Universe Age: Why Is There a Set Number?

[TheScienceOrca]

If this is true, why is there a set number of years that the "universe" has existed for? Aren't different parts of the universe traveling at EXTREMELY different relative speeds? Doesn't this mean that the universe is a different age depending on your location in the universe, which you are of course part of the universe itself.

 

[PeterDonis]

why is there a set number of years that the "universe" has existed for?

The numbers usually quoted for the "age" of the universe assume a "comoving" observer, i.e., one who is at rest in the standard cosmological coordinates (alternatively, one to whom the universe looks homogenous and isotropic). An observer who is not comoving would observe a different universe age (i.e., a different elapsed time since the Big Bang, or since some other universe-wide event such as the surface of last scattering of the CMB).

For example, we on Earth are not comoving observers--we observe a significant dipole anisotropy in the CMB (as well as other indications of our motion relative to the average motion of all the galaxies). So the actual proper time elapsed on our Earth worldline since the Big Bang (assuming for the sake of argument that we can extrapolate Earth's worldline back that far) would be a bit smaller than the standard figure quoted for the "age" of the universe, which, as noted above, assumes a comoving observer. The difference would not be very much because Earth is moving very slowly, in relativistic terms, compared to a comoving observer; our velocity is a few hundred km/sec, or about 1/1000 of the speed of light.

Aren't different parts of the universe traveling at EXTREMELY different relative speeds?

Not in the sense that matters for the question you are asking. (Btw, in a recent thread you appeared to believe that objects at rest in cosmological coordinates were "not moving" relative to each other; that makes the question I just quoted above seem rather odd coming from you.)