> That kills any notion of simultaneity. That's not actually true. Assuming there's a distinguished "initial" event [the big bang], and your universe does not

> That kills any notion of simultaneity. That's not actually true. Assuming there's a distinguished "initial" event [the big bang], and your universe does not contain any closed timelike curves [citation needed], you can define a Longest Proper Time from the initial event to any other event as the (obviously) longest proper time that any object or test particle could experience along a (timelike) path from the initial event to the later event. Simultaneous events have equal LPTs and are always spacelike-separated (if there was a timelike path from A to B then B.lpt (>)= A.lpt + len(path) > A.lpt, so B.lpt!=A.lpt and they aren't actually simultaneous).