many universes, each governed by a differing set of numbers, there will be one where there is a particular set of numbers suitable to life. We are in that one.
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. Thats not because it would take too long to get there-though of course it would-but because even if you traveled outward and outward in a straight line, indefinitely and pugnaciously, you would never arrive at an outer boundary. Instead, you would come back to where you began (at which point, presumably, you would rather lose heart in the exercise and give up). The reason for this is that the universe bends, in a way we cant adequately imagine, in conformance with Einsteins theory of relativity (which we will get to in due course). For the moment it is enough to know that we are not adrift in some large, ever-expanding bubble. Rather, space curves, in a way that allows it to be boundless but finite. Space cannot even properly be said to be expanding because, as the physicist and Nobel laureate Steven Weinberg notes, solar systems and galaxies are not expanding, and space itself is not expanding. Rather, the galaxies are rushing apart. It is all something of a challenge to intuition. Or as the biologist J. B. S. Haldane once famously observed: The universe is not only queerer than we suppose; it is queerer than we can suppose.
The analogy that is usually given for explaining the curvature of space is to try to imagine someone from a universe of flat surfaces, who had never seen a sphere, being brought to Earth. No matter how far he roamed across the planets surface, he would never find an edge. He might eventually return to the spot where he had started, and would of course be utterly confounded to explain how that had happened. Well, we are in the same position in space as our puzzled flatlander, only we are flummoxed by a higher dimension.
Just as there is no place where you can find the edge of the universe, so there is no place where you can stand at the center and say: This is where it all began. This is the centermost point of it all. We are all at the center of it all. Actually, we dont know that for sure; we cant prove it mathematically. Scientists just assume that we cant really be the center of the universe-think what that would imply-but that the phenomenon must be the same for all observers in all places. Still, we dont actually know.
For us, the universe goes only as far as light has traveled in the billions of years since the universe was formed. This visible universe-the universe we know and can talk about-is a million million million million (thats 1,000,000,000,000,000,000,000,000)
miles across. But according to most theories the universe at large-the meta-universe, as it is sometimes called-is vastly roomier still. According to Rees, the number of light-years to the edge of this larger, unseen universe would be written not with ten zeroes, not even with a hundred, but with millions. In short, theres more space than you can imagine already without going to the trouble of trying to envision some additional beyond.