Science Advisor

2025 Award

Is the question "is simultaneity still relative in a universe with a CMB that provides a global clock"?

If so, yes it's still relative. For example, our GPS clocks do not use the same concept of simultaneity as the CMB one, and they work perfectly. There is no experiment you can do that will tell you that you are wrong to use either the GPS or CMB simultaneity, or any other.

The CMB provides an obvious way to define simultaneity on cosmological scales, and using any other scheme makes the maths harder while obscuring the symmetries. So nobody does it. But it is just convenience - the same kind of convenience as road traffic laws specifying speed relative to the local Earth surface rest frame rather than a Sun-centric frame. You could do the latter and nothing would change except how easy it is to understand what's being said.