he moment of the universe's birth created both matter and an expanding space-time in which this matter could exist. While gravity pulled the matter together, the expansion of space drew particles of matter apart - and the further apart they drifted, the weaker their mutual attraction became.

It turns out that the struggle between these two was balanced on a knife-edge. If the expansion of space had overwhelmed the pull of gravity in the newborn universe, stars, galaxies and humans would never have been able to form. If, on the other hand, gravity had been much stronger, stars and galaxies might have formed, but they would have quickly collapsed in on themselves and each other. What's more, the gravitational distortion of space-time would have folded up the universe in a big crunch. Our cosmic history could have been over by now.

Only the middle ground, where the expansion and the gravitational strength balance to within 1 part in 1015 at 1 second after the big bang, allows life to form. That is down to the size of the gravitational constant G, also known as Big G.

G is the least well-defined of all the constants of nature. It has been pinned down to only 1 part in 10,000, which makes it look pretty rough and ready next to the fundamental number called the Planck constant, which is accurate to 2.5 parts in 100 million. It's gravity's weakness that makes G difficult to measure more accurately - but that's just a laboratory issue. The important question is, where does this value come from? Why does G have the value that allowed life to form in the cosmos?

The simple but unsatisfying answer is that we could not be here to observe it if it were any different. As to the deeper answer - no one knows. "We can make measurements that determine its size, but we have no idea where this value comes from," says John Barrow of the University of Cambridge. "We have never explained any basic constant of nature."