Observational Cosmology

Observational Cosmology , Stephen SerjeantCambridge U. Press, New York, 2010. $60.00 (324 pp.). ISBN 978-0-521-15715-5

It’s an exciting time to study our weird universe. In the past decade, rapid progress in observational cosmology has yielded rich and bizarre findings. For example, we now know that less than 5% of the universe’s energy density is in normal baryonic matter. The other main components—dark matter and dark energy—contain enough energy density to render the universe spatially flat and are mysterious beyond their basic properties. The field’s next decade will be dedicated to probing how dark matter and energy influence the expansion of the universe and control the assembly of structure.

Observational Cosmology provides upper-level undergraduate students a wide-ranging and compact introduction to the field. Its author, Stephen Serjeant, is a reader in cosmology at the Open University, a distance-learning institution based in the UK. He studies active and star-forming galaxies, often working with IR- and submillimeter-wavelength telescopes, like the Herschel Space Observatory, that observe the dust heated by newborn stars or active galactic nuclei in those systems. Serjeant also consults on the production of documentaries, such as Bang Goes the Theory, for the British Broadcasting Corporation.

Serjeant’s book is beautiful, filled with full-color photographs and cleanly rendered plots and diagrams. For a dynamic field, his treatment is contemporary and addresses a broad range of topics in informal, enthusiastic prose. His goal is to assemble the knowledge an ideal student would have when starting PhD research in cosmology; we would be fortunate indeed if students were so prepared. Regarding its title, the book focuses on astrophysical phenomena and their observable properties, rather than the mechanics of making observations; there’s no in-depth coverage of observational astronomy methods or, perhaps more surprisingly, of statistical techniques for interpreting data.

Serjeant’s research focus comes through in the text, which emphasizes extragalactic astrophysics, particularly star-forming galaxies, much more than other cosmology textbooks do. The treatments of the homogeneous universe, inflation, perturbations, and the cosmic microwave background are undergraduate friendly. Other main topics include supermassive black holes and their host galaxies, gravitational lensing, Lyman-α absorbers, and reionization. A student or even a sheepish professor who hears in a seminar of an unfamiliar phenomenon or cosmological test will likely find it here, briefly explained with useful tips to find out more.

Embedded in the text of each chapter of this lean book are a half dozen or so exercises that reinforce the narrative. Every exercise is fully solved in an appendix, so none remain for homework problems. Throughout, chapter introductions are brief, and sometimes concepts come and go so quickly that the point gets lost in the onrush. The reader sometimes has the feeling of running up a descending escalator—making progress, but working too hard and running out of breath.

Another consequence of the book’s brevity is a slight organizational weakness. Such a comprehensive survey must be ordered with care so that students are adequately prepared for the concepts they encounter. Serjeant often sends the reader backward and forward in the book for explanations of unfamiliar ideas. A single paragraph on the distribution of baryons and dark matter in galactic halos provides an extreme but not isolated example. It refers to related concepts—the distribution of Milky Way satellites, feedback from star formation, feedback from active galactic nuclei, cuspy dark-matter halo profiles—drawn from four different chapters, three of which the reader has yet to encounter.

Also, although Observational Cosmology addresses research at the forefront of our knowledge, where some phenomena are poorly understood, the student reader is left on unsteady ground when told the following: Here is an object; some theories say this, others conclude that, but this theory might be model-dependent. Such muddying of fact, hypothesis, and speculation can lead to the mistaken notion that cosmology is a collection of loosely related concepts about poorly understood objects rather than an integrated study of the large-scale universe. The chapters that best illustrate the integrated viewpoint are the ones organized by physical phenomena. Those are the strongest ones: They include material on spacetime, the Robertson–Walker metric, the microwave background, and inflation. Some of the weaker chapters, in which topics are too often presented one after another with little overarching theme, are organized by observational means. They include the ones on optical and multiwavelength views of the distant universe.

Observational Cosmology would be most useful as a classroom tool, especially in a course emphasizing the role of star formation in shaping our understanding of cosmology. Students familiar with introductory astronomical concepts will be most comfortable with this text. Physics majors without that background may need a primer on the properties of stars and other basic concepts in astrophysics.