I’ve been reading up on gravity for a bit of a diversion and came across this interesting quote from David Toback of Texas A & M University:
General relativity can predict what would have happened in a slightly different universe. For example, if the universe had the right mass back then it would give us the critical density now. However, if a short time after the bang the mass was tiny fraction larger, just 10–50 percent larger, the outcome would have been starkly different: the extra mass in such a universe would have caused a big crunch long ago. If the density were smaller by the same amount, the universe would have expanded so quickly that galaxies and stars would never have formed. This is troublesome. It suggests that the early universe had to be set up exactly correctly for us to come to be the way we are now.
10–50 is 0.00000000000000000000000000000000000000000000000001, a fairly small number! Alternatively, it amounts to the addition or removal of a single chlorine atom to the mass of the Earth.