system
Given the following post, respond as if you're commenting on Reddit:
human
How can the universe have a single age?

According to wikipedia, the universe is estimated at 13.3 to 13.9 billion years old, but I haven't been able to figure out what exactly that means. If you take two objects and ask them how long ago an event they both witnessed was, you would get two different answers depending on gravitaty and velocity. Objects that have been trapped inside of a star since the big bang would say the universe is much younger then an object free floating between galaxies. What is the age of the universe measured relative to, and what's the significance of that age over any other given age?
gpt
When you zoom out to the cosmological scale, where anything smaller than a galaxy is too small to resolve, it turns out that nothing moves very fast. It's true that the proper time in a moving reference frame will differ from the proper time in a notionally stationary reference frame, but unless the relative velocity between those reference frames is large, it doesn't differ *much.*

Cosmologists consider a special reference frame in which the cosmic microwave background appears, *and always has appeared,* isotropic. That is, there's no significant difference in the frequency of the microwave background radiation coming from *this* direction and the radiation coming from *that* direction. This gives cosmologists a reference frame to work from.

Conveniently, Earth is in this reference frame, within certain limits. The dipole anisotropy we see in the sky indicates that the Earth is moving relative to the background at about 600 kilometers a second, a velocity so small that to a cosmologist it rounds down to zero.

So any two clocks in reference frames in which the CMBR is fairly isotropic will agree within an acceptable margin of error.

Considering the error bounds on the age of the universe encompass something like *half a billion years,* "close enough" is, indeed, close enough.