Recipe for the Universe - Just Six Numbers

"The most incomprehensible thing about the Universe is that it is comprehensible" is one of Albert Einstein's best-known aphorisms. It expresses his amazement that the laws of physics, which our minds are somehow attuned to understand, apply not just here on Earth but also in the remotest galaxy. Newton taught us that the same force that makes apples fall holds the Moon and planets in their courses. We now know that this same force binds the galaxies, makes some stars collapse into black holes, and may eventually cause the Andromeda galaxy to collapse on top of us. Atoms in the most distant galaxies are identical to those we can study in our laboratories. All parts of the universe seem to be evolving in a similar way, as though they shared a common origin. Without this uniformity, cosmology would have got nowhere.

Recent advances bring into focus new mysteries about the origin of our Universe, the laws governing it, and even its eventual fate. These pertain to the first tiny fraction of a second after the Big Bang, when conditions were so extreme that the relevant physics isn't understood - where we wonder about the nature of time, the number of dimensions, and the origin of matter. In this initial instant, everything was squeezed to such immense densities that the problems of the cosmos and the micro-world overlap.

Space can't be indefinitely divided. The details are still mysterious, but most physicists suspect that there is some kind of granularity on a scale of 10^-33 centimetres. This is twenty powers of ten smaller than an atomic nucleus: as big a decrease as the increase in scale from an atomic nucleus to a major city. We then encounter a barrier: even if there were still tinier structures, they would transcend our concepts of space and time.

Other universes

0.7 Measuring the sixth number, lambda, was the biggest scientific news of 1998, though its precise value is still uncertain. An unsuspected new force - a cosmic 'antigravity' - controls the expansion of our Universe. Fortunately for us, lambda 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.

What about the largest scales? Are there domains whose light has not yet had time to reach us in the ten billion years or so since the Big Bang? We plainly have no direct evidence. However, there are no theoretical bounds on the extent of our Universe (in space, and in future time), and on what may come into view in the remote future - indeed, it may stretch not just millions of times farther than our currently observable domain, but millions of powers of ten further.

And even that isn't all. Our Universe, extending immensely far beyond our present horizon, may itself be just one member of a possibly infinite ensemble. This 'multiverse' concept, though specula-tive, is a natural extension of current cosmological theories, which gain credence because they account for things that we do observe. The physical laws and geometry could be different in other universes.

What distinguishes our Universe from all those others may be just six numbers.