Derek,

Yes I think I agree. Pondering on possible worlds that in effect cannot be possible at all because of violation of large scale physical constraints like galaxy formation, is pointless. I wonder why one would do that. Is it Wittgensteinian 'crossing the language border' methodology?

Moreover, bi-sector cosmology is explicitly single universe cosmology. Bear that in mind please.

You 'phillies' are strange folk if you forgive me my pun. You labor at the front of what can be thought and experiment with concepts. That is perfectly in order. Then some scientific fact comes along and restricts somewhat the mental liberty. Then all of the sudden one is affraid that analytic philosophy is the slave of cosmology. 'Aux contraire' I would like to say. The 'grammar' of the thinkable is somewhat restricted but that leaves the philosopher quite some lattitude to investigate possible universae.
 
I do not believe in the necessity of multiple universae. But I am still wrestling with my quantum cosmology and Robertson, Friedman and Walker leapfrog algorithm for Baby Universes. I am lagging behind ways.

You can philosophize about possible worlds as long as you e.g. do not take a bi-sector universe with a mirror sector that is hotter and more dense than our common matter. Because in that case we would not have seen galaxies.  

For the rest, but do read some  cosmology please (e.g. P. Ciarcelluti, Int. J. Mod. Phys. D. (14)  187-231 (2005) ), ... it is free ... enjoy. Perhaps better ask Paolo to guide you through the mirror sector cosmology himself. He has gmail: [email protected],  give him my regards and my wish he returns to active cosmological research.

Han