The probability measure is the one that's conserved by physical time-evolution of the system, no? It would be a bit weird to have an ethical system where universe A was worth the same as universe 1 and then a few minutes later it was only worth half as much.
I think it's past time for another Stupid Questions thread, so here we go.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Please respect people trying to fix any ignorance they might have, rather than mocking that ignorance.