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
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.
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.
From the perspective of an observer comoving with the cosmic microwave background. That’s the oldest age that the universe could possibly be. You are correct that some observers would measure the universe to be younger, however it’s important to note that almost all of the universe is very close to being comoving with the CMB, so most observers would agree that the age of the universe is close to that. You’d have to have spent a significant amount of time very close to a massive object like a black hole or moving at very close to c do disagree, and there just aren’t that many places or things in the universe that fit that criteria.
That’s 13.787 billion years, according to Earth’s current reference frame. True, that reference frame didn’t exist for roughly 2/3 of that time period, but we have to start somewhere. And what better place to start than where we are right now?
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.
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