If you want to bake a cake you need flour and yeast. In Sanders' cosmic cake dark energy and dark matter are the two ingredients to which he directs his main attention. But a cake is more than flour and yeast. The same applies to Sanders' cosmic cake. Already in the introduction, he warns us that not all will be sweetness. Although he takes a basically positive attitude to the present cosmological mainstream ΛCDM (Lambda Cold Dark Matter) model, he warns against categorically excluding alternatives, such as modified Newtonian dynamics (MOND). But he also assures us that the term “deconstructing” is not meant to express a negative attitude towards the study of cosmology, but that it expresses the intention to take the standard paradigms apart, in order to reveal possible flaws, biases, or inconsistencies. Indeed, he pursues this aim in a very scientific and undogmatic way.
The book begins by reminding us that already the ancient creation myths reflect, just as today's cosmology, the human desire for an understanding of the origin and evolution of the Universe. Chapter 2 gives a brief summary of the early cosmology up to the 1990s. In chapter 3, he introduces inflation. In chapters 4 and 5 follow the description of the standard cold dark matter (CDM) cosmology and the concordance model. Dark energy and dark matter follow in chapters 6 and 7, at which point we are at page 95 of the total of 145. These first two-thirds give a comprehensive description of modern cosmology. He emphasises the importance of the modern high-resolution observations of the cosmic microwave background for the construction of the present standard ΛCDM model. Indeed, there is an enormous qualitative jump from the first observations of traces of the cosmic background radiation in 1965 and the highly sensitive ground and space observations since the 1990s.
Although Sanders agrees that by and large there is good agreement between observations and the ΛCDM model, he warns us that its success has led to a triumphalism that may be premature. Indeed, although there is strong observational evidence for dark matter and dark energy—basic ingredients of modern cosmology—we have hardly a clue about their nature. This is an exceedingly unsatisfactory situation. What is the remedy? Sanders devotes a special section to the “sociology of dark matter detection.” He points to the considerable investment in money and brainpower to experimentally detect dark matter; this involves a vast community of astronomers and high-energy physicists with vested career interests. Moreover, there is also a general hesitation amongst astronomers and physicists to tamper with historically established laws of physics, such as Newton's gravitation. Sanders wants to overcome that general reluctance. He pleads for including the search for an alternative to dark matter into the cosmological mainstream research, and this leads to the last third of the book: MOND.
It has been known for more than 30 years that the visible mass of spiral galaxies does not explain their rotation curves. It is natural to assume that the bulk of the mass in a galaxy lies in the same volume, where the bulk of the stars is seen. Newton's 1/r2 law of gravitation demands that the velocity of rotation around the galactic center should diminish outside that central bulk. But observations show flat rotation curves out to large distances. The solution to the puzzle was seen in dark matter. Dark matter had already been invoked by Fritz Zwicky in 1933 in connection with clusters of galaxies. The virial theorem gives a relation between the velocity distribution of the individual galaxies within the cluster and the cluster's total mass. The observed mass was much below the theoretically required amount. Zwicky postulated dark matter to explain the discrepancy: matter that did not radiate but manifested itself through its gravitation. Similarly, the flat rotation curves of galaxies could be explained if the galaxy was placed in a dark matter halo.
However, in 1983, Mordehai Milgrom suggested that the observed galactic rotation curves could be explained even better by a “Modification of the Newtonian Dynamics,” now known as MOND. Basically MOND says that Newton's law of gravitation, F = mGM/r2 = ma, should be modified to F = mGM/r2 = mμ(a/a0)a. As with Newtonian dynamics, M is the mass of the galaxy, m is the mass of the star at a distance r from the center of the galaxy and with acceleration a, and G is the gravitational constant. The function μ(x) is undetermined and the quantity a0 represents a fundamental constant that marks the transition between the Newtonian and the deep-MOND regimes. Of course, to be consistent with Newtonian dynamics, μ(x) must take the value 1 within the solar system. But at great distances the acceleration a decreases below the value a0 and μ(x) changes into a form for which the rotation velocity no longer depends on distance, but instead depends only on the mass of the galaxy. There are different versions of μ(x) that can fulfill this task.
Sanders himself has contributed to MOND, which he considers a serious challenge to dark matter and its role in cosmology. It is a great quality of his book that he discusses MOND in a detached way. He locates its successes and failures, and at the same time discusses the successes and failures of its competitor, the CDM (cold dark matter) hypothesis. This results in a well-balanced book about modern cosmology, including a non-fashionable alternative to CDM. Of course, a book of 145 pages cannot include all that present day cosmology is dealing with; it has to be fragmentary. But for those looking for more in-depth information, an extensive list of competent further literature is provided.
The author has a gift for presenting complex matters in a very lucid way, and the book makes for pleasant reading; he seldom has recourse to formulae. Here, someone is writing who knows what he is talking about, and knows how to present it to those who have a general notion about cosmology but would like to be better informed about today's main issues. Even if the book may not convince the majority of the astronomical community that MOND will be a necessary ingredient of cosmology, one hopes that it will at least keep them alert to the possibility that the ΛCDM model may not be the final answer either.
Harry Nussbaumer is professor emeritus at the ETH Zurich. His research was in atomic physics needed in astrophysical spectroscopy, as well as in modelling of symbiotic stars. He has also published on the history of astronomy, in particular, the much-noticed Discovering the Expanding Universe, where with co-author Lydia Bieri he traces from first sources the history of the discovery of the expanding universe.
