The dark corners of the heavens

Patrick Moore praises two efforts to explain the great cosmic enigmas and fill in the black holes in our understanding of the universe and its history.

In some ways these two books about cosmology complement each other. Parts of them cover the same ground - the history of the universe - but the approaches are different, and so are some of the conclusions. For example, Joseph Silk spends some time in discussing concepts such as parallel universes, while Eric Chaisson gives the impression of being decidedly sceptical about them.

Silk is Savilian professor of astronomy at Oxford University; he is one of the world's leading cosmologists and a prolific author. The subtitle of his new book is Questions from the Frontiers of Cosmology and certainly there are many questions to be answered; a very strong point of the book is that the author is quite ready to admit our ignorance in some subjects, such as dark matter and the even more enigmatic dark energy.

After an introductory chapter, Silk launches into a description of the universe as we see it: our neighbour planets, then the stars, the galaxies and galaxy clusters, leading us out to the furthest depths of the observable universe. He then turns to the geometry of space, and this is bound to involve a certain amount of relativity theory, but it is so clearly put that the reader will not be daunted - and there are no mathematical formulae. He says that "our understanding of cosmology appears to be converging toward the idea that the universe is infinite", but can anyone really comprehend infinity?

I doubt whether anyone can explain these difficult topics better than Silk does. He describes how we use supernovae as "standard candles" to measure the distances of remote galaxy clusters; it is these supernovae that have told us that the expansion of the universe is accelerating rather than slowing down, as logically it would be expected to do because of the relentless pull of gravity. This brings Silk on to what Einstein described as his own greatest blunder - the introduction of the so-called cosmological constant, which he added to his equations in an effort to make the universe stable. Einstein later abandoned it, but recent unexpected discoveries seem to show that he may have been right after all.

The existence of dark matter is undeniable, and its mass must be far greater than the combined mass of all visible objects combined, but what makes it up? Silk does not pretend that we have any real idea. The matter we know is made up of baryons, protons, neutrons and the like. It may well be that dark matter is non-baryonic, so that with our present instruments we are quite unable to detect it. As for dark energy, well, he says: "It is bizarre stuff. It is uniform and always stays uniform. It does not cluster like ordinary matter under the influence of gravity... it is only detectable via its effect on the acceleration of the expansion of the universe."

The chapter on "Origins" takes the reader back to the very moment of creation. "There are live major episodes in cosmology when the rate of expansion of the universe undergoes a change. These are the moment of the singularity, the epoch of inflation, the period of radiation domination, the domain of matter domination when large structures form, and the epoch of the late acceleration when the cosmological constant dominates the energy density." At least we can detect a diffuse microwave background glow that seems to be the last manifestation of the Big Bang, and this indicates that with regard to the history of the universe we are on the right track.

Yet there are dissentients. For example, there is some support for the idea that we live in a cyclic universe, so that the present phase of expansion will be followed by a period of contraction and a "Big Crunch", after which the whole sequence of events will be repeated. There could be "an infinite number of cycles of an infinite universe", but "no one has the remotest idea of how this could work". Not many authorities take this seriously nowadays, but Silk is not afraid to bring it into the story. And he adds:

"Who is to say that the universe is infinite? This is unprovable, and it may simply be very large."

The last two chapters of the book are different, since they deal with subjects such as time travel and extraterrestrial life. Obviously there must be a great deal of speculation. If other civilisations exist, why have they not contacted us? Perhaps they regard us as unworthy of consideration, but we can do little more than guess, because the rules of the game are unknown. The final section brings in the role of God in cosmology, which does seem to be a departure from the book's main theme. This is an outstanding work, suited to readers of all ages and all backgrounds, and is recommended without the slightest hesitation.

Chaisson, who directs the Wright Centre for Science Education at Tufts University in the US, also deals with the history of the universe, though - unlike Silk - he does not peer into the far future, and much of the book is devoted to the nature and evolution of life. He divides the story into seven epochs: particle, galactic, stellar, planetary, chemical, biological and cultural. His book is much longer than Silk's; he, too, explains difficult concepts as simply as possible, but inevitably there are some chapters, particularly those concerned with biology, that will make the reader concentrate hard. This is not intended to be any sort of criticism; one cannot simplify too much without "dumbing down", and Chaisson has skilfully avoided this trap.

He begins with a "cosmological overview", which leads him on to the particle epoch - the very start of the universe. The Big Bang happened almost 14 billion years ago, but we do not really understand it even with the help of our immensely powerful computers. "Most computer models suggest that in the beginning there was chaos. But, frankly, we are sometimes unsure if the chaos was in the universe or is now in our computer codes."

Chaisson has little patience with concepts such as parallel universe theories, which at present can be neither proved nor disproved - and he is by no means enthusiastic about dark energy, which many cosmologists consider to be of vital importance, but which Chaisson mentions only three times in his book. "Even if the mysterious 'dark energy' noted at the end of the Prologue does exist, its presence was not likely of much consequence in the early universe." This is a statement that will make some of his colleagues raise their eyebrows.

There is an admirable account of how the universe developed. Galaxies formed from the original material; quasars were much in evidence - and can they be the ancestors of all (or most of) the galaxies we know today? If so, "then maybe even our own Milky Way Galaxy was once a brilliant quasar".

Stars blazed out; there is a section on stellar evolution, and modern astronomers readily identify "red giant stars, planetary nebulae and white dwarf stars in the nearby cosmos". The subject matter covered is much the same as in Silk's book, but thereafter the two diverge. Chaisson pays much more attention to the planets of our solar system, and then comes what he calls the chemical epoch: "Let's first ask, what is life? And immediately we are stumped." Biologists have "failed to offer a clear, concise, standard definition of life". But at least we know how living things are made up and we know that life on Earth has been around for a long time - in fact, since our world was very young. Chaisson looks at the possibilities of life on other bodies, such as Europa and Titan. "If life forms exist, or have ever existed, on alien icy worlds, they are probably quite unlike those populating the sea ice on Earth today."

In the cultural epoch we come to consider the ancestors of modern humans; we have replaced the Cro-Magnons, just as they replaced the Neanderthals 300 centuries ago. At present we rule the world; how long we will continue to do so is debatable. Certainly the Earth cannot survive for ever, and when the Sun swells out to become a red giant, we will no longer have a home.

All this is fascinating, but I confess to a slight feeling of regret that Chaisson did not continue his account of evolution beyond the present day.

He would have written it so well.

There are a few controversial points - for example, not many people will give credence to the idea that Sirius changed colour from red to white in historic times (it is much too stable a star) - and a few quibbles (Alfred Wegener, of continental drift fame, was Austrian, not German as Chaisson claims). But if I were asked which book I would recommend, Silk's or Chaisson's, I would give a very prompt and firm reply: "Read both!"

Sir Patrick Moore is a fellow of the Royal Society and the author of more than 60 books, mainly on astronomy.