Dark Energy: Theory and Observations; Dark Energy

Dark Energy: Theory and Observations, Luca Amendola and Shinji Tsujikawa Cambridge U. Press, New York, 2010. $75.00 (491 pp.). ISBN 978-0-521-51600-6

Dark Energy, Yun Wang Wiley-VCH, Weinheim, Germany, 2010. $79.00 (244 pp.). ISBN 978-3-527-40941-9

Two teams of astronomers studying distant type Ia supernovae presented evidence in 1998 that the expansion of the universe is speeding up. From the time of Edwin Hubble, cosmologists had been trying to measure the slowing of the expansion due to gravity, so the discovery of cosmic acceleration instantly became one of the field’s most important developments. Subsequent observations, including more detailed studies of supernovae, along with independent evidence from clusters of galaxies, large-scale structure, and the cosmic microwave background, have now firmly established this remarkable finding.

Unraveling the physical origin of cosmic acceleration will be an important goal for fundamental physics in the coming years. Is the acceleration caused by dark energy, a new form of stuff that dominates the mass–energy of the universe, or does it indicate that on cosmic scales, Einstein’s theory of general relativity must be replaced by a new theory of gravity? If the answer is dark energy, is it the energy of the vacuum—or, equivalently, Einstein’s cosmological constant—or something else, perhaps an ultralight scalar field sometimes called quintessence? Weirder still, could the observations be telling us that despite the near-isotropy of the cosmic microwave background, the universe is inhomogeneous on large scales and our Milky Way galaxy is located near the center of a very large void?

Although dark energy has been the subject of several excellent review articles in recent years, until now this young and rapidly maturing field has lacked a focused textbook. Two recent graduate-level texts by experienced practitioners go a substantial way toward filling that hole: One of those books is Dark Energy: Theory and Observations by Luca Amendola of the University of Heidelberg and the National Institute for Astrophysics in Rome and Shinji Tsujikawa of Tokyo University. The other is Dark Energy by Yun Wang of the University of Oklahoma.

Dark Energy: Theory and Observations is the more comprehensive of the two. It begins with chapters about the basics of physical cosmology, measures of large-scale structure, the evolution of cosmological perturbations, and the current observational evidence for cosmic acceleration. The bulk of the book focuses on theoretical models for acceleration, including the cosmological constant, quintessence and its variations, modified-gravity models, and void models. The final segment of the book covers the impact of dark energy on the evolution of cosmological perturbations, discusses statistical analysis methods for cosmological surveys, and gives an overview of techniques that will be used by upcoming surveys to probe dark energy.

One strength of Amendola and Tsujikawa’s book is the level of detail it provides on dark-energy models. The authors do a good job of describing the theoretical challenges to incorporating dark energy into the framework of elementary-particle physics. The discussion, though, is not always self-contained; for example, the authors assume a familiarity with supersymmetry and supergravity models. However, the book would have grown too unwieldy had the authors tried to present that supplementary material at the requisite level.

My main quibbles with Dark Energy: Theory and Observations are that the authors missed the opportunity to cover increasingly standard analysis techniques such as Markov Chain Monte Carlo and omitted discussing sources of systematic errors in the major techniques for probing dark energy. And a brief survey of the landscape of current and upcoming experimental projects in the field would have been helpful to provide context for students considering observational dark-energy research.

Many of those shortcomings are addressed in Wang’s Dark Energy. Although Wang also covers the basic cosmology background and includes a short chapter on models to explain cosmic acceleration, her book focuses much more on the major observational methods for probing dark energy—supernovae, large-scale structure, weak lensing, and clusters. As an example, Wang goes into substantially more detail on the nuances and systematics of estimating supernova distances. Her book will therefore be of more practical interest to those contemplating or involved in analysis of cosmological data. It also includes a chapter on instrumentation for dark-energy experiments and a brief discussion of ongoing and future projects. My main concerns with Wang’s book are that it is rather compact for a standalone text and that the choices of subtopics and references to the literature are occasionally idiosyncratic.

Our best tools for understanding the fundamental physics driving cosmic acceleration are measurements of the history of the expansion rate and of the growth of large-scale structure. If general relativity plus dark energy is the correct paradigm, there is a definite correlation between the expansion rate and the growth of structure. Moreover, if dark energy is vacuum energy, then the expansion rate and structure will each have a specific dependence on cosmic time that can be tested. Currently, the data are consistent with a universe comprising about 71% vacuum energy, 25% dark matter, and 4% ordinary matter, but we need more precise measurements to draw definitive conclusions about the nature of dark energy and the consistency of the paradigm of general relativity plus dark energy.

Ongoing, planned, and proposed experiments will employ several complementary techniques to make greatly improved measurements of expansion and structure growth and thereby help uncover the cause of cosmic acceleration. For those needing a useful introduction to this exciting area of research, these two textbooks, taken together, provide just that.