in response to the statement that:
Because FRW pertains to the "observable universe", there would be a limit on d, because the observable universe is finite, although it does not have an edge. FRW does not model a universe of infinite dimension.
And with regard to this statement:
While I would agree that there is no "center" where the explosion originated (or more usefully, due to the "stretching of space" we are all still in the center), the speed of radiation does define the distance of the big bang "horizon" which defines the "observable universe" which is the object described by FRW. Put another way, there is no "d" in the universe described by FRW at the present time with a length of 50 billion light years.
See also: http://odin.physastro.mnsu.edu/~eskr...25/week14.html
As I understand it, even in FRW with a flat topology, "stuff" only exists out to the "horizon" even though "stuff" will expand infinitely with the horizon.
Neither FRW, nor any mainstream Big Bang theory makes any statement about the "Universe" as opposed to the "universe".