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

Ours, of course. The concept of years is a human construct based on the current period of orbit of earth around the sun. But the theory and the math that suggests that age is xxx years is almost always from our perspective. Since the earth and the sun is only 4.5 billion years old, this measure of course doesn’t literally means that the earth has orbited 13.787 billion times around the sun since the universe began.