Astroparticle Physics , Claus Grupen , Springer, New York, 2005. $59.95 (441 pp.). ISBN 3-540-25312-2
Astroparticle Physics by Claus Grupen attempts to provide a broad overview of particle astrophysics, from studies of cosmic rays to the early universe. A book with such a broad scope would challenge any author. It should present a unified picture of the subjects and a reason why each is included. Unfortunately Grupen does not meet the challenges. Instead, his Astroparticle Physics is a set of disconnected short articles introducing disparate topics, often with no clear progression of material.
The book’s lack of development results in repetition and little in-depth coverage. The deficiencies are most noticeable in the first half of the book, which covers high-energy gamma-ray and cosmic-ray astrophysics. The second portion, on cosmology, is substantially better, which left me wondering why the author dealt with many of the topics in the first half. One major complaint is the paucity of references, an especially problematic feature because the author includes controversial results that have proved to be incorrect, sometimes even before the book was written.
With the recent results from the high-energy stereoscopic system telescope (HESS) in Namibia, the field of high-energy gamma-ray astrophysics is now undergoing a revolution. Unfortunately, Grupen’s book was written before that development, and only minor updates were made during the English translation. Although not completely the author’s fault, the omission leaves the reader with a somewhat obsolete view of the field.
Remarkably, the discussion of high-energy gamma-ray astrophysics makes no mention of the Whipple telescope. The telescope’s imaging technique enabled researchers to witness the first convincing observation of a source of TeV gamma rays—the Crab Nebula. The results were published by Trevor C. Weekes’s group in 1989, and the discovery came after decades of struggle over false and contradictory results. The author does offer an abbreviated version of some of the more controversial claims from early work involving the observations of Cygnus X-3. But the discussion is so perfunctory that it does not do justice to the story. Similar problems arise in Grupen’s coverage of ultra-high-energy cosmic rays. Some suspicious results— “hints” of an excess from the supergalactic plane—are discussed without an appropriate level of skepticism. Why are results of questionable credibility included in a graduate textbook?
The book omits many topics, and some that it includes are of dubious value. One notable omission is any discussion of the absorption of TeV gamma rays through interactions with optical and infrared photons as those rays propagate through the universe. Although Grupen presents a detailed study on absorption of higher-energy gamma rays via interactions with the cosmic microwave background radiation, the figure demonstrating the effect is misleading because it shows the lower-energy photons as unattenuated. In fact this is an active area of research, and the absorption spectra from active galaxies have been used to set stringent limits on the density of infrared photons in intergalactic space.
The author states that no measurements of the cosmic-ray composition have been made above the energy reach of satellite-borne detectors, about 1015 eV. That statement completely ignores the important 1990 results from the High Resolution Fly’s Eye (HiRes) group in Utah, which measured a changing composition from heavy nuclei to light nuclei at energies near 1017.5 eV. Moreover, in chapter 7 there is a rather detailed discussion of radiation belts and the composition of the secondary cosmic-ray particles produced in the atmosphere. Although the information is of some interest, it bears little relation to the remainder of the book. The space could have been better used to further develop the theoretical aspects of high-energy astrophysics.
Beginning with the chapter discussing cosmology and the early universe, many of the problems mentioned above disappear. These later chapters present a unified view and a straightforward topical progression of the field. From the Big Bang to the development of the Friedmann equations, Grupen takes readers on a quick but relatively complete tour of the state of the field. His treatment of inflation is a good introduction for the nonexpert, as is his coverage of Big Bang nucleosynthesis and the classic derivation of the limit to the number of neutrino flavors— though the last is without reference or attribution to the pioneering work of Gary Steigman, David Schramm, and James Gunn. The author completes the overview in the penultimate chapter with a discussion of dark matter—and that is where the book should have ended. Unfortunately, an additional four-page chapter on astrobiology reminds readers of the problems in the first part of the book. Again, I wondered—Why?
The second half of Astroparticle Physics could be used as an introduction to cosmology in an undergraduate class. Exercises at the end of each chapter contain useful information and are an important part of the book. For an introduction to high-energy astrophysics, it is difficult to do better than Malcolm Longair’s High Energy Astrophysics (Cambridge U. Press, 1992); for a study of cosmic-ray physics, Thomas Gaisser’s Cosmic Rays and Particle Physics (Cambridge U. Press, 1990) is tough to beat. Grupen’s book does not improve upon those efforts.
Gus Sinnis, Los Alamos National Laboratory, Los Alamos, New Mexico
