The age of the universe

It’s often stated as fact that the universe is about 13.8 billion years old. In fact, I heard a scientist at a planetarium claim that we’re more confident about our estimate of the age of the universe than we are about the age of our earth.

My question is, does the measurement of the age of universe vary depending on where you are in the universe. Does the 13.8 billion years only apply to Milky Way? I ask because, as I understand it, time itself is relative.

The 13.8 billion number is the same as the number of light years light has traveled since the Big Bang. Since the Big Bang happened everywhere in space at once, that distance is the same everywhere.

I think the “time relativity” you mention is what happens when you send one object off at a high speed in an accelerating system and find that when it returns its clock is not the same as the clock that stayed behind. That doesn’t apply here because, again, the Big Bang took place everywhere in space and all space is expanding uniformly (at least for this discussion).

You’ve heard that there is no single preferred reference frame. Well, when you’re looking at a cosmological scale, that has to be qualified a bit. At any given point in the Universe, the cosmic expansion does define a preferred reference frame at that point. The catch is that the preferred frame is different at every point, so there’s still no single preferred frame that applies to the whole Universe. And it’s in the preferred reference frame for any given point that the 13.8 billion years value is given.

It is in a single reference frames. Tbh you needn’t even consider global frames of reference as, ignoring the the ‘wrinkles’ in the universe’s graviational field caused by small-scale inhomogeneties, it’s just the local inertial reference frame at any point where the universe has it’s maximal age.