The Wraparound Universe by Jean-Pierre Luminet, A K Peters. Hardback ISBN 9781568813097 $39.
The first thing that impressed me about this book by Jean-Pierre Luminet is the way that the text is organized. Although the author seems to suggest that the reader uses the references as a way to jump between different chapters – rather like hyperlinks on the web – what I really appreciated is that the book has 45 short chapters, each just a few pages long. This allows the reader to taste and digest it bit by bit, following their own rhythm.
Luminet has done his best to explain everything in a simple but never simplistic way, so it may take different people a different amount of time, and this structure makes it easier to adapt to each reader’s needs. This could be one of the reasons why the book has had two editions in the original French as well as this English translation, and will probably be translated into other languages.
If you know a little of modern cosmology, you will read this book avidly from the first page to the last. I should admit that I felt ignorant at the beginning because I had never realized the importance of topology when discussing cosmic evolution (the fact that apparently I was not alone is not a good excuse). Consequently I’m grateful to Luminet for having made the role of topology in modern cosmology clear.
For example, I learned that the finite or infinite character of space depends both on its curvature and on its topology, though the latter is often neglected, even in refereed papers published by important journals. In addition, I realized that Einstein’s equations do not set constraints a priori on the universe’s topology. Rather, they can be solved for different boundary conditions, which include the specification of the 3D space topology.
Space may have positive, null or negative curvature – a property of the "metric" that encodes the machinery to measure distances. We say that the geometry is spherical, Euclidean or hyperbolic in these three cases. The metric is the subject of Einstein’s equations, which express it as a function of the total energy content of the universe through the energy-momentum tensor and the cosmological constant. On the other hand, the equations do not constrain the topology of the universe at all.
The simplest topology is "simply connected" (i.e. we can shrink all closed loops down to points without "exiting" from the space), and this is often implicitly assumed in books and articles about cosmology. However, "multiply connected" topologies are also possible (with any curvature), and if their "volume" is smaller than the visible universe, they may leave distinct signatures on the cosmic microwave background radiation, which could be experimentally detectable.
In general, a multiply connected universe would produce several images of each galaxy, and different topologies would produce different patterns, although searching for them is not an easy task. Luminet shows that the most recent data about the cosmic background radiation from the Wilkinson Microwave Anisotropy Probe are fitted better by "well-proportioned spaces", among which the best seems to be the "spherical dodecahedral Poincaré space", the volume of which is a 120th of the hypersphere (i.e. the simply connected topology) with the same curvature. This conclusion has been criticized by many researchers, but cannot be falsified with the present data. However, the Planck satellite should be able to provide measurements precise enough to discard this possibility if wrong and, possibly, identify the topology of our universe.
Lumin<i>et al</i>so considers the sociology of science. Comments about the impact of different ideas or even about the same ideas in different historical and geographical conditions are scattered throughout, but especially occur at the end. In conclusion, the book is well within the reach of the general public but still offers valuable insights to more expert people. It raises a number of questions and tries to provide a few answers in one of the most fascinating subjects of modern research.
Diego Casadei, New York University and CERN.
The Formation of the Solar System: Theories Old and New by Michael Woolfson, Imperial College Press. Hardback ISBN 9781860948244, £49 ($95).
Starting from ancient times, this book looks at various ideas about the solar system, and details how theories about planet formation evolved over time. The author spells out the constraints that each theory had to fit (and which observations were less crucial), along with the problems and benefits of each model. With observations continually improving, some theories were discarded, others were refined and revisited decades or centuries later, and new ones also emerged. The past 100 to 200 years especially have seen much more detailed observations and new insights into what bodies are to be found in our solar system and the different features that they exhibit.
The author looks in particular at the solar nebula theory, which has been developed extensively during the past decades and now offers mechanisms for many aspects of planet formation. However there are still theoretical difficulties and some new observations that do not fit with this model. The author has therefore developed a different model, the capture theory, which he first proposed in 1962 and has since developed. Although this model describes the detailed structure of the solar system and its formation, the author does not claim to have the found the final answer, only a plausible one.
The book is written for a lay audience: the author avoids formulas – only in the second half are there a few – and uses simple illustrations and two-dimensional models to explain the concepts that he is describing. He also tries to connect to the reader’s everyday experience. For example, he realises that zodiacal light cannot usually be seen in cities because of light pollution, and that the term "isotopes" usually appears in articles about nuclear reactors rather than astronomy. I also very much appreciated that he tried to put a face to the different theories by having many portraits of astronomers.
All in all, the book gives a good overview for both specialist and non-specialist readers. The non-specialists will need to invest some effort towards the end, where the author describes the current dominating theories in detail; and for those who want to go deeper into the field there are extensive references for each chapter.
Hannelore Hämmerle, MPG Munich.