A thought I've had about this is that perhaps the speed limit within a universe (299,792,458 m/s in ours) is relative to a constant at its Big Bang moment such

A thought I've had about this is that perhaps the speed limit within a universe (299,792,458 m/s in ours) is relative to a constant at its Big Bang moment such as total energy contained or maximum rate of expansion of space-time. If the smallest particles of our universe are smaller universes and our universe is a tiny particle within a larger universe, then the speed limit within each could be different. This theory would allow FTL travel in a larger universe relative to the speed limit of our universe, but not FTL communication. FTL travel within one universe may be the escape velocity required to enter the next larger universe outside of the origin universe's boundary. But a transmission, originally traveling FTL, sent into a smaller universe may be slowed by its speed limit (similar to light propagating through a non-vacuum medium). In a universe expanding like ours, the transmission would never reach the destination.