Regarding Relativity of Simultaneity, is non-local "now" impossible?

[OmniThoughts]

TL;DR

"Why do some people claim against that, although not Relativity-fundamental, "there's a "non-local cosmic/universal non-privileged non-absolute 'now' that entails "co-existing/co-occuring but not "being in absolute sync""" that doesn't override nor cancel out anything"" when the light of the CMB is seen having the same patterns/temp in every direction & when light from "spacelike distance away" reach us but we can match when that light emission occured with during when in our local-time history?

Regarding Relativity of Simultaneity, is a non-local "now" impossible? "Why do some people claim against that, although not Relativity-fundamental, "there is a "non-local cosmic/universal non-privileged non-absolute 'now' that entails "co-existing/co-occuring but not "being in absolute sync""" that doesn't override nor cancel out anything"" when the light of the CMB is seen having the same patterns and temp in every direction and when light from "spacelike distance away" reach us but we can match when that light emission occured with during when in our local-time history?

 

[Dale]

Why do some people claim against that

Who specifically claims that, and where did they claim it?

Usually if you look in context then you can see why someone said something.

 

[Ibix]

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.