calling that the beginning of time.