system
Given the following post, respond as if you're commenting on Reddit:
human
If Einstein showed that time is relative and not an absolute then how can we talk about the universe having an absolute age of around 13.7 billion years?
gpt
You're indeed correct that there's no absolute reference frame, hence no absolute time. But the fact that the Universe is, on large scales, homogeneous and isotropic (i.e., the same everywhere) allows us to define a cosmic rest frame, in which the Universe *is* homogenous and isotropic on constant time slices.

What does this mean? Let's say "homogenous and isotropic" means "at a given time, the density of matter/radiation/etc. is the same everywhere." This means on a 3D "slice" through spacetime (i.e., a surface of constant time) the density is the same at every point. But how we pick our slices is fairly arbitrary, since there is no absolute time - we could just as well "tilt" the slice a bit, so that at faraway distances, times a little bit later or earlier are now considered to be simultaneous with us.

Since the Universe is expanding, densities decrease over time, so if the density was constant on our initial 3D slice, they won't be on any other slice. This original slice, on which the Universe is homogenous and isotropic, is our cosmic rest frame. There's nothing more "correct" about it, it's just a very natural and convenient one to use.

The Universe is roughly 13.7 billion years old in this reference frame. What it means is that if you were a particle at rest relative to this frame - that is, you always saw the Universe being the same in all directions - then a clock you carried with you would measure 13.7 billion years today.