The cosmos is so vast because there is one crucially important huge number N in nature, equal to 1,000,000,000,000,000,000,000,000,000,000,000,000. This number measures the strength of the electrical forces that hold atoms together, divided by the force of gravity between them. If N had a few less zeros, only a short-lived miniature universe could exist: no creatures could grow larger than insects, and there would be no time for biological evolution.

Another number, e, whose value is 0.007, defines how firmly atomic nuclei bind together and how all the atoms on Earth were made. Its value controls the power from the Sun and, more sensitively, how stars transmute hydrogen into all the atoms of the periodic table. Carbon and oxygen are common, whereas gold and uranium are rare, because of what happens in the stars. If e were 0.006 or 0.008, we could not exist.

The cosmic number W (omega) measures the amount of material in our universe--galaxies, diffuse gas, and "dark matter." W tells us the relative importance of gravity and expansion energy in the universe. If this ratio were too high relative to a particular "critical" value, the universe would have collapsed long ago; had it been too low, no galaxies or stars would have formed. The initial expansion speed seems to have been finely tuned.

Measuring the fourth number, l (lambda), was the biggest scientific news of 1998. An unsuspected new force--a cosmic "antigravity"--controls the expansion of our universe, even though it has no discernible effect on scales less than a billion light-years. It is destined to become ever more dominant over gravity and other forces as our universe becomes ever darker and emptier. Fortunately for us (and very surprisingly to theorists), l is very small. Otherwise its effect would have stopped galaxies and stars from forming, and cosmic evolution would have been stifled before it could even begin.

The seeds for all cosmic structures--stars, galaxies, and clusters of galaxies--were all imprinted in the Big Bang. The fabric of our universe depends on one number, Q, which represents the ratio of two fundamental energies and is about 1/100,000 in value. If Q were even smaller, the universe would be inert and structureless; if Q were much larger, it would be a violent place, in which no stars or solar systems could survive, dominated by vast black holes.

The sixth crucial number has been known for centuries, although it is now viewed in a new perspective. It is the number of spatial dimensions of our world, D, and equals three. Life could not exist if D were two or four. Time is a fourth dimension, but distinctively different from the others in that it has a built-in arrow: we "move" only towards the future. Near black holes, space is so warped that light moves in circles, and time can stand still. Furthermore, close to the Big Bang, and also on microscopic scales, space may reveal its deepest underlying structure of all: the vibrations and harmonies of objects called "superstrings," in a ten-dimensional arena.

--Martin Rees, Just Six Numbers: The Deep Forces That Shape the Universe, pp. 2-3