Science: The Ultimate Answer ; Will We Ever Understand How Our Universe Was Created, or Is the Problem Too Complex to Comprehend? in the Second Part of His Examination of Cosmology, Martin Rees (Left) Charts the Search for a `Final Theory'

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Cosmological thought stretches back for thousands of years, but the conceptual excitement has never been more intense than it is at the start of the 21st century. Recent technical advances have enriched our cosmic perspective. Space probes have beamed back pictures from all the planets of our Solar System: new technology enables a worldwide public to share this vicarious cosmic exploration. Pictures of a comet crashing into Jupiter, made with the Hubble Space Telescope, were viewed almost in real time by more than a million people on the internet. During this decade, probes will trundle across the surface of Mars; they will land on Titan, Saturn's giant moon; samples of Martian soil may be collected and brought back to Earth.

Our universe extends millions of times beyond the remotest stars we can see - out to galaxies so far away that their light has taken billions of years to reach us. Bizarre cosmic objects - quasars, black holes, and neutron stars - have entered the general vocabulary, if not the common understanding.

Classically, everything on Earth was thought to be made of earth, air, fire and water: the firmament was made of a different substance, the "fifth essence". In modern guise, this disjunction has reappeared. We've learnt that all the stars and galaxies in the universe, made of atoms, are a trace constituent - 95 per cent of the stuff in the universe is not in the form of ordinary atoms at all, it consists of mysterious dark particles or energy latent in space.

Through the efforts of geologists and biologists we understand, at least in outline, how our Earth and its biosphere have evolved over their 4.5 billion-year history. We now envision our Earth in an evolutionary context stretching back before the birth of our Solar System. By observing the most distant galaxies whose light has spent 10 billion years in transit, we can probe the past, and see what galaxies looked like when they were newly formed. We can confidently trace cosmic history back to a universal fireball far hotter than the centre of the Sun - this much is now as well established as anything geologists can tell us about the early history of the Earth, because we can detect the faint afterglow of this intense heat.

In an earlier book, I expressed 90 per cent confidence in the evidence for this one-second-old fireball with a temperature of 10 billion degrees. Recent measurements have firmed up the theory, and I'd now raise my confidence level to 99 per cent. But I would still prudently leave the other 1 per cent for the possibility that our satisfaction is just as illusory as that of a Ptolemaic astronomer who had successfully fitted some more epicycles into his theory.

Cosmologists are sometimes chided for being "often in error but never in doubt". But when we try to probe back further still, into the first tiny fraction of a second, we enter the realm of conjecture. The temperatures and energies would then have been so high that they are beyond anything we can study in our laboratories, so we lose a firm foothold in experiment.

According to the favoured current guess, our entire universe started off as an infinitesimal speck. It underwent a period of "inflationary" expansion, driven by energy latent in space. The scale doubled, then doubled, and then doubled again. Within about a trillionth of a trillionth of a trillionth of a second, it is claimed, an embryo universe could have inflated to become large enough to encompass everything that we now see. And then the fierce repulsion switched off; some of the energy converted into heat, initiating the more familiar expansion process that has led to our present habitat.

Indeed, the process would be likely to overshoot, inflating by far more than is needed to account for the 10 billion-light-year dimensions of our observable universe: the distance to the edge could be a number with millions of zeros. …