Postcards depicting life at the wilder shores of the cosmos

Cosmology is an old preoccupation but a young science. Only in the middle of the past century did questions of the origin and structure of the whole Universe really become scientifically respectable. By its end, there was a reasonably well-attested story describing a visible Universe with an agreed starting point, about 14 billion years ago. Its evolution to its present state was accounted for by laws of physics that united understanding of the very large - the whole cosmos - and the very small - the micro-worlds of elementary particles.

But within, beneath and behind this apparently satisfying picture, uncertainty swirls. There may be unseen dimensions, hidden mass, hitherto undetected forces and perhaps myriad other universes. This is the scientific front line of Marcus Chown's subtitle and, as front lines go, it is pretty scary. The prolonged search for the elusive theory to unify the great incommensurables of general relativity and quantum mechanics and the emergence of a number of embarrassing anomalies in the cosmologists' world picture have prompted a great deal of speculation, much of it of the kind that science writer John Horgan dubbed "baroque science".

There is an emphasis on fun, far-out ideas, even if they are not obviously testable. This lends itself well to Chown's breezy brand of popular science, and the intellectual exuberance on display here is refreshing. His explanations of the weirdest ideas, which range over mathematics and complexity theory as well as cosmology, are admirably clear, although the book's episodic structure leads to some repetition. And it is easy to be beguiled by the creativity of the thoughts on offer. Whether explaining the subjective flow of time by considering a robot information processor or considering whether an all-powerful past intelligence might have inscribed the ultimate "we woz 'ere" in the cosmic microwave radiation that echoes from the Big Bang, there is much here to entertain anyone prepared to make a bit of effort to follow the often abstruse notions in play.

On the other hand, the raft of what he cheerfully refers to as bolt-on ideas - such as the existence of enormous quantities of otherwise undetectable so-called dark matter to account for the gravitational forces apparently at work in our universe - does make even mainstream cosmology seem a rather ramshackle affair. And the wilder shores of the subject throw up notions that, while diverting, stretch credulity even for the seasoned suspender of disbelief. This peaks at the end, where Chown summarises Frank Tipler's rather silly book The Physics of Immortality and its argument that it will please some future intelligence to recreate all past lives, but in a nicer version: a born-again physicist's vision of heaven.

But even before then, many of the scientists Chown selects seem stuck in a kind of metaphysical free-for-all, competing to come up with new ideas but in reality mostly revisiting earlier cosmologies from Lucretius to Leibniz. Monads and vortices still have their place in these speculations. So do endlessly recreated universes, multiverses, infinite universes that contain an unlimited number of perfect copies of the one we see, simulated universes, and a universe that functions as a cosmic computer. It is often hard to judge whether the notions being bandied about by these playful moderns are profound, or profoundly trivial, and the references to The Hitch-Hiker's Guide to the Galaxy , which are de rigueur in this kind of writing, do not help much in that regard.

Still, as in his previous books, Chown does an excellent job of conveying the flavour of the ideas without the benefit of mathematics. And cosmology's current fertility, even if it is largely restricted to dreaming up things that might be true, seems likely to keep him going for plenty more.

Jon Turney is a science writer and editor, and is writing The Rough Guide to the Future .