Rees maintains that six numbers in particular govern our universe, and that if any of these values were changed even very slightly things could not be as they are. For example, for the universe to exist as it does requires that hydrogen be converted to helium in a precise but comparatively stately manner-specifically, in a way that converts seven one-thousandths of its mass to energy. Lower that value very slightly-from 0.007 percent to 0.006 percent, say-and no transformation could take place: the universe would consist of hydrogen and nothing else. Raise the value very slightly-to 0.008 percent-and bonding would be so wildly prolific that the hydrogen would long since have been exhausted. In either case, with the slightest tweaking of the numbers the universe as we know and need it would not be here.
I should say that everything is just right so far. In the long term, gravity may turn out to be a little too strong, and one day it may halt the expansion of the universe and bring it collapsing in upon itself, till it crushes itself down into another singularity, possibly to start the whole process over again. On the other hand it may be too weak and the universe will keep racing away forever until everything is so far apart that there is no chance of material interactions, so that the universe becomes a place that is inert and dead, but very roomy. The third option is that gravity is just right-“critical density” is the cosmologists’ term for it-and that it will hold the universe together at just the right dimensions to allow things to go on indefinitely. Cosmologists in their lighter moments sometimes call this the Goldilocks effect-that everything is just right. (For the record, these three possible universes are known respectively as closed, open, and flat.)
Now the question that has occurred to all of us at some point is: what would happen if you traveled out to the edge of the universe and, as it were, put your head through the curtains? Where would your head be if it were no longer in the universe? What would you find beyond? The answer, disappointingly, is that you can never get to the edge of the universe.