Martin Rees

JUST SIX NUMBERS

The Deep Forces that Shape the Universe

Our existence depends on very fine tuning of certain cosmological numbers. This is the theme of Rees's latest book. As the title indicates, he discusses six of these.

The first measures the strength of the electrical forces that hold atoms together, divided by the force of gravity between them. This is a huge number; if it were only a little smaller, only a short-lived miniature universe could exist, no creatures could grow larger than insects, and there would be no time for biological evolution.

The second, whose value is 0.007, defines how firmly atomic nuclei bind together and how all the atoms on earth were made. If the value were either 0.006 or 0.008, we could not exist.

The third is omega, the cosmic number. It measures the amount of material in the universe. If it was too high or too low the universe would either have collapsed long ago or would contain no stars or galaxies.

The fourth, lambda, was measured only in 1998. It relates to a newly discovered cosmic force, antigravity, which controls the expansion of the universe, though only on the huge scale (a billion light years). It is very small; if it were bigger, the present universe could not exist. (In fact, Rees cautions that the evidence for the existence of lambda is not yet overwhelming, but he is fairly confident that it is real.)

The fifth number, Q, is the ratio of two fundamental energies. Its value is about 1/100,000. If it were much smaller the universe would be inert and structureless; if it were bigger, it would be dominated by vast black holes and there would be no stars or galaxies.

The sixth number is the number of spatial dimensions in our universe, and is, of course, 3. (Time, the fourth dimension, differs from the others in having a built-in arrow of direction.) Life couldn't exist if there were either 2 or 4 spatial dimensions.

These six numbers, and their implications for cosmology, are what this book is about. To quite a large extent it repeats, in a smaller compass, material that was contained in Rees's earlier book, Before the Beginning, although in some respects it is an update (as noted above, the value of lambda was measured after the earlier book was written). There is a particularly good discussion of the question of dark matter, the mysterious 'something' that is required to account for the gravitation that prevents the galaxies from disintegrating. This matter has to be present, and indeed makes up 90 per cent of the universe, but we don't know what it is; it could be anything from neutrinos to heavy black holes. It's unlikely to be ordinary atoms.

Perhaps the most startling idea in the book arises from the theory of 'inflation' that has been proposed to explain the early expansion of the universe after the Big Bang. Most versions of the theory imply that inflation may proceed much further than is needed to account for our observable universe. It takes about ten billion years for light to reach us from the 'edge' of the universe we can see with telescopes. But the real 'edge' may be inconceivably distant, so that it would take millions of zeros to write the number of light years that would be needed for light to reach us from there.

Even this, however, may not be all there is. Rees is one of those cosmologists who incline to the view that our universe is merely one event in an infinite ensemble of universes, which he calls the multiverse, and which may have an infinite past. Though this is still a tentative hypothesis, Rees thinks it is genuine refutable science, not metaphysics. The existence of a variety of universes, each beginning from its own Big Bang but evolving in diverse ways with diverse laws, could account for the apparent fine tuning that characterizes our own home universe. Alternatively, there may be profound reasons why the six numbers must always have just the values they do.

This is an authoritative and readable account of one of the most fascinating questions in science today.