A small group of "if"f"s from Scienific American for December 1999:
"Our universe could not have become structured if it were not expanding at a special rate.
If the big bang had produced fewer density fluctuations, the universe would have remained
dark and featureless, with no galaxies or stars. And there are other prerequisites for complexity.
If our universe had more than three spatial dimensions planets could not stay in orbits around
their stars. If gravity were much stronger, it would crush organisms of human size, and stars would
be small and short lived. If nuclear forces were a few percent weaker, only hydrogen would be stable:
there would be no periodic table, no chemistry and no life. On the other hand, if nuclear forces were
slightly stronger, hydrogen itself could not exist.