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

From the perspective of the frame of reference where there is zero net motion in the universe.

There is a bit of mathematics called the FLRW metric. This allows us to choose any large-scale conditions for a model universe, and let us calculate how it evolves over time. We then tweak those model conditions until the result matches what we observe.